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On the Origin of Species, 6th Edition
by Charles Darwin
December, 1999 [Etext #2009]
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The Origin of Species by means of Natural Selection;
or, the
Preservation of Favoured Races in the Struggle for Life.
By Charles Darwin, M.A., F.R.S.,
Author of "The Descent of Man," etc., etc.
Sixth London Edition, with all Additions and Corrections.
The 6th Edition is often considered the definititive edition.
Also see Project Gutenberg Etext #1228 for an earlier edition.
"But with regard to the material world, we can at least go so far as this--
we can perceive that events are brought about not by insulated
interpositions of Divine power, exerted in each particular case, but by the
establishment of general laws."--Whewell: "Bridgewater Treatise".
"The only distinct meaning of the word 'natural' is STATED, FIXED or
SETTLED; since what is natural as much requires and presupposes an
intelligent agent to render it so, i.e., to effect it continually or at
stated times, as what is supernatural or miraculous does to effect it for
once."--Butler: "Analogy of Revealed Religion".
"To conclude, therefore, let no man out of a weak conceit of sobriety, or
an ill-applied moderation, think or maintain, that a man can search too far
or be too well studied in the book of God's word, or in the book of God's
works; divinity or philosophy; but rather let men endeavour an endless
progress or proficience in both."--Bacon: "Advancement of Learning".
AN HISTORICAL SKETCH
OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES,
PREVIOUSLY TO THE PUBLICATION OF THE FIRST EDITION OF THIS WORK.
I will here give a brief sketch of the progress of opinion on the Origin of
Species. Until recently the great majority of naturalists believed that
species were immutable productions, and had been separately created. This
view has been ably maintained by many authors. Some few naturalists, on
the other hand, have believed that species undergo modification, and that
the existing forms of life are the descendants by true generation of pre
existing forms. Passing over allusions to the subject in the classical
writers (Aristotle, in his "Physicae Auscultationes" (lib.2, cap.8, s.2),
after remarking that rain does not fall in order to make the corn grow, any
more than it falls to spoil the farmer's corn when threshed out of doors,
applies the same argument to organisation; and adds (as translated by Mr.
Clair Grece, who first pointed out the passage to me), "So what hinders the
different parts (of the body) from having this merely accidental relation
in nature? as the teeth, for example, grow by necessity, the front ones
sharp, adapted for dividing, and the grinders flat, and serviceable for
masticating the food; since they were not made for the sake of this, but it
was the result of accident. And in like manner as to other parts in which
there appears to exist an adaptation to an end. Wheresoever, therefore,
all things together (that is all the parts of one whole) happened like as
if they were made for the sake of something, these were preserved, having
been appropriately constituted by an internal spontaneity; and whatsoever
things were not thus constituted, perished and still perish." We here see
the principle of natural selection shadowed forth, but how little Aristotle
fully comprehended the principle, is shown by his remarks on the formation
of the teeth.), the first author who in modern times has treated it in a
scientific spirit was Buffon. But as his opinions fluctuated greatly at
different periods, and as he does not enter on the causes or means of the
transformation of species, I need not here enter on details.
Lamarck was the first man whose conclusions on the subject excited much
attention. This justly celebrated naturalist first published his views in
1801; he much enlarged them in 1809 in his "Philosophie Zoologique", and
subsequently, 1815, in the Introduction to his "Hist. Nat. des Animaux sans
Vertebres". In these works he up holds the doctrine that all species,
including man, are descended from other species. He first did the eminent
service of arousing attention to the probability of all change in the
organic, as well as in the inorganic world, being the result of law, and
not of miraculous interposition. Lamarck seems to have been chiefly led to
his conclusion on the gradual change of species, by the difficulty of
distinguishing species and varieties, by the almost perfect gradation of
forms in certain groups, and by the analogy of domestic productions. With
respect to the means of modification, he attributed something to the direct
action of the physical conditions of life, something to the crossing of
already existing forms, and much to use and disuse, that is, to the effects
of habit. To this latter agency he seems to attribute all the beautiful
adaptations in nature; such as the long neck of the giraffe for browsing on
the branches of trees. But he likewise believed in a law of progressive
development, and as all the forms of life thus tend to progress, in order
to account for the existence at the present day of simple productions, he
maintains that such forms are now spontaneously generated. (I have taken
the date of the first publication of Lamarck from Isidore Geoffroy Saint-
Hilaire's ("Hist. Nat. Generale", tom. ii. page 405, 1859) excellent
history of opinion on this subject. In this work a full account is given
of Buffon's conclusions on the same subject. It is curious how largely my
grandfather, Dr. Erasmus Darwin, anticipated the views and erroneous
grounds of opinion of Lamarck in his "Zoonomia" (vol. i. pages 500-510),
published in 1794. According to Isid. Geoffroy there is no doubt that
Goethe was an extreme partisan of similar views, as shown in the
introduction to a work written in 1794 and 1795, but not published till
long afterward; he has pointedly remarked ("Goethe als Naturforscher", von
Dr. Karl Meding, s. 34) that the future question for naturalists will be
how, for instance, cattle got their horns and not for what they are used.
It is rather a singular instance of the manner in which similar views arise
at about the same time, that Goethe in Germany, Dr. Darwin in England, and
Geoffroy Saint-Hilaire (as we shall immediately see) in France, came to the
same conclusion on the origin of species, in the years 1794-5.)
Geoffroy Saint-Hilaire, as is stated in his "Life", written by his son,
suspected, as early as 1795, that what we call species are various
degenerations of the same type. It was not until 1828 that he published
his conviction that the same forms have not been perpetuated since the
origin of all things. Geoffroy seems to have relied chiefly on the
conditions of life, or the "monde ambiant" as the cause of change. He was
cautious in drawing conclusions, and did not believe that existing species
are now undergoing modification; and, as his son adds, "C'est donc un
probleme a reserver entierement a l'avenir, suppose meme que l'avenir doive
avoir prise sur lui."
In 1813 Dr. W.C. Wells read before the Royal Society "An Account of a White
Female, part of whose skin resembles that of a Negro"; but his paper was
not published until his famous "Two Essays upon Dew and Single Vision"
appeared in 1818. In this paper he distinctly recognises the principle of
natural selection, and this is the first recognition which has been
indicated; but he applies it only to the races of man, and to certain
characters alone. After remarking that negroes and mulattoes enjoy an
immunity from certain tropical diseases, he observes, firstly, that all
animals tend to vary in some degree, and, secondly, that agriculturists
improve their domesticated animals by selection; and then, he adds, but
what is done in this latter case "by art, seems to be done with equal
efficacy, though more slowly, by nature, in the formation of varieties of
mankind, fitted for the country which they inhabit. Of the accidental
varieties of man, which would occur among the first few and scattered
inhabitants of the middle regions of Africa, some one would be better
fitted than others to bear the diseases of the country. This race would
consequently multiply, while the others would decrease; not only from their
in ability to sustain the attacks of disease, but from their incapacity of
contending with their more vigorous neighbours. The colour of this
vigorous race I take for granted, from what has been already said, would be
dark. But the same disposition to form varieties still existing, a darker
and a darker race would in the course of time occur: and as the darkest
would be the best fitted for the climate, this would at length become the
most prevalent, if not the only race, in the particular country in which it
had originated." He then extends these same views to the white inhabitants
of colder climates. I am indebted to Mr. Rowley, of the United States, for
having called my attention, through Mr. Brace, to the above passage of Dr.
Wells' work.
The Hon. and Rev. W. Herbert, afterward Dean of Manchester, in the fourth
volume of the "Horticultural Transactions", 1822, and in his work on the
"Amaryllidaceae" (1837, pages 19, 339), declares that "horticultural
experiments have established, beyond the possibility of refutation, that
botanical species are only a higher and more permanent class of varieties."
He extends the same view to animals. The dean believes that single species
of each genus were created in an originally highly plastic condition, and
that these have produced, chiefly by inter-crossing, but likewise by
variation, all our existing species.
In 1826 Professor Grant, in the concluding paragraph in his well-known
paper ("Edinburgh Philosophical Journal", vol. XIV, page 283) on the
Spongilla, clearly declares his belief that species are descended from
other species, and that they become improved in the course of modification.
This same view was given in his Fifty-fifth Lecture, published in the
"Lancet" in 1834.
In 1831 Mr. Patrick Matthew published his work on "Naval Timber and
Arboriculture", in which he gives precisely the same view on the origin of
species as that (presently to be alluded to) propounded by Mr. Wallace and
myself in the "Linnean Journal", and as that enlarged in the present
volume. Unfortunately the view was given by Mr. Matthew very briefly in
scattered passages in an appendix to a work on a different subject, so that
it remained unnoticed until Mr. Matthew himself drew attention to it in the
"Gardeners' Chronicle", on April 7, 1860. The differences of Mr. Matthew's
views from mine are not of much importance: he seems to consider that the
world was nearly depopulated at successive periods, and then restocked; and
he gives as an alternative, that new forms may be generated "without the
presence of any mold or germ of former aggregates." I am not sure that I
understand some passages; but it seems that he attributes much influence to
the direct action of the conditions of life. He clearly saw, however, the
full force of the principle of natural selection.
The celebrated geologist and naturalist, Von Buch, in his excellent
"Description Physique des Isles Canaries" (1836, page 147), clearly
expresses his belief that varieties slowly become changed into permanent
species, which are no longer capable of intercrossing.
Rafinesque, in his "New Flora of North America", published in 1836, wrote
(page 6) as follows: "All species might have been varieties once, and many
varieties are gradually becoming species by assuming constant and peculiar
characters;" but further on (page 18) he adds, "except the original types
or ancestors of the genus."
In 1843-44 Professor Haldeman ("Boston Journal of Nat. Hist. U. States",
vol. iv, page 468) has ably given the arguments for and against the
hypothesis of the development and modification of species: he seems to
lean toward the side of change.
The "Vestiges of Creation" appeared in 1844. In the tenth and much
improved edition (1853) the anonymous author says (page 155): "The
proposition determined on after much consideration is, that the several
series of animated beings, from the simplest and oldest up to the highest
and most recent, are, under the providence of God, the results, FIRST, of
an impulse which has been imparted to the forms of life, advancing them, in
definite times, by generation, through grades of organisation terminating
in the highest dicotyledons and vertebrata, these grades being few in
number, and generally marked by intervals of organic character, which we
find to be a practical difficulty in ascertaining affinities; SECOND, of
another impulse connected with the vital forces, tending, in the course of
generations, to modify organic structures in accordance with external
circumstances, as food, the nature of the habitat, and the meteoric
agencies, these being the 'adaptations' of the natural theologian." The
author apparently believes that organisation progresses by sudden leaps,
but that the effects produced by the conditions of life are gradual. He
argues with much force on general grounds that species are not immutable
productions. But I cannot see how the two supposed "impulses" account in a
scientific sense for the numerous and beautiful coadaptations which we see
throughout nature; I cannot see that we thus gain any insight how, for
instance, a woodpecker has become adapted to its peculiar habits of life.
The work, from its powerful and brilliant style, though displaying in the
early editions little accurate knowledge and a great want of scientific
caution, immediately had a very wide circulation. In my opinion it has
done excellent service in this country in calling attention to the subject,
in removing prejudice, and in thus preparing the ground for the reception
of analogous views.
In 1846 the veteran geologist M.J. d'Omalius d'Halloy published in an
excellent though short paper ("Bulletins de l'Acad. Roy. Bruxelles", tom.
xiii, page 581) his opinion that it is more probable that new species have
been produced by descent with modification than that they have been
separately created: the author first promulgated this opinion in 1831.
Professor Owen, in 1849 ("Nature of Limbs", page 86), wrote as follows:
"The archetypal idea was manifested in the flesh under diverse such
modifications, upon this planet, long prior to the existence of those
animal species that actually exemplify it. To what natural laws or
secondary causes the orderly succession and progression of such organic
phenomena may have been committed, we, as yet, are ignorant." In his
address to the British Association, in 1858, he speaks (page li) of "the
axiom of the continuous operation of creative power, or of the ordained
becoming of living things." Further on (page xc), after referring to
geographical distribution, he adds, "These phenomena shake our confidence
in the conclusion that the Apteryx of New Zealand and the Red Grouse of
England were distinct creations in and for those islands respectively.
Always, also, it may be well to bear in mind that by the word 'creation'
the zoologist means 'a process he knows not what.'" He amplifies this idea
by adding that when such cases as that of the Red Grouse are "enumerated by
the zoologist as evidence of distinct creation of the bird in and for such
islands, he chiefly expresses that he knows not how the Red Grouse came to
be there, and there exclusively; signifying also, by this mode of
expressing such ignorance, his belief that both the bird and the islands
owed their origin to a great first Creative Cause." If we interpret these
sentences given in the same address, one by the other, it appears that this
eminent philosopher felt in 1858 his confidence shaken that the Apteryx and
the Red Grouse first appeared in their respective homes "he knew not how,"
or by some process "he knew not what."
This address was delivered after the papers by Mr. Wallace and myself on
the Origin of Species, presently to be referred to, had been read before
the Linnean Society. When the first edition of this work was published, I
was so completely deceived, as were many others, by such expressions as
"the continuous operation of creative power," that I included Professor
Owen with other palaeontologists as being firmly convinced of the
immutability of species; but it appears ("Anat. of Vertebrates", vol. iii,
page 796) that this was on my part a preposterous error. In the last
edition of this work I inferred, and the inference still seems to me
perfectly just, from a passage beginning with the words "no doubt the type-
form," etc.(Ibid., vol. i, page xxxv), that Professor Owen admitted that
natural selection may have done something in the formation of a new
species; but this it appears (Ibid., vol. iii. page 798) is inaccurate and
without evidence. I also gave some extracts from a correspondence between
Professor Owen and the editor of the "London Review", from which it
appeared manifest to the editor as well as to myself, that Professor Owen
claimed to have promulgated the theory of natural selection before I had
done so; and I expressed my surprise and satisfaction at this announcement;
but as far as it is possible to understand certain recently published
passages (Ibid., vol. iii. page 798) I have either partially or wholly
again fallen into error. It is consolatory to me that others find
Professor Owen's controversial writings as difficult to understand and to
reconcile with each other, as I do. As far as the mere enunciation of the
principle of natural selection is concerned, it is quite immaterial whether
or not Professor Owen preceded me, for both of us, as shown in this
historical sketch, were long ago preceded by Dr. Wells and Mr. Matthews.
M. Isidore Geoffroy Saint-Hilaire, in his lectures delivered in 1850 (of
which a Resume appeared in the "Revue et Mag. de Zoolog.", Jan., 1851),
briefly gives his reason for believing that specific characters "sont
fixes, pour chaque espece, tant qu'elle se perpetue au milieu des memes
circonstances: ils se modifient, si les circonstances ambiantes viennent a
changer. En resume, L'OBSERVATION des animaux sauvages demontre deja la
variabilite LIMITEE des especes. Les EXPERIENCES sur les animaux sauvages
devenus domestiques, et sur les animaux domestiques redevenus sauvages, la
demontrent plus clairment encore. Ces memes experiences prouvent, de plus,
que les differences produites peuvent etre de VALEUR GENERIQUE." In his
"Hist. Nat. Generale" (tom. ii, page 430, 1859) he amplifies analogous
conclusions.
>From a circular lately issued it appears that Dr. Freke, in 1851 ("Dublin
Medical Press", page 322), propounded the doctrine that all organic beings
have descended from one primordial form. His grounds of belief and
treatment of the subject are wholly different from mine; but as Dr. Freke
has now (1861) published his Essay on the "Origin of Species by means of
Organic Affinity", the difficult attempt to give any idea of his views
would be superfluous on my part.
Mr. Herbert Spencer, in an Essay (originally published in the "Leader",
March, 1852, and republished in his "Essays", in 1858), has contrasted the
theories of the Creation and the Development of organic beings with
remarkable skill and force. He argues from the analogy of domestic
productions, from the changes which the embryos of many species undergo,
from the difficulty of distinguishing species and varieties, and from the
principle of general gradation, that species have been modified; and he
attributes the modification to the change of circumstances. The author
(1855) has also treated Psychology on the principle of the necessary
acquirement of each mental power and capacity by gradation.
In 1852 M. Naudin, a distinguished botanist, expressly stated, in an
admirable paper on the Origin of Species ("Revue Horticole", page 102;
since partly republished in the "Nouvelles Archives du Museum", tom. i,
page 171), his belief that species are formed in an analogous manner as
varieties are under cultivation; and the latter process he attributes to
man's power of selection. But he does not show how selection acts under
nature. He believes, like Dean Herbert, that species, when nascent, were
more plastic than at present. He lays weight on what he calls the
principle of finality, "puissance mysterieuse, indeterminee; fatalite pour
les uns; pour les autres volonte providentielle, dont l'action incessante
sur les etres vivantes determine, a toutes les epoques de l'existence du
monde, la forme, le volume, et la duree de chacun d'eux, en raison de sa
destinee dans l'ordre de choses dont il fait partie. C'est cette puissance
qui harmonise chaque membre a l'ensemble, en l'appropriant a la fonction
qu'il doit remplir dans l'organisme general de la nature, fonction qui est
pour lui sa raison d'etre." (From references in Bronn's "Untersuchungen
uber die Entwickelungs-Gesetze", it appears that the celebrated botanist
and palaeontologist Unger published, in 1852, his belief that species
undergo development and modification. Dalton, likewise, in Pander and
Dalton's work on Fossil Sloths, expressed, in 1821, a similar belief.
Similar views have, as is well known, been maintained by Oken in his
mystical "Natur-Philosophie". From other references in Godron's work "Sur
l'Espece", it seems that Bory St. Vincent, Burdach, Poiret and Fries, have
all admitted that new species are continually being produced. I may add,
that of the thirty-four authors named in this Historical Sketch, who
believe in the modification of species, or at least disbelieve in separate
acts of creation, twenty-seven have written on special branches of natural
history or geology.)
In 1853 a celebrated geologist, Count Keyserling ("Bulletin de la Soc.
Geolog.", 2nd Ser., tom. x, page 357), suggested that as new diseases,
supposed to have been caused by some miasma have arisen and spread over the
world, so at certain periods the germs of existing species may have been
chemically affected by circumambient molecules of a particular nature, and
thus have given rise to new forms.
In this same year, 1853, Dr. Schaaffhausen published an excellent pamphlet
("Verhand. des Naturhist. Vereins der Preuss. Rheinlands", etc.), in which
he maintains the development of organic forms on the earth. He infers that
many species have kept true for long periods, whereas a few have become
modified. The distinction of species he explains by the destruction of
intermediate graduated forms. "Thus living plants and animals are not
separated from the extinct by new creations, but are to be regarded as
their descendants through continued reproduction."
A well-known French botanist, M. Lecoq, writes in 1854 ("Etudes sur
Geograph. Bot. tom. i, page 250), "On voit que nos recherches sur la fixite
ou la variation de l'espece, nous conduisent directement aux idees emises
par deux hommes justement celebres, Geoffroy Saint-Hilaire et Goethe."
Some other passages scattered through M. Lecoq's large work make it a
little doubtful how far he extends his views on the modification of
species.
The "Philosophy of Creation" has been treated in a masterly manner by the
Rev. Baden Powell, in his "Essays on the Unity of Worlds", 1855. Nothing
can be more striking than the manner in which he shows that the
introduction of new species is "a regular, not a casual phenomenon," or, as
Sir John Herschel expresses it, "a natural in contradistinction to a
miraculous process."
The third volume of the "Journal of the Linnean Society" contains papers,
read July 1, 1858, by Mr. Wallace and myself, in which, as stated in the
introductory remarks to this volume, the theory of Natural Selection is
promulgated by Mr. Wallace with admirable force and clearness.
Von Baer, toward whom all zoologists feel so profound a respect, expressed
about the year 1859 (see Prof. Rudolph Wagner, "Zoologisch-Anthropologische
Untersuchungen", 1861, s. 51) his conviction, chiefly grounded on the laws
of geographical distribution, that forms now perfectly distinct have
descended from a single parent-form.
In June, 1859, Professor Huxley gave a lecture before the Royal Institution
on the "Persistent Types of Animal Life". Referring to such cases, he
remarks, "It is difficult to comprehend the meaning of such facts as these,
if we suppose that each species of animal and plant, or each great type of
organisation, was formed and placed upon the surface of the globe at long
intervals by a distinct act of creative power; and it is well to recollect
that such an assumption is as unsupported by tradition or revelation as it
is opposed to the general analogy of nature. If, on the other hand, we
view "Persistent Types" in relation to that hypothesis which supposes the
species living at any time to be the result of the gradual modification of
pre-existing species, a hypothesis which, though unproven, and sadly
damaged by some of its supporters, is yet the only one to which physiology
lends any countenance; their existence would seem to show that the amount
of modification which living beings have undergone during geological time
is but very small in relation to the whole series of changes which they
have suffered."
In December, 1859, Dr. Hooker published his "Introduction to the Australian
Flora". In the first part of this great work he admits the truth of the
descent and modification of species, and supports this doctrine by many
original observations.
The first edition of this work was published on November 24, 1859, and the
second edition on January 7, 1860.
CONTENTS.
INTRODUCTION
CHAPTER I.
VARIATION UNDER DOMESTICATION.
Causes of Variability -- Effects of Habit and the use or disuse of Parts --
Correlated Variation -- Inheritance -- Character of Domestic Varieties --
Difficulty of distinguishing between Varieties and Species -- Origin of
Domestic Varieties from one or more Species -- Domestic Pigeons, their
Differences and Origin -- Principles of Selection, anciently followed,
their Effects -- Methodical and Unconscious Selection -- Unknown Origin of
our Domestic Productions -- Circumstances favourable to Man's power of
Selection.
CHAPTER II.
VARIATION UNDER NATURE.
Variability -- Individual Differences -- Doubtful species -- Wide ranging,
much diffused, and common species, vary most -- Species of the larger
genera in each country vary more frequently than the species of the smaller
genera -- Many of the species of the larger genera resemble varieties in
being very closely, but unequally, related to each other, and in having
restricted ranges.
CHAPTER III.
STRUGGLE FOR EXISTENCE.
Its bearing on natural selection -- The term used in a wide sense --
Geometrical ratio of increase -- Rapid increase of naturalised animals and
plants -- Nature of the checks to increase -- Competition universal --
Effects of climate -- Protection from the number of individuals -- Complex
relations of all animals and plants throughout nature -- Struggle for life
most severe between individuals and varieties of the same species; often
severe between species of the same genus -- The relation of organism to
organism the most important of all relations.
CHAPTER IV.
NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
Natural Selection -- its power compared with man's selection -- its power
on characters of trifling importance -- its power at all ages and on both
sexes -- Sexual Selection -- On the generality of intercrosses between
individuals of the same species -- Circumstances favourable and
unfavourable to the results of Natural Selection, namely, intercrossing,
isolation, number of individuals -- Slow action -- Extinction caused by
Natural Selection -- Divergence of Character, related to the diversity of
inhabitants of any small area and to naturalisation -- Action of Natural
Selection, through Divergence of Character and Extinction, on the
descendants from a common parent -- Explains the Grouping of all organic
beings -- Advance in organisation -- Low forms preserved -- Convergence of
character -- Indefinite multiplication of species -- Summary.
CHAPTER V.
LAWS OF VARIATION.
Effects of changed conditions -- Use and disuse, combined with natural
selection; organs of flight and of vision -- Acclimatisation -- Correlated
variation -- Compensation and economy of growth -- False correlations --
Multiple, rudimentary, and lowly organised structures variable -- Parts
developed in an unusual manner are highly variable; specific characters
more variable than generic; secondary sexual characters variable -- Species
of the same genus vary in an analogous manner -- Reversions to long-lost
characters -- Summary.
CHAPTER VI.
DIFFICULTIES OF THE THEORY.
Difficulties of the theory of descent with modification -- Absence or
rarity of transitional varieties -- Transitions in habits of life --
Diversified habits in the same species -- Species with habits widely
different from those of their allies -- Organs of extreme perfection --
Modes of transition -- Cases of difficulty -- Natura non facit saltum --
Organs of small importance -- Organs not in all cases absolutely perfect --
The law of Unity of Type and of the Conditions of Existence embraced by the
theory of Natural Selection.
CHAPTER VII.
MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.
Longevity -- Modifications not necessarily simultaneous -- Modifications
apparently of no direct service -- Progressive development -- Characters of
small functional importance, the most constant -- Supposed incompetence of
natural selection to account for the incipient stages of useful structures
-- Causes which interfere with the acquisition through natural selection of
useful structures -- Gradations of structure with changed functions --
Widely different organs in members of the same class, developed from one
and the same source -- Reasons for disbelieving in great and abrupt
modifications.
CHAPTER VIII.
INSTINCT.
Instincts comparable with habits, but different in their origin --
Instincts graduated -- Aphides and ants -- Instincts variable -- Domestic
instincts, their origin -- Natural instincts of the cuckoo, molothrus,
ostrich, and parasitic bees -- Slave-making ants -- Hive-bee, its cell-
making instinct -- Changes of instinct and structure not necessarily
simultaneous -- Difficulties on the theory of the Natural Selection of
instincts -- Neuter or sterile insects -- Summary.
CHAPTER IX.
HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids --
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication -- Laws governing the sterility of
hybrids -- Sterility not a special endowment, but incidental on other
differences, not accumulated by natural selection -- Causes of the
sterility of first crosses and of hybrids -- Parallelism between the
effects of changed conditions of life and of crossing -- Dimorphism and
Trimorphism -- Fertility of varieties when crossed and of their mongrel
offspring not universal -- Hybrids and mongrels compared independently of
their fertility -- Summary.
CHAPTER X.
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day -- On the
nature of extinct intermediate varieties; on their number -- On the lapse
of time, as inferred from the rate of denudation and of deposition -- On
the lapse of time as estimated in years -- On the poorness of our
palaeontological collections -- On the intermittence of geological
formations -- On the denudation of granitic areas -- On the absence of
intermediate varieties in any one formation -- On the sudden appearance of
groups of species -- On their sudden appearance in the lowest known
fossiliferous strata -- Antiquity of the habitable earth.
CHAPTER XI.
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
On the slow and successive appearance of new species -- On their different
rates of change -- Species once lost do not reappear -- Groups of species
follow the same general rules in their appearance and disappearance as do
single species -- On extinction -- On simultaneous changes in the forms of
life throughout the world -- On the affinities of extinct species to each
other and to living species -- On the state of development of ancient forms
-- On the succession of the same types within the same areas -- Summary of
preceding and present chapter.
CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in physical
conditions -- Importance of barriers -- Affinity of the productions of the
same continent -- Centres of creation -- Means of dispersal by changes of
climate and of the level of the land, and by occasional means -- Dispersal
during the Glacial period -- Alternate Glacial periods in the north and
south.
CHAPTER XIII.
GEOGRAPHICAL DISTRIBUTION -- CONTINUED.
Distribution of fresh-water productions -- On the inhabitants of oceanic
islands -- Absence of Batrachians and of terrestrial Mammals -- On the
relation of the inhabitants of islands to those of the nearest mainland --
On colonisation from the nearest source with subsequent modification --
Summary of the last and present chapter.
CHAPTER XIV.
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY --
RUDIMENTARY ORGANS.
Classification, groups subordinate to groups -- Natural system -- Rules and
difficulties in classification, explained on the theory of descent with
modification -- Classification of varieties -- Descent always used in
classification -- Analogical or adaptive characters -- Affinities, general,
complex and radiating -- Extinction separates and defines groups --
Morphology, between members of the same class, between parts of the same
individual -- Embryology, laws of, explained by variations not supervening
at an early age, and being inherited at a corresponding age -- Rudimentary
Organs; their origin explained -- Summary.
CHAPTER XV.
RECAPITULATION AND CONCLUSION.
Recapitulation of the objections to the theory of Natural Selection --
Recapitulation of the general and special circumstances in its favour --
Causes of the general belief in the immutability of species -- How far the
theory of Natural Selection may be extended -- Effects of its adoption on
the study of Natural history -- Concluding remarks.
GLOSSARY OF SCIENTIFIC TERMS.
INDEX.
ORIGIN OF SPECIES.
INTRODUCTION.
When on board H.M.S. Beagle, as naturalist, I was much struck with certain
facts in the distribution of the organic beings inhabiting South America,
and in the geological relations of the present to the past inhabitants of
that continent. These facts, as will be seen in the latter chapters of
this volume, seemed to throw some light on the origin of species--that
mystery of mysteries, as it has been called by one of our greatest
philosophers. On my return home, it occurred to me, in 1837, that
something might perhaps be made out on this question by patiently
accumulating and reflecting on all sorts of facts which could possibly have
any bearing on it. After five years' work I allowed myself to speculate on
the subject, and drew up some short notes; these I enlarged in 1844 into a
sketch of the conclusions, which then seemed to me probable: from that
period to the present day I have steadily pursued the same object. I hope
that I may be excused for entering on these personal details, as I give
them to show that I have not been hasty in coming to a decision.
My work is now (1859) nearly finished; but as it will take me many more
years to complete it, and as my health is far from strong, I have been
urged to publish this abstract. I have more especially been induced to do
this, as Mr. Wallace, who is now studying the natural history of the Malay
Archipelago, has arrived at almost exactly the same general conclusions
that I have on the origin of species. In 1858 he sent me a memoir on this
subject, with a request that I would forward it to Sir Charles Lyell, who
sent it to the Linnean Society, and it is published in the third volume of
the Journal of that Society. Sir C. Lyell and Dr. Hooker, who both knew of
my work--the latter having read my sketch of 1844--honoured me by thinking
it advisable to publish, with Mr. Wallace's excellent memoir, some brief
extracts from my manuscripts.
This abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements; and
I must trust to the reader reposing some confidence in my accuracy. No
doubt errors may have crept in, though I hope I have always been cautious
in trusting to good authorities alone. I can here give only the general
conclusions at which I have arrived, with a few facts in illustration, but
which, I hope, in most cases will suffice. No one can feel more sensible
than I do of the necessity of hereafter publishing in detail all the facts,
with references, on which my conclusions have been grounded; and I hope in
a future work to do this. For I am well aware that scarcely a single point
is discussed in this volume on which facts cannot be adduced, often
apparently leading to conclusions directly opposite to those at which I
have arrived. A fair result can be obtained only by fully stating and
balancing the facts and arguments on both sides of each question; and this
is here impossible.
I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very many
naturalists, some of them personally unknown to me. I cannot, however, let
this opportunity pass without expressing my deep obligations to Dr. Hooker,
who, for the last fifteen years, has aided me in every possible way by his
large stores of knowledge and his excellent judgment.
In considering the origin of species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings, on their
embryological relations, their geographical distribution, geological
succession, and other such facts, might come to the conclusion that species
had not been independently created, but had descended, like varieties, from
other species. Nevertheless, such a conclusion, even if well founded,
would be unsatisfactory, until it could be shown how the innumerable
species, inhabiting this world have been modified, so as to acquire that
perfection of structure and coadaptation which justly excites our
admiration. Naturalists continually refer to external conditions, such as
climate, food, etc., as the only possible cause of variation. In one
limited sense, as we shall hereafter see, this may be true; but it is
preposterous to attribute to mere external conditions, the structure, for
instance, of the woodpecker, with its feet, tail, beak, and tongue, so
admirably adapted to catch insects under the bark of trees. In the case of
the mistletoe, which draws its nourishment from certain trees, which has
seeds that must be transported by certain birds, and which has flowers with
separate sexes absolutely requiring the agency of certain insects to bring
pollen from one flower to the other, it is equally preposterous to account
for the structure of this parasite, with its relations to several distinct
organic beings, by the effects of external conditions, or of habit, or of
the volition of the plant itself.
It is, therefore, of the highest importance to gain a clear insight into
the means of modification and coadaptation. At the commencement of my
observations it seemed to me probable that a careful study of domesticated
animals and of cultivated plants would offer the best chance of making out
this obscure problem. Nor have I been disappointed; in this and in all
other perplexing cases I have invariably found that our knowledge,
imperfect though it be, of variation under domestication, afforded the best
and safest clue. I may venture to express my conviction of the high value
of such studies, although they have been very commonly neglected by
naturalists.
>From these considerations, I shall devote the first chapter of this
abstract to variation under domestication. We shall thus see that a large
amount of hereditary modification is at least possible; and, what is
equally or more important, we shall see how great is the power of man in
accumulating by his selection successive slight variations. I will then
pass on to the variability of species in a state of nature; but I shall,
unfortunately, be compelled to treat this subject far too briefly, as it
can be treated properly only by giving long catalogues of facts. We shall,
however, be enabled to discuss what circumstances are most favourable to
variation. In the next chapter the struggle for existence among all
organic beings throughout the world, which inevitably follows from the high
geometrical ratio of their increase, will be considered. This is the
doctrine of Malthus, applied to the whole animal and vegetable kingdoms.
As many more individuals of each species are born than can possibly
survive; and as, consequently, there is a frequently recurring struggle for
existence, it follows that any being, if it vary however slightly in any
manner profitable to itself, under the complex and sometimes varying
conditions of life, will have a better chance of surviving, and thus be
NATURALLY SELECTED. From the strong principle of inheritance, any selected
variety will tend to propagate its new and modified form.
This fundamental subject of natural selection will be treated at some
length in the fourth chapter; and we shall then see how natural selection
almost inevitably causes much extinction of the less improved forms of
life, and leads to what I have called divergence of character. In the next
chapter I shall discuss the complex and little known laws of variation. In
the five succeeding chapters, the most apparent and gravest difficulties in
accepting the theory will be given: namely, first, the difficulties of
transitions, or how a simple being or a simple organ can be changed and
perfected into a highly developed being or into an elaborately constructed
organ; secondly the subject of instinct, or the mental powers of animals;
thirdly, hybridism, or the infertility of species and the fertility of
varieties when intercrossed; and fourthly, the imperfection of the
geological record. In the next chapter I shall consider the geological
succession of organic beings throughout time; in the twelfth and
thirteenth, their geographical distribution throughout space; in the
fourteenth, their classification or mutual affinities, both when mature and
in an embryonic condition. In the last chapter I shall give a brief
recapitulation of the whole work, and a few concluding remarks.
No one ought to feel surprise at much remaining as yet unexplained in
regard to the origin of species and varieties, if he make due allowance for
our profound ignorance in regard to the mutual relations of the many beings
which live around us. Who can explain why one species ranges widely and is
very numerous, and why another allied species has a narrow range and is
rare? Yet these relations are of the highest importance, for they
determine the present welfare and, as I believe, the future success and
modification of every inhabitant of this world. Still less do we know of
the mutual relations of the innumerable inhabitants of the world during the
many past geological epochs in its history. Although much remains obscure,
and will long remain obscure, I can entertain no doubt, after the most
deliberate study and dispassionate judgment of which I am capable, that the
view which most naturalists until recently entertained, and which I
formerly entertained--namely, that each species has been independently
created--is erroneous. I am fully convinced that species are not
immutable; but that those belonging to what are called the same genera are
lineal descendants of some other and generally extinct species, in the same
manner as the acknowledged varieties of any one species are the descendants
of that species. Furthermore, I am convinced that natural selection has
been the most important, but not the exclusive, means of modification.
CHAPTER I.
VARIATION UNDER DOMESTICATION.
Causes of Variability -- Effects of Habit and the use and disuse of Parts
-- Correlated Variation -- Inheritance -- Character of Domestic Varieties
-- Difficulty of distinguishing between Varieties and Species -- Origin of
Domestic Varieties from one or more Species -- Domestic Pigeons, their
Differences and Origin -- Principles of Selection, anciently followed,
their Effects -- Methodical and Unconscious Selection -- Unknown Origin of
our Domestic Productions -- Circumstances favourable to Man's power of
Selection.
CAUSES OF VARIABILITY.
When we compare the individuals of the same variety or sub-variety of our
older cultivated plants and animals, one of the first points which strikes
us is, that they generally differ more from each other than do the
individuals of any one species or variety in a state of nature. And if we
reflect on the vast diversity of the plants and animals which have been
cultivated, and which have varied during all ages under the most different
climates and treatment, we are driven to conclude that this great
variability is due to our domestic productions having been raised under
conditions of life not so uniform as, and somewhat different from, those to
which the parent species had been exposed under nature. There is, also,
some probability in the view propounded by Andrew Knight, that this
variability may be partly connected with excess of food. It seems clear
that organic beings must be exposed during several generations to new
conditions to cause any great amount of variation; and that, when the
organisation has once begun to vary, it generally continues varying for
many generations. No case is on record of a variable organism ceasing to
vary under cultivation. Our oldest cultivated plants, such as wheat, still
yield new varieties: our oldest domesticated animals are still capable of
rapid improvement or modification.
As far as I am able to judge, after long attending to the subject, the
conditions of life appear to act in two ways--directly on the whole
organisation or on certain parts alone and in directly by affecting the
reproductive system. With respect to the direct action, we must bear in
mind that in every case, as Professor Weismann has lately insisted, and as
I have incidently shown in my work on "Variation under Domestication,"
there are two factors: namely, the nature of the organism and the nature
of the conditions. The former seems to be much the more important; for
nearly similar variations sometimes arise under, as far as we can judge,
dissimilar conditions; and, on the other hand, dissimilar variations arise
under conditions which appear to be nearly uniform. The effects on the
offspring are either definite or in definite. They may be considered as
definite when all or nearly all the offspring of individuals exposed to
certain conditions during several generations are modified in the same
manner. It is extremely difficult to come to any conclusion in regard to
the extent of the changes which have been thus definitely induced. There
can, however, be little doubt about many slight changes, such as size from
the amount of food, colour from the nature of the food, thickness of the
skin and hair from climate, etc. Each of the endless variations which we
see in the plumage of our fowls must have had some efficient cause; and if
the same cause were to act uniformly during a long series of generations on
many individuals, all probably would be modified in the same manner. Such
facts as the complex and extraordinary out growths which variably follow
from the insertion of a minute drop of poison by a gall-producing insect,
shows us what singular modifications might result in the case of plants
from a chemical change in the nature of the sap.
In definite variability is a much more common result of changed conditions
than definite variability, and has probably played a more important part in
the formation of our domestic races. We see in definite variability in the
endless slight peculiarities which distinguish the individuals of the same
species, and which cannot be accounted for by inheritance from either
parent or from some more remote ancestor. Even strongly-marked differences
occasionally appear in the young of the same litter, and in seedlings from
the same seed-capsule. At long intervals of time, out of millions of
individuals reared in the same country and fed on nearly the same food,
deviations of structure so strongly pronounced as to deserve to be called
monstrosities arise; but monstrosities cannot be separated by any distinct
line from slighter variations. All such changes of structure, whether
extremely slight or strongly marked, which appear among many individuals
living together, may be considered as the in definite effects of the
conditions of life on each individual organism, in nearly the same manner
as the chill effects different men in an in definite manner, according to
their state of body or constitution, causing coughs or colds, rheumatism,
or inflammation of various organs.
With respect to what I have called the in direct action of changed
conditions, namely, through the reproductive system of being affected, we
may infer that variability is thus induced, partly from the fact of this
system being extremely sensitive to any change in the conditions, and
partly from the similarity, as Kolreuter and others have remarked, between
the variability which follows from the crossing of distinct species, and
that which may be observed with plants and animals when reared under new or
unnatural conditions. Many facts clearly show how eminently susceptible
the reproductive system is to very slight changes in the surrounding
conditions. Nothing is more easy than to tame an animal, and few things
more difficult than to get it to breed freely under confinement, even when
the male and female unite. How many animals there are which will not
breed, though kept in an almost free state in their native country! This
is generally, but erroneously attributed to vitiated instincts. Many
cultivated plants display the utmost vigour, and yet rarely or never seed!
In some few cases it has been discovered that a very trifling change, such
as a little more or less water at some particular period of growth, will
determine whether or not a plant will produce seeds. I cannot here give
the details which I have collected and elsewhere published on this curious
subject; but to show how singular the laws are which determine the
reproduction of animals under confinement, I may mention that carnivorous
animals, even from the tropics, breed in this country pretty freely under
confinement, with the exception of the plantigrades or bear family, which
seldom produce young; whereas, carnivorous birds, with the rarest
exception, hardly ever lay fertile eggs. Many exotic plants have pollen
utterly worthless, in the same condition as in the most sterile hybrids.
When, on the one hand, we see domesticated animals and plants, though often
weak and sickly, breeding freely under confinement; and when, on the other
hand, we see individuals, though taken young from a state of nature
perfectly tamed, long-lived, and healthy (of which I could give numerous
instances), yet having their reproductive system so seriously affected by
unperceived causes as to fail to act, we need not be surprised at this
system, when it does act under confinement, acting irregularly, and
producing offspring somewhat unlike their parents. I may add that as some
organisms breed freely under the most unnatural conditions--for instance,
rabbits and ferrets kept in hutches--showing that their reproductive organs
are not easily affected; so will some animals and plants withstand
domestication or cultivation, and vary very slightly--perhaps hardly more
than in a state of nature.
Some naturalists have maintained that all variations are connected with the
act of sexual reproduction; but this is certainly an error; for I have
given in another work a long list of "sporting plants;" as they are called
by gardeners; that is, of plants which have suddenly produced a single bud
with a new and sometimes widely different character from that of the other
buds on the same plant. These bud variations, as they may be named, can be
propagated by grafts, offsets, etc., and sometimes by seed. They occur
rarely under nature, but are far from rare under culture. As a single bud
out of many thousands produced year after year on the same tree under
uniform conditions, has been known suddenly to assume a new character; and
as buds on distinct trees, growing under different conditions, have
sometimes yielded nearly the same variety--for instance, buds on peach-
trees producing nectarines, and buds on common roses producing moss-roses--
we clearly see that the nature of the conditions is of subordinate
importance in comparison with the nature of the organism in determining
each particular form of variation; perhaps of not more importance than the
nature of the spark, by which a mass of combustible matter is ignited, has
in determining the nature of the flames.
EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS; CORRELATED VARIATION;
INHERITANCE.
Changed habits produce an inherited effect as in the period of the
flowering of plants when transported from one climate to another. With
animals the increased use or disuse of parts has had a more marked
influence; thus I find in the domestic duck that the bones of the wing
weigh less and the bones of the leg more, in proportion to the whole
skeleton, than do the same bones in the wild duck; and this change may be
safely attributed to the domestic duck flying much less, and walking more,
than its wild parents. The great and inherited development of the udders
in cows and goats in countries where they are habitually milked, in
comparison with these organs in other countries, is probably another
instance of the effects of use. Not one of our domestic animals can be
named which has not in some country drooping ears; and the view which has
been suggested that the drooping is due to disuse of the muscles of the
ear, from the animals being seldom much alarmed, seems probable.
Many laws regulate variation, some few of which can be dimly seen, and will
hereafter be briefly discussed. I will here only allude to what may be
called correlated variation. Important changes in the embryo or larva will
probably entail changes in the mature animal. In monstrosities, the
correlations between quite distinct parts are very curious; and many
instances are given in Isidore Geoffroy St. Hilaire's great work on this
subject. Breeders believe that long limbs are almost always accompanied by
an elongated head. Some instances of correlation are quite whimsical; thus
cats which are entirely white and have blue eyes are generally deaf; but it
has been lately stated by Mr. Tait that this is confined to the males.
Colour and constitutional peculiarities go together, of which many
remarkable cases could be given among animals and plants. From facts
collected by Heusinger, it appears that white sheep and pigs are injured by
certain plants, while dark-coloured individuals escape: Professor Wyman
has recently communicated to me a good illustration of this fact; on asking
some farmers in Virginia how it was that all their pigs were black, they
informed him that the pigs ate the paint-root (Lachnanthes), which coloured
their bones pink, and which caused the hoofs of all but the black varieties
to drop off; and one of the "crackers" (i.e. Virginia squatters) added, "we
select the black members of a litter for raising, as they alone have a good
chance of living." Hairless dogs have imperfect teeth; long-haired and
coarse-haired animals are apt to have, as is asserted, long or many horns;
pigeons with feathered feet have skin between their outer toes; pigeons
with short beaks have small feet, and those with long beaks large feet.
Hence if man goes on selecting, and thus augmenting, any peculiarity, he
will almost certainly modify unintentionally other parts of the structure,
owing to the mysterious laws of correlation.
The results of the various, unknown, or but dimly understood laws of
variation are infinitely complex and diversified. It is well worth while
carefully to study the several treatises on some of our old cultivated
plants, as on the hyacinth, potato, even the dahlia, etc.; and it is really
surprising to note the endless points of structure and constitution in
which the varieties and sub-varieties differ slightly from each other. The
whole organisation seems to have become plastic, and departs in a slight
degree from that of the parental type.
Any variation which is not inherited is unimportant for us. But the number
and diversity of inheritable deviations of structure, both those of slight
and those of considerable physiological importance, are endless. Dr.
Prosper Lucas' treatise, in two large volumes, is the fullest and the best
on this subject. No breeder doubts how strong is the tendency to
inheritance; that like produces like is his fundamental belief: doubts
have been thrown on this principle only by theoretical writers. When any
deviation of structure often appears, and we see it in the father and
child, we cannot tell whether it may not be due to the same cause having
acted on both; but when among individuals, apparently exposed to the same
conditions, any very rare deviation, due to some extraordinary combination
of circumstances, appears in the parent--say, once among several million
individuals--and it reappears in the child, the mere doctrine of chances
almost compels us to attribute its reappearance to inheritance. Every one
must have heard of cases of albinism, prickly skin, hairy bodies, etc.,
appearing in several members of the same family. If strange and rare
deviations of structure are truly inherited, less strange and commoner
deviations may be freely admitted to be inheritable. Perhaps the correct
way of viewing the whole subject would be, to look at the inheritance of
every character whatever as the rule, and non-inheritance as the anomaly.
The laws governing inheritance are for the most part unknown; no one can
say why the same peculiarity in different individuals of the same species,
or in different species, is sometimes inherited and sometimes not so; why
the child often reverts in certain characteristics to its grandfather or
grandmother or more remote ancestor; why a peculiarity is often transmitted
from one sex to both sexes, or to one sex alone, more commonly but not
exclusively to the like sex. It is a fact of some importance to us, that
peculiarities appearing in the males of our domestic breeds are often
transmitted, either exclusively or in a much greater degree, to the males
alone. A much more important rule, which I think may be trusted, is that,
at whatever period of life a peculiarity first appears, it tends to
reappear in the offspring at a corresponding age, though sometimes earlier.
In many cases this could not be otherwise; thus the inherited peculiarities
in the horns of cattle could appear only in the offspring when nearly
mature; peculiarities in the silk-worm are known to appear at the
corresponding caterpillar or cocoon stage. But hereditary diseases and
some other facts make me believe that the rule has a wider extension, and
that, when there is no apparent reason why a peculiarity should appear at
any particular age, yet that it does tend to appear in the offspring at the
same period at which it first appeared in the parent. I believe this rule
to be of the highest importance in explaining the laws of embryology.
These remarks are of course confined to the first APPEARANCE of the
peculiarity, and not to the primary cause which may have acted on the
ovules or on the male element; in nearly the same manner as the increased
length of the horns in the offspring from a short-horned cow by a
long-horned bull, though appearing late in life, is clearly due to the male
element.
Having alluded to the subject of reversion, I may here refer to a statement
often made by naturalists--namely, that our domestic varieties, when run
wild, gradually but invariably revert in character to their aboriginal
stocks. Hence it has been argued that no deductions can be drawn from
domestic races to species in a state of nature. I have in vain endeavoured
to discover on what decisive facts the above statement has so often and so
boldly been made. There would be great difficulty in proving its truth:
we may safely conclude that very many of the most strongly marked domestic
varieties could not possibly live in a wild state. In many cases we do not
know what the aboriginal stock was, and so could not tell whether or not
nearly perfect reversion had ensued. It would be necessary, in order to
prevent the effects of intercrossing, that only a single variety should be
turned loose in its new home. Nevertheless, as our varieties certainly do
occasionally revert in some of their characters to ancestral forms, it
seems to me not improbable that if we could succeed in naturalising, or
were to cultivate, during many generations, the several races, for
instance, of the cabbage, in very poor soil--in which case, however, some
effect would have to be attributed to the DEFINITE action of the poor soil
--that they would, to a large extent, or even wholly, revert to the wild
aboriginal stock. Whether or not the experiment would succeed is not of
great importance for our line of argument; for by the experiment itself the
conditions of life are changed. If it could be shown that our domestic
varieties manifested a strong tendency to reversion--that is, to lose their
acquired characters, while kept under the same conditions and while kept in
a considerable body, so that free intercrossing might check, by blending
together, any slight deviations in their structure, in such case, I grant
that we could deduce nothing from domestic varieties in regard to species.
But there is not a shadow of evidence in favour of this view: to assert
that we could not breed our cart and race-horses, long and short-horned
cattle, and poultry of various breeds, and esculent vegetables, for an
unlimited number of generations, would be opposed to all experience.
CHARACTER OF DOMESTIC VARIETIES; DIFFICULTY OF DISTINGUISHING BETWEEN
VARIETIES AND SPECIES; ORIGIN OF DOMESTIC VARIETIES FROM ONE OR MORE
SPECIES.
When we look to the hereditary varieties or races of our domestic animals
and plants, and compare them with closely allied species, we generally
perceive in each domestic race, as already remarked, less uniformity of
character than in true species. Domestic races often have a somewhat
monstrous character; by which I mean, that, although differing from each
other and from other species of the same genus, in several trifling
respects, they often differ in an extreme degree in some one part, both
when compared one with another, and more especially when compared with the
species under nature to which they are nearest allied. With these
exceptions (and with that of the perfect fertility of varieties when
crossed--a subject hereafter to be discussed), domestic races of the same
species differ from each other in the same manner as do the closely allied
species of the same genus in a state of nature, but the differences in most
cases are less in degree. This must be admitted as true, for the domestic
races of many animals and plants have been ranked by some competent judges
as the descendants of aboriginally distinct species, and by other competent
judges as mere varieties. If any well marked distinction existed between a
domestic race and a species, this source of doubt would not so perpetually
recur. It has often been stated that domestic races do not differ from
each other in characters of generic value. It can be shown that this
statement is not correct; but naturalists differ much in determining what
characters are of generic value; all such valuations being at present
empirical. When it is explained how genera originate under nature, it will
be seen that we have no right to expect often to find a generic amount of
difference in our domesticated races.
In attempting to estimate the amount of structural difference between
allied domestic races, we are soon involved in doubt, from not knowing
whether they are descended from one or several parent species. This point,
if it could be cleared up, would be interesting; if, for instance, it could
be shown that the greyhound, bloodhound, terrier, spaniel and bull-dog,
which we all know propagate their kind truly, were the offspring of any
single species, then such facts would have great weight in making us doubt
about the immutability of the many closely allied natural species--for
instance, of the many foxes--inhabiting the different quarters of the
world. I do not believe, as we shall presently see, that the whole amount
of difference between the several breeds of the dog has been produced under
domestication; I believe that a small part of the difference is due to
their being descended from distinct species. In the case of strongly
marked races of some other domesticated species, there is presumptive or
even strong evidence that all are descended from a single wild stock.
It has often been assumed that man has chosen for domestication animals and
plants having an extraordinary inherent tendency to vary, and likewise to
withstand diverse climates. I do not dispute that these capacities have
added largely to the value of most of our domesticated productions; but how
could a savage possibly know, when he first tamed an animal, whether it
would vary in succeeding generations, and whether it would endure other
climates? Has the little variability of the ass and goose, or the small
power of endurance of warmth by the reindeer, or of cold by the common
camel, prevented their domestication? I cannot doubt that if other animals
and plants, equal in number to our domesticated productions, and belonging
to equally diverse classes and countries, were taken from a state of
nature, and could be made to breed for an equal number of generations under
domestication, they would on an average vary as largely as the parent
species of our existing domesticated productions have varied.
In the case of most of our anciently domesticated animals and plants, it is
not possible to come to any definite conclusion, whether they are descended
from one or several wild species. The argument mainly relied on by those
who believe in the multiple origin of our domestic animals is, that we find
in the most ancient times, on the monuments of Egypt, and in the lake-
habitations of Switzerland, much diversity in the breeds; and that some of
these ancient breeds closely resemble, or are even identical with, those
still existing. But this only throws far backward the history of
civilisation, and shows that animals were domesticated at a much earlier
period than has hitherto been supposed. The lake-inhabitants of
Switzerland cultivated several kinds of wheat and barley, the pea, the
poppy for oil and flax; and they possessed several domesticated animals.
They also carried on commerce with other nations. All this clearly shows,
as Heer has remarked, that they had at this early age progressed
considerably in civilisation; and this again implies a long continued
previous period of less advanced civilisation, during which the
domesticated animals, kept by different tribes in different districts,
might have varied and given rise to distinct races. Since the discovery of
flint tools in the superficial formations of many parts of the world, all
geologists believe that barbarian men existed at an enormously remote
period; and we know that at the present day there is hardly a tribe so
barbarous as not to have domesticated at least the dog.
The origin of most of our domestic animals will probably forever remain
vague. But I may here state that, looking to the domestic dogs of the
whole world, I have, after a laborious collection of all known facts, come
to the conclusion that several wild species of Canidae have been tamed, and
that their blood, in some cases mingled together, flows in the veins of our
domestic breeds. In regard to sheep and goats I can form no decided
opinion. From facts communicated to me by Mr. Blyth, on the habits, voice,
constitution and structure of the humped Indian cattle, it is almost
certain that they are descended from a different aboriginal stock from our
European cattle; and some competent judges believe that these latter have
had two or three wild progenitors, whether or not these deserve to be
called species. This conclusion, as well as that of the specific
distinction between the humped and common cattle, may, indeed, be looked
upon as established by the admirable researches of Professor Rutimeyer.
With respect to horses, from reasons which I cannot here give, I am
doubtfully inclined to believe, in opposition to several authors, that all
the races belong to the same species. Having kept nearly all the English
breeds of the fowl alive, having bred and crossed them, and examined their
skeletons, it appears to me almost certain that all are the descendants of
the wild Indian fowl, Gallus bankiva; and this is the conclusion of Mr.
Blyth, and of others who have studied this bird in India. In regard to
ducks and rabbits, some breeds of which differ much from each other, the
evidence is clear that they are all descended from the common duck and wild
rabbit.
The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some authors.
They believe that every race which breeds true, let the distinctive
characters be ever so slight, has had its wild prototype. At this rate
there must have existed at least a score of species of wild cattle, as many
sheep, and several goats, in Europe alone, and several even within Great
Britain. One author believes that there formerly existed eleven wild
species of sheep peculiar to Great Britain! When we bear in mind that
Britain has now not one peculiar mammal, and France but few distinct from
those of Germany, and so with Hungary, Spain, etc., but that each of these
kingdoms possesses several peculiar breeds of cattle, sheep, etc., we must
admit that many domestic breeds must have originated in Europe; for whence
otherwise could they have been derived? So it is in India. Even in the
case of the breeds of the domestic dog throughout the world, which I admit
are descended from several wild species, it cannot be doubted that there
has been an immense amount of inherited variation; for who will believe
that animals closely resembling the Italian greyhound, the bloodhound, the
bull-dog, pug-dog, or Blenheim spaniel, etc.--so unlike all wild
Canidae--ever existed in a state of nature? It has often been loosely said
that all our races of dogs have been produced by the crossing of a few
aboriginal species; but by crossing we can only get forms in some degree
intermediate between their parents; and if we account for our several
domestic races by this process, we must admit the former existence of the
most extreme forms, as the Italian greyhound, bloodhound, bull-dog, etc.,
in the wild state. Moreover, the possibility of making distinct races by
crossing has been greatly exaggerated. Many cases are on record showing
that a race may be modified by occasional crosses if aided by the careful
selection of the individuals which present the desired character; but to
obtain a race intermediate between two quite distinct races would be very
difficult. Sir J. Sebright expressly experimented with this object and
failed. The offspring from the first cross between two pure breeds is
tolerably and sometimes (as I have found with pigeons) quite uniform in
character, and every thing seems simple enough; but when these mongrels are
crossed one with another for several generations, hardly two of them are
alike, and then the difficulty of the task becomes manifest.
BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES AND ORIGIN.
Believing that it is always best to study some special group, I have, after
deliberation, taken up domestic pigeons. I have kept every breed which I
could purchase or obtain, and have been most kindly favoured with skins
from several quarters of the world, more especially by the Hon. W. Elliot
from India, and by the Hon. C. Murray from Persia. Many treatises in
different languages have been published on pigeons, and some of them are
very important, as being of considerable antiquity. I have associated with
several eminent fanciers, and have been permitted to join two of the London
Pigeon Clubs. The diversity of the breeds is something astonishing.
Compare the English carrier and the short-faced tumbler, and see the
wonderful difference in their beaks, entailing corresponding differences in
their skulls. The carrier, more especially the male bird, is also
remarkable from the wonderful development of the carunculated skin about
the head, and this is accompanied by greatly elongated eyelids, very large
external orifices to the nostrils, and a wide gape of mouth. The
short-faced tumbler has a beak in outline almost like that of a finch; and
the common tumbler has the singular inherited habit of flying at a great
height in a compact flock, and tumbling in the air head over heels. The
runt is a bird of great size, with long, massive beak and large feet; some
of the sub-breeds of runts have very long necks, others very long wings and
tails, others singularly short tails. The barb is allied to the carrier,
but, instead of a long beak, has a very short and broad one. The pouter
has a much elongated body, wings, and legs; and its enormously developed
crop, which it glories in inflating, may well excite astonishment and even
laughter. The turbit has a short and conical beak, with a line of reversed
feathers down the breast; and it has the habit of continually expanding,
slightly, the upper part of the oesophagus. The Jacobin has the feathers
so much reversed along the back of the neck that they form a hood, and it
has, proportionally to its size, elongated wing and tail feathers. The
trumpeter and laugher, as their names express, utter a very different coo
from the other breeds. The fantail has thirty or even forty tail-feathers,
instead of twelve or fourteen, the normal number in all the members of the
great pigeon family: these feathers are kept expanded and are carried so
erect that in good birds the head and tail touch: the oil-gland is quite
aborted. Several other less distinct breeds might be specified.
In the skeletons of the several breeds, the development of the bones of the
face, in length and breadth and curvature, differs enormously. The shape,
as well as the breadth and length of the ramus of the lower jaw, varies in
a highly remarkable manner. The caudal and sacral vertebrae vary in
number; as does the number of the ribs, together with their relative
breadth and the presence of processes. The size and shape of the apertures
in the sternum are highly variable; so is the degree of divergence and
relative size of the two arms of the furcula. The proportional width of
the gape of mouth, the proportional length of the eyelids, of the orifice
of the nostrils, of the tongue (not always in strict correlation with the
length of beak), the size of the crop and of the upper part of the
oesophagus; the development and abortion of the oil-gland; the number of
the primary wing and caudal feathers; the relative length of the wing and
tail to each other and to the body; the relative length of the leg and
foot; the number of scutellae on the toes, the development of skin between
the toes, are all points of structure which are variable. The period at
which the perfect plumage is acquired varies, as does the state of the down
with which the nestling birds are clothed when hatched. The shape and size
of the eggs vary. The manner of flight, and in some breeds the voice and
disposition, differ remarkably. Lastly, in certain breeds, the males and
females have come to differ in a slight degree from each other.
Altogether at least a score of pigeons might be chosen, which, if shown to
an ornithologist, and he were told that they were wild birds, would
certainly be ranked by him as well-defined species. Moreover, I do not
believe that any ornithologist would in this case place the English
carrier, the short-faced tumbler, the runt, the barb, pouter, and fantail
in the same genus; more especially as in each of these breeds several
truly-inherited sub-breeds, or species, as he would call them, could be
shown him.
Great as are the differences between the breeds of the pigeon, I am fully
convinced that the common opinion of naturalists is correct, namely, that
all are descended from the rock-pigeon (Columba livia), including under
this term several geographical races or sub-species, which differ from each
other in the most trifling respects. As several of the reasons which have
led me to this belief are in some degree applicable in other cases, I will
here briefly give them. If the several breeds are not varieties, and have
not proceeded from the rock-pigeon, they must have descended from at least
seven or eight aboriginal stocks; for it is impossible to make the present
domestic breeds by the crossing of any lesser number: how, for instance,
could a pouter be produced by crossing two breeds unless one of the
parent-stocks possessed the characteristic enormous crop? The supposed
aboriginal stocks must all have been rock-pigeons, that is, they did not
breed or willingly perch on trees. But besides C. livia, with its
geographical sub-species, only two or three other species of rock-pigeons
are known; and these have not any of the characters of the domestic breeds.
Hence the supposed aboriginal stocks must either still exist in the
countries where they were originally domesticated, and yet be unknown to
ornithologists; and this, considering their size, habits and remarkable
characters, seems improbable; or they must have become extinct in the wild
state. But birds breeding on precipices, and good flyers, are unlikely to
be exterminated; and the common rock-pigeon, which has the same habits with
the domestic breeds, has not been exterminated even on several of the
smaller British islets, or on the shores of the Mediterranean. Hence the
supposed extermination of so many species having similar habits with the
rock-pigeon seems a very rash assumption. Moreover, the several
above-named domesticated breeds have been transported to all parts of the
world, and, therefore, some of them must have been carried back again into
their native country; but not one has become wild or feral, though the
dovecot-pigeon, which is the rock-pigeon in a very slightly altered state,
has become feral in several places. Again, all recent experience shows
that it is difficult to get wild animals to breed freely under
domestication; yet on the hypothesis of the multiple origin of our pigeons,
it must be assumed that at least seven or eight species were so thoroughly
domesticated in ancient times by half-civilized man, as to be quite
prolific under confinement.
An argument of great weight, and applicable in several other cases, is,
that the above-specified breeds, though agreeing generally with the wild
rock-pigeon in constitution, habits, voice, colouring, and in most parts of
their structure, yet are certainly highly abnormal in other parts; we may
look in vain through the whole great family of Columbidae for a beak like
that of the English carrier, or that of the short-faced tumbler, or barb;
for reversed feathers like those of the Jacobin; for a crop like that of
the pouter; for tail-feathers like those of the fantail. Hence it must be
assumed, not only that half-civilized man succeeded in thoroughly
domesticating several species, but that he intentionally or by chance
picked out extraordinarily abnormal species; and further, that these very
species have since all become extinct or unknown. So many strange
contingencies are improbable in the highest degree.
Some facts in regard to the colouring of pigeons well deserve
consideration. The rock-pigeon is of a slaty-blue, with white loins; but
the Indian sub-species, C. intermedia of Strickland, has this part bluish.
The tail has a terminal dark bar, with the outer feathers externally edged
at the base with white. The wings have two black bars. Some semi-domestic
breeds, and some truly wild breeds, have, besides the two black bars, the
wings chequered with black. These several marks do not occur together in
any other species of the whole family. Now, in every one of the domestic
breeds, taking thoroughly well-bred birds, all the above marks, even to the
white edging of the outer tail-feathers, sometimes concur perfectly
developed. Moreover, when birds belonging to two or more distinct breeds
are crossed, none of which are blue or have any of the above-specified
marks, the mongrel offspring are very apt suddenly to acquire these
characters. To give one instance out of several which I have observed: I
crossed some white fantails, which breed very true, with some black barbs--
and it so happens that blue varieties of barbs are so rare that I never
heard of an instance in England; and the mongrels were black, brown and
mottled. I also crossed a barb with a spot, which is a white bird with a
red tail and red spot on the forehead, and which notoriously breeds very
true; the mongrels were dusky and mottled. I then crossed one of the
mongrel barb-fantails with a mongrel barb-spot, and they produced a bird of
as beautiful a blue colour, with the white loins, double black wing-bar,
and barred and white-edged tail-feathers, as any wild rock-pigeon! We can
understand these facts, on the well-known principle of reversion to
ancestral characters, if all the domestic breeds are descended from the
rock-pigeon. But if we deny this, we must make one of the two following
highly improbable suppositions. Either, first, that all the several
imagined aboriginal stocks were coloured and marked like the rock-pigeon,
although no other existing species is thus coloured and marked, so that in
each separate breed there might be a tendency to revert to the very same
colours and markings. Or, secondly, that each breed, even the purest, has
within a dozen, or at most within a score, of generations, been crossed by
the rock-pigeon: I say within a dozen or twenty generations, for no
instance is known of crossed descendants reverting to an ancestor of
foreign blood, removed by a greater number of generations. In a breed
which has been crossed only once the tendency to revert to any character
derived from such a cross will naturally become less and less, as in each
succeeding generation there will be less of the foreign blood; but when
there has been no cross, and there is a tendency in the breed to revert to
a character which was lost during some former generation, this tendency,
for all that we can see to the contrary, may be transmitted undiminished
for an indefinite number of generations. These two distinct cases of
reversion are often confounded together by those who have written on
inheritance.
Lastly, the hybrids or mongrels from between all the breeds of the pigeon
are perfectly fertile, as I can state from my own observations, purposely
made, on the most distinct breeds. Now, hardly any cases have been
ascertained with certainty of hybrids from two quite distinct species of
animals being perfectly fertile. Some authors believe that long-continued
domestication eliminates this strong tendency to sterility in species.
>From the history of the dog, and of some other domestic animals, this
conclusion is probably quite correct, if applied to species closely related
to each other. But to extend it so far as to suppose that species,
aboriginally as distinct as carriers, tumblers, pouters, and fantails now
are, should yield offspring perfectly fertile, inter se, seems to me rash
in the extreme.
>From these several reasons, namely, the improbability of man having
formerly made seven or eight supposed species of pigeons to breed freely
under domestication--these supposed species being quite unknown in a wild
state, and their not having become anywhere feral--these species presenting
certain very abnormal characters, as compared with all other Columbidae,
though so like the rock-pigeon in most other respects--the occasional
reappearance of the blue colour and various black marks in all the breeds,
both when kept pure and when crossed--and lastly, the mongrel offspring
being perfectly fertile--from these several reasons, taken together, we may
safely conclude that all our domestic breeds are descended from the rock-
pigeon or Columba livia with its geographical sub-species.
In favour of this view, I may add, firstly, that the wild C. livia has been
found capable of domestication in Europe and in India; and that it agrees
in habits and in a great number of points of structure with all the
domestic breeds. Secondly, that although an English carrier or a
short-faced tumbler differs immensely in certain characters from the
rock-pigeon, yet that by comparing the several sub-breeds of these two
races, more especially those brought from distant countries, we can make,
between them and the rock-pigeon, an almost perfect series; so we can in
some other cases, but not with all the breeds. Thirdly, those characters
which are mainly distinctive of each breed are in each eminently variable,
for instance, the wattle and length of beak of the carrier, the shortness
of that of the tumbler, and the number of tail-feathers in the fantail; and
the explanation of this fact will be obvious when we treat of selection.
Fourthly, pigeons have been watched and tended with the utmost care, and
loved by many people. They have been domesticated for thousands of years
in several quarters of the world; the earliest known record of pigeons is
in the fifth Aegyptian dynasty, about 3000 B.C., as was pointed out to me
by Professor Lepsius; but Mr. Birch informs me that pigeons are given in a
bill of fare in the previous dynasty. In the time of the Romans, as we
hear from Pliny, immense prices were given for pigeons; "nay, they are come
to this pass, that they can reckon up their pedigree and race." Pigeons
were much valued by Akber Khan in India, about the year 1600; never less
than 20,000 pigeons were taken with the court. "The monarchs of Iran and
Turan sent him some very rare birds;" and, continues the courtly historian,
"His Majesty, by crossing the breeds, which method was never practised
before, has improved them astonishingly." About this same period the Dutch
were as eager about pigeons as were the old Romans. The paramount
importance of these considerations in explaining the immense amount of
variation which pigeons have undergone, will likewise be obvious when we
treat of selection. We shall then, also, see how it is that the several
breeds so often have a somewhat monstrous character. It is also a most
favourable circumstance for the production of distinct breeds, that male
and female pigeons can be easily mated for life; and thus different breeds
can be kept together in the same aviary.
I have discussed the probable origin of domestic pigeons at some, yet quite
insufficient, length; because when I first kept pigeons and watched the
several kinds, well knowing how truly they breed, I felt fully as much
difficulty in believing that since they had been domesticated they had all
proceeded from a common parent, as any naturalist could in coming to a
similar conclusion in regard to the many species of finches, or other
groups of birds, in nature. One circumstance has struck me much; namely,
that nearly all the breeders of the various domestic animals and the
cultivators of plants, with whom I have conversed, or whose treatises I
have read, are firmly convinced that the several breeds to which each has
attended, are descended from so many aboriginally distinct species. Ask,
as I have asked, a celebrated raiser of Hereford cattle, whether his cattle
might not have descended from Long-horns, or both from a common parent-
stock, and he will laugh you to scorn. I have never met a pigeon, or
poultry, or duck, or rabbit fancier, who was not fully convinced that each
main breed was descended from a distinct species. Van Mons, in his
treatise on pears and apples, shows how utterly he disbelieves that the
several sorts, for instance a Ribston-pippin or Codlin-apple, could ever
have proceeded from the seeds of the same tree. Innumerable other examples
could be given. The explanation, I think, is simple: from long-continued
study they are strongly impressed with the differences between the several
races; and though they well know that each race varies slightly, for they
win their prizes by selecting such slight differences, yet they ignore all
general arguments, and refuse to sum up in their minds slight differences
accumulated during many successive generations. May not those naturalists
who, knowing far less of the laws of inheritance than does the breeder, and
knowing no more than he does of the intermediate links in the long lines of
descent, yet admit that many of our domestic races are descended from the
same parents--may they not learn a lesson of caution, when they deride the
idea of species in a state of nature being lineal descendants of other
species?
PRINCIPLES OF SELECTION ANCIENTLY FOLLOWED, AND THEIR EFFECTS.
Let us now briefly consider the steps by which domestic races have been
produced, either from one or from several allied species. Some effect may
be attributed to the direct and definite action of the external conditions
of life, and some to habit; but he would be a bold man who would account by
such agencies for the differences between a dray and race-horse, a
greyhound and bloodhound, a carrier and tumbler pigeon. One of the most
remarkable features in our domesticated races is that we see in them
adaptation, not indeed to the animal's or plant's own good, but to man's
use or fancy. Some variations useful to him have probably arisen suddenly,
or by one step; many botanists, for instance, believe that the fuller's
teasel, with its hooks, which can not be rivalled by any mechanical
contrivance, is only a variety of the wild Dipsacus; and this amount of
change may have suddenly arisen in a seedling. So it has probably been
with the turnspit dog; and this is known to have been the case with the
ancon sheep. But when we compare the dray-horse and race-horse, the
dromedary and camel, the various breeds of sheep fitted either for
cultivated land or mountain pasture, with the wool of one breed good for
one purpose, and that of another breed for another purpose; when we compare
the many breeds of dogs, each good for man in different ways; when we
compare the game-cock, so pertinacious in battle, with other breeds so
little quarrelsome, with "everlasting layers" which never desire to sit,
and with the bantam so small and elegant; when we compare the host of
agricultural, culinary, orchard, and flower-garden races of plants, most
useful to man at different seasons and for different purposes, or so
beautiful in his eyes, we must, I think, look further than to mere
variability. We can not suppose that all the breeds were suddenly produced
as perfect and as useful as we now see them; indeed, in many cases, we know
that this has not been their history. The key is man's power of
accumulative selection: nature gives successive variations; man adds them
up in certain directions useful to him. In this sense he may be said to
have made for himself useful breeds.
The great power of this principle of selection is not hypothetical. It is
certain that several of our eminent breeders have, even within a single
lifetime, modified to a large extent their breeds of cattle and sheep. In
order fully to realise what they have done it is almost necessary to read
several of the many treatises devoted to this subject, and to inspect the
animals. Breeders habitually speak of an animal's organisation as
something plastic, which they can model almost as they please. If I had
space I could quote numerous passages to this effect from highly competent
authorities. Youatt, who was probably better acquainted with the works of
agriculturalists than almost any other individual, and who was himself a
very good judge of animals, speaks of the principle of selection as "that
which enables the agriculturist, not only to modify the character of his
flock, but to change it altogether. It is the magician's wand, by means of
which he may summon into life whatever form and mould he pleases." Lord
Somerville, speaking of what breeders have done for sheep, says: "It would
seem as if they had chalked out upon a wall a form perfect in itself, and
then had given it existence." In Saxony the importance of the principle of
selection in regard to merino sheep is so fully recognised, that men follow
it as a trade: the sheep are placed on a table and are studied, like a
picture by a connoisseur; this is done three times at intervals of months,
and the sheep are each time marked and classed, so that the very best may
ultimately be selected for breeding.
What English breeders have actually effected is proved by the enormous
prices given for animals with a good pedigree; and these have been exported
to almost every quarter of the world. The improvement is by no means
generally due to crossing different breeds; all the best breeders are
strongly opposed to this practice, except sometimes among closely allied
sub-breeds. And when a cross has been made, the closest selection is far
more indispensable even than in ordinary cases. If selection consisted
merely in separating some very distinct variety and breeding from it, the
principle would be so obvious as hardly to be worth notice; but its
importance consists in the great effect produced by the accumulation in one
direction, during successive generations, of differences absolutely
inappreciable by an uneducated eye--differences which I for one have vainly
attempted to appreciate. Not one man in a thousand has accuracy of eye and
judgment sufficient to become an eminent breeder. If gifted with these
qualities, and he studies his subject for years, and devotes his lifetime
to it with indomitable perseverance, he will succeed, and may make great
improvements; if he wants any of these qualities, he will assuredly fail.
Few would readily believe in the natural capacity and years of practice
requisite to become even a skilful pigeon-fancier.
The same principles are followed by horticulturists; but the variations are
here often more abrupt. No one supposes that our choicest productions have
been produced by a single variation from the aboriginal stock. We have
proofs that this is not so in several cases in which exact records have
been kept; thus, to give a very trifling instance, the steadily increasing
size of the common gooseberry may be quoted. We see an astonishing
improvement in many florists' flowers, when the flowers of the present day
are compared with drawings made only twenty or thirty years ago. When a
race of plants is once pretty well established, the seed-raisers do not
pick out the best plants, but merely go over their seed-beds, and pull up
the "rogues," as they call the plants that deviate from the proper
standard. With animals this kind of selection is, in fact, likewise
followed; for hardly any one is so careless as to breed from his worst
animals.
In regard to plants, there is another means of observing the accumulated
effects of selection--namely, by comparing the diversity of flowers in the
different varieties of the same species in the flower-garden; the diversity
of leaves, pods, or tubers, or whatever part is valued, in the
kitchen-garden, in comparison with the flowers of the same varieties; and
the diversity of fruit of the same species in the orchard, in comparison
with the leaves and flowers of the same set of varieties. See how
different the leaves of the cabbage are, and how extremely alike the
flowers; how unlike the flowers of the heartsease are, and how alike the
leaves; how much the fruit of the different kinds of gooseberries differ in
size, colour, shape, and hairiness, and yet the flowers present very slight
differences. It is not that the varieties which differ largely in some one
point do not differ at all in other points; this is hardly ever--I speak
after careful observation--perhaps never, the case. The law of correlated
variation, the importance of which should never be overlooked, will ensure
some differences; but, as a general rule, it cannot be doubted that the
continued selection of slight variations, either in the leaves, the
flowers, or the fruit, will produce races differing from each other chiefly
in these characters.
It may be objected that the principle of selection has been reduced to
methodical practice for scarcely more than three-quarters of a century; it
has certainly been more attended to of late years, and many treatises have
been published on the subject; and the result has been, in a corresponding
degree, rapid and important. But it is very far from true that the
principle is a modern discovery. I could give several references to works
of high antiquity, in which the full importance of the principle is
acknowledged. In rude and barbarous periods of English history choice
animals were often imported, and laws were passed to prevent their
exportation: the destruction of horses under a certain size was ordered,
and this may be compared to the "roguing" of plants by nurserymen. The
principle of selection I find distinctly given in an ancient Chinese
encyclopaedia. Explicit rules are laid down by some of the Roman classical
writers. From passages in Genesis, it is clear that the colour of domestic
animals was at that early period attended to. Savages now sometimes cross
their dogs with wild canine animals, to improve the breed, and they
formerly did so, as is attested by passages in Pliny. The savages in South
Africa match their draught cattle by colour, as do some of the Esquimaux
their teams of dogs. Livingstone states that good domestic breeds are
highly valued by the negroes in the interior of Africa who have not
associated with Europeans. Some of these facts do not show actual
selection, but they show that the breeding of domestic animals was
carefully attended to in ancient times, and is now attended to by the
lowest savages. It would, indeed, have been a strange fact, had attention
not been paid to breeding, for the inheritance of good and bad qualities is
so obvious.
UNCONSCIOUS SELECTION.
At the present time, eminent breeders try by methodical selection, with a
distinct object in view, to make a new strain or sub-breed, superior to
anything of the kind in the country. But, for our purpose, a form of
selection, which may be called unconscious, and which results from every
one trying to possess and breed from the best individual animals, is more
important. Thus, a man who intends keeping pointers naturally tries to get
as good dogs as he can, and afterwards breeds from his own best dogs, but
he has no wish or expectation of permanently altering the breed.
Nevertheless we may infer that this process, continued during centuries,
would improve and modify any breed, in the same way as Bakewell, Collins,
etc., by this very same process, only carried on more methodically, did
greatly modify, even during their lifetimes, the forms and qualities of
their cattle. Slow and insensible changes of this kind could never be
recognised unless actual measurements or careful drawings of the breeds in
question have been made long ago, which may serve for comparison. In some
cases, however, unchanged, or but little changed, individuals of the same
breed exist in less civilised districts, where the breed has been less
improved. There is reason to believe that King Charles' spaniel has been
unconsciously modified to a large extent since the time of that monarch.
Some highly competent authorities are convinced that the setter is directly
derived from the spaniel, and has probably been slowly altered from it. It
is known that the English pointer has been greatly changed within the last
century, and in this case the change has, it is believed, been chiefly
effected by crosses with the foxhound; but what concerns us is, that the
change has been effected unconsciously and gradually, and yet so
effectually that, though the old Spanish pointer certainly came from Spain,
Mr. Borrow has not seen, as I am informed by him, any native dog in Spain
like our pointer.
By a similar process of selection, and by careful training, English race-
horses have come to surpass in fleetness and size the parent Arabs, so that
the latter, by the regulations for the Goodwood Races, are favoured in the
weights which they carry. Lord Spencer and others have shown how the
cattle of England have increased in weight and in early maturity, compared
with the stock formerly kept in this country. By comparing the accounts
given in various old treatises of the former and present state of carrier
and tumbler pigeons in Britain, India, and Persia, we can trace the stages
through which they have insensibly passed, and come to differ so greatly
from the rock-pigeon.
Youatt gives an excellent illustration of the effects of a course of
selection which may be considered as unconscious, in so far that the
breeders could never have expected, or even wished, to produce the result
which ensued--namely, the production of the distinct strains. The two
flocks of Leicester sheep kept by Mr. Buckley and Mr. Burgess, as Mr.
Youatt remarks, "Have been purely bred from the original stock of Mr.
Bakewell for upwards of fifty years. There is not a suspicion existing in
the mind of any one at all acquainted with the subject that the owner of
either of them has deviated in any one instance from the pure blood of Mr.
Bakewell's flock, and yet the difference between the sheep possessed by
these two gentlemen is so great that they have the appearance of being
quite different varieties."
If there exist savages so barbarous as never to think of the inherited
character of the offspring of their domestic animals, yet any one animal
particularly useful to them, for any special purpose, would be carefully
preserved during famines and other accidents, to which savages are so
liable, and such choice animals would thus generally leave more offspring
than the inferior ones; so that in this case there would be a kind of
unconscious selection going on. We see the value set on animals even by
the barbarians of Tierra del Fuego, by their killing and devouring their
old women, in times of dearth, as of less value than their dogs.
In plants the same gradual process of improvement through the occasional
preservation of the best individuals, whether or not sufficiently distinct
to be ranked at their first appearance as distinct varieties, and whether
or not two or more species or races have become blended together by
crossing, may plainly be recognised in the increased size and beauty which
we now see in the varieties of the heartsease, rose, pelargonium, dahlia,
and other plants, when compared with the older varieties or with their
parent-stocks. No one would ever expect to get a first-rate heartsease or
dahlia from the seed of a wild plant. No one would expect to raise a
first-rate melting pear from the seed of a wild pear, though he might
succeed from a poor seedling growing wild, if it had come from a
garden-stock. The pear, though cultivated in classical times, appears,
from Pliny's description, to have been a fruit of very inferior quality. I
have seen great surprise expressed in horticultural works at the wonderful
skill of gardeners in having produced such splendid results from such poor
materials; but the art has been simple, and, as far as the final result is
concerned, has been followed almost unconsciously. It has consisted in
always cultivating the best known variety, sowing its seeds, and, when a
slightly better variety chanced to appear, selecting it, and so onwards.
But the gardeners of the classical period, who cultivated the best pears
which they could procure, never thought what splendid fruit we should eat;
though we owe our excellent fruit in some small degree to their having
naturally chosen and preserved the best varieties they could anywhere find.
A large amount of change, thus slowly and unconsciously accumulated,
explains, as I believe, the well-known fact, that in a number of cases we
cannot recognise, and therefore do not know, the wild parent-stocks of the
plants which have been longest cultivated in our flower and kitchen
gardens. If it has taken centuries or thousands of years to improve or
modify most of our plants up to their present standard of usefulness to
man, we can understand how it is that neither Australia, the Cape of Good
Hope, nor any other region inhabited by quite uncivilised man, has afforded
us a single plant worth culture. It is not that these countries, so rich
in species, do not by a strange chance possess the aboriginal stocks of any
useful plants, but that the native plants have not been improved by
continued selection up to a standard of perfection comparable with that
acquired by the plants in countries anciently civilised.
In regard to the domestic animals kept by uncivilised man, it should not be
overlooked that they almost always have to struggle for their own food, at
least during certain seasons. And in two countries very differently
circumstanced, individuals of the same species, having slightly different
constitutions or structure, would often succeed better in the one country
than in the other, and thus by a process of "natural selection," as will
hereafter be more fully explained, two sub-breeds might be formed. This,
perhaps, partly explains why the varieties kept by savages, as has been
remarked by some authors, have more of the character of true species than
the varieties kept in civilised countries.
On the view here given of the important part which selection by man has
played, it becomes at once obvious, how it is that our domestic races show
adaptation in their structure or in their habits to man's wants or fancies.
We can, I think, further understand the frequently abnormal character of
our domestic races, and likewise their differences being so great in
external characters, and relatively so slight in internal parts or organs.
Man can hardly select, or only with much difficulty, any deviation of
structure excepting such as is externally visible; and indeed he rarely
cares for what is internal. He can never act by selection, excepting on
variations which are first given to him in some slight degree by nature.
No man would ever try to make a fantail till he saw a pigeon with a tail
developed in some slight degree in an unusual manner, or a pouter till he
saw a pigeon with a crop of somewhat unusual size; and the more abnormal or
unusual any character was when it first appeared, the more likely it would
be to catch his attention. But to use such an expression as trying to make
a fantail is, I have no doubt, in most cases, utterly incorrect. The man
who first selected a pigeon with a slightly larger tail, never dreamed what
the descendants of that pigeon would become through long-continued, partly
unconscious and partly methodical, selection. Perhaps the parent bird of
all fantails had only fourteen tail-feathers somewhat expanded, like the
present Java fantail, or like individuals of other and distinct breeds, in
which as many as seventeen tail-feathers have been counted. Perhaps the
first pouter-pigeon did not inflate its crop much more than the turbit now
does the upper part of its oesophagus--a habit which is disregarded by all
fanciers, as it is not one of the points of the breed.
Nor let it be thought that some great deviation of structure would be
necessary to catch the fancier's eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however
slight, in one's own possession. Nor must the value which would formerly
have been set on any slight differences in the individuals of the same
species, be judged of by the value which is now set on them, after several
breeds have fairly been established. It is known that with pigeons many
slight variations now occasionally appear, but these are rejected as faults
or deviations from the standard of perfection in each breed. The common
goose has not given rise to any marked varieties; hence the Toulouse and
the common breed, which differ only in colour, that most fleeting of
characters, have lately been exhibited as distinct at our poultry-shows.
These views appear to explain what has sometimes been noticed, namely, that
we know hardly anything about the origin or history of any of our domestic
breeds. But, in fact, a breed, like a dialect of a language, can hardly be
said to have a distinct origin. A man preserves and breeds from an
individual with some slight deviation of structure, or takes more care than
usual in matching his best animals, and thus improves them, and the
improved animals slowly spread in the immediate neighbourhood. But they
will as yet hardly have a distinct name, and from being only slightly
valued, their history will have been disregarded. When further improved by
the same slow and gradual process, they will spread more widely, and will
be recognised as something distinct and valuable, and will then probably
first receive a provincial name. In semi-civilised countries, with little
free communication, the spreading of a new sub-breed will be a slow
process. As soon as the points of value are once acknowledged, the
principle, as I have called it, of unconscious selection will always
tend--perhaps more at one period than at another, as the breed rises or
falls in fashion--perhaps more in one district than in another, according
to the state of civilisation of the inhabitants--slowly to add to the
characteristic features of the breed, whatever they may be. But the chance
will be infinitely small of any record having been preserved of such slow,
varying, and insensible changes.
CIRCUMSTANCES FAVOURABLE TO MAN'S POWER OF SELECTION.
I will now say a few words on the circumstances, favourable or the reverse,
to man's power of selection. A high degree of variability is obviously
favourable, as freely giving the materials for selection to work on; not
that mere individual differences are not amply sufficient, with extreme
care, to allow of the accumulation of a large amount of modification in
almost any desired direction. But as variations manifestly useful or
pleasing to man appear only occasionally, the chance of their appearance
will be much increased by a large number of individuals being kept. Hence
number is of the highest importance for success. On this principle
Marshall formerly remarked, with respect to the sheep of part of Yorkshire,
"As they generally belong to poor people, and are mostly IN SMALL LOTS,
they never can be improved." On the other hand, nurserymen, from keeping
large stocks of the same plant, are generally far more successful than
amateurs in raising new and valuable varieties. A large number of
individuals of an animal or plant can be reared only where the conditions
for its propagation are favourable. When the individuals are scanty all
will be allowed to breed, whatever their quality may be, and this will
effectually prevent selection. But probably the most important element is
that the animal or plant should be so highly valued by man, that the
closest attention is paid to even the slightest deviations in its qualities
or structure. Unless such attention be paid nothing can be effected. I
have seen it gravely remarked, that it was most fortunate that the
strawberry began to vary just when gardeners began to attend to this plant.
No doubt the strawberry had always varied since it was cultivated, but the
slight varieties had been neglected. As soon, however, as gardeners picked
out individual plants with slightly larger, earlier, or better fruit, and
raised seedlings from them, and again picked out the best seedlings and
bred from them, then (with some aid by crossing distinct species) those
many admirable varieties of the strawberry were raised which have appeared
during the last half-century.
With animals, facility in preventing crosses is an important element in the
formation of new races--at least, in a country which is already stocked
with other races. In this respect enclosure of the land plays a part.
Wandering savages or the inhabitants of open plains rarely possess more
than one breed of the same species. Pigeons can be mated for life, and
this is a great convenience to the fancier, for thus many races may be
improved and kept true, though mingled in the same aviary; and this
circumstance must have largely favoured the formation of new breeds.
Pigeons, I may add, can be propagated in great numbers and at a very quick
rate, and inferior birds may be freely rejected, as when killed they serve
for food. On the other hand, cats, from their nocturnal rambling habits,
can not be easily matched, and, although so much valued by women and
children, we rarely see a distinct breed long kept up; such breeds as we do
sometimes see are almost always imported from some other country. Although
I do not doubt that some domestic animals vary less than others, yet the
rarity or absence of distinct breeds of the cat, the donkey, peacock,
goose, etc., may be attributed in main part to selection not having been
brought into play: in cats, from the difficulty in pairing them; in
donkeys, from only a few being kept by poor people, and little attention
paid to their breeding; for recently in certain parts of Spain and of the
United States this animal has been surprisingly modified and improved by
careful selection; in peacocks, from not being very easily reared and a
large stock not kept; in geese, from being valuable only for two purposes,
food and feathers, and more especially from no pleasure having been felt in
the display of distinct breeds; but the goose, under the conditions to
which it is exposed when domesticated, seems to have a singularly
inflexible organisation, though it has varied to a slight extent, as I have
elsewhere described.
Some authors have maintained that the amount of variation in our domestic
productions is soon reached, and can never afterward be exceeded. It would
be somewhat rash to assert that the limit has been attained in any one
case; for almost all our animals and plants have been greatly improved in
many ways within a recent period; and this implies variation. It would be
equally rash to assert that characters now increased to their utmost limit,
could not, after remaining fixed for many centuries, again vary under new
conditions of life. No doubt, as Mr. Wallace has remarked with much truth,
a limit will be at last reached. For instance, there must be a limit to
the fleetness of any terrestrial animal, as this will be determined by the
friction to be overcome, the weight of the body to be carried, and the
power of contraction in the muscular fibres. But what concerns us is that
the domestic varieties of the same species differ from each other in almost
every character, which man has attended to and selected, more than do the
distinct species of the same genera. Isidore Geoffroy St. Hilaire has
proved this in regard to size, and so it is with colour, and probably with
the length of hair. With respect to fleetness, which depends on many
bodily characters, Eclipse was far fleeter, and a dray-horse is comparably
stronger, than any two natural species belonging to the same genus. So
with plants, the seeds of the different varieties of the bean or maize
probably differ more in size than do the seeds of the distinct species in
any one genus in the same two families. The same remark holds good in
regard to the fruit of the several varieties of the plum, and still more
strongly with the melon, as well as in many other analogous cases.
To sum up on the origin of our domestic races of animals and plants.
Changed conditions of life are of the highest importance in causing
variability, both by acting directly on the organisation, and indirectly by
affecting the reproductive system. It is not probable that variability is
an inherent and necessary contingent, under all circumstances. The greater
or less force of inheritance and reversion determine whether variations
shall endure. Variability is governed by many unknown laws, of which
correlated growth is probably the most important. Something, but how much
we do not know, may be attributed to the definite action of the conditions
of life. Some, perhaps a great, effect may be attributed to the increased
use or disuse of parts. The final result is thus rendered infinitely
complex. In some cases the intercrossing of aboriginally distinct species
appears to have played an important part in the origin of our breeds. When
several breeds have once been formed in any country, their occasional
intercrossing, with the aid of selection, has, no doubt, largely aided in
the formation of new sub-breeds; but the importance of crossing has been
much exaggerated, both in regard to animals and to those plants which are
propagated by seed. With plants which are temporarily propagated by
cuttings, buds, etc., the importance of crossing is immense; for the
cultivator may here disregard the extreme variability both of hybrids and
of mongrels, and the sterility of hybrids; but plants not propagated by
seed are of little importance to us, for their endurance is only temporary.
Over all these causes of change, the accumulative action of selection,
whether applied methodically and quickly, or unconsciously and slowly, but
more efficiently, seems to have been the predominant power.
CHAPTER II.
VARIATION UNDER NATURE.
Variability -- Individual differences -- Doubtful species -- Wide ranging,
much diffused, and common species, vary most -- Species of the larger
genera in each country vary more frequently than the species of the smaller
genera -- Many of the species of the larger genera resemble varieties in
being very closely, but unequally, related to each other, and in having
restricted ranges.
Before applying the principles arrived at in the last chapter to organic
beings in a state of nature, we must briefly discuss whether these latter
are subject to any variation. To treat this subject properly, a long
catalogue of dry facts ought to be given; but these I shall reserve for a
future work. Nor shall I here discuss the various definitions which have
been given of the term species. No one definition has satisfied all
naturalists; yet every naturalist knows vaguely what he means when he
speaks of a species. Generally the term includes the unknown element of a
distinct act of creation. The term "variety" is almost equally difficult
to define; but here community of descent is almost universally implied,
though it can rarely be proved. We have also what are called
monstrosities; but they graduate into varieties. By a monstrosity I
presume is meant some considerable deviation of structure, generally
injurious, or not useful to the species. Some authors use the term
"variation" in a technical sense, as implying a modification directly due
to the physical conditions of life; and "variations" in this sense are
supposed not to be inherited; but who can say that the dwarfed condition of
shells in the brackish waters of the Baltic, or dwarfed plants on Alpine
summits, or the thicker fur of an animal from far northwards, would not in
some cases be inherited for at least a few generations? And in this case I
presume that the form would be called a variety.
It may be doubted whether sudden and considerable deviations of structure,
such as we occasionally see in our domestic productions, more especially
with plants, are ever permanently propagated in a state of nature. Almost
every part of every organic being is so beautifully related to its complex
conditions of life that it seems as improbable that any part should have
been suddenly produced perfect, as that a complex machine should have been
invented by man in a perfect state. Under domestication monstrosities
sometimes occur which resemble normal structures in widely different
animals. Thus pigs have occasionally been born with a sort of proboscis,
and if any wild species of the same genus had naturally possessed a
proboscis, it might have been argued that this had appeared as a
monstrosity; but I have as yet failed to find, after diligent search, cases
of monstrosities resembling normal structures in nearly allied forms, and
these alone bear on the question. If monstrous forms of this kind ever do
appear in a state of nature and are capable of reproduction (which is not
always the case), as they occur rarely and singly, their preservation would
depend on unusually favourable circumstances. They would, also, during the
first and succeeding generations cross with the ordinary form, and thus
their abnormal character would almost inevitably be lost. But I shall have
to return in a future chapter to the preservation and perpetuation of
single or occasional variations.
INDIVIDUAL DIFFERENCES.
The many slight differences which appear in the offspring from the same
parents, or which it may be presumed have thus arisen, from being observed
in the individuals of the same species inhabiting the same confined
locality, may be called individual differences. No one supposes that all
the individuals of the same species are cast in the same actual mould.
These individual differences are of the highest importance for us, for they
are often inherited, as must be familiar to every one; and they thus afford
materials for natural selection to act on and accumulate, in the same
manner as man accumulates in any given direction individual differences in
his domesticated productions. These individual differences generally
affect what naturalists consider unimportant parts; but I could show, by a
long catalogue of facts, that parts which must be called important, whether
viewed under a physiological or classificatory point of view, sometimes
vary in the individuals of the same species. I am convinced that the most
experienced naturalist would be surprised at the number of the cases of
variability, even in important parts of structure, which he could collect
on good authority, as I have collected, during a course of years. It
should be remembered that systematists are far from being pleased at
finding variability in important characters, and that there are not many
men who will laboriously examine internal and important organs, and compare
them in many specimens of the same species. It would never have been
expected that the branching of the main nerves close to the great central
ganglion of an insect would have been variable in the same species; it
might have been thought that changes of this nature could have been
effected only by slow degrees; yet Sir J. Lubbock has shown a degree of
variability in these main nerves in Coccus, which may almost be compared to
the irregular branching of the stem of a tree. This philosophical
naturalist, I may add, has also shown that the muscles in the larvae of
certain insects are far from uniform. Authors sometimes argue in a circle
when they state that important organs never vary; for these same authors
practically rank those parts as important (as some few naturalists have
honestly confessed) which do not vary; and, under this point of view, no
instance will ever be found of an important part varying; but under any
other point of view many instances assuredly can be given.
There is one point connected with individual differences which is extremely
perplexing: I refer to those genera which have been called "protean" or
"polymorphic," in which species present an inordinate amount of variation.
With respect to many of these forms, hardly two naturalists agree whether
to rank them as species or as varieties. We may instance Rubus, Rosa, and
Hieracium among plants, several genera of insects, and of Brachiopod
shells. In most polymorphic genera some of the species have fixed and
definite characters. Genera which are polymorphic in one country seem to
be, with a few exceptions, polymorphic in other countries, and likewise,
judging from Brachiopod shells, at former periods of time. These facts are
very perplexing, for they seem to show that this kind of variability is
independent of the conditions of life. I am inclined to suspect that we
see, at least in some of these polymorphic genera, variations which are of
no service or disservice to the species, and which consequently have not
been seized on and rendered definite by natural selection, as hereafter to
be explained.
Individuals of the same species often present, as is known to every one,
great differences of structure, independently of variation, as in the two
sexes of various animals, in the two or three castes of sterile females or
workers among insects, and in the immature and larval states of many of the
lower animals. There are, also, cases of dimorphism and trimorphism, both
with animals and plants. Thus, Mr. Wallace, who has lately called
attention to the subject, has shown that the females of certain species of
butterflies, in the Malayan Archipelago, regularly appear under two or even
three conspicuously distinct forms, not connected by intermediate
varieties. Fritz Muller has described analogous but more extraordinary
cases with the males of certain Brazilian Crustaceans: thus, the male of a
Tanais regularly occurs under two distinct forms; one of these has strong
and differently shaped pincers, and the other has antennae much more
abundantly furnished with smelling-hairs. Although in most of these cases,
the two or three forms, both with animals and plants, are not now connected
by intermediate gradations, it is possible that they were once thus
connected. Mr. Wallace, for instance, describes a certain butterfly which
presents in the same island a great range of varieties connected by
intermediate links, and the extreme links of the chain closely resemble the
two forms of an allied dimorphic species inhabiting another part of the
Malay Archipelago. Thus also with ants, the several worker-castes are
generally quite distinct; but in some cases, as we shall hereafter see, the
castes are connected together by finely graduated varieties. So it is, as
I have myself observed, with some dimorphic plants. It certainly at first
appears a highly remarkable fact that the same female butterfly should have
the power of producing at the same time three distinct female forms and a
male; and that an hermaphrodite plant should produce from the same seed-
capsule three distinct hermaphrodite forms, bearing three different kinds
of females and three or even six different kinds of males. Nevertheless
these cases are only exaggerations of the common fact that the female
produces offspring of two sexes which sometimes differ from each other in a
wonderful manner.
DOUBTFUL SPECIES.
The forms which possess in some considerable degree the character of
species, but which are so closely similar to other forms, or are so closely
linked to them by intermediate gradations, that naturalists do not like to
rank them as distinct species, are in several respects the most important
for us. We have every reason to believe that many of these doubtful and
closely allied forms have permanently retained their characters for a long
time; for as long, as far as we know, as have good and true species.
Practically, when a naturalist can unite by means of intermediate links any
two forms, he treats the one as a variety of the other, ranking the most
common, but sometimes the one first described as the species, and the other
as the variety. But cases of great difficulty, which I will not here
enumerate, sometimes arise in deciding whether or not to rank one form as a
variety of another, even when they are closely connected by intermediate
links; nor will the commonly assumed hybrid nature of the intermediate
forms always remove the difficulty. In very many cases, however, one form
is ranked as a variety of another, not because the intermediate links have
actually been found, but because analogy leads the observer to suppose
either that they do now somewhere exist, or may formerly have existed; and
here a wide door for the entry of doubt and conjecture is opened.
Hence, in determining whether a form should be ranked as a species or a
variety, the opinion of naturalists having sound judgment and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as species by
at least some competent judges.
That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France, or of
the United States, drawn up by different botanists, and see what a
surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr. H.C. Watson, to whom I lie
under deep obligation for assistance of all kinds, has marked for me 182
British plants, which are generally considered as varieties, but which have
all been ranked by botanists as species; and in making this list he has
omitted many trifling varieties, but which nevertheless have been ranked by
some botanists as species, and he has entirely omitted several highly
polymorphic genera. Under genera, including the most polymorphic forms,
Mr. Babington gives 251 species, whereas Mr. Bentham gives only 112--a
difference of 139 doubtful forms! Among animals which unite for each
birth, and which are highly locomotive, doubtful forms, ranked by one
zoologist as a species and by another as a variety, can rarely be found
within the same country, but are common in separated areas. How many of
the birds and insects in North America and Europe, which differ very
slightly from each other, have been ranked by one eminent naturalist as
undoubted species, and by another as varieties, or, as they are often
called, geographical races! Mr. Wallace, in several valuable papers on the
various animals, especially on the Lepidoptera, inhabiting the islands of
the great Malayan Archipelago, shows that they may be classed under four
heads, namely, as variable forms, as local forms, as geographical races or
sub-species, and as true representative species. The first or variable
forms vary much within the limits of the same island. The local forms are
moderately constant and distinct in each separate island; but when all from
the several islands are compared together, the differences are seen to be
so slight and graduated that it is impossible to define or describe them,
though at the same time the extreme forms are sufficiently distinct. The
geographical races or sub-species are local forms completely fixed and
isolated; but as they do not differ from each other by strongly marked and
important characters, "There is no possible test but individual opinion to
determine which of them shall be considered as species and which as
varieties." Lastly, representative species fill the same place in the
natural economy of each island as do the local forms and sub-species; but
as they are distinguished from each other by a greater amount of difference
than that between the local forms and sub-species, they are almost
universally ranked by naturalists as true species. Nevertheless, no
certain criterion can possibly be given by which variable forms, local
forms, sub species and representative species can be recognised.
Many years ago, when comparing, and seeing others compare, the birds from
the closely neighbouring islands of the Galapagos Archipelago, one with
another, and with those from the American mainland, I was much struck how
entirely vague and arbitrary is the distinction between species and
varieties. On the islets of the little Madeira group there are many
insects which are characterized as varieties in Mr. Wollaston's admirable
work, but which would certainly be ranked as distinct species by many
entomologists. Even Ireland has a few animals, now generally regarded as
varieties, but which have been ranked as species by some zoologists.
Several experienced ornithologists consider our British red grouse as only
a strongly marked race of a Norwegian species, whereas the greater number
rank it as an undoubted species peculiar to Great Britain. A wide distance
between the homes of two doubtful forms leads many naturalists to rank them
as distinct species; but what distance, it has been well asked, will
suffice if that between America and Europe is ample, will that between
Europe and the Azores, or Madeira, or the Canaries, or between the several
islets of these small archipelagos, be sufficient?
Mr. B.D. Walsh, a distinguished entomologist of the United States, has
described what he calls Phytophagic varieties and Phytophagic species.
Most vegetable-feeding insects live on one kind of plant or on one group of
plants; some feed indiscriminately on many kinds, but do not in consequence
vary. In several cases, however, insects found living on different plants,
have been observed by Mr. Walsh to present in their larval or mature state,
or in both states, slight, though constant differences in colour, size, or
in the nature of their secretions. In some instances the males alone, in
other instances, both males and females, have been observed thus to differ
in a slight degree. When the differences are rather more strongly marked,
and when both sexes and all ages are affected, the forms are ranked by all
entomologists as good species. But no observer can determine for another,
even if he can do so for himself, which of these Phytophagic forms ought to
be called species and which varieties. Mr. Walsh ranks the forms which it
may be supposed would freely intercross, as varieties; and those which
appear to have lost this power, as species. As the differences depend on
the insects having long fed on distinct plants, it cannot be expected that
intermediate links connecting the several forms should now be found. The
naturalist thus loses his best guide in determining whether to rank
doubtful forms as varieties or species. This likewise necessarily occurs
with closely allied organisms, which inhabit distinct continents or
islands. When, on the other hand, an animal or plant ranges over the same
continent, or inhabits many islands in the same archipelago, and presents
different forms in the different areas, there is always a good chance that
intermediate forms will be discovered which will link together the extreme
states; and these are then degraded to the rank of varieties.
Some few naturalists maintain that animals never present varieties; but
then these same naturalists rank the slightest difference as of specific
value; and when the same identical form is met with in two distant
countries, or in two geological formations, they believe that two distinct
species are hidden under the same dress. The term species thus comes to be
a mere useless abstraction, implying and assuming a separate act of
creation. It is certain that many forms, considered by highly competent
judges to be varieties, resemble species so completely in character that
they have been thus ranked by other highly competent judges. But to
discuss whether they ought to be called species or varieties, before any
definition of these terms has been generally accepted, is vainly to beat
the air.
Many of the cases of strongly marked varieties or doubtful species well
deserve consideration; for several interesting lines of argument, from
geographical distribution, analogical variation, hybridism, etc., have been
brought to bear in the attempt to determine their rank; but space does not
here permit me to discuss them. Close investigation, in many cases, will
no doubt bring naturalists to agree how to rank doubtful forms. Yet it
must be confessed that it is in the best known countries that we find the
greatest number of them. I have been struck with the fact that if any
animal or plant in a state of nature be highly useful to man, or from any
cause closely attracts his attention, varieties of it will almost
universally be found recorded. These varieties, moreover, will often be
ranked by some authors as species. Look at the common oak, how closely it
has been studied; yet a German author makes more than a dozen species out
of forms, which are almost universally considered by other botanists to be
varieties; and in this country the highest botanical authorities and
practical men can be quoted to show that the sessile and pedunculated oaks
are either good and distinct species or mere varieties.
I may here allude to a remarkable memoir lately published by A. de
Candolle, on the oaks of the whole world. No one ever had more ample
materials for the discrimination of the species, or could have worked on
them with more zeal and sagacity. He first gives in detail all the many
points of structure which vary in the several species, and estimates
numerically the relative frequency of the variations. He specifies above a
dozen characters which may be found varying even on the same branch,
sometimes according to age or development, sometimes without any assignable
reason. Such characters are not of course of specific value, but they are,
as Asa Gray has remarked in commenting on this memoir, such as generally
enter into specific definitions. De Candolle then goes on to say that he
gives the rank of species to the forms that differ by characters never
varying on the same tree, and never found connected by intermediate states.
After this discussion, the result of so much labour, he emphatically
remarks: "They are mistaken, who repeat that the greater part of our
species are clearly limited, and that the doubtful species are in a feeble
minority. This seemed to be true, so long as a genus was imperfectly
known, and its species were founded upon a few specimens, that is to say,
were provisional. Just as we come to know them better, intermediate forms
flow in, and doubts as to specific limits augment." He also adds that it
is the best known species which present the greatest number of spontaneous
varieties and sub-varieties. Thus Quercus robur has twenty-eight
varieties, all of which, excepting six, are clustered round three sub-
species, namely Q. pedunculata, sessiliflora and pubescens. The forms
which connect these three sub-species are comparatively rare; and, as Asa
Gray again remarks, if these connecting forms which are now rare were to
become totally extinct the three sub-species would hold exactly the same
relation to each other as do the four or five provisionally admitted
species which closely surround the typical Quercus robur. Finally, De
Candolle admits that out of the 300 species, which will be enumerated in
his Prodromus as belonging to the oak family, at least two-thirds are
provisional species, that is, are not known strictly to fulfil the
definition above given of a true species. It should be added that De
Candolle no longer believes that species are immutable creations, but
concludes that the derivative theory is the most natural one, "and the most
accordant with the known facts in palaeontology, geographical botany and
zoology, of anatomical structure and classification."
When a young naturalist commences the study of a group of organisms quite
unknown to him he is at first much perplexed in determining what
differences to consider as specific and what as varietal; for he knows
nothing of the amount and kind of variation to which the group is subject;
and this shows, at least, how very generally there is some variation. But
if he confine his attention to one class within one country he will soon
make up his mind how to rank most of the doubtful forms. His general
tendency will be to make many species, for he will become impressed, just
like the pigeon or poultry fancier before alluded to, with the amount of
difference in the forms which he is continually studying; and he has little
general knowledge of analogical variation in other groups and in other
countries by which to correct his first impressions. As he extends the
range of his observations he will meet with more cases of difficulty; for
he will encounter a greater number of closely-allied forms. But if his
observations be widely extended he will in the end generally be able to
make up his own mind; but he will succeed in this at the expense of
admitting much variation, and the truth of this admission will often be
disputed by other naturalists. When he comes to study allied forms brought
from countries not now continuous, in which case he cannot hope to find
intermediate links, he will be compelled to trust almost entirely to
analogy, and his difficulties will rise to a climax.
Certainly no clear line of demarcation has as yet been drawn between
species and sub-species--that is, the forms which in the opinion of some
naturalists come very near to, but do not quite arrive at, the rank of
species; or, again, between sub-species and well-marked varieties, or
between lesser varieties and individual differences. These differences
blend into each other by an insensible series; and a series impresses the
mind with the idea of an actual passage.
Hence I look at individual differences, though of small interest to the
systematist, as of the highest importance for us, as being the first step
towards such slight varieties as are barely thought worth recording in
works on natural history. And I look at varieties which are in any degree
more distinct and permanent, as steps toward more strongly marked and
permanent varieties; and at the latter, as leading to sub-species, and then
to species. The passage from one stage of difference to another may, in
many cases, be the simple result of the nature of the organism and of the
different physical conditions to which it has long been exposed; but with
respect to the more important and adaptive characters, the passage from one
stage of difference to another may be safely attributed to the cumulative
action of natural selection, hereafter to be explained, and to the effects
of the increased use or disuse of parts. A well-marked variety may
therefore be called an incipient species; but whether this belief is
justifiable must be judged by the weight of the various facts and
considerations to be given throughout this work.
It need not be supposed that all varieties or incipient species attain the
rank of species. They may become extinct, or they may endure as varieties
for very long periods, as has been shown to be the case by Mr. Wollaston
with the varieties of certain fossil land-shells in Madeira, and with
plants by Gaston de Saporta. If a variety were to flourish so as to exceed
in numbers the parent species, it would then rank as the species, and the
species as the variety; or it might come to supplant and exterminate the
parent species; or both might co-exist, and both rank as independent
species. But we shall hereafter return to this subject.
>From these remarks it will be seen that I look at the term species as one
arbitrarily given, for the sake of convenience, to a set of individuals
closely resembling each other, and that it does not essentially differ from
the term variety, which is given to less distinct and more fluctuating
forms. The term variety, again, in comparison with mere individual
differences, is also applied arbitrarily, for convenience sake.
WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST.
Guided by theoretical considerations, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H.C.
Watson, to whom I am much indebted for valuable advice and assistance on
this subject, soon convinced me that there were many difficulties, as did
subsequently Dr. Hooker, even in stronger terms. I shall reserve for a
future work the discussion of these difficulties, and the tables of the
proportional numbers of the varying species. Dr. Hooker permits me to add
that after having carefully read my manuscript, and examined the tables, he
thinks that the following statements are fairly well established. The
whole subject, however, treated as it necessarily here is with much
brevity, is rather perplexing, and allusions cannot be avoided to the
"struggle for existence," "divergence of character," and other questions,
hereafter to be discussed.
Alphonse de Candolle and others have shown that plants which have very wide
ranges generally present varieties; and this might have been expected, as
they are exposed to diverse physical conditions, and as they come into
competition (which, as we shall hereafter see, is a far more important
circumstance) with different sets of organic beings. But my tables further
show that, in any limited country, the species which are the most common,
that is abound most in individuals, and the species which are most widely
diffused within their own country (and this is a different consideration
from wide range, and to a certain extent from commonness), oftenest give
rise to varieties sufficiently well-marked to have been recorded in
botanical works. Hence it is the most flourishing, or, as they may be
called, the dominant species--those which range widely, are the most
diffused in their own country, and are the most numerous in
individuals--which oftenest produce well-marked varieties, or, as I
consider them, incipient species. And this, perhaps, might have been
anticipated; for, as varieties, in order to become in any degree permanent,
necessarily have to struggle with the other inhabitants of the country, the
species which are already dominant will be the most likely to yield
offspring, which, though in some slight degree modified, still inherit
those advantages that enabled their parents to become dominant over their
compatriots. In these remarks on predominence, it should be understood
that reference is made only to the forms which come into competition with
each other, and more especially to the members of the same genus or class
having nearly similar habits of life. With respect to the number of
individuals or commonness of species, the comparison of course relates only
to the members of the same group. One of the higher plants may be said to
be dominant if it be more numerous in individuals and more widely diffused
than the other plants of the same country, which live under nearly the same
conditions. A plant of this kind is not the less dominant because some
conferva inhabiting the water or some parasitic fungus is infinitely more
numerous in individuals, and more widely diffused. But if the conferva or
parasitic fungus exceeds its allies in the above respects, it will then be
dominant within its own class.
SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY MORE FREQUENTLY THAN THE
SPECIES OF THE SMALLER GENERA.
If the plants inhabiting a country as described in any Flora, be divided
into two equal masses, all those in the larger genera (i.e., those
including many species) being placed on one side, and all those in the
smaller genera on the other side, the former will be found to include a
somewhat larger number of the very common and much diffused or dominant
species. This might have been anticipated, for the mere fact of many
species of the same genus inhabiting any country, shows that there is
something in the organic or inorganic conditions of that country favourable
to the genus; and, consequently, we might have expected to have found in
the larger genera, or those including many species, a larger proportional
number of dominant species. But so many causes tend to obscure this
result, that I am surprised that my tables show even a small majority on
the side of the larger genera. I will here allude to only two causes of
obscurity. Fresh water and salt-loving plants generally have very wide
ranges and are much diffused, but this seems to be connected with the
nature of the stations inhabited by them, and has little or no relation to
the size of the genera to which the species belong. Again, plants low in
the scale of organisation are generally much more widely diffused than
plants higher in the scale; and here again there is no close relation to
the size of the genera. The cause of lowly-organised plants ranging widely
will be discussed in our chapter on Geographical Distribution.
>From looking at species as only strongly marked and well-defined varieties,
I was led to anticipate that the species of the larger genera in each
country would oftener present varieties, than the species of the smaller
genera; for wherever many closely related species (i.e., species of the
same genus) have been formed, many varieties or incipient species ought, as
a general rule, to be now forming. Where many large trees grow, we expect
to find saplings. Where many species of a genus have been formed through
variation, circumstances have been favourable for variation; and hence we
might expect that the circumstances would generally still be favourable to
variation. On the other hand, if we look at each species as a special act
of creation, there is no apparent reason why more varieties should occur in
a group having many species, than in one having few.
To test the truth of this anticipation I have arranged the plants of twelve
countries, and the coleopterous insects of two districts, into two nearly
equal masses, the species of the larger genera on one side, and those of
the smaller genera on the other side, and it has invariably proved to be
the case that a larger proportion of the species on the side of the larger
genera presented varieties, than on the side of the smaller genera.
Moreover, the species of the large genera which present any varieties,
invariably present a larger average number of varieties than do the species
of the small genera. Both these results follow when another division is
made, and when all the least genera, with from only one to four species,
are altogether excluded from the tables. These facts are of plain
signification on the view that species are only strongly marked and
permanent varieties; for wherever many species of the same genus have been
formed, or where, if we may use the expression, the manufactory of species
has been active, we ought generally to find the manufactory still in
action, more especially as we have every reason to believe the process of
manufacturing new species to be a slow one. And this certainly holds true
if varieties be looked at as incipient species; for my tables clearly show,
as a general rule, that, wherever many species of a genus have been formed,
the species of that genus present a number of varieties, that is, of
incipient species, beyond the average. It is not that all large genera are
now varying much, and are thus increasing in the number of their species,
or that no small genera are now varying and increasing; for if this had
been so, it would have been fatal to my theory; inasmuch as geology plainly
tells us that small genera have in the lapse of time often increased
greatly in size; and that large genera have often come to their maxima,
declined, and disappeared. All that we want to show is, that where many
species of a genus have been formed, on an average many are still forming;
and this certainly holds good.
MANY OF THE SPECIES INCLUDED WITHIN THE LARGER GENERA RESEMBLE VARIETIES IN
BEING VERY CLOSELY, BUT UNEQUALLY, RELATED TO EACH OTHER, AND IN HAVING
RESTRICTED RANGES.
There are other relations between the species of large genera and their
recorded varieties which deserve notice. We have seen that there is no
infallible criterion by which to distinguish species and well-marked
varieties; and when intermediate links have not been found between doubtful
forms, naturalists are compelled to come to a determination by the amount
of difference between them, judging by analogy whether or not the amount
suffices to raise one or both to the rank of species. Hence the amount of
difference is one very important criterion in settling whether two forms
should be ranked as species or varieties. Now Fries has remarked in regard
to plants, and Westwood in regard to insects, that in large genera the
amount of difference between the species is often exceedingly small. I
have endeavoured to test this numerically by averages, and, as far as my
imperfect results go, they confirm the view. I have also consulted some
sagacious and experienced observers, and, after deliberation, they concur
in this view. In this respect, therefore, the species of the larger genera
resemble varieties, more than do the species of the smaller genera. Or the
case may be put in another way, and it may be said, that in the larger
genera, in which a number of varieties or incipient species greater than
the average are now manufacturing, many of the species already manufactured
still to a certain extent resemble varieties, for they differ from each
other by a less than the usual amount of difference.
Moreover, the species of the larger genera are related to each other, in
the same manner as the varieties of any one species are related to each
other. No naturalist pretends that all the species of a genus are equally
distinct from each other; they may generally be divided into sub-genera, or
sections, or lesser groups. As Fries has well remarked, little groups of
species are generally clustered like satellites around other species. And
what are varieties but groups of forms, unequally related to each other,
and clustered round certain forms--that is, round their parent-species.
Undoubtedly there is one most important point of difference between
varieties and species, namely, that the amount of difference between
varieties, when compared with each other or with their parent-species, is
much less than that between the species of the same genus. But when we
come to discuss the principle, as I call it, of divergence of character, we
shall see how this may be explained, and how the lesser differences between
varieties tend to increase into the greater differences between species.
There is one other point which is worth notice. Varieties generally have
much restricted ranges. This statement is indeed scarcely more than a
truism, for if a variety were found to have a wider range than that of its
supposed parent-species, their denominations would be reversed. But there
is reason to believe that the species which are very closely allied to
other species, and in so far resemble varieties, often have much restricted
ranges. For instance, Mr. H.C. Watson has marked for me in the well-sifted
London catalogue of Plants (4th edition) sixty-three plants which are
therein ranked as species, but which he considers as so closely allied to
other species as to be of doubtful value: these sixty-three reputed
species range on an average over 6.9 of the provinces into which Mr. Watson
has divided Great Britain. Now, in this same catalogue, fifty-three
acknowledged varieties are recorded, and these range over 7.7 provinces;
whereas, the species to which these varieties belong range over 14.3
provinces. So that the acknowledged varieties have very nearly the same
restricted average range, as have the closely allied forms, marked for me
by Mr. Watson as doubtful species, but which are almost universally ranked
by British botanists as good and true species.
SUMMARY.
Finally, varieties cannot be distinguished from species--except, first, by
the discovery of intermediate linking forms; and, secondly, by a certain
indefinite amount of difference between them; for two forms, if differing
very little, are generally ranked as varieties, notwithstanding that they
cannot be closely connected; but the amount of difference considered
necessary to give to any two forms the rank of species cannot be defined.
In genera having more than the average number of species in any country,
the species of these genera have more than the average number of varieties.
In large genera the species are apt to be closely but unequally allied
together, forming little clusters round other species. Species very
closely allied to other species apparently have restricted ranges. In all
these respects the species of large genera present a strong analogy with
varieties. And we can clearly understand these analogies, if species once
existed as varieties, and thus originated; whereas, these analogies are
utterly inexplicable if species are independent creations.
We have also seen that it is the most flourishing or dominant species of
the larger genera within each class which on an average yield the greatest
number of varieties, and varieties, as we shall hereafter see, tend to
become converted into new and distinct species. Thus the larger genera
tend to become larger; and throughout nature the forms of life which are
now dominant tend to become still more dominant by leaving many modified
and dominant descendants. But, by steps hereafter to be explained, the
larger genera also tend to break up into smaller genera. And thus, the
forms of life throughout the universe become divided into groups
subordinate to groups.
CHAPTER III.
STRUGGLE FOR EXISTENCE.
Its bearing on natural selection -- The term used in a wide sense --
Geometrical ratio of increase -- Rapid increase of naturalised animals and
plants -- Nature of the checks to increase -- Competition universal --
Effects of climate -- Protection from the number of individuals -- Complex
relations of all animals and plants throughout nature -- Struggle for life
most severe between individuals and varieties of the same species: often
severe between species of the same genus -- The relation of organism to
organism the most important of all relations.
Before entering on the subject of this chapter I must make a few
preliminary remarks to show how the struggle for existence bears on natural
selection. It has been seen in the last chapter that among organic beings
in a state of nature there is some individual variability: indeed I am not
aware that this has ever been disputed. It is immaterial for us whether a
multitude of doubtful forms be called species or sub-species or varieties;
what rank, for instance, the two or three hundred doubtful forms of British
plants are entitled to hold, if the existence of any well-marked varieties
be admitted. But the mere existence of individual variability and of some
few well-marked varieties, though necessary as the foundation for the work,
helps us but little in understanding how species arise in nature. How have
all those exquisite adaptations of one part of the organisation to another
part, and to the conditions of life and of one organic being to another
being, been perfected? We see these beautiful co-adaptations most plainly
in the woodpecker and the mistletoe; and only a little less plainly in the
humblest parasite which clings to the hairs of a quadruped or feathers of a
bird; in the structure of the beetle which dives through the water; in the
plumed seed which is wafted by the gentlest breeze; in short, we see
beautiful adaptations everywhere and in every part of the organic world.
Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more than
do the varieties of the same species? How do those groups of species,
which constitute what are called distinct genera and which differ from each
other more than do the species of the same genus, arise? All these
results, as we shall more fully see in the next chapter, follow from the
struggle for life. Owing to this struggle, variations, however slight and
from whatever cause proceeding, if they be in any degree profitable to the
individuals of a species, in their infinitely complex relations to other
organic beings and to their physical conditions of life, will tend to the
preservation of such individuals, and will generally be inherited by the
offspring. The offspring, also, will thus have a better chance of
surviving, for, of the many individuals of any species which are
periodically born, but a small number can survive. I have called this
principle, by which each slight variation, if useful, is preserved, by the
term natural selection, in order to mark its relation to man's power of
selection. But the expression often used by Mr. Herbert Spencer, of the
Survival of the Fittest, is more accurate, and is sometimes equally
convenient. We have seen that man by selection can certainly produce great
results, and can adapt organic beings to his own uses, through the
accumulation of slight but useful variations, given to him by the hand of
Nature. But Natural Selection, we shall hereafter see, is a power
incessantly ready for action, and is as immeasurably superior to man's
feeble efforts, as the works of Nature are to those of Art.
We will now discuss in a little more detail the struggle for existence. In
my future work this subject will be treated, as it well deserves, at
greater length. The elder De Candolle and Lyell have largely and
philosophically shown that all organic beings are exposed to severe
competition. In regard to plants, no one has treated this subject with
more spirit and ability than W. Herbert, Dean of Manchester, evidently the
result of his great horticultural knowledge. Nothing is easier than to
admit in words the truth of the universal struggle for life, or more
difficult--at least I found it so--than constantly to bear this conclusion
in mind. Yet unless it be thoroughly engrained in the mind, the whole
economy of nature, with every fact on distribution, rarity, abundance,
extinction, and variation, will be dimly seen or quite misunderstood. We
behold the face of nature bright with gladness, we often see superabundance
of food; we do not see or we forget that the birds which are idly singing
round us mostly live on insects or seeds, and are thus constantly
destroying life; or we forget how largely these songsters, or their eggs,
or their nestlings, are destroyed by birds and beasts of prey; we do not
always bear in mind, that, though food may be now superabundant, it is not
so at all seasons of each recurring year.
THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE SENSE.
I should premise that I use this term in a large and metaphorical sense,
including dependence of one being on another, and including (which is more
important) not only the life of the individual, but success in leaving
progeny. Two canine animals, in a time of dearth, may be truly said to
struggle with each other which shall get food and live. But a plant on the
edge of a desert is said to struggle for life against the drought, though
more properly it should be said to be dependent on the moisture. A plant
which annually produces a thousand seeds, of which only one of an average
comes to maturity, may be more truly said to struggle with the plants of
the same and other kinds which already clothe the ground. The mistletoe is
dependent on the apple and a few other trees, but can only in a far-fetched
sense be said to struggle with these trees, for, if too many of these
parasites grow on the same tree, it languishes and dies. But several
seedling mistletoes, growing close together on the same branch, may more
truly be said to struggle with each other. As the mistletoe is
disseminated by birds, its existence depends on them; and it may
metaphorically be said to struggle with other fruit-bearing plants, in
tempting the birds to devour and thus disseminate its seeds. In these
several senses, which pass into each other, I use for convenience sake the
general term of Struggle for Existence.
GEOMETRICAL RATIO OF INCREASE.
A struggle for existence inevitably follows from the high rate at which all
organic beings tend to increase. Every being, which during its natural
lifetime produces several eggs or seeds, must suffer destruction during
some period of its life, and during some season or occasional year,
otherwise, on the principle of geometrical increase, its numbers would
quickly become so inordinately great that no country could support the
product. Hence, as more individuals are produced than can possibly
survive, there must in every case be a struggle for existence, either one
individual with another of the same species, or with the individuals of
distinct species, or with the physical conditions of life. It is the
doctrine of Malthus applied with manifold force to the whole animal and
vegetable kingdoms; for in this case there can be no artificial increase of
food, and no prudential restraint from marriage. Although some species may
be now increasing, more or less rapidly, in numbers, all cannot do so, for
the world would not hold them.
There is no exception to the rule that every organic being naturally
increases at so high a rate, that, if not destroyed, the earth would soon
be covered by the progeny of a single pair. Even slow-breeding man has
doubled in twenty-five years, and at this rate, in less than a thousand
years, there would literally not be standing room for his progeny.
Linnaeus has calculated that if an annual plant produced only two
seeds--and there is no plant so unproductive as this--and their seedlings
next year produced two, and so on, then in twenty years there would be a
million plants. The elephant is reckoned the slowest breeder of all known
animals, and I have taken some pains to estimate its probable minimum rate
of natural increase; it will be safest to assume that it begins breeding
when thirty years old, and goes on breeding till ninety years old, bringing
forth six young in the interval, and surviving till one hundred years old;
if this be so, after a period of from 740 to 750 years there would be
nearly nineteen million elephants alive descended from the first pair.
But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when circumstances
have been favourable to them during two or three following seasons. Still
more striking is the evidence from our domestic animals of many kinds which
have run wild in several parts of the world; if the statements of the rate
of increase of slow-breeding cattle and horses in South America, and
latterly in Australia, had not been well authenticated, they would have
been incredible. So it is with plants; cases could be given of introduced
plants which have become common throughout whole islands in a period of
less than ten years. Several of the plants, such as the cardoon and a tall
thistle, which are now the commonest over the wide plains of La Plata,
clothing square leagues of surface almost to the exclusion of every other
plant, have been introduced from Europe; and there are plants which now
range in India, as I hear from Dr. Falconer, from Cape Comorin to the
Himalaya, which have been imported from America since its discovery. In
such cases, and endless others could be given, no one supposes that the
fertility of the animals or plants has been suddenly and temporarily
increased in any sensible degree. The obvious explanation is that the
conditions of life have been highly favourable, and that there has
consequently been less destruction of the old and young and that nearly all
the young have been enabled to breed. Their geometrical ratio of increase,
the result of which never fails to be surprising, simply explains their
extraordinarily rapid increase and wide diffusion in their new homes.
In a state of nature almost every full-grown plant annually produces seed,
and among animals there are very few which do not annually pair. Hence we
may confidently assert that all plants and animals are tending to increase
at a geometrical ratio--that all would rapidly stock every station in which
they could any how exist, and that this geometrical tendency to increase
must be checked by destruction at some period of life. Our familiarity
with the larger domestic animals tends, I think, to mislead us; we see no
great destruction falling on them, and we do not keep in mind that
thousands are annually slaughtered for food, and that in a state of nature
an equal number would have somehow to be disposed of.
The only difference between organisms which annually produce eggs or seeds
by the thousand, and those which produce extremely few, is, that the slow
breeders would require a few more years to people, under favourable
conditions, a whole district, let it be ever so large. The condor lays a
couple of eggs and the ostrich a score, and yet in the same country the
condor may be the more numerous of the two. The Fulmar petrel lays but one
egg, yet it is believed to be the most numerous bird in the world. One fly
deposits hundreds of eggs, and another, like the hippobosca, a single one.
But this difference does not determine how many individuals of the two
species can be supported in a district. A large number of eggs is of some
importance to those species which depend on a fluctuating amount of food,
for it allows them rapidly to increase in number. But the real importance
of a large number of eggs or seeds is to make up for much destruction at
some period of life; and this period in the great majority of cases is an
early one. If an animal can in any way protect its own eggs or young, a
small number may be produced, and yet the average stock be fully kept up;
but if many eggs or young are destroyed, many must be produced or the
species will become extinct. It would suffice to keep up the full number
of a tree, which lived on an average for a thousand years, if a single seed
were produced once in a thousand years, supposing that this seed were never
destroyed and could be ensured to germinate in a fitting place; so that, in
all cases, the average number of any animal or plant depends only
indirectly on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind--never to forget that every single organic
being may be said to be striving to the utmost to increase in numbers; that
each lives by a struggle at some period of its life; that heavy destruction
inevitably falls either on the young or old during each generation or at
recurrent intervals. Lighten any check, mitigate the destruction ever so
little, and the number of the species will almost instantaneously increase
to any amount.
NATURE OF THE CHECKS TO INCREASE.
The causes which check the natural tendency of each species to increase are
most obscure. Look at the most vigorous species; by as much as it swarms
in numbers, by so much will it tend to increase still further. We know not
exactly what the checks are even in a single instance. Nor will this
surprise any one who reflects how ignorant we are on this head, even in
regard to mankind, although so incomparably better known than any other
animal. This subject of the checks to increase has been ably treated by
several authors, and I hope in a future work to discuss it at considerable
length, more especially in regard to the feral animals of South America.
Here I will make only a few remarks, just to recall to the reader's mind
some of the chief points. Eggs or very young animals seem generally to
suffer most, but this is not invariably the case. With plants there is a
vast destruction of seeds, but from some observations which I have made it
appears that the seedlings suffer most from germinating in ground already
thickly stocked with other plants. Seedlings, also, are destroyed in vast
numbers by various enemies; for instance, on a piece of ground three feet
long and two wide, dug and cleared, and where there could be no choking
from other plants, I marked all the seedlings of our native weeds as they
came up, and out of 357 no less than 295 were destroyed, chiefly by slugs
and insects. If turf which has long been mown, and the case would be the
same with turf closely browsed by quadrupeds, be let to grow, the more
vigorous plants gradually kill the less vigorous, though fully grown
plants; thus out of twenty species grown on a little plot of mown turf
(three feet by four) nine species perished, from the other species being
allowed to grow up freely.
The amount of food for each species, of course, gives the extreme limit to
which each can increase; but very frequently it is not the obtaining food,
but the serving as prey to other animals, which determines the average
number of a species. Thus, there seems to be little doubt that the stock
of partridges, grouse, and hares on any large estate depends chiefly on the
destruction of vermin. If not one head of game were shot during the next
twenty years in England, and, at the same time, if no vermin were
destroyed, there would, in all probability, be less game than at present,
although hundreds of thousands of game animals are now annually shot. On
the other hand, in some cases, as with the elephant, none are destroyed by
beasts of prey; for even the tiger in India most rarely dares to attack a
young elephant protected by its dam.
Climate plays an important part in determining the average numbers of a
species, and periodical seasons of extreme cold or drought seem to be the
most effective of all checks. I estimated (chiefly from the greatly
reduced numbers of nests in the spring) that the winter of 1854-5 destroyed
four-fifths of the birds in my own grounds; and this is a tremendous
destruction, when we remember that ten per cent. is an extraordinarily
severe mortality from epidemics with man. The action of climate seems at
first sight to be quite independent of the struggle for existence; but in
so far as climate chiefly acts in reducing food, it brings on the most
severe struggle between the individuals, whether of the same or of distinct
species, which subsist on the same kind of food. Even when climate, for
instance, extreme cold, acts directly, it will be the least vigorous
individuals, or those which have got least food through the advancing
winter, which will suffer the most. When we travel from south to north, or
from a damp region to a dry, we invariably see some species gradually
getting rarer and rarer, and finally disappearing; and the change of
climate being conspicuous, we are tempted to attribute the whole effect to
its direct action. But this is a false view; we forget that each species,
even where it most abounds, is constantly suffering enormous destruction at
some period of its life, from enemies or from competitors for the same
place and food; and if these enemies or competitors be in the least degree
favoured by any slight change of climate, they will increase in numbers;
and as each area is already fully stocked with inhabitants, the other
species must decrease. When we travel southward and see a species
decreasing in numbers, we may feel sure that the cause lies quite as much
in other species being favoured, as in this one being hurt. So it is when
we travel northward, but in a somewhat lesser degree, for the number of
species of all kinds, and therefore of competitors, decreases northward;
hence in going northward, or in ascending a mountain, we far oftener meet
with stunted forms, due to the DIRECTLY injurious action of climate, than
we do in proceeding southward or in descending a mountain. When we reach
the Arctic regions, or snow-capped summits, or absolute deserts, the
struggle for life is almost exclusively with the elements.
That climate acts in main part indirectly by favouring other species we
clearly see in the prodigious number of plants which in our gardens can
perfectly well endure our climate, but which never become naturalised, for
they cannot compete with our native plants nor resist destruction by our
native animals.
When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics--at least, this seems
generally to occur with our game animals--often ensue; and here we have a
limiting check independent of the struggle for life. But even some of
these so-called epidemics appear to be due to parasitic worms, which have
from some cause, possibly in part through facility of diffusion among the
crowded animals, been disproportionally favoured: and here comes in a sort
of struggle between the parasite and its prey.
On the other hand, in many cases, a large stock of individuals of the same
species, relatively to the numbers of its enemies, is absolutely necessary
for its preservation. Thus we can easily raise plenty of corn and
rape-seed, etc., in our fields, because the seeds are in great excess
compared with the number of birds which feed on them; nor can the birds,
though having a superabundance of food at this one season, increase in
number proportionally to the supply of seed, as their numbers are checked
during the winter; but any one who has tried knows how troublesome it is to
get seed from a few wheat or other such plants in a garden; I have in this
case lost every single seed. This view of the necessity of a large stock
of the same species for its preservation, explains, I believe, some
singular facts in nature such as that of very rare plants being sometimes
extremely abundant, in the few spots where they do exist; and that of some
social plants being social, that is abounding in individuals, even on the
extreme verge of their range. For in such cases, we may believe, that a
plant could exist only where the conditions of its life were so favourable
that many could exist together, and thus save the species from utter
destruction. I should add that the good effects of intercrossing, and the
ill effects of close interbreeding, no doubt come into play in many of
these cases; but I will not here enlarge on this subject.
COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER IN THE STRUGGLE
FOR EXISTENCE.
Many cases are on record showing how complex and unexpected are the checks
and relations between organic beings, which have to struggle together in
the same country. I will give only a single instance, which, though a
simple one, interested me. In Staffordshire, on the estate of a relation,
where I had ample means of investigation, there was a large and extremely
barren heath, which had never been touched by the hand of man; but several
hundred acres of exactly the same nature had been enclosed twenty-five
years previously and planted with Scotch fir. The change in the native
vegetation of the planted part of the heath was most remarkable, more than
is generally seen in passing from one quite different soil to another: not
only the proportional numbers of the heath-plants were wholly changed, but
twelve species of plants (not counting grasses and carices) flourished in
the plantations, which could not be found on the heath. The effect on the
insects must have been still greater, for six insectivorous birds were very
common in the plantations, which were not to be seen on the heath; and the
heath was frequented by two or three distinct insectivorous birds. Here we
see how potent has been the effect of the introduction of a single tree,
nothing whatever else having been done, with the exception of the land
having been enclosed, so that cattle could not enter. But how important an
element enclosure is, I plainly saw near Farnham, in Surrey. Here there
are extensive heaths, with a few clumps of old Scotch firs on the distant
hill-tops: within the last ten years large spaces have been enclosed, and
self-sown firs are now springing up in multitudes, so close together that
all cannot live. When I ascertained that these young trees had not been
sown or planted I was so much surprised at their numbers that I went to
several points of view, whence I could examine hundreds of acres of the
unenclosed heath, and literally I could not see a single Scotch fir, except
the old planted clumps. But on looking closely between the stems of the
heath, I found a multitude of seedlings and little trees, which had been
perpetually browsed down by the cattle. In one square yard, at a point
some hundred yards distant from one of the old clumps, I counted thirty-two
little trees; and one of them, with twenty-six rings of growth, had, during
many years tried to raise its head above the stems of the heath, and had
failed. No wonder that, as soon as the land was enclosed, it became
thickly clothed with vigorously growing young firs. Yet the heath was so
extremely barren and so extensive that no one would ever have imagined that
cattle would have so closely and effectually searched it for food.
Here we see that cattle absolutely determine the existence of the Scotch
fir; but in several parts of the world insects determine the existence of
cattle. Perhaps Paraguay offers the most curious instance of this; for
here neither cattle nor horses nor dogs have ever run wild, though they
swarm southward and northward in a feral state; and Azara and Rengger have
shown that this is caused by the greater number in Paraguay of a certain
fly, which lays its eggs in the navels of these animals when first born.
The increase of these flies, numerous as they are, must be habitually
checked by some means, probably by other parasitic insects. Hence, if
certain insectivorous birds were to decrease in Paraguay, the parasitic
insects would probably increase; and this would lessen the number of the
navel-frequenting flies--then cattle and horses would become feral, and
this would certainly greatly alter (as indeed I have observed in parts of
South America) the vegetation: this again would largely affect the
insects; and this, as we have just seen in Staffordshire, the insectivorous
birds, and so onwards in ever-increasing circles of complexity. Not that
under nature the relations will ever be as simple as this. Battle within
battle must be continually recurring with varying success; and yet in the
long-run the forces are so nicely balanced that the face of nature remains
for long periods of time uniform, though assuredly the merest trifle would
give the victory to one organic being over another. Nevertheless, so
profound is our ignorance, and so high our presumption, that we marvel when
we hear of the extinction of an organic being; and as we do not see the
cause, we invoke cataclysms to desolate the world, or invent laws on the
duration of the forms of life!
I am tempted to give one more instance showing how plants and animals,
remote in the scale of nature, are bound together by a web of complex
relations. I shall hereafter have occasion to show that the exotic Lobelia
fulgens is never visited in my garden by insects, and consequently, from
its peculiar structure, never sets a seed. Nearly all our orchidaceous
plants absolutely require the visits of insects to remove their
pollen-masses and thus to fertilise them. I find from experiments that
humble-bees are almost indispensable to the fertilisation of the heartsease
(Viola tricolor), for other bees do not visit this flower. I have also
found that the visits of bees are necessary for the fertilisation of some
kinds of clover; for instance twenty heads of Dutch clover (Trifolium
repens) yielded 2,290 seeds, but twenty other heads, protected from bees,
produced not one. Again, 100 heads of red clover (T. pratense) produced
2,700 seeds, but the same number of protected heads produced not a single
seed. Humble bees alone visit red clover, as other bees cannot reach the
nectar. It has been suggested that moths may fertilise the clovers; but I
doubt whether they could do so in the case of the red clover, from their
weight not being sufficient to depress the wing petals. Hence we may infer
as highly probable that, if the whole genus of humble-bees became extinct
or very rare in England, the heartsease and red clover would become very
rare, or wholly disappear. The number of humble-bees in any district
depends in a great measure upon the number of field-mice, which destroy
their combs and nests; and Colonel Newman, who has long attended to the
habits of humble-bees, believes that "more than two-thirds of them are thus
destroyed all over England." Now the number of mice is largely dependent,
as every one knows, on the number of cats; and Colonel Newman says, "Near
villages and small towns I have found the nests of humble-bees more
numerous than elsewhere, which I attribute to the number of cats that
destroy the mice." Hence it is quite credible that the presence of a
feline animal in large numbers in a district might determine, through the
intervention first of mice and then of bees, the frequency of certain
flowers in that district!
In the case of every species, many different checks, acting at different
periods of life, and during different seasons or years, probably come into
play; some one check or some few being generally the most potent, but all
will concur in determining the average number, or even the existence of the
species. In some cases it can be shown that widely-different checks act on
the same species in different districts. When we look at the plants and
bushes clothing an entangled bank, we are tempted to attribute their
proportional numbers and kinds to what we call chance. But how false a
view is this! Every one has heard that when an American forest is cut
down, a very different vegetation springs up; but it has been observed that
ancient Indian ruins in the Southern United States, which must formerly
have been cleared of trees, now display the same beautiful diversity and
proportion of kinds as in the surrounding virgin forests. What a struggle
must have gone on during long centuries between the several kinds of trees,
each annually scattering its seeds by the thousand; what war between insect
and insect--between insects, snails, and other animals with birds and
beasts of prey--all striving to increase, all feeding on each other, or on
the trees, their seeds and seedlings, or on the other plants which first
clothed the ground and thus checked the growth of the trees. Throw up a
handful of feathers, and all fall to the ground according to definite laws;
but how simple is the problem where each shall fall compared to that of the
action and reaction of the innumerable plants and animals which have
determined, in the course of centuries, the proportional numbers and kinds
of trees now growing on the old Indian ruins!
The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in the scale of nature. This is
likewise sometimes the case with those which may strictly be said to
struggle with each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle will almost invariably be most
severe between the individuals of the same species, for they frequent the
same districts, require the same food, and are exposed to the same dangers.
In the case of varieties of the same species, the struggle will generally
be almost equally severe, and we sometimes see the contest soon decided:
for instance, if several varieties of wheat be sown together, and the mixed
seed be resown, some of the varieties which best suit the soil or climate,
or are naturally the most fertile, will beat the others and so yield more
seed, and will consequently in a few years supplant the other varieties.
To keep up a mixed stock of even such extremely close varieties as the
variously coloured sweet-peas, they must be each year harvested separately,
and the seed then mixed in due proportion, otherwise the weaker kinds will
steadily decrease in number and disappear. So again with the varieties of
sheep: it has been asserted that certain mountain-varieties will starve
out other mountain-varieties, so that they cannot be kept together. The
same result has followed from keeping together different varieties of the
medicinal leech. It may even be doubted whether the varieties of any of
our domestic plants or animals have so exactly the same strength, habits,
and constitution, that the original proportions of a mixed stock (crossing
being prevented) could be kept up for half-a-dozen generations, if they
were allowed to struggle together, in the same manner as beings in a state
of nature, and if the seed or young were not annually preserved in due
proportion.
STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF THE SAME
SPECIES.
As the species of the same genus usually have, though by no means
invariably, much similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between them, if they
come into competition with each other, than between the species of distinct
genera. We see this in the recent extension over parts of the United
States of one species of swallow having caused the decrease of another
species. The recent increase of the missel-thrush in parts of Scotland has
caused the decrease of the song-thrush. How frequently we hear of one
species of rat taking the place of another species under the most different
climates! In Russia the small Asiatic cockroach has everywhere driven
before it its great congener. In Australia the imported hive-bee is
rapidly exterminating the small, stingless native bee. One species of
charlock has been known to supplant another species; and so in other cases.
We can dimly see why the competition should be most severe between allied
forms, which fill nearly the same place in the economy of nature; but
probably in no one case could we precisely say why one species has been
victorious over another in the great battle of life.
A corollary of the highest importance may be deduced from the foregoing
remarks, namely, that the structure of every organic being is related, in
the most essential yet often hidden manner, to that of all other organic
beings, with which it comes into competition for food or residence, or from
which it has to escape, or on which it preys. This is obvious in the
structure of the teeth and talons of the tiger; and in that of the legs and
claws of the parasite which clings to the hair on the tiger's body. But in
the beautifully plumed seed of the dandelion, and in the flattened and
fringed legs of the water-beetle, the relation seems at first confined to
the elements of air and water. Yet the advantage of the plumed seeds no
doubt stands in the closest relation to the land being already thickly
clothed with other plants; so that the seeds may be widely distributed and
fall on unoccupied ground. In the water-beetle, the structure of its legs,
so well adapted for diving, allows it to compete with other aquatic
insects, to hunt for its own prey, and to escape serving as prey to other
animals.
The store of nutriment laid up within the seeds of many plants seems at
first sight to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds, as peas and beans,
when sown in the midst of long grass, it may be suspected that the chief
use of the nutriment in the seed is to favour the growth of the seedlings,
whilst struggling with other plants growing vigorously all around.
Look at a plant in the midst of its range! Why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges into
slightly hotter or colder, damper or drier districts. In this case we can
clearly see that if we wish in imagination to give the plant the power of
increasing in numbers, we should have to give it some advantage over its
competitors, or over the animals which prey on it. On the confines of its
geographical range, a change of constitution with respect to climate would
clearly be an advantage to our plant; but we have reason to believe that
only a few plants or animals range so far, that they are destroyed
exclusively by the rigour of the climate. Not until we reach the extreme
confines of life, in the Arctic regions or on the borders of an utter
desert, will competition cease. The land may be extremely cold or dry, yet
there will be competition between some few species, or between the
individuals of the same species, for the warmest or dampest spots.
Hence we can see that when a plant or animal is placed in a new country,
among new competitors, the conditions of its life will generally be changed
in an essential manner, although the climate may be exactly the same as in
its former home. If its average numbers are to increase in its new home,
we should have to modify it in a different way to what we should have had
to do in its native country; for we should have to give it some advantage
over a different set of competitors or enemies.
It is good thus to try in imagination to give any one species an advantage
over another. Probably in no single instance should we know what to do.
This ought to convince us of our ignorance on the mutual relations of all
organic beings; a conviction as necessary, as it is difficult to acquire.
All that we can do is to keep steadily in mind that each organic being is
striving to increase in a geometrical ratio; that each, at some period of
its life, during some season of the year, during each generation, or at
intervals, has to struggle for life and to suffer great destruction. When
we reflect on this struggle we may console ourselves with the full belief
that the war of nature is not incessant, that no fear is felt, that death
is generally prompt, and that the vigorous, the healthy, and the happy
survive and multiply.
CHAPTER IV.
NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
Natural Selection -- its power compared with man's selection -- its power
on characters of trifling importance -- its power at all ages and on both
sexes -- Sexual Selection -- On the generality of intercrosses between
individuals of the same species -- Circumstances favourable and
unfavourable to the results of Natural Selection, namely, intercrossing,
isolation, number of individuals -- Slow action -- Extinction caused by
Natural Selection -- Divergence of Character, related to the diversity of
inhabitants of any small area and to naturalisation -- Action of Natural
Selection, through Divergence of Character and Extinction, on the
descendants from a common parent -- Explains the Grouping of all organic
beings -- Advance in organisation -- Low forms preserved -- Convergence of
character -- Indefinite multiplication of species -- Summary.
How will the struggle for existence, briefly discussed in the last chapter,
act in regard to variation? Can the principle of selection, which we have
seen is so potent in the hands of man, apply under nature? I think we
shall see that it can act most efficiently. Let the endless number of
slight variations and individual differences occurring in our domestic
productions, and, in a lesser degree, in those under nature, be borne in
mind; as well as the strength of the hereditary tendency. Under
domestication, it may truly be said that the whole organisation becomes in
some degree plastic. But the variability, which we almost universally meet
with in our domestic productions is not directly produced, as Hooker and
Asa Gray have well remarked, by man; he can neither originate varieties nor
prevent their occurrence; he can only preserve and accumulate such as do
occur. Unintentionally he exposes organic beings to new and changing
conditions of life, and variability ensues; but similar changes of
conditions might and do occur under nature. Let it also be borne in mind
how infinitely complex and close-fitting are the mutual relations of all
organic beings to each other and to their physical conditions of life; and
consequently what infinitely varied diversities of structure might be of
use to each being under changing conditions of life. Can it then be
thought improbable, seeing that variations useful to man have undoubtedly
occurred, that other variations useful in some way to each being in the
great and complex battle of life, should occur in the course of many
successive generations? If such do occur, can we doubt (remembering that
many more individuals are born than can possibly survive) that individuals
having any advantage, however slight, over others, would have the best
chance of surviving and procreating their kind? On the other hand, we may
feel sure that any variation in the least degree injurious would be rigidly
destroyed. This preservation of favourable individual differences and
variations, and the destruction of those which are injurious, I have called
Natural Selection, or the Survival of the Fittest. Variations neither
useful nor injurious would not be affected by natural selection, and would
be left either a fluctuating element, as perhaps we see in certain
polymorphic species, or would ultimately become fixed, owing to the nature
of the organism and the nature of the conditions.
Several writers have misapprehended or objected to the term Natural
Selection. Some have even imagined that natural selection induces
variability, whereas it implies only the preservation of such variations as
arise and are beneficial to the being under its conditions of life. No one
objects to agriculturists speaking of the potent effects of man's
selection; and in this case the individual differences given by nature,
which man for some object selects, must of necessity first occur. Others
have objected that the term selection implies conscious choice in the
animals which become modified; and it has even been urged that, as plants
have no volition, natural selection is not applicable to them! In the
literal sense of the word, no doubt, natural selection is a false term; but
who ever objected to chemists speaking of the elective affinities of the
various elements?--and yet an acid cannot strictly be said to elect the
base with which it in preference combines. It has been said that I speak
of natural selection as an active power or Deity; but who objects to an
author speaking of the attraction of gravity as ruling the movements of the
planets? Every one knows what is meant and is implied by such metaphorical
expressions; and they are almost necessary for brevity. So again it is
difficult to avoid personifying the word Nature; but I mean by nature, only
the aggregate action and product of many natural laws, and by laws the
sequence of events as ascertained by us. With a little familiarity such
superficial objections will be forgotten.
We shall best understand the probable course of natural selection by taking
the case of a country undergoing some slight physical change, for instance,
of climate. The proportional numbers of its inhabitants will almost
immediately undergo a change, and some species will probably become
extinct. We may conclude, from what we have seen of the intimate and
complex manner in which the inhabitants of each country are bound together,
that any change in the numerical proportions of the inhabitants,
independently of the change of climate itself, would seriously affect the
others. If the country were open on its borders, new forms would certainly
immigrate, and this would likewise seriously disturb the relations of some
of the former inhabitants. Let it be remembered how powerful the influence
of a single introduced tree or mammal has been shown to be. But in the
case of an island, or of a country partly surrounded by barriers, into
which new and better adapted forms could not freely enter, we should then
have places in the economy of nature which would assuredly be better filled
up if some of the original inhabitants were in some manner modified; for,
had the area been open to immigration, these same places would have been
seized on by intruders. In such cases, slight modifications, which in any
way favoured the individuals of any species, by better adapting them to
their altered conditions, would tend to be preserved; and natural selection
would have free scope for the work of improvement.
We have good reason to believe, as shown in the first chapter, that changes
in the conditions of life give a tendency to increased variability; and in
the foregoing cases the conditions the changed, and this would manifestly
be favourable to natural selection, by affording a better chance of the
occurrence of profitable variations. Unless such occur, natural selection
can do nothing. Under the term of "variations," it must never be forgotten
that mere individual differences are included. As man can produce a great
result with his domestic animals and plants by adding up in any given
direction individual differences, so could natural selection, but far more
easily from having incomparably longer time for action. Nor do I believe
that any great physical change, as of climate, or any unusual degree of
isolation, to check immigration, is necessary in order that new and
unoccupied places should be left for natural selection to fill up by
improving some of the varying inhabitants. For as all the inhabitants of
each country are struggling together with nicely balanced forces, extremely
slight modifications in the structure or habits of one species would often
give it an advantage over others; and still further modifications of the
same kind would often still further increase the advantage, as long as the
species continued under the same conditions of life and profited by similar
means of subsistence and defence. No country can be named in which all the
native inhabitants are now so perfectly adapted to each other and to the
physical conditions under which they live, that none of them could be still
better adapted or improved; for in all countries, the natives have been so
far conquered by naturalised productions that they have allowed some
foreigners to take firm possession of the land. And as foreigners have
thus in every country beaten some of the natives, we may safely conclude
that the natives might have been modified with advantage, so as to have
better resisted the intruders.
As man can produce, and certainly has produced, a great result by his
methodical and unconscious means of selection, what may not natural
selection effect? Man can act only on external and visible characters:
Nature, if I may be allowed to personify the natural preservation or
survival of the fittest, cares nothing for appearances, except in so far as
they are useful to any being. She can act on every internal organ, on
every shade of constitutional difference, on the whole machinery of life.
Man selects only for his own good; Nature only for that of the being which
she tends. Every selected character is fully exercised by her, as is
implied by the fact of their selection. Man keeps the natives of many
climates in the same country. He seldom exercises each selected character
in some peculiar and fitting manner; he feeds a long and a short-beaked
pigeon on the same food; he does not exercise a long-backed or long-legged
quadruped in any peculiar manner; he exposes sheep with long and short wool
to the same climate; does not allow the most vigorous males to struggle for
the females; he does not rigidly destroy all inferior animals, but protects
during each varying season, as far as lies in his power, all his
productions. He often begins his selection by some half-monstrous form, or
at least by some modification prominent enough to catch the eye or to be
plainly useful to him. Under nature, the slightest differences of
structure or constitution may well turn the nicely-balanced scale in the
struggle for life, and so be preserved. How fleeting are the wishes and
efforts of man! How short his time, and consequently how poor will be his
results, compared with those accumulated by Nature during whole geological
periods! Can we wonder, then, that Nature's productions should be far
"truer" in character than man's productions; that they should be infinitely
better adapted to the most complex conditions of life, and should plainly
bear the stamp of far higher workmanship?
It may metaphorically be said that natural selection is daily and hourly
scrutinising, throughout the world, the slightest variations; rejecting
those that are bad, preserving and adding up all that are good; silently
and insensibly working, WHENEVER AND WHEREVER OPPORTUNITY OFFERS, at the
improvement of each organic being in relation to its organic and inorganic
conditions of life. We see nothing of these slow changes in progress,
until the hand of time has marked the long lapse of ages, and then so
imperfect is our view into long-past geological ages that we see only that
the forms of life are now different from what they formerly were.
In order that any great amount of modification should be effected in a
species, a variety, when once formed must again, perhaps after a long
interval of time, vary or present individual differences of the same
favourable nature as before; and these must again be preserved, and so
onward, step by step. Seeing that individual differences of the same kind
perpetually recur, this can hardly be considered as an unwarrantable
assumption. But whether it is true, we can judge only by seeing how far
the hypothesis accords with and explains the general phenomena of nature.
On the other hand, the ordinary belief that the amount of possible
variation is a strictly limited quantity, is likewise a simple assumption.
Although natural selection can act only through and for the good of each
being, yet characters and structures, which we are apt to consider as of
very trifling importance, may thus be acted on. When we see leaf-eating
insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in
winter, the red-grouse the colour of heather, we must believe that these
tints are of service to these birds and insects in preserving them from
danger. Grouse, if not destroyed at some period of their lives, would
increase in countless numbers; they are known to suffer largely from birds
of prey; and hawks are guided by eyesight to their prey,--so much so that
on parts of the continent persons are warned not to keep white pigeons, as
being the most liable to destruction. Hence natural selection might be
effective in giving the proper colour to each kind of grouse, and in
keeping that colour, when once acquired, true and constant. Nor ought we
to think that the occasional destruction of an animal of any particular
colour would produce little effect; we should remember how essential it is
in a flock of white sheep to destroy a lamb with the faintest trace of
black. We have seen how the colour of hogs, which feed on the "paint-root"
in Virginia, determines whether they shall live or die. In plants, the
down on the fruit and the colour of the flesh are considered by botanists
as characters of the most trifling importance; yet we hear from an
excellent horticulturist, Downing, that in the United States smooth-skinned
fruits suffer far more from a beetle, a Curculio, than those with down;
that purple plums suffer far more from a certain disease than yellow plums;
whereas another disease attacks yellow-fleshed peaches far more than those
with other coloured flesh. If, with all the aids of art, these slight
differences make a great difference in cultivating the several varieties,
assuredly, in a state of nature, where the trees would have to struggle
with other trees and with a host of enemies, such differences would
effectually settle which variety, whether a smooth or downy, a yellow or a
purple-fleshed fruit, should succeed.
In looking at many small points of difference between species, which, as
far as our ignorance permits us to judge, seem quite unimportant, we must
not forget that climate, food, etc., have no doubt produced some direct
effect. It is also necessary to bear in mind that, owing to the law of
correlation, when one part varies and the variations are accumulated
through natural selection, other modifications, often of the most
unexpected nature, will ensue.
As we see that those variations which, under domestication, appear at any
particular period of life, tend to reappear in the offspring at the same
period; for instance, in the shape, size and flavour of the seeds of the
many varieties of our culinary and agricultural plants; in the caterpillar
and cocoon stages of the varieties of the silkworm; in the eggs of poultry,
and in the colour of the down of their chickens; in the horns of our sheep
and cattle when nearly adult; so in a state of nature natural selection
will be enabled to act on and modify organic beings at any age, by the
accumulation of variations profitable at that age, and by their inheritance
at a corresponding age. If it profit a plant to have its seeds more and
more widely disseminated by the wind, I can see no greater difficulty in
this being effected through natural selection, than in the cotton-planter
increasing and improving by selection the down in the pods on his
cotton-trees. Natural selection may modify and adapt the larva of an
insect to a score of contingencies, wholly different from those which
concern the mature insect; and these modifications may affect, through
correlation, the structure of the adult. So, conversely, modifications in
the adult may affect the structure of the larva; but in all cases natural
selection will ensure that they shall not be injurious: for if they were
so, the species would become extinct.
Natural selection will modify the structure of the young in relation to the
parent and of the parent in relation to the young. In social animals it
will adapt the structure of each individual for the benefit of the whole
community; if the community profits by the selected change. What natural
selection cannot do, is to modify the structure of one species, without
giving it any advantage, for the good of another species; and though
statements to this effect may be found in works of natural history, I
cannot find one case which will bear investigation. A structure used only
once in an animal's life, if of high importance to it, might be modified to
any extent by natural selection; for instance, the great jaws possessed by
certain insects, used exclusively for opening the cocoon--or the hard tip
to the beak of unhatched birds, used for breaking the eggs. It has been
asserted, that of the best short-beaked tumbler-pigeons a greater number
perish in the egg than are able to get out of it; so that fanciers assist
in the act of hatching. Now, if nature had to make the beak of a
full-grown pigeon very short for the bird's own advantage, the process of
modification would be very slow, and there would be simultaneously the most
rigorous selection of all the young birds within the egg, which had the
most powerful and hardest beaks, for all with weak beaks would inevitably
perish: or, more delicate and more easily broken shells might be selected,
the thickness of the shell being known to vary like every other structure.
It may be well here to remark that with all beings there must be much
fortuitous destruction, which can have little or no influence on the course
of natural selection. For instance, a vast number of eggs or seeds are
annually devoured, and these could be modified through natural selection
only if they varied in some manner which protected them from their enemies.
Yet many of these eggs or seeds would perhaps, if not destroyed, have
yielded individuals better adapted to their conditions of life than any of
those which happened to survive. So again a vast number of mature animals
and plants, whether or not they be the best adapted to their conditions,
must be annually destroyed by accidental causes, which would not be in the
least degree mitigated by certain changes of structure or constitution
which would in other ways be beneficial to the species. But let the
destruction of the adults be ever so heavy, if the number which can exist
in any district be not wholly kept down by such causes--or again let the
destruction of eggs or seeds be so great that only a hundredth or a
thousandth part are developed--yet of those which do survive, the best
adapted individuals, supposing that there is any variability in a
favourable direction, will tend to propagate their kind in larger numbers
than the less well adapted. If the numbers be wholly kept down by the
causes just indicated, as will often have been the case, natural selection
will be powerless in certain beneficial directions; but this is no valid
objection to its efficiency at other times and in other ways; for we are
far from having any reason to suppose that many species ever undergo
modification and improvement at the same time in the same area.
SEXUAL SELECTION.
Inasmuch as peculiarities often appear under domestication in one sex and
become hereditarily attached to that sex, so no doubt it will be under
nature. Thus it is rendered possible for the two sexes to be modified
through natural selection in relation to different habits of life, as is
sometimes the case; or for one sex to be modified in relation to the other
sex, as commonly occurs. This leads me to say a few words on what I have
called sexual selection. This form of selection depends, not on a struggle
for existence in relation to other organic beings or to external
conditions, but on a struggle between the individuals of one sex, generally
the males, for the possession of the other sex. The result is not death to
the unsuccessful competitor, but few or no offspring. Sexual selection is,
therefore, less rigorous than natural selection. Generally, the most
vigorous males, those which are best fitted for their places in nature,
will leave most progeny. But in many cases victory depends not so much on
general vigour, but on having special weapons, confined to the male sex. A
hornless stag or spurless cock would have a poor chance of leaving numerous
offspring. Sexual selection, by always allowing the victor to breed, might
surely give indomitable courage, length of spur, and strength to the wing
to strike in the spurred leg, in nearly the same manner as does the brutal
cockfighter by the careful selection of his best cocks. How low in the
scale of nature the law of battle descends I know not; male alligators have
been described as fighting, bellowing, and whirling round, like Indians in
a war-dance, for the possession of the females; male salmons have been
observed fighting all day long; male stag-beetles sometimes bear wounds
from the huge mandibles of other males; the males of certain hymenopterous
insects have been frequently seen by that inimitable observer M. Fabre,
fighting for a particular female who sits by, an apparently unconcerned
beholder of the struggle, and then retires with the conqueror. The war is,
perhaps, severest between the males of polygamous animals, and these seem
oftenest provided with special weapons. The males of carnivorous animals
are already well armed; though to them and to others, special means of
defence may be given through means of sexual selection, as the mane of the
lion, and the hooked jaw to the male salmon; for the shield may be as
important for victory as the sword or spear.
Among birds, the contest is often of a more peaceful character. All those
who have attended to the subject, believe that there is the severest
rivalry between the males of many species to attract, by singing, the
females. The rock-thrush of Guiana, birds of paradise, and some others,
congregate, and successive males display with the most elaborate care, and
show off in the best manner, their gorgeous plumage; they likewise perform
strange antics before the females, which, standing by as spectators, at
last choose the most attractive partner. Those who have closely attended
to birds in confinement well know that they often take individual
preferences and dislikes: thus Sir R. Heron has described how a pied
peacock was eminently attractive to all his hen birds. I cannot here enter
on the necessary details; but if man can in a short time give beauty and an
elegant carriage to his bantams, according to his standard of beauty, I can
see no good reason to doubt that female birds, by selecting, during
thousands of generations, the most melodious or beautiful males, according
to their standard of beauty, might produce a marked effect. Some
well-known laws, with respect to the plumage of male and female birds, in
comparison with the plumage of the young, can partly be explained through
the action of sexual selection on variations occurring at different ages,
and transmitted to the males alone or to both sexes at corresponding ages;
but I have not space here to enter on this subject.
Thus it is, as I believe, that when the males and females of any animal
have the same general habits of life, but differ in structure, colour, or
ornament, such differences have been mainly caused by sexual selection:
that is, by individual males having had, in successive generations, some
slight advantage over other males, in their weapons, means of defence, or
charms; which they have transmitted to their male offspring alone. Yet, I
would not wish to attribute all sexual differences to this agency: for we
see in our domestic animals peculiarities arising and becoming attached to
the male sex, which apparently have not been augmented through selection by
man. The tuft of hair on the breast of the wild turkey-cock cannot be of
any use, and it is doubtful whether it can be ornamental in the eyes of the
female bird; indeed, had the tuft appeared under domestication it would
have been called a monstrosity.
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE SURVIVAL OF THE
FITTEST.
In order to make it clear how, as I believe, natural selection acts, I must
beg permission to give one or two imaginary illustrations. Let us take the
case of a wolf, which preys on various animals, securing some by craft,
some by strength, and some by fleetness; and let us suppose that the
fleetest prey, a deer for instance, had from any change in the country
increased in numbers, or that other prey had decreased in numbers, during
that season of the year when the wolf was hardest pressed for food. Under
such circumstances the swiftest and slimmest wolves have the best chance of
surviving, and so be preserved or selected, provided always that they
retained strength to master their prey at this or some other period of the
year, when they were compelled to prey on other animals. I can see no more
reason to doubt that this would be the result, than that man should be able
to improve the fleetness of his greyhounds by careful and methodical
selection, or by that kind of unconscious selection which follows from each
man trying to keep the best dogs without any thought of modifying the
breed. I may add that, according to Mr. Pierce, there are two varieties of
the wolf inhabiting the Catskill Mountains, in the United States, one with
a light greyhound-like form, which pursues deer, and the other more bulky,
with shorter legs, which more frequently attacks the shepherd's flocks.
Even without any change in the proportional numbers of the animals on which
our wolf preyed, a cub might be born with an innate tendency to pursue
certain kinds of prey. Nor can this be thought very improbable; for we
often observe great differences in the natural tendencies of our domestic
animals; one cat, for instance, taking to catch rats, another mice; one
cat, according to Mr. St. John, bringing home winged game, another hares or
rabbits, and another hunting on marshy ground and almost nightly catching
woodcocks or snipes. The tendency to catch rats rather than mice is known
to be inherited. Now, if any slight innate change of habit or of structure
benefited an individual wolf, it would have the best chance of surviving
and of leaving offspring. Some of its young would probably inherit the
same habits or structure, and by the repetition of this process, a new
variety might be formed which would either supplant or coexist with the
parent-form of wolf. Or, again, the wolves inhabiting a mountainous
district, and those frequenting the lowlands, would naturally be forced to
hunt different prey; and from the continued preservation of the individuals
best fitted for the two sites, two varieties might slowly be formed. These
varieties would cross and blend where they met; but to this subject of
intercrossing we shall soon have to return. I may add, that, according to
Mr. Pierce, there are two varieties of the wolf inhabiting the Catskill
Mountains in the United States, one with a light greyhound-like form, which
pursues deer, and the other more bulky, with shorter legs, which more
frequently attacks the shepherd's flocks.
It should be observed that in the above illustration, I speak of the
slimmest individual wolves, and not of any single strongly marked variation
having been preserved. In former editions of this work I sometimes spoke
as if this latter alternative had frequently occurred. I saw the great
importance of individual differences, and this led me fully to discuss the
results of unconscious selection by man, which depends on the preservation
of all the more or less valuable individuals, and on the destruction of the
worst. I saw, also, that the preservation in a state of nature of any
occasional deviation of structure, such as a monstrosity, would be a rare
event; and that, if at first preserved, it would generally be lost by
subsequent intercrossing with ordinary individuals. Nevertheless, until
reading an able and valuable article in the "North British Review" (1867),
I did not appreciate how rarely single variations, whether slight or
strongly marked, could be perpetuated. The author takes the case of a pair
of animals, producing during their lifetime two hundred offspring, of
which, from various causes of destruction, only two on an average survive
to pro-create their kind. This is rather an extreme estimate for most of
the higher animals, but by no means so for many of the lower organisms. He
then shows that if a single individual were born, which varied in some
manner, giving it twice as good a chance of life as that of the other
individuals, yet the chances would be strongly against its survival.
Supposing it to survive and to breed, and that half its young inherited the
favourable variation; still, as the Reviewer goes onto show, the young
would have only a slightly better chance of surviving and breeding; and
this chance would go on decreasing in the succeeding generations. The
justice of these remarks cannot, I think, be disputed. If, for instance, a
bird of some kind could procure its food more easily by having its beak
curved, and if one were born with its beak strongly curved, and which
consequently flourished, nevertheless there would be a very poor chance of
this one individual perpetuating its kind to the exclusion of the common
form; but there can hardly be a doubt, judging by what we see taking place
under domestication, that this result would follow from the preservation
during many generations of a large number of individuals with more or less
strongly curved beaks, and from the destruction of a still larger number
with the straightest beaks.
It should not, however, be overlooked that certain rather strongly marked
variations, which no one would rank as mere individual differences,
frequently recur owing to a similar organisation being similarly acted on--
of which fact numerous instances could be given with our domestic
productions. In such cases, if the varying individual did not actually
transmit to its offspring its newly-acquired character, it would
undoubtedly transmit to them, as long as the existing conditions remained
the same, a still stronger tendency to vary in the same manner. There can
also be little doubt that the tendency to vary in the same manner has often
been so strong that all the individuals of the same species have been
similarly modified without the aid of any form of selection. Or only a
third, fifth, or tenth part of the individuals may have been thus affected,
of which fact several instances could be given. Thus Graba estimates that
about one-fifth of the guillemots in the Faroe Islands consist of a variety
so well marked, that it was formerly ranked as a distinct species under the
name of Uria lacrymans. In cases of this kind, if the variation were of a
beneficial nature, the original form would soon be supplanted by the
modified form, through the survival of the fittest.
To the effects of intercrossing in eliminating variations of all kinds, I
shall have to recur; but it may be here remarked that most animals and
plants keep to their proper homes, and do not needlessly wander about; we
see this even with migratory birds, which almost always return to the same
spot. Consequently each newly-formed variety would generally be at first
local, as seems to be the common rule with varieties in a state of nature;
so that similarly modified individuals would soon exist in a small body
together, and would often breed together. If the new variety were
successful in its battle for life, it would slowly spread from a central
district, competing with and conquering the unchanged individuals on the
margins of an ever-increasing circle.
It may be worth while to give another and more complex illustration of the
action of natural selection. Certain plants excrete sweet juice,
apparently for the sake of eliminating something injurious from the sap:
this is effected, for instance, by glands at the base of the stipules in
some Leguminosae, and at the backs of the leaves of the common laurel.
This juice, though small in quantity, is greedily sought by insects; but
their visits do not in any way benefit the plant. Now, let us suppose that
the juice or nectar was excreted from the inside of the flowers of a
certain number of plants of any species. Insects in seeking the nectar
would get dusted with pollen, and would often transport it from one flower
to another. The flowers of two distinct individuals of the same species
would thus get crossed; and the act of crossing, as can be fully proved,
gives rise to vigorous seedlings, which consequently would have the best
chance of flourishing and surviving. The plants which produced flowers
with the largest glands or nectaries, excreting most nectar, would oftenest
be visited by insects, and would oftenest be crossed; and so in the
long-run would gain the upper hand and form a local variety. The flowers,
also, which had their stamens and pistils placed, in relation to the size
and habits of the particular insect which visited them, so as to favour in
any degree the transportal of the pollen, would likewise be favoured. We
might have taken the case of insects visiting flowers for the sake of
collecting pollen instead of nectar; and as pollen is formed for the sole
purpose of fertilisation, its destruction appears to be a simple loss to
the plant; yet if a little pollen were carried, at first occasionally and
then habitually, by the pollen-devouring insects from flower to flower, and
a cross thus effected, although nine-tenths of the pollen were destroyed it
might still be a great gain to the plant to be thus robbed; and the
individuals which produced more and more pollen, and had larger anthers,
would be selected.
When our plant, by the above process long continued, had been rendered
highly attractive to insects, they would, unintentionally on their part,
regularly carry pollen from flower to flower; and that they do this
effectually I could easily show by many striking facts. I will give only
one, as likewise illustrating one step in the separation of the sexes of
plants. Some holly-trees bear only male flowers, which have four stamens
producing a rather small quantity of pollen, and a rudimentary pistil;
other holly-trees bear only female flowers; these have a full-sized pistil,
and four stamens with shrivelled anthers, in which not a grain of pollen
can be detected. Having found a female tree exactly sixty yards from a
male tree, I put the stigmas of twenty flowers, taken from different
branches, under the microscope, and on all, without exception, there were a
few pollen-grains, and on some a profusion. As the wind had set for
several days from the female to the male tree, the pollen could not thus
have been carried. The weather had been cold and boisterous and therefore
not favourable to bees, nevertheless every female flower which I examined
had been effectually fertilised by the bees, which had flown from tree to
tree in search of nectar. But to return to our imaginary case; as soon as
the plant had been rendered so highly attractive to insects that pollen was
regularly carried from flower to flower, another process might commence.
No naturalist doubts the advantage of what has been called the
"physiological division of labour;" hence we may believe that it would be
advantageous to a plant to produce stamens alone in one flower or on one
whole plant, and pistils alone in another flower or on another plant. In
plants under culture and placed under new conditions of life, sometimes the
male organs and sometimes the female organs become more or less impotent;
now if we suppose this to occur in ever so slight a degree under nature,
then, as pollen is already carried regularly from flower to flower, and as
a more complete separation of the sexes of our plant would be advantageous
on the principle of the division of labour, individuals with this tendency
more and more increased, would be continually favoured or selected, until
at last a complete separation of the sexes might be effected. It would
take up too much space to show the various steps, through dimorphism and
other means, by which the separation of the sexes in plants of various
kinds is apparently now in progress; but I may add that some of the species
of holly in North America are, according to Asa Gray, in an exactly
intermediate condition, or, as he expresses it, are more or less
dioeciously polygamous.
Let us now turn to the nectar-feeding insects; we may suppose the plant of
which we have been slowly increasing the nectar by continued selection, to
be a common plant; and that certain insects depended in main part on its
nectar for food. I could give many facts showing how anxious bees are to
save time: for instance, their habit of cutting holes and sucking the
nectar at the bases of certain flowers, which with a very little more
trouble they can enter by the mouth. Bearing such facts in mind, it may be
believed that under certain circumstances individual differences in the
curvature or length of the proboscis, etc., too slight to be appreciated by
us, might profit a bee or other insect, so that certain individuals would
be able to obtain their food more quickly than others; and thus the
communities to which they belonged would flourish and throw off many swarms
inheriting the same peculiarities. The tubes of the corolla of the common
red or incarnate clovers (Trifolium pratense and incarnatum) do not on a
hasty glance appear to differ in length; yet the hive-bee can easily suck
the nectar out of the incarnate clover, but not out of the common red
clover, which is visited by humble-bees alone; so that whole fields of the
red clover offer in vain an abundant supply of precious nectar to the
hive-bee. That this nectar is much liked by the hive-bee is certain; for I
have repeatedly seen, but only in the autumn, many hive-bees sucking the
flowers through holes bitten in the base of the tube by humble bees. The
difference in the length of the corolla in the two kinds of clover, which
determines the visits of the hive-bee, must be very trifling; for I have
been assured that when red clover has been mown, the flowers of the second
crop are somewhat smaller, and that these are visited by many hive-bees. I
do not know whether this statement is accurate; nor whether another
published statement can be trusted, namely, that the Ligurian bee, which is
generally considered a mere variety of the common hive-bee, and which
freely crosses with it, is able to reach and suck the nectar of the red
clover. Thus, in a country where this kind of clover abounded, it might be
a great advantage to the hive-bee to have a slightly longer or differently
constructed proboscis. On the other hand, as the fertility of this clover
absolutely depends on bees visiting the flowers, if humble-bees were to
become rare in any country, it might be a great advantage to the plant to
have a shorter or more deeply divided corolla, so that the hive-bees should
be enabled to suck its flowers. Thus I can understand how a flower and a
bee might slowly become, either simultaneously or one after the other,
modified and adapted to each other in the most perfect manner, by the
continued preservation of all the individuals which presented slight
deviations of structure mutually favourable to each other.
I am well aware that this doctrine of natural selection, exemplified in the
above imaginary instances, is open to the same objections which were first
urged against Sir Charles Lyell's noble views on "the modern changes of the
earth, as illustrative of geology;" but we now seldom hear the agencies
which we see still at work, spoken of as trifling and insignificant, when
used in explaining the excavation of the deepest valleys or the formation
of long lines of inland cliffs. Natural selection acts only by the
preservation and accumulation of small inherited modifications, each
profitable to the preserved being; and as modern geology has almost
banished such views as the excavation of a great valley by a single
diluvial wave, so will natural selection banish the belief of the continued
creation of new organic beings, or of any great and sudden modification in
their structure.
ON THE INTERCROSSING OF INDIVIDUALS.
I must here introduce a short digression. In the case of animals and
plants with separated sexes, it is of course obvious that two individuals
must always (with the exception of the curious and not well understood
cases of parthenogenesis) unite for each birth; but in the case of
hermaphrodites this is far from obvious. Nevertheless there is reason to
believe that with all hermaphrodites two individuals, either occasionally
or habitually, concur for the reproduction of their kind. This view was
long ago doubtfully suggested by Sprengel, Knight and Kolreuter. We shall
presently see its importance; but I must here treat the subject with
extreme brevity, though I have the materials prepared for an ample
discussion. All vertebrate animals, all insects and some other large
groups of animals, pair for each birth. Modern research has much
diminished the number of supposed hermaphrodites and of real hermaphrodites
a large number pair; that is, two individuals regularly unite for
reproduction, which is all that concerns us. But still there are many
hermaphrodite animals which certainly do not habitually pair, and a vast
majority of plants are hermaphrodites. What reason, it may be asked, is
there for supposing in these cases that two individuals ever concur in
reproduction? As it is impossible here to enter on details, I must trust
to some general considerations alone.
In the first place, I have collected so large a body of facts, and made so
many experiments, showing, in accordance with the almost universal belief
of breeders, that with animals and plants a cross between different
varieties, or between individuals of the same variety but of another
strain, gives vigour and fertility to the offspring; and on the other hand,
that CLOSE interbreeding diminishes vigour and fertility; that these facts
alone incline me to believe that it is a general law of nature that no
organic being fertilises itself for a perpetuity of generations; but that a
cross with another individual is occasionally--perhaps at long intervals of
time--indispensable.
On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any other
view are inexplicable. Every hybridizer knows how unfavourable exposure to
wet is to the fertilisation of a flower, yet what a multitude of flowers
have their anthers and stigmas fully exposed to the weather! If an
occasional cross be indispensable, notwithstanding that the plant's own
anthers and pistil stand so near each other as almost to ensure self-
fertilisation, the fullest freedom for the entrance of pollen from another
individual will explain the above state of exposure of the organs. Many
flowers, on the other hand, have their organs of fructification closely
enclosed, as in the great papilionaceous or pea-family; but these almost
invariably present beautiful and curious adaptations in relation to the
visits of insects. So necessary are the visits of bees to many
papilionaceous flowers, that their fertility is greatly diminished if these
visits be prevented. Now, it is scarcely possible for insects to fly from
flower to flower, and not to carry pollen from one to the other, to the
great good of the plant. Insects act like a camel-hair pencil, and it is
sufficient, to ensure fertilisation, just to touch with the same brush the
anthers of one flower and then the stigma of another; but it must not be
supposed that bees would thus produce a multitude of hybrids between
distinct species; for if a plant's own pollen and that from another species
are placed on the same stigma, the former is so prepotent that it
invariably and completely destroys, as has been shown by Gartner, the
influence of the foreign pollen.
When the stamens of a flower suddenly spring towards the pistil, or slowly
move one after the other towards it, the contrivance seems adapted solely
to ensure self-fertilisation; and no doubt it is useful for this end: but
the agency of insects is often required to cause the stamens to spring
forward, as Kolreuter has shown to be the case with the barberry; and in
this very genus, which seems to have a special contrivance for
self-fertilisation, it is well known that, if closely-allied forms or
varieties are planted near each other, it is hardly possible to raise pure
seedlings, so largely do they naturally cross. In numerous other cases,
far from self-fertilisation being favoured, there are special contrivances
which effectually prevent the stigma receiving pollen from its own flower,
as I could show from the works of Sprengel and others, as well as from my
own observations: for instance, in Lobelia fulgens, there is a really
beautiful and elaborate contrivance by which all the infinitely numerous
pollen-granules are swept out of the conjoined anthers of each flower,
before the stigma of that individual flower is ready to receive them; and
as this flower is never visited, at least in my garden, by insects, it
never sets a seed, though by placing pollen from one flower on the stigma
of another, I raise plenty of seedlings. Another species of Lobelia, which
is visited by bees, seeds freely in my garden. In very many other cases,
though there is no special mechanical contrivance to prevent the stigma
receiving pollen from the same flower, yet, as Sprengel, and more recently
Hildebrand and others have shown, and as I can confirm, either the anthers
burst before the stigma is ready for fertilisation, or the stigma is ready
before the pollen of that flower is ready, so that these so-named
dichogamous plants have in fact separated sexes, and must habitually be
crossed. So it is with the reciprocally dimorphic and trimorphic plants
previously alluded to. How strange are these facts! How strange that the
pollen and stigmatic surface of the same flower, though placed so close
together, as if for the very purpose of self-fertilisation, should be in so
many cases mutually useless to each other! How simply are these facts
explained on the view of an occasional cross with a distinct individual
being advantageous or indispensable!
If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority of the
seedlings thus raised turn out, as I found, mongrels: for instance, I
raised 233 seedling cabbages from some plants of different varieties
growing near each other, and of these only 78 were true to their kind, and
some even of these were not perfectly true. Yet the pistil of each
cabbage-flower is surrounded not only by its own six stamens but by those
of the many other flowers on the same plant; and the pollen of each flower
readily gets on its stigma without insect agency; for I have found that
plants carefully protected from insects produce the full number of pods.
How, then, comes it that such a vast number of the seedlings are
mongrelized? It must arise from the pollen of a distinct VARIETY having a
prepotent effect over the flower's own pollen; and that this is part of the
general law of good being derived from the intercrossing of distinct
individuals of the same species. When distinct SPECIES are crossed the
case is reversed, for a plant's own pollen is always prepotent over foreign
pollen; but to this subject we shall return in a future chapter.
In the case of a large tree covered with innumerable flowers, it may be
objected that pollen could seldom be carried from tree to tree, and at most
only from flower to flower on the same tree; and flowers on the same tree
can be considered as distinct individuals only in a limited sense. I
believe this objection to be valid, but that nature has largely provided
against it by giving to trees a strong tendency to bear flowers with
separated sexes. When the sexes are separated, although the male and
female flowers may be produced on the same tree, pollen must be regularly
carried from flower to flower; and this will give a better chance of pollen
being occasionally carried from tree to tree. That trees belonging to all
orders have their sexes more often separated than other plants, I find to
be the case in this country; and at my request Dr. Hooker tabulated the
trees of New Zealand, and Dr. Asa Gray those of the United States, and the
result was as I anticipated. On the other hand, Dr. Hooker informs me that
the rule does not hold good in Australia: but if most of the Australian
trees are dichogamous, the same result would follow as if they bore flowers
with separated sexes. I have made these few remarks on trees simply to
call attention to the subject.
Turning for a brief space to animals: various terrestrial species are
hermaphrodites, such as the land-mollusca and earth-worms; but these all
pair. As yet I have not found a single terrestrial animal which can
fertilise itself. This remarkable fact, which offers so strong a contrast
with terrestrial plants, is intelligible on the view of an occasional cross
being indispensable; for owing to the nature of the fertilising element
there are no means, analogous to the action of insects and of the wind with
plants, by which an occasional cross could be effected with terrestrial
animals without the concurrence of two individuals. Of aquatic animals,
there are many self-fertilising hermaphrodites; but here the currents of
water offer an obvious means for an occasional cross. As in the case of
flowers, I have as yet failed, after consultation with one of the highest
authorities, namely, Professor Huxley, to discover a single hermaphrodite
animal with the organs of reproduction so perfectly enclosed that access
from without, and the occasional influence of a distinct individual, can be
shown to be physically impossible. Cirripedes long appeared to me to
present, under this point of view, a case of great difficulty; but I have
been enabled, by a fortunate chance, to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly that, both with
animals and plants, some species of the same family and even of the same
genus, though agreeing closely with each other in their whole organisation,
are hermaphrodites, and some unisexual. But if, in fact, all
hermaphrodites do occasionally intercross, the difference between them and
unisexual species is, as far as function is concerned, very small.
>From these several considerations and from the many special facts which I
have collected, but which I am unable here to give, it appears that with
animals and plants an occasional intercross between distinct individuals is
a very general, if not universal, law of nature.
CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW FORMS THROUGH NATURAL
SELECTION.
This is an extremely intricate subject. A great amount of variability,
under which term individual differences are always included, will evidently
be favourable. A large number of individuals, by giving a better chance
within any given period for the appearance of profitable variations, will
compensate for a lesser amount of variability in each individual, and is, I
believe, a highly important element of success. Though nature grants long
periods of time for the work of natural selection, she does not grant an
indefinite period; for as all organic beings are striving to seize on each
place in the economy of nature, if any one species does not become modified
and improved in a corresponding degree with its competitors it will be
exterminated. Unless favourable variations be inherited by some at least
of the offspring, nothing can be effected by natural selection. The
tendency to reversion may often check or prevent the work; but as this
tendency has not prevented man from forming by selection numerous domestic
races, why should it prevail against natural selection?
In the case of methodical selection, a breeder selects for some definite
object, and if the individuals be allowed freely to intercross, his work
will completely fail. But when many men, without intending to alter the
breed, have a nearly common standard of perfection, and all try to procure
and breed from the best animals, improvement surely but slowly follows from
this unconscious process of selection, notwithstanding that there is no
separation of selected individuals. Thus it will be under nature; for
within a confined area, with some place in the natural polity not perfectly
occupied, all the individuals varying in the right direction, though in
different degrees, will tend to be preserved. But if the area be large,
its several districts will almost certainly present different conditions of
life; and then, if the same species undergoes modification in different
districts, the newly formed varieties will intercross on the confines of
each. But we shall see in the sixth chapter that intermediate varieties,
inhabiting intermediate districts, will in the long run generally be
supplanted by one of the adjoining varieties. Intercrossing will chiefly
affect those animals which unite for each birth and wander much, and which
do not breed at a very quick rate. Hence with animals of this nature, for
instance birds, varieties will generally be confined to separated
countries; and this I find to be the case. With hermaphrodite organisms
which cross only occasionally, and likewise with animals which unite for
each birth, but which wander little and can increase at a rapid rate, a new
and improved variety might be quickly formed on any one spot, and might
there maintain itself in a body and afterward spread, so that the
individuals of the new variety would chiefly cross together. On this
principle nurserymen always prefer saving seed from a large body of plants,
as the chance of intercrossing is thus lessened.
Even with animals which unite for each birth, and which do not propagate
rapidly, we must not assume that free intercrossing would always eliminate
the effects of natural selection; for I can bring forward a considerable
body of facts showing that within the same area two varieties of the same
animal may long remain distinct, from haunting different stations, from
breeding at slightly different seasons, or from the individuals of each
variety preferring to pair together.
Intercrossing plays a very important part in nature by keeping the
individuals of the same species, or of the same variety, true and uniform
in character. It will obviously thus act far more efficiently with those
animals which unite for each birth; but, as already stated, we have reason
to believe that occasional intercrosses take place with all animals and
plants. Even if these take place only at long intervals of time, the young
thus produced will gain so much in vigour and fertility over the offspring
from long-continued self-fertilisation, that they will have a better chance
of surviving and propagating their kind; and thus in the long run the
influence of crosses, even at rare intervals, will be great. With respect
to organic beings extremely low in the scale, which do not propagate
sexually, nor conjugate, and which cannot possibly intercross, uniformity
of character can be retained by them under the same conditions of life,
only through the principle of inheritance, and through natural selection
which will destroy any individuals departing from the proper type. If the
conditions of life change and the form undergoes modification, uniformity
of character can be given to the modified offspring, solely by natural
selection preserving similar favourable variations.
Isolation also is an important element in the modification of species
through natural selection. In a confined or isolated area, if not very
large, the organic and inorganic conditions of life will generally be
almost uniform; so that natural selection will tend to modify all the
varying individuals of the same species in the same manner. Intercrossing
with the inhabitants of the surrounding districts, will also be thus
prevented. Moritz Wagner has lately published an interesting essay on this
subject, and has shown that the service rendered by isolation in preventing
crosses between newly-formed varieties is probably greater even than I
supposed. But from reasons already assigned I can by no means agree with
this naturalist, that migration and isolation are necessary elements for
the formation of new species. The importance of isolation is likewise
great in preventing, after any physical change in the conditions, such as
of climate, elevation of the land, etc., the immigration of better adapted
organisms; and thus new places in the natural economy of the district will
be left open to be filled up by the modification of the old inhabitants.
Lastly, isolation will give time for a new variety to be improved at a slow
rate; and this may sometimes be of much importance. If, however, an
isolated area be very small, either from being surrounded by barriers, or
from having very peculiar physical conditions, the total number of the
inhabitants will be small; and this will retard the production of new
species through natural selection, by decreasing the chances of favourable
variations arising.
The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously asserted
that the element of time has been assumed by me to play an all-important
part in modifying species, as if all the forms of life were necessarily
undergoing change through some innate law. Lapse of time is only so far
important, and its importance in this respect is great, that it gives a
better chance of beneficial variations arising and of their being selected,
accumulated, and fixed. It likewise tends to increase the direct action of
the physical conditions of life, in relation to the constitution of each
organism.
If we turn to nature to test the truth of these remarks, and look at any
small isolated area, such as an oceanic island, although the number of the
species inhabiting it is small, as we shall see in our chapter on
Geographical Distribution; yet of these species a very large proportion are
endemic,--that is, have been produced there and nowhere else in the world.
Hence an oceanic island at first sight seems to have been highly favourable
for the production of new species. But we may thus deceive ourselves, for
to ascertain whether a small isolated area, or a large open area like a
continent, has been most favourable for the production of new organic
forms, we ought to make the comparison within equal times; and this we are
incapable of doing.
Although isolation is of great importance in the production of new species,
on the whole I am inclined to believe that largeness of area is still more
important, especially for the production of species which shall prove
capable of enduring for a long period, and of spreading widely. Throughout
a great and open area, not only will there be a better chance of favourable
variations, arising from the large number of individuals of the same
species there supported, but the conditions of life are much more complex
from the large number of already existing species; and if some of these
many species become modified and improved, others will have to be improved
in a corresponding degree, or they will be exterminated. Each new form,
also, as soon as it has been much improved, will be able to spread over the
open and continuous area, and will thus come into competition with many
other forms. Moreover, great areas, though now continuous, will often,
owing to former oscillations of level, have existed in a broken condition,
so that the good effects of isolation will generally, to a certain extent,
have concurred. Finally, I conclude that, although small isolated areas
have been in some respects highly favourable for the production of new
species, yet that the course of modification will generally have been more
rapid on large areas; and what is more important, that the new forms
produced on large areas, which already have been victorious over many
competitors, will be those that will spread most widely, and will give rise
to the greatest number of new varieties and species. They will thus play a
more important part in the changing history of the organic world.
In accordance with this view, we can, perhaps, understand some facts which
will be again alluded to in our chapter on Geographical Distribution; for
instance, the fact of the productions of the smaller continent of Australia
now yielding before those of the larger Europaeo-Asiatic area. Thus, also,
it is that continental productions have everywhere become so largely
naturalised on islands. On a small island, the race for life will have
been less severe, and there will have been less modification and less
extermination. Hence, we can understand how it is that the flora of
Madeira, according to Oswald Heer, resembles to a certain extent the
extinct tertiary flora of Europe. All fresh water basins, taken together,
make a small area compared with that of the sea or of the land.
Consequently, the competition between fresh water productions will have
been less severe than elsewhere; new forms will have been more slowly
produced, and old forms more slowly exterminated. And it is in fresh water
basins that we find seven genera of Ganoid fishes, remnants of a once
preponderant order: and in fresh water we find some of the most anomalous
forms now known in the world, as the Ornithorhynchus and Lepidosiren,
which, like fossils, connect to a certain extent orders at present widely
separated in the natural scale. These anomalous forms may be called living
fossils; they have endured to the present day, from having inhabited a
confined area, and from having been exposed to less varied, and therefore
less severe, competition.
To sum up, as far as the extreme intricacy of the subject permits, the
circumstances favourable and unfavourable for the production of new species
through natural selection. I conclude that for terrestrial productions a
large continental area, which has undergone many oscillations of level,
will have been the most favourable for the production of many new forms of
life, fitted to endure for a long time and to spread widely. While the
area existed as a continent the inhabitants will have been numerous in
individuals and kinds, and will have been subjected to severe competition.
When converted by subsidence into large separate islands there will still
have existed many individuals of the same species on each island:
intercrossing on the confines of the range of each new species will have
been checked: after physical changes of any kind immigration will have
been prevented, so that new places in the polity of each island will have
had to be filled up by the modification of the old inhabitants; and time
will have been allowed for the varieties in each to become well modified
and perfected. When, by renewed elevation, the islands were reconverted
into a continental area, there will again have been very severe
competition; the most favoured or improved varieties will have been enabled
to spread; there will have been much extinction of the less improved forms,
and the relative proportional numbers of the various inhabitants of the
reunited continent will again have been changed; and again there will have
been a fair field for natural selection to improve still further the
inhabitants, and thus to produce new species.
That natural selection generally act with extreme slowness I fully admit.
It can act only when there are places in the natural polity of a district
which can be better occupied by the modification of some of its existing
inhabitants. The occurrence of such places will often depend on physical
changes, which generally take place very slowly, and on the immigration of
better adapted forms being prevented. As some few of the old inhabitants
become modified the mutual relations of others will often be disturbed; and
this will create new places, ready to be filled up by better adapted forms;
but all this will take place very slowly. Although all the individuals of
the same species differ in some slight degree from each other, it would
often be long before differences of the right nature in various parts of
the organisation might occur. The result would often be greatly retarded
by free intercrossing. Many will exclaim that these several causes are
amply sufficient to neutralise the power of natural selection. I do not
believe so. But I do believe that natural selection will generally act
very slowly, only at long intervals of time, and only on a few of the
inhabitants of the same region. I further believe that these slow,
intermittent results accord well with what geology tells us of the rate and
manner at which the inhabitants of the world have changed.
Slow though the process of selection may be, if feeble man can do much by
artificial selection, I can see no limit to the amount of change, to the
beauty and complexity of the coadaptations between all organic beings, one
with another and with their physical conditions of life, which may have
been effected in the long course of time through nature's power of
selection, that is by the survival of the fittest.
EXTINCTION CAUSED BY NATURAL SELECTION.
This subject will be more fully discussed in our chapter on Geology; but it
must here be alluded to from being intimately connected with natural
selection. Natural selection acts solely through the preservation of
variations in some way advantageous, which consequently endure. Owing to
the high geometrical rate of increase of all organic beings, each area is
already fully stocked with inhabitants, and it follows from this, that as
the favoured forms increase in number, so, generally, will the less
favoured decrease and become rare. Rarity, as geology tells us, is the
precursor to extinction. We can see that any form which is represented by
few individuals will run a good chance of utter extinction, during great
fluctuations in the nature or the seasons, or from a temporary increase in
the number of its enemies. But we may go further than this; for as new
forms are produced, unless we admit that specific forms can go on
indefinitely increasing in number, many old forms must become extinct.
That the number of specific forms has not indefinitely increased, geology
plainly tells us; and we shall presently attempt to show why it is that the
number of species throughout the world has not become immeasurably great.
We have seen that the species which are most numerous in individuals have
the best chance of producing favourable variations within any given period.
We have evidence of this, in the facts stated in the second chapter,
showing that it is the common and diffused or dominant species which offer
the greatest number of recorded varieties. Hence, rare species will be
less quickly modified or improved within any given period; they will
consequently be beaten in the race for life by the modified and improved
descendants of the commoner species.
>From these several considerations I think it inevitably follows, that as
new species in the course of time are formed through natural selection,
others will become rarer and rarer, and finally extinct. The forms which
stand in closest competition with those undergoing modification and
improvement, will naturally suffer most. And we have seen in the chapter
on the Struggle for Existence that it is the most closely-allied
forms,--varieties of the same species, and species of the same genus or
related genera,--which, from having nearly the same structure, constitution
and habits, generally come into the severest competition with each other.
Consequently, each new variety or species, during the progress of its
formation, will generally press hardest on its nearest kindred, and tend to
exterminate them. We see the same process of extermination among our
domesticated productions, through the selection of improved forms by man.
Many curious instances could be given showing how quickly new breeds of
cattle, sheep and other animals, and varieties of flowers, take the place
of older and inferior kinds. In Yorkshire, it is historically known that
the ancient black cattle were displaced by the long-horns, and that these
"were swept away by the short-horns" (I quote the words of an agricultural
writer) "as if by some murderous pestilence."
DIVERGENCE OF CHARACTER.
The principle, which I have designated by this term, is of high importance,
and explains, as I believe, several important facts. In the first place,
varieties, even strongly-marked ones, though having somewhat of the
character of species--as is shown by the hopeless doubts in many cases how
to rank them--yet certainly differ far less from each other than do good
and distinct species. Nevertheless according to my view, varieties are
species in the process of formation, or are, as I have called them,
incipient species. How, then, does the lesser difference between varieties
become augmented into the greater difference between species? That this
does habitually happen, we must infer from most of the innumerable species
throughout nature presenting well-marked differences; whereas varieties,
the supposed prototypes and parents of future well-marked species, present
slight and ill-defined differences. Mere chance, as we may call it, might
cause one variety to differ in some character from its parents, and the
offspring of this variety again to differ from its parent in the very same
character and in a greater degree; but this alone would never account for
so habitual and large a degree of difference as that between the species of
the same genus.
As has always been my practice, I have sought light on this head from our
domestic productions. We shall here find something analogous. It will be
admitted that the production of races so different as short-horn and
Hereford cattle, race and cart horses, the several breeds of pigeons, etc.,
could never have been effected by the mere chance accumulation of similar
variations during many successive generations. In practice, a fancier is,
for instance, struck by a pigeon having a slightly shorter beak; another
fancier is struck by a pigeon having a rather longer beak; and on the
acknowledged principle that "fanciers do not and will not admire a medium
standard, but like extremes," they both go on (as has actually occurred
with the sub-breeds of the tumbler-pigeon) choosing and breeding from birds
with longer and longer beaks, or with shorter and shorter beaks. Again, we
may suppose that at an early period of history, the men of one nation or
district required swifter horses, while those of another required stronger
and bulkier horses. The early differences would be very slight; but, in
the course of time, from the continued selection of swifter horses in the
one case, and of stronger ones in the other, the differences would become
greater, and would be noted as forming two sub-breeds. Ultimately after
the lapse of centuries, these sub-breeds would become converted into two
well-established and distinct breeds. As the differences became greater,
the inferior animals with intermediate characters, being neither very swift
nor very strong, would not have been used for breeding, and will thus have
tended to disappear. Here, then, we see in man's productions the action of
what may be called the principle of divergence, causing differences, at
first barely appreciable, steadily to increase, and the breeds to diverge
in character, both from each other and from their common parent.
But how, it may be asked, can any analogous principle apply in nature? I
believe it can and does apply most efficiently (though it was a long time
before I saw how), from the simple circumstance that the more diversified
the descendants from any one species become in structure, constitution, and
habits, by so much will they be better enabled to seize on many and widely
diversified places in the polity of nature, and so be enabled to increase
in numbers.
We can clearly discern this in the case of animals with simple habits.
Take the case of a carnivorous quadruped, of which the number that can be
supported in any country has long ago arrived at its full average. If its
natural power of increase be allowed to act, it can succeed in increasing
(the country not undergoing any change in conditions) only by its varying
descendants seizing on places at present occupied by other animals: some
of them, for instance, being enabled to feed on new kinds of prey, either
dead or alive; some inhabiting new stations, climbing trees, frequenting
water, and some perhaps becoming less carnivorous. The more diversified in
habits and structure the descendants of our carnivorous animals become, the
more places they will be enabled to occupy. What applies to one animal
will apply throughout all time to all animals--that is, if they vary--for
otherwise natural selection can effect nothing. So it will be with plants.
It has been experimentally proved, that if a plot of ground be sown with
one species of grass, and a similar plot be sown with several distinct
genera of grasses, a greater number of plants and a greater weight of dry
herbage can be raised in the latter than in the former case. The same has
been found to hold good when one variety and several mixed varieties of
wheat have been sown on equal spaces of ground. Hence, if any one species
of grass were to go on varying, and the varieties were continually selected
which differed from each other in the same manner, though in a very slight
degree, as do the distinct species and genera of grasses, a greater number
of individual plants of this species, including its modified descendants,
would succeed in living on the same piece of ground. And we know that each
species and each variety of grass is annually sowing almost countless
seeds; and is thus striving, as it may be said, to the utmost to increase
in number. Consequently, in the course of many thousand generations, the
most distinct varieties of any one species of grass would have the best
chance of succeeding and of increasing in numbers, and thus of supplanting
the less distinct varieties; and varieties, when rendered very distinct
from each other, take the rank of species.
The truth of the principle that the greatest amount of life can be
supported by great diversification of structure, is seen under many natural
circumstances. In an extremely small area, especially if freely open to
immigration, and where the contest between individual and individual must
be very severe, we always find great diversity in its inhabitants. For
instance, I found that a piece of turf, three feet by four in size, which
had been exposed for many years to exactly the same conditions, supported
twenty species of plants, and these belonged to eighteen genera and to
eight orders, which shows how much these plants differed from each other.
So it is with the plants and insects on small and uniform islets: also in
small ponds of fresh water. Farmers find that they can raise more food by
a rotation of plants belonging to the most different orders: nature
follows what may be called a simultaneous rotation. Most of the animals
and plants which live close round any small piece of ground, could live on
it (supposing its nature not to be in any way peculiar), and may be said to
be striving to the utmost to live there; but, it is seen, that where they
come into the closest competition, the advantages of diversification of
structure, with the accompanying differences of habit and constitution,
determine that the inhabitants, which thus jostle each other most closely,
shall, as a general rule, belong to what we call different genera and
orders.
The same principle is seen in the naturalisation of plants through man's
agency in foreign lands. It might have been expected that the plants which
would succeed in becoming naturalised in any land would generally have been
closely allied to the indigenes; for these are commonly looked at as
specially created and adapted for their own country. It might also,
perhaps, have been expected that naturalised plants would have belonged to
a few groups more especially adapted to certain stations in their new
homes. But the case is very different; and Alph. de Candolle has well
remarked, in his great and admirable work, that floras gain by
naturalisation, proportionally with the number of the native genera and
species, far more in new genera than in new species. To give a single
instance: in the last edition of Dr. Asa Gray's "Manual of the Flora of
the Northern United States," 260 naturalised plants are enumerated, and
these belong to 162 genera. We thus see that these naturalised plants are
of a highly diversified nature. They differ, moreover, to a large extent,
from the indigenes, for out of the 162 naturalised genera, no less than 100
genera are not there indigenous, and thus a large proportional addition is
made to the genera now living in the United States.
By considering the nature of the plants or animals which have in any
country struggled successfully with the indigenes, and have there become
naturalised, we may gain some crude idea in what manner some of the natives
would have had to be modified in order to gain an advantage over their
compatriots; and we may at least infer that diversification of structure,
amounting to new generic differences, would be profitable to them.
The advantage of diversification of structure in the inhabitants of the
same region is, in fact, the same as that of the physiological division of
labour in the organs of the same individual body--a subject so well
elucidated by Milne Edwards. No physiologist doubts that a stomach by
being adapted to digest vegetable matter alone, or flesh alone, draws most
nutriment from these substances. So in the general economy of any land,
the more widely and perfectly the animals and plants are diversified for
different habits of life, so will a greater number of individuals be
capable of there supporting themselves. A set of animals, with their
organisation but little diversified, could hardly compete with a set more
perfectly diversified in structure. It may be doubted, for instance,
whether the Australian marsupials, which are divided into groups differing
but little from each other, and feebly representing, as Mr. Waterhouse and
others have remarked, our carnivorous, ruminant, and rodent mammals, could
successfully compete with these well-developed orders. In the Australian
mammals, we see the process of diversification in an early and incomplete
stage of development.
THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH DIVERGENCE
OF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF A COMMON ANCESTOR.
After the foregoing discussion, which has been much compressed, we may
assume that the modified descendants of any one species will succeed so
much the better as they become more diversified in structure, and are thus
enabled to encroach on places occupied by other beings. Now let us see how
this principle of benefit being derived from divergence of character,
combined with the principles of natural selection and of extinction, tends
to act.
The accompanying diagram will aid us in understanding this rather
perplexing subject. Let A to L represent the species of a genus large in
its own country; these species are supposed to resemble each other in
unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal distances. I
have said a large genus, because as we saw in the second chapter, on an
average more species vary in large genera than in small genera; and the
varying species of the large genera present a greater number of varieties.
We have, also, seen that the species, which are the commonest and most
widely-diffused, vary more than do the rare and restricted species. Let
(A) be a common, widely-diffused, and varying species, belonging to a genus
large in its own country. The branching and diverging dotted lines of
unequal lengths proceeding from (A), may represent its varying offspring.
The variations are supposed to be extremely slight, but of the most
diversified nature; they are not supposed all to appear simultaneously, but
often after long intervals of time; nor are they all supposed to endure for
equal periods. Only those variations which are in some way profitable will
be preserved or naturally selected. And here the importance of the
principle of benefit derived from divergence of character comes in; for
this will generally lead to the most different or divergent variations
(represented by the outer dotted lines) being preserved and accumulated by
natural selection. When a dotted line reaches one of the horizontal lines,
and is there marked by a small numbered letter, a sufficient amount of
variation is supposed to have been accumulated to form it into a fairly
well-marked variety, such as would be thought worthy of record in a
systematic work.
The intervals between the horizontal lines in the diagram, may represent
each a thousand or more generations. After a thousand generations, species
(A) is supposed to have produced two fairly well-marked varieties, namely
a1 and m1. These two varieties will generally still be exposed to the same
conditions which made their parents variable, and the tendency to
variability is in itself hereditary; consequently they will likewise tend
to vary, and commonly in nearly the same manner as did their parents.
Moreover, these two varieties, being only slightly modified forms, will
tend to inherit those advantages which made their parent (A) more numerous
than most of the other inhabitants of the same country; they will also
partake of those more general advantages which made the genus to which the
parent-species belonged, a large genus in its own country. And all these
circumstances are favourable to the production of new varieties.
If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety a1 is supposed in the
diagram to have produced variety a2, which will, owing to the principle of
divergence, differ more from (A) than did variety a1. Variety m1 is
supposed to have produced two varieties, namely m2 and s2, differing from
each other, and more considerably from their common parent (A). We may
continue the process by similar steps for any length of time; some of the
varieties, after each thousand generations, producing only a single
variety, but in a more and more modified condition, some producing two or
three varieties, and some failing to produce any. Thus the varieties or
modified descendants of the common parent (A), will generally go on
increasing in number and diverging in character. In the diagram the
process is represented up to the ten-thousandth generation, and under a
condensed and simplified form up to the fourteen-thousandth generation.
But I must here remark that I do not suppose that the process ever goes on
so regularly as is represented in the diagram, though in itself made
somewhat irregular, nor that it goes on continuously; it is far more
probable that each form remains for long periods unaltered, and then again
undergoes modification. Nor do I suppose that the most divergent varieties
are invariably preserved: a medium form may often long endure, and may or
may not produce more than one modified descendant; for natural selection
will always act according to the nature of the places which are either
unoccupied or not perfectly occupied by other beings; and this will depend
on infinitely complex relations. But as a general rule, the more
diversified in structure the descendants from any one species can be
rendered, the more places they will be enabled to seize on, and the more
their modified progeny will increase. In our diagram the line of
succession is broken at regular intervals by small numbered letters marking
the successive forms which have become sufficiently distinct to be recorded
as varieties. But these breaks are imaginary, and might have been inserted
anywhere, after intervals long enough to allow the accumulation of a
considerable amount of divergent variation.
As all the modified descendants from a common and widely-diffused species,
belonging to a large genus, will tend to partake of the same advantages
which made their parent successful in life, they will generally go on
multiplying in number as well as diverging in character: this is
represented in the diagram by the several divergent branches proceeding
from (A). The modified offspring from the later and more highly improved
branches in the lines of descent, will, it is probable, often take the
place of, and so destroy, the earlier and less improved branches: this is
represented in the diagram by some of the lower branches not reaching to
the upper horizontal lines. In some cases no doubt the process of
modification will be confined to a single line of descent, and the number
of modified descendants will not be increased; although the amount of
divergent modification may have been augmented. This case would be
represented in the diagram, if all the lines proceeding from (A) were
removed, excepting that from a1 to a10. In the same way the English
racehorse and English pointer have apparently both gone on slowly diverging
in character from their original stocks, without either having given off
any fresh branches or races.
After ten thousand generations, species (A) is supposed to have produced
three forms, a10, f10, and m10, which, from having diverged in character
during the successive generations, will have come to differ largely, but
perhaps unequally, from each other and from their common parent. If we
suppose the amount of change between each horizontal line in our diagram to
be excessively small, these three forms may still be only well-marked
varieties; but we have only to suppose the steps in the process of
modification to be more numerous or greater in amount, to convert these
three forms into doubtful or at least into well-defined species: thus the
diagram illustrates the steps by which the small differences distinguishing
varieties are increased into the larger differences distinguishing species.
By continuing the same process for a greater number of generations (as
shown in the diagram in a condensed and simplified manner), we get eight
species, marked by the letters between a14 and m14, all descended from (A).
Thus, as I believe, species are multiplied and genera are formed.
In a large genus it is probable that more than one species would vary. In
the diagram I have assumed that a second species (I) has produced, by
analogous steps, after ten thousand generations, either two well-marked
varieties (w10 and z10) or two species, according to the amount of change
supposed to be represented between the horizontal lines. After fourteen
thousand generations, six new species, marked by the letters n14 to z14,
are supposed to have been produced. In any genus, the species which are
already very different in character from each other, will generally tend to
produce the greatest number of modified descendants; for these will have
the best chance of seizing on new and widely different places in the polity
of nature: hence in the diagram I have chosen the extreme species (A), and
the nearly extreme species (I), as those which have largely varied, and
have given rise to new varieties and species. The other nine species
(marked by capital letters) of our original genus, may for long but unequal
periods continue to transmit unaltered descendants; and this is shown in
the diagram by the dotted lines unequally prolonged upwards.
But during the process of modification, represented in the diagram, another
of our principles, namely that of extinction, will have played an important
part. As in each fully stocked country natural selection necessarily acts
by the selected form having some advantage in the struggle for life over
other forms, there will be a constant tendency in the improved descendants
of any one species to supplant and exterminate in each stage of descent
their predecessors and their original progenitor. For it should be
remembered that the competition will generally be most severe between those
forms which are most nearly related to each other in habits, constitution
and structure. Hence all the intermediate forms between the earlier and
later states, that is between the less and more improved states of a the
same species, as well as the original parent-species itself, will generally
tend to become extinct. So it probably will be with many whole collateral
lines of descent, which will be conquered by later and improved lines. If,
however, the modified offspring of a species get into some distinct
country, or become quickly adapted to some quite new station, in which
offspring and progenitor do not come into competition, both may continue to
exist.
If, then, our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have become
extinct, being replaced by eight new species (a14 to m14); and species (I)
will be replaced by six (n14 to z14) new species.
But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally the
case in nature; species (A) being more nearly related to B, C, and D than
to the other species; and species (I) more to G, H, K, L, than to the
others. These two species (A and I), were also supposed to be very common
and widely diffused species, so that they must originally have had some
advantage over most of the other species of the genus. Their modified
descendants, fourteen in number at the fourteen-thousandth generation, will
probably have inherited some of the same advantages: they have also been
modified and improved in a diversified manner at each stage of descent, so
as to have become adapted to many related places in the natural economy of
their country. It seems, therefore, extremely probable that they will have
taken the places of, and thus exterminated, not only their parents (A) and
(I), but likewise some of the original species which were most nearly
related to their parents. Hence very few of the original species will have
transmitted offspring to the fourteen-thousandth generation. We may
suppose that only one (F) of the two species (E and F) which were least
closely related to the other nine original species, has transmitted
descendants to this late stage of descent.
The new species in our diagram, descended from the original eleven species,
will now be fifteen in number. Owing to the divergent tendency of natural
selection, the extreme amount of difference in character between species
a14 and z14 will be much greater than that between the most distinct of the
original eleven species. The new species, moreover, will be allied to each
other in a widely different manner. Of the eight descendants from (A) the
three marked a14, q14, p14, will be nearly related from having recently
branched off from a10; b14 and f14, from having diverged at an earlier
period from a5, will be in some degree distinct from the three first-named
species; and lastly, o14, e14, and m14, will be nearly related one to the
other, but, from having diverged at the first commencement of the process
of modification, will be widely different from the other five species, and
may constitute a sub-genus or a distinct genus.
The six descendants from (I) will form two sub-genera or genera. But as
the original species (I) differed largely from (A), standing nearly at the
extreme end of the original genus, the six descendants from (I) will, owing
to inheritance alone, differ considerably from the eight descendants from
(A); the two groups, moreover, are supposed to have gone on diverging in
different directions. The intermediate species, also (and this is a very
important consideration), which connected the original species (A) and (I),
have all become, except (F), extinct, and have left no descendants. Hence
the six new species descended from (I), and the eight descendants from (A),
will have to be ranked as very distinct genera, or even as distinct
sub-families.
Thus it is, as I believe, that two or more genera are produced by descent
with modification, from two or more species of the same genus. And the two
or more parent-species are supposed to be descended from some one species
of an earlier genus. In our diagram this is indicated by the broken lines
beneath the capital letters, converging in sub-branches downwards towards a
single point; this point represents a species, the supposed progenitor of
our several new sub-genera and genera.
It is worth while to reflect for a moment on the character of the new
species F14, which is supposed not to have diverged much in character, but
to have retained the form of (F), either unaltered or altered only in a
slight degree. In this case its affinities to the other fourteen new
species will be of a curious and circuitous nature. Being descended from a
form that stood between the parent-species (A) and (I), now supposed to be
extinct and unknown, it will be in some degree intermediate in character
between the two groups descended from these two species. But as these two
groups have gone on diverging in character from the type of their parents,
the new species (F14) will not be directly intermediate between them, but
rather between types of the two groups; and every naturalist will be able
to call such cases before his mind.
In the diagram each horizontal line has hitherto been supposed to represent
a thousand generations, but each may represent a million or more
generations; it may also represent a section of the successive strata of
the earth's crust including extinct remains. We shall, when we come to our
chapter on geology, have to refer again to this subject, and I think we
shall then see that the diagram throws light on the affinities of extinct
beings, which, though generally belonging to the same orders, families, or
genera, with those now living, yet are often, in some degree, intermediate
in character between existing groups; and we can understand this fact, for
the extinct species lived at various remote epochs when the branching lines
of descent had diverged less.
I see no reason to limit the process of modification, as now explained, to
the formation of genera alone. If, in the diagram, we suppose the amount
of change represented by each successive group of diverging dotted lines to
be great, the forms marked a14 to p14, those marked b14 and f14, and those
marked o14 to m14, will form three very distinct genera. We shall also
have two very distinct genera descended from (I), differing widely from the
descendants of (A). These two groups of genera will thus form two distinct
families, or orders, according to the amount of divergent modification
supposed to be represented in the diagram. And the two new families, or
orders, are descended from two species of the original genus; and these are
supposed to be descended from some still more ancient and unknown form.
We have seen that in each country it is the species belonging to the larger
genera which oftenest present varieties or incipient species. This,
indeed, might have been expected; for as natural selection acts through one
form having some advantage over other forms in the struggle for existence,
it will chiefly act on those which already have some advantage; and the
largeness of any group shows that its species have inherited from a common
ancestor some advantage in common. Hence, the struggle for the production
of new and modified descendants will mainly lie between the larger groups,
which are all trying to increase in number. One large group will slowly
conquer another large group, reduce its number, and thus lessen its chance
of further variation and improvement. Within the same large group, the
later and more highly perfected sub-groups, from branching out and seizing
on many new places in the polity of nature, will constantly tend to
supplant and destroy the earlier and less improved sub-groups. Small and
broken groups and sub-groups will finally disappear. Looking to the
future, we can predict that the groups of organic beings which are now
large and triumphant, and which are least broken up, that is, which have as
yet suffered least extinction, will, for a long period, continue to
increase. But which groups will ultimately prevail, no man can predict;
for we know that many groups, formerly most extensively developed, have now
become extinct. Looking still more remotely to the future, we may predict
that, owing to the continued and steady increase of the larger groups, a
multitude of smaller groups will become utterly extinct, and leave no
modified descendants; and consequently that, of the species living at any
one period, extremely few will transmit descendants to a remote futurity.
I shall have to return to this subject in the chapter on classification,
but I may add that as, according to this view, extremely few of the more
ancient species have transmitted descendants to the present day, and, as
all the descendants of the same species form a class, we can understand how
it is that there exist so few classes in each main division of the animal
and vegetable kingdoms. Although few of the most ancient species have left
modified descendants, yet, at remote geological periods, the earth may have
been almost as well peopled with species of many genera, families, orders
and classes, as at the present day.
ON THE DEGREE TO WHICH ORGANISATION TENDS TO ADVANCE.
Natural selection acts exclusively by the preservation and accumulation of
variations, which are beneficial under the organic and inorganic conditions
to which each creature is exposed at all periods of life. The ultimate
result is that each creature tends to become more and more improved in
relation to its conditions. This improvement inevitably leads to the
gradual advancement of the organisation of the greater number of living
beings throughout the world. But here we enter on a very intricate
subject, for naturalists have not defined to each other's satisfaction what
is meant by an advance in organisation. Among the vertebrata the degree of
intellect and an approach in structure to man clearly come into play. It
might be thought that the amount of change which the various parts and
organs pass through in their development from embryo to maturity would
suffice as a standard of comparison; but there are cases, as with certain
parasitic crustaceans, in which several parts of the structure become less
perfect, so that the mature animal cannot be called higher than its larva.
Von Baer's standard seems the most widely applicable and the best, namely,
the amount of differentiation of the parts of the same organic being, in
the adult state, as I should be inclined to add, and their specialisation
for different functions; or, as Milne Edwards would express it, the
completeness of the division of physiological labour. But we shall see how
obscure this subject is if we look, for instance, to fishes, among which
some naturalists rank those as highest which, like the sharks, approach
nearest to amphibians; while other naturalists rank the common bony or
teleostean fishes as the highest, inasmuch as they are most strictly fish-
like, and differ most from the other vertebrate classes. We see still more
plainly the obscurity of the subject by turning to plants, among which the
standard of intellect is of course quite excluded; and here some botanists
rank those plants as highest which have every organ, as sepals, petals,
stamens and pistils, fully developed in each flower; whereas other
botanists, probably with more truth, look at the plants which have their
several organs much modified and reduced in number as the highest.
If we take as the standard of high organisation, the amount of
differentiation and specialisation of the several organs in each being when
adult (and this will include the advancement of the brain for intellectual
purposes), natural selection clearly leads towards this standard: for all
physiologists admit that the specialisation of organs, inasmuch as in this
state they perform their functions better, is an advantage to each being;
and hence the accumulation of variations tending towards specialisation is
within the scope of natural selection. On the other hand, we can see,
bearing in mind that all organic beings are striving to increase at a high
ratio and to seize on every unoccupied or less well occupied place in the
economy of nature, that it is quite possible for natural selection
gradually to fit a being to a situation in which several organs would be
superfluous or useless: in such cases there would be retrogression in the
scale of organisation. Whether organisation on the whole has actually
advanced from the remotest geological periods to the present day will be
more conveniently discussed in our chapter on Geological Succession.
But it may be objected that if all organic beings thus tend to rise in the
scale, how is it that throughout the world a multitude of the lowest forms
still exist; and how is it that in each great class some forms are far more
highly developed than others? Why have not the more highly developed forms
every where supplanted and exterminated the lower? Lamarck, who believed
in an innate and inevitable tendency towards perfection in all organic
beings, seems to have felt this difficulty so strongly that he was led to
suppose that new and simple forms are continually being produced by
spontaneous generation. Science has not as yet proved the truth of this
belief, whatever the future may reveal. On our theory the continued
existence of lowly organisms offers no difficulty; for natural selection,
or the survival of the fittest, does not necessarily include progressive
development--it only takes advantage of such variations as arise and are
beneficial to each creature under its complex relations of life. And it
may be asked what advantage, as far as we can see, would it be to an
infusorian animalcule--to an intestinal worm--or even to an earth-worm, to
be highly organised. If it were no advantage, these forms would be left,
by natural selection, unimproved or but little improved, and might remain
for indefinite ages in their present lowly condition. And geology tells us
that some of the lowest forms, as the infusoria and rhizopods, have
remained for an enormous period in nearly their present state. But to
suppose that most of the many now existing low forms have not in the least
advanced since the first dawn of life would be extremely rash; for every
naturalist who has dissected some of the beings now ranked as very low in
the scale, must have been struck with their really wondrous and beautiful
organisation.
Nearly the same remarks are applicable, if we look to the different grades
of organisation within the same great group; for instance, in the
vertebrata, to the co-existence of mammals and fish--among mammalia, to the
co-existence of man and the ornithorhynchus--among fishes, to the co-
existence of the shark and the lancelet (Amphioxus), which latter fish in
the extreme simplicity of its structure approaches the invertebrate
classes. But mammals and fish hardly come into competition with each
other; the advancement of the whole class of mammals, or of certain members
in this class, to the highest grade would not lead to their taking the
place of fishes. Physiologists believe that the brain must be bathed by
warm blood to be highly active, and this requires aerial respiration; so
that warm-blooded mammals when inhabiting the water lie under a
disadvantage in having to come continually to the surface to breathe. With
fishes, members of the shark family would not tend to supplant the
lancelet; for the lancelet, as I hear from Fritz Muller, has as sole
companion and competitor on the barren sandy shore of South Brazil, an
anomalous annelid. The three lowest orders of mammals, namely, marsupials,
edentata, and rodents, co-exist in South America in the same region with
numerous monkeys, and probably interfere little with each other. Although
organisation, on the whole, may have advanced and be still advancing
throughout the world, yet the scale will always present many degrees of
perfection; for the high advancement of certain whole classes, or of
certain members of each class, does not at all necessarily lead to the
extinction of those groups with which they do not enter into close
competition. In some cases, as we shall hereafter see, lowly organised
forms appear to have been preserved to the present day, from inhabiting
confined or peculiar stations, where they have been subjected to less
severe competition, and where their scanty numbers have retarded the chance
of favourable variations arising.
Finally, I believe that many lowly organised forms now exist throughout the
world, from various causes. In some cases variations or individual
differences of a favourable nature may never have arisen for natural
selection to act on and accumulate. In no case, probably, has time
sufficed for the utmost possible amount of development. In some few cases
there has been what we must call retrogression or organisation. But the
main cause lies in the fact that under very simple conditions of life a
high organisation would be of no service--possibly would be of actual
disservice, as being of a more delicate nature, and more liable to be put
out of order and injured.
Looking to the first dawn of life, when all organic beings, as we may
believe, presented the simplest structure, how, it has been asked, could
the first step in the advancement or differentiation of parts have arisen?
Mr. Herbert Spencer would probably answer that, as soon as simple
unicellular organisms came by growth or division to be compounded of
several cells, or became attached to any supporting surface, his law "that
homologous units of any order become differentiated in proportion as their
relations to incident forces become different" would come into action. But
as we have no facts to guide us, speculation on the subject is almost
useless. It is, however, an error to suppose that there would be no
struggle for existence, and, consequently, no natural selection, until many
forms had been produced: variations in a single species inhabiting an
isolated station might be beneficial, and thus the whole mass of
individuals might be modified, or two distinct forms might arise. But, as
I remarked towards the close of the introduction, no one ought to feel
surprise at much remaining as yet unexplained on the origin of species, if
we make due allowance for our profound ignorance on the mutual relations of
the inhabitants of the world at the present time, and still more so during
past ages.
CONVERGENCE OF CHARACTER.
Mr. H.C. Watson thinks that I have overrated the importance of divergence
of character (in which, however, he apparently believes), and that
convergence, as it may be called, has likewise played a part. If two
species belonging to two distinct though allied genera, had both produced a
large number of new and divergent forms, it is conceivable that these might
approach each other so closely that they would have all to be classed under
the same genus; and thus the descendants of two distinct genera would
converge into one. But it would in most cases be extremely rash to
attribute to convergence a close and general similarity of structure in the
modified descendants of widely distinct forms. The shape of a crystal is
determined solely by the molecular forces, and it is not surprising that
dissimilar substances should sometimes assume the same form; but with
organic beings we should bear in mind that the form of each depends on an
infinitude of complex relations, namely on the variations which have
arisen, these being due to causes far too intricate to be followed out--on
the nature of the variations which have been preserved or selected, and
this depends on the surrounding physical conditions, and in a still higher
degree on the surrounding organisms with which each being has come into
competition--and lastly, on inheritance (in itself a fluctuating element)
from innumerable progenitors, all of which have had their forms determined
through equally complex relations. It is incredible that the descendants
of two organisms, which had originally differed in a marked manner, should
ever afterwards converge so closely as to lead to a near approach to
identity throughout their whole organisation. If this had occurred, we
should meet with the same form, independently of genetic connection,
recurring in widely separated geological formations; and the balance of
evidence is opposed to any such an admission.
Mr. Watson has also objected that the continued action of natural
selection, together with divergence of character, would tend to make an
indefinite number of specific forms. As far as mere inorganic conditions
are concerned, it seems probable that a sufficient number of species would
soon become adapted to all considerable diversities of heat, moisture,
etc.; but I fully admit that the mutual relations of organic beings are
more important; and as the number of species in any country goes on
increasing, the organic conditions of life must become more and more
complex. Consequently there seems at first no limit to the amount of
profitable diversification of structure, and therefore no limit to the
number of species which might be produced. We do not know that even the
most prolific area is fully stocked with specific forms: at the Cape of
Good Hope and in Australia, which support such an astonishing number of
species, many European plants have become naturalised. But geology shows
us, that from an early part of the tertiary period the number of species of
shells, and that from the middle part of this same period, the number of
mammals has not greatly or at all increased. What then checks an
indefinite increase in the number of species? The amount of life (I do not
mean the number of specific forms) supported on an area must have a limit,
depending so largely as it does on physical conditions; therefore, if an
area be inhabited by very many species, each or nearly each species will be
represented by few individuals; and such species will be liable to
extermination from accidental fluctuations in the nature of the seasons or
in the number of their enemies. The process of extermination in such cases
would be rapid, whereas the production of new species must always be slow.
Imagine the extreme case of as many species as individuals in England, and
the first severe winter or very dry summer would exterminate thousands on
thousands of species. Rare species, and each species will become rare if
the number of species in any country becomes indefinitely increased, will,
on the principal often explained, present within a given period few
favourable variations; consequently, the process of giving birth to new
specific forms would thus be retarded. When any species becomes very rare,
close interbreeding will help to exterminate it; authors have thought that
this comes into play in accounting for the deterioration of the aurochs in
Lithuania, of red deer in Scotland and of bears in Norway, etc. Lastly,
and this I am inclined to think is the most important element, a dominant
species, which has already beaten many competitors in its own home, will
tend to spread and supplant many others. Alph. de Candolle has shown that
those species which spread widely tend generally to spread VERY widely,
consequently they will tend to supplant and exterminate several species in
several areas, and thus check the inordinate increase of specific forms
throughout the world. Dr. Hooker has recently shown that in the southeast
corner of Australia, where, apparently, there are many invaders from
different quarters of the globe, the endemic Australian species have been
greatly reduced in number. How much weight to attribute to these several
considerations I will not pretend to say; but conjointly they must limit in
each country the tendency to an indefinite augmentation of specific forms.
SUMMARY OF CHAPTER.
If under changing conditions of life organic beings present individual
differences in almost every part of their structure, and this cannot be
disputed; if there be, owing to their geometrical rate of increase, a
severe struggle for life at some age, season or year, and this certainly
cannot be disputed; then, considering the infinite complexity of the
relations of all organic beings to each other and to their conditions of
life, causing an infinite diversity in structure, constitution, and habits,
to be advantageous to them, it would be a most extraordinary fact if no
variations had ever occurred useful to each being's own welfare, in the
same manner as so many variations have occurred useful to man. But if
variations useful to any organic being ever do occur, assuredly individuals
thus characterised will have the best chance of being preserved in the
struggle for life; and from the strong principle of inheritance, these will
tend to produce offspring similarly characterised. This principle of
preservation, or the survival of the fittest, I have called natural
selection. It leads to the improvement of each creature in relation to its
organic and inorganic conditions of life; and consequently, in most cases,
to what must be regarded as an advance in organisation. Nevertheless, low
and simple forms will long endure if well fitted for their simple
conditions of life.
Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young as easily as the
adult. Among many animals sexual selection will have given its aid to
ordinary selection by assuring to the most vigorous and best adapted males
the greatest number of offspring. Sexual selection will also give
characters useful to the males alone in their struggles or rivalry with
other males; and these characters will be transmitted to one sex or to both
sexes, according to the form of inheritance which prevails.
Whether natural selection has really thus acted in adapting the various
forms of life to their several conditions and stations, must be judged by
the general tenour and balance of evidence given in the following chapters.
But we have already seen how it entails extinction; and how largely
extinction has acted in the world's history, geology plainly declares.
Natural selection, also, leads to divergence of character; for the more
organic beings diverge in structure, habits and constitution, by so much
the more can a large number be supported on the area, of which we see proof
by looking to the inhabitants of any small spot, and to the productions
naturalised in foreign lands. Therefore, during the modification of the
descendants of any one species, and during the incessant struggle of all
species to increase in numbers, the more diversified the descendants
become, the better will be their chance of success in the battle for life.
Thus the small differences distinguishing varieties of the same species,
steadily tend to increase, till they equal the greater differences between
species of the same genus, or even of distinct genera.
We have seen that it is the common, the widely diffused, and widely ranging
species, belonging to the larger genera within each class, which vary most;
and these tend to transmit to their modified offspring that superiority
which now makes them dominant in their own countries. Natural selection,
as has just been remarked, leads to divergence of character and to much
extinction of the less improved and intermediate forms of life. On these
principles, the nature of the affinities, and the generally well defined
distinctions between the innumerable organic beings in each class
throughout the world, may be explained. It is a truly wonderful fact--the
wonder of which we are apt to overlook from familiarity--that all animals
and all plants throughout all time and space should be related to each
other in groups, subordinate to groups, in the manner which we everywhere
behold--namely, varieties of the same species most closely related, species
of the same genus less closely and unequally related, forming sections and
sub-genera, species of distinct genera much less closely related, and
genera related in different degrees, forming sub-families, families,
orders, sub-classes, and classes. The several subordinate groups in any
class cannot be ranked in a single file, but seem clustered round points,
and these round other points, and so on in almost endless cycles. If
species had been independently created, no explanation would have been
possible of this kind of classification; but it is explained through
inheritance and the complex action of natural selection, entailing
extinction and divergence of character, as we have seen illustrated in the
diagram.
The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during former years may represent the long succession of
extinct species. At each period of growth all the growing twigs have tried
to branch out on all sides, and to overtop and kill the surrounding twigs
and branches, in the same manner as species and groups of species have at
all times overmastered other species in the great battle for life. The
limbs divided into great branches, and these into lesser and lesser
branches, were themselves once, when the tree was young, budding twigs; and
this connexion of the former and present buds by ramifying branches may
well represent the classification of all extinct and living species in
groups subordinate to groups. Of the many twigs which flourished when the
tree was a mere bush, only two or three, now grown into great branches, yet
survive and bear the other branches; so with the species which lived during
long-past geological periods, very few have left living and modified
descendants. From the first growth of the tree, many a limb and branch has
decayed and dropped off; and these fallen branches of various sizes may
represent those whole orders, families, and genera which have now no living
representatives, and which are known to us only in a fossil state. As we
here and there see a thin, straggling branch springing from a fork low down
in a tree, and which by some chance has been favoured and is still alive on
its summit, so we occasionally see an animal like the Ornithorhynchus or
Lepidosiren, which in some small degree connects by its affinities two
large branches of life, and which has apparently been saved from fatal
competition by having inhabited a protected station. As buds give rise by
growth to fresh buds, and these, if vigorous, branch out and overtop on all
sides many a feebler branch, so by generation I believe it has been with
the great Tree of Life, which fills with its dead and broken branches the
crust of the earth, and covers the surface with its ever-branching and
beautiful ramifications.
CHAPTER V.
LAWS OF VARIATION.
Effects of changed conditions -- Use and disuse, combined with natural
selection; organs of flight and of vision -- Acclimatisation -- Correlated
variation -- Compensation and economy of growth -- False correlations --
Multiple, rudimentary, and lowly organised structures variable -- Parts
developed in an unusual manner are highly variable: specific characters
more variable than generic: secondary sexual characters variable --
Species of the same genus vary in an analogous manner -- Reversions to
long-lost characters -- Summary.
I have hitherto sometimes spoken as if the variations--so common and
multiform with organic beings under domestication, and in a lesser degree
with those under nature--were due to chance. This, of course is a wholly
incorrect expression, but it serves to acknowledge plainly our ignorance of
the cause of each particular variation. Some authors believe it to be as
much the function of the reproductive system to produce individual
differences, or slight deviations of structure, as to make the child like
its parents. But the fact of variations and monstrosities occurring much
more frequently under domestication than under nature, and the greater
variability of species having wide ranges than of those with restricted
ranges, lead to the conclusion that variability is generally related to the
conditions of life to which each species has been exposed during several
successive generations. In the first chapter I attempted to show that
changed conditions act in two ways, directly on the whole organisation or
on certain parts alone, and indirectly through the reproductive system. In
all cases there are two factors, the nature of the organism, which is much
the most important of the two, and the nature of the conditions. The
direct action of changed conditions leads to definite or indefinite
results. In the latter case the organisation seems to become plastic, and
we have much fluctuating variability. In the former case the nature of the
organism is such that it yields readily, when subjected to certain
conditions, and all, or nearly all, the individuals become modified in the
same way.
It is very difficult to decide how far changed conditions, such as of
climate, food, etc., have acted in a definite manner. There is reason to
believe that in the course of time the effects have been greater than can
be proved by clear evidence. But we may safely conclude that the
innumerable complex co-adaptations of structure, which we see throughout
nature between various organic beings, cannot be attributed simply to such
action. In the following cases the conditions seem to have produced some
slight definite effect: E. Forbes asserts that shells at their southern
limit, and when living in shallow water, are more brightly coloured than
those of the same species from further north or from a greater depth; but
this certainly does not always hold good. Mr. Gould believes that birds of
the same species are more brightly coloured under a clear atmosphere, than
when living near the coast or on islands; and Wollaston is convinced that
residence near the sea affects the colours of insects. Moquin-Tandon gives
a list of plants which, when growing near the sea-shore, have their leaves
in some degree fleshy, though not elsewhere fleshy. These slightly varying
organisms are interesting in as far as they present characters analogous to
those possessed by the species which are confined to similar conditions.
When a variation is of the slightest use to any being, we cannot tell how
much to attribute to the accumulative action of natural selection, and how
much to the definite action of the conditions of life. Thus, it is well
known to furriers that animals of the same species have thicker and better
fur the further north they live; but who can tell how much of this
difference may be due to the warmest-clad individuals having been favoured
and preserved during many generations, and how much to the action of the
severe climate? For it would appear that climate has some direct action on
the hair of our domestic quadrupeds.
Instances could be given of similar varieties being produced from the same
species under external conditions of life as different as can well be
conceived; and, on the other hand, of dissimilar varieties being produced
under apparently the same external conditions. Again, innumerable
instances are known to every naturalist, of species keeping true, or not
varying at all, although living under the most opposite climates. Such
considerations as these incline me to lay less weight on the direct action
of the surrounding conditions, than on a tendency to vary, due to causes of
which we are quite ignorant.
In one sense the conditions of life may be said, not only to cause
variability, either directly or indirectly, but likewise to include natural
selection, for the conditions determine whether this or that variety shall
survive. But when man is the selecting agent, we clearly see that the two
elements of change are distinct; variability is in some manner excited, but
it is the will of man which accumulates the variations in certain
direction; and it is this latter agency which answers to the survival of
the fittest under nature.
EFFECTS OF THE INCREASED USE AND DISUSE OF PARTS, AS CONTROLLED BY NATURAL
SELECTION.
>From the facts alluded to in the first chapter, I think there can be no
doubt that use in our domestic animals has strengthened and enlarged
certain parts, and disuse diminished them; and that such modifications are
inherited. Under free nature we have no standard of comparison by which to
judge of the effects of long-continued use or disuse, for we know not the
parent-forms; but many animals possess structures which can be best
explained by the effects of disuse. As Professor Owen has remarked, there
is no greater anomaly in nature than a bird that cannot fly; yet there are
several in this state. The logger-headed duck of South America can only
flap along the surface of the water, and has its wings in nearly the same
condition as the domestic Aylesbury duck: it is a remarkable fact that the
young birds, according to Mr. Cunningham, can fly, while the adults have
lost this power. As the larger ground-feeding birds seldom take flight
except to escape danger, it is probable that the nearly wingless condition
of several birds, now inhabiting or which lately inhabited several oceanic
islands, tenanted by no beasts of prey, has been caused by disuse. The
ostrich indeed inhabits continents, and is exposed to danger from which it
cannot escape by flight, but it can defend itself, by kicking its enemies,
as efficiently as many quadrupeds. We may believe that the progenitor of
the ostrich genus had habits like those of the bustard, and that, as the
size and weight of its body were increased during successive generations,
its legs were used more and its wings less, until they became incapable of
flight.
Kirby has remarked (and I have observed the same fact) that the anterior
tarsi, or feet, of many male dung-feeding beetles are often broken off; he
examined seventeen specimens in his own collection, and not one had even a
relic left. In the Onites apelles the tarsi are so habitually lost that
the insect has been described as not having them. In some other genera
they are present, but in a rudimentary condition. In the Ateuchus or
sacred beetle of the Egyptians, they are totally deficient. The evidence
that accidental mutilations can be inherited is at present not decisive;
but the remarkable cases observed by Brown-Sequard in guinea-pigs, of the
inherited effects of operations, should make us cautious in denying this
tendency. Hence, it will perhaps be safest to look at the entire absence
of the anterior tarsi in Ateuchus, and their rudimentary condition in some
other genera, not as cases of inherited mutilations, but as due to the
effects of long-continued disuse; for as many dung-feeding beetles are
generally found with their tarsi lost, this must happen early in life;
therefore the tarsi cannot be of much importance or be much used by these
insects.
In some cases we might easily put down to disuse modifications of structure
which are wholly, or mainly due to natural selection. Mr. Wollaston has
discovered the remarkable fact that 200 beetles, out of the 550 species
(but more are now known) inhabiting Madeira, are so far deficient in wings
that they cannot fly; and that, of the twenty-nine endemic genera, no less
than twenty-three have all their species in this condition! Several facts,
namely, that beetles in many parts of the world are very frequently blown
to sea and perish; that the beetles in Madeira, as observed by Mr.
Wollaston, lie much concealed, until the wind lulls and the sun shines;
that the proportion of wingless beetles is larger on the exposed Desertas
than in Madeira itself; and especially the extraordinary fact, so strongly
insisted on by Mr. Wollaston, that certain large groups of beetles,
elsewhere excessively numerous, which absolutely require the use of their
wings, are here almost entirely absent. These several considerations make
me believe that the wingless condition of so many Madeira beetles is mainly
due to the action of natural selection, combined probably with disuse. For
during many successive generations each individual beetle which flew least,
either from its wings having been ever so little less perfectly developed
or from indolent habit, will have had the best chance of surviving from not
being blown out to sea; and, on the other hand, those beetles which most
readily took to flight would oftenest have been blown to sea, and thus
destroyed.
The insects in Madeira which are not ground-feeders, and which, as certain
flower-feeding coleoptera and lepidoptera, must habitually use their wings
to gain their subsistence, have, as Mr. Wollaston suspects, their wings not
at all reduced, but even enlarged. This is quite compatible with the
action of natural selection. For when a new insect first arrived on the
island, the tendency of natural selection to enlarge or to reduce the
wings, would depend on whether a greater number of individuals were saved
by successfully battling with the winds, or by giving up the attempt and
rarely or never flying. As with mariners shipwrecked near a coast, it
would have been better for the good swimmers if they had been able to swim
still further, whereas it would have been better for the bad swimmers if
they had not been able to swim at all and had stuck to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary in size,
and in some cases are quite covered by skin and fur. This state of the
eyes is probably due to gradual reduction from disuse, but aided perhaps by
natural selection. In South America, a burrowing rodent, the tuco-tuco, or
Ctenomys, is even more subterranean in its habits than the mole; and I was
assured by a Spaniard, who had often caught them, that they were frequently
blind. One which I kept alive was certainly in this condition, the cause,
as appeared on dissection, having been inflammation of the nictitating
membrane. As frequent inflammation of the eyes must be injurious to any
animal, and as eyes are certainly not necessary to animals having
subterranean habits, a reduction in their size, with the adhesion of the
eyelids and growth of fur over them, might in such case be an advantage;
and if so, natural selection would aid the effects of disuse.
It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Carniola and Kentucky, are blind. In
some of the crabs the foot-stalk for the eye remains, though the eye is
gone; the stand for the telescope is there, though the telescope with its
glasses has been lost. As it is difficult to imagine that eyes, though
useless, could be in any way injurious to animals living in darkness, their
loss may be attributed to disuse. In one of the blind animals, namely, the
cave-rat (Neotoma), two of which were captured by Professor Silliman at
above half a mile distance from the mouth of the cave, and therefore not in
the profoundest depths, the eyes were lustrous and of large size; and these
animals, as I am informed by Professor Silliman, after having been exposed
for about a month to a graduated light, acquired a dim perception of
objects.
It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that, in accordance
with the old view of the blind animals having been separately created for
the American and European caverns, very close similarity in their
organisation and affinities might have been expected. This is certainly
not the case if we look at the two whole faunas; with respect to the
insects alone, Schiodte has remarked: "We are accordingly prevented from
considering the entire phenomenon in any other light than something purely
local, and the similarity which is exhibited in a few forms between the
Mammoth Cave (in Kentucky) and the caves in Carniola, otherwise than as a
very plain expression of that analogy which subsists generally between the
fauna of Europe and of North America." On my view we must suppose that
American animals, having in most cases ordinary powers of vision, slowly
migrated by successive generations from the outer world into the deeper and
deeper recesses of the Kentucky caves, as did European animals into the
caves of Europe. We have some evidence of this gradation of habit; for, as
Schiodte remarks: "We accordingly look upon the subterranean faunas as
small ramifications which have penetrated into the earth from the
geographically limited faunas of the adjacent tracts, and which, as they
extended themselves into darkness, have been accommodated to surrounding
circumstances. Animals not far remote from ordinary forms, prepare the
transition from light to darkness. Next follow those that are constructed
for twilight; and, last of all, those destined for total darkness, and
whose formation is quite peculiar." These remarks of Schiodte's it should
be understood, apply not to the same, but to distinct species. By the time
that an animal had reached, after numberless generations, the deepest
recesses, disuse will on this view have more or less perfectly obliterated
its eyes, and natural selection will often have effected other changes,
such as an increase in the length of the antennae or palpi, as a
compensation for blindness. Notwithstanding such modifications, we might
expect still to see in the cave-animals of America, affinities to the other
inhabitants of that continent, and in those of Europe to the inhabitants of
the European continent. And this is the case with some of the American
cave-animals, as I hear from Professor Dana; and some of the European
cave-insects are very closely allied to those of the surrounding country.
It would be difficult to give any rational explanation of the affinities of
the blind cave-animals to the other inhabitants of the two continents on
the ordinary view of their independent creation. That several of the
inhabitants of the caves of the Old and New Worlds should be closely
related, we might expect from the well-known relationship of most of their
other productions. As a blind species of Bathyscia is found in abundance
on shady rocks far from caves, the loss of vision in the cave species of
this one genus has probably had no relation to its dark habitation; for it
is natural that an insect already deprived of vision should readily become
adapted to dark caverns. Another blind genus (Anophthalmus) offers this
remarkable peculiarity, that the species, as Mr. Murray observes, have not
as yet been found anywhere except in caves; yet those which inhabit the
several caves of Europe and America are distinct; but it is possible that
the progenitors of these several species, while they were furnished with
eyes, may formerly have ranged over both continents, and then have become
extinct, excepting in their present secluded abodes. Far from feeling
surprise that some of the cave-animals should be very anomalous, as Agassiz
has remarked in regard to the blind fish, the Amblyopsis, and as is the
case with the blind Proteus, with reference to the reptiles of Europe, I am
only surprised that more wrecks of ancient life have not been preserved,
owing to the less severe competition to which the scanty inhabitants of
these dark abodes will have been exposed.
ACCLIMATISATION.
Habit is hereditary with plants, as in the period of flowering, in the time
of sleep, in the amount of rain requisite for seeds to germinate, etc., and
this leads me to say a few words on acclimatisation. As it is extremely
common for distinct species belonging to the same genus to inhabit hot and
cold countries, if it be true that all the species of the same genus are
descended from a single parent-form, acclimatisation must be readily
effected during a long course of descent. It is notorious that each
species is adapted to the climate of its own home: species from an arctic
or even from a temperate region cannot endure a tropical climate, or
conversely. So again, many succulent plants cannot endure a damp climate.
But the degree of adaptation of species to the climates under which they
live is often overrated. We may infer this from our frequent inability to
predict whether or not an imported plant will endure our climate, and from
the number of plants and animals brought from different countries which are
here perfectly healthy. We have reason to believe that species in a state
of nature are closely limited in their ranges by the competition of other
organic beings quite as much as, or more than, by adaptation to particular
climates. But whether or not this adaptation is in most cases very close,
we have evidence with some few plants, of their becoming, to a certain
extent, naturally habituated to different temperatures; that is, they
become acclimatised: thus the pines and rhododendrons, raised from seed
collected by Dr. Hooker from the same species growing at different heights
on the Himalayas, were found to possess in this country different
constitutional powers of resisting cold. Mr. Thwaites informs me that he
has observed similar facts in Ceylon; analogous observations have been made
by Mr. H.C. Watson on European species of plants brought from the Azores to
England; and I could give other cases. In regard to animals, several
authentic instances could be adduced of species having largely extended,
within historical times, their range from warmer to colder latitudes, and
conversely; but we do not positively know that these animals were strictly
adapted to their native climate, though in all ordinary cases we assume
such to be the case; nor do we know that they have subsequently become
specially acclimatised to their new homes, so as to be better fitted for
them than they were at first.
As we may infer that our domestic animals were originally chosen by
uncivilised man because they were useful, and because they bred readily
under confinement, and not because they were subsequently found capable of
far-extended transportation, the common and extraordinary capacity in our
domestic animals of not only withstanding the most different climates, but
of being perfectly fertile (a far severer test) under them, may be used as
an argument that a large proportion of other animals now in a state of
nature could easily be brought to bear widely different climates. We must
not, however, push the foregoing argument too far, on account of the
probable origin of some of our domestic animals from several wild stocks:
the blood, for instance, of a tropical and arctic wolf may perhaps be
mingled in our domestic breeds. The rat and mouse cannot be considered as
domestic animals, but they have been transported by man to many parts of
the world, and now have a far wider range than any other rodent; for they
live under the cold climate of Faroe in the north and of the Falklands in
the south, and on many an island in the torrid zones. Hence adaptation to
any special climate may be looked at as a quality readily grafted on an
innate wide flexibility of constitution, common to most animals. On this
view, the capacity of enduring the most different climates by man himself
and by his domestic animals, and the fact of the extinct elephant and
rhinoceros having formerly endured a glacial climate, whereas the living
species are now all tropical or sub-tropical in their habits, ought not to
be looked at as anomalies, but as examples of a very common flexibility of
constitution, brought, under peculiar circumstances, into action.
How much of the acclimatisation of species to any peculiar climate is due
to mere habit, and how much to the natural selection of varieties having
different innate constitutions, and how much to both means combined, is an
obscure question. That habit or custom has some influence, I must believe,
both from analogy and from the incessant advice given in agricultural
works, even in the ancient Encyclopaedias of China, to be very cautious in
transporting animals from one district to another. And as it is not likely
that man should have succeeded in selecting so many breeds and sub-breeds
with constitutions specially fitted for their own districts, the result
must, I think, be due to habit. On the other hand, natural selection would
inevitably tend to preserve those individuals which were born with
constitutions best adapted to any country which they inhabited. In
treatises on many kinds of cultivated plants, certain varieties are said to
withstand certain climates better than others; this is strikingly shown in
works on fruit-trees published in the United States, in which certain
varieties are habitually recommended for the northern and others for the
southern states; and as most of these varieties are of recent origin, they
cannot owe their constitutional differences to habit. The case of the
Jerusalem artichoke, which is never propagated in England by seed, and of
which, consequently, new varieties have not been produced, has even been
advanced, as proving that acclimatisation cannot be effected, for it is now
as tender as ever it was! The case, also, of the kidney-bean has been
often cited for a similar purpose, and with much greater weight; but until
some one will sow, during a score of generations, his kidney-beans so early
that a very large proportion are destroyed by frost, and then collect seed
from the few survivors, with care to prevent accidental crosses, and then
again get seed from these seedlings, with the same precautions, the
experiment cannot be said to have been even tried. Nor let it be supposed
that differences in the constitution of seedling kidney-beans never appear,
for an account has been published how much more hardy some seedlings are
than others; and of this fact I have myself observed striking instances.
On the whole, we may conclude that habit, or use and disuse, have, in some
cases, played a considerable part in the modification of the constitution
and structure; but that the effects have often been largely combined with,
and sometimes overmastered by, the natural selection of innate variations.
CORRELATED VARIATION.
I mean by this expression that the whole organisation is so tied together,
during its growth and development, that when slight variations in any one
part occur and are accumulated through natural selection, other parts
become modified. This is a very important subject, most imperfectly
understood, and no doubt wholly different classes of facts may be here
easily confounded together. We shall presently see that simple inheritance
often gives the false appearance of correlation. One of the most obvious
real cases is, that variations of structure arising in the young or larvae
naturally tend to affect the structure of the mature animal. The several
parts which are homologous, and which, at an early embryonic period, are
identical in structure, and which are necessarily exposed to similar
conditions, seem eminently liable to vary in a like manner: we see this in
the right and left sides of the body varying in the same manner; in the
front and hind legs, and even in the jaws and limbs, varying together, for
the lower jaw is believed by some anatomists to be homologous with the
limbs. These tendencies, I do not doubt, may be mastered more or less
completely by natural selection: thus a family of stags once existed with
an antler only on one side; and if this had been of any great use to the
breed, it might probably have been rendered permanent by natural selection.
Homologous parts, as has been remarked by some authors, tend to cohere;
this is often seen in monstrous plants: and nothing is more common than
the union of homologous parts in normal structures, as in the union of the
petals into a tube. Hard parts seem to affect the form of adjoining soft
parts; it is believed by some authors that with birds the diversity in the
shape of the pelvis causes the remarkable diversity in the shape of the
kidneys. Others believe that the shape of the pelvis in the human mother
influences by pressure the shape of the head of the child. In snakes,
according to Schlegel, the shape of the body and the manner of swallowing
determine the position and form of several of the most important viscera.
The nature of the bond is frequently quite obscure. M. Is. Geoffroy St.
Hilaire has forcibly remarked that certain malconformations frequently, and
that others rarely, coexist without our being able to assign any reason.
What can be more singular than the relation in cats between complete
whiteness and blue eyes with deafness, or between the tortoise-shell colour
and the female sex; or in pigeons, between their feathered feet and skin
betwixt the outer toes, or between the presence of more or less down on the
young pigeon when first hatched, with the future colour of its plumage; or,
again, the relation between the hair and the teeth in the naked Turkish
dog, though here no doubt homology comes into play? With respect to this
latter case of correlation, I think it can hardly be accidental that the
two orders of mammals which are most abnormal in their dermal covering,
viz., Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters, etc.),
are likewise on the whole the most abnormal in their teeth, but there are
so many exceptions to this rule, as Mr. Mivart has remarked, that it has
little value.
I know of no case better adapted to show the importance of the laws of
correlation and variation, independently of utility, and therefore of
natural selection, than that of the difference between the outer and inner
flowers in some Compositous and Umbelliferous plants. Everyone is familiar
with the difference between the ray and central florets of, for instance,
the daisy, and this difference is often accompanied with the partial or
complete abortion of the reproductive organs. But in some of these plants
the seeds also differ in shape and sculpture. These differences have
sometimes been attributed to the pressure of the involucra on the florets,
or to their mutual pressure, and the shape of the seeds in the ray-florets
of some Compositae countenances this idea; but with the Umbelliferae it is
by no means, as Dr. Hooker informs me, the species with the densest heads
which most frequently differ in their inner and outer flowers. It might
have been thought that the development of the ray-petals, by drawing
nourishment from the reproductive organs causes their abortion; but this
can hardly be the sole case, for in some Compositae the seeds of the outer
and inner florets differ, without any difference in the corolla. Possibly
these several differences may be connected with the different flow of
nutriment towards the central and external flowers. We know, at least,
that with irregular flowers those nearest to the axis are most subject to
peloria, that is to become abnormally symmetrical. I may add, as an
instance of this fact, and as a striking case of correlation, that in many
pelargoniums the two upper petals in the central flower of the truss often
lose their patches of darker colour; and when this occurs, the adherent
nectary is quite aborted, the central flower thus becoming peloric or
regular. When the colour is absent from only one of the two upper petals,
the nectary is not quite aborted but is much shortened.
With respect to the development of the corolla, Sprengel's idea that the
ray-florets serve to attract insects, whose agency is highly advantageous,
or necessary for the fertilisation of these plants, is highly probable; and
if so, natural selection may have come into play. But with respect to the
seeds, it seems impossible that their differences in shape, which are not
always correlated with any difference in the corolla, can be in any way
beneficial; yet in the Umbelliferae these differences are of such apparent
importance--the seeds being sometimes orthospermous in the exterior flowers
and coelospermous in the central flowers--that the elder De Candolle
founded his main divisions in the order on such characters. Hence
modifications of structure, viewed by systematists as of high value, may be
wholly due to the laws of variation and correlation, without being, as far
as we can judge, of the slightest service to the species.
We may often falsely attribute to correlated variation structures which are
common to whole groups of species, and which in truth are simply due to
inheritance; for an ancient progenitor may have acquired through natural
selection some one modification in structure, and, after thousands of
generations, some other and independent modification; and these two
modifications, having been transmitted to a whole group of descendants with
diverse habits, would naturally be thought to be in some necessary manner
correlated. Some other correlations are apparently due to the manner in
which natural selection can alone act. For instance, Alph. De Candolle has
remarked that winged seeds are never found in fruits which do not open; I
should explain this rule by the impossibility of seeds gradually becoming
winged through natural selection, unless the capsules were open; for in
this case alone could the seeds, which were a little better adapted to be
wafted by the wind, gain an advantage over others less well fitted for wide
dispersal.
COMPENSATION AND ECONOMY OF GROWTH.
The elder Geoffroy and Goethe propounded, at about the same time, their law
of compensation or balancement of growth; or, as Goethe expressed it, "in
order to spend on one side, nature is forced to economise on the other
side." I think this holds true to a certain extent with our domestic
productions: if nourishment flows to one part or organ in excess, it
rarely flows, at least in excess, to another part; thus it is difficult to
get a cow to give much milk and to fatten readily. The same varieties of
the cabbage do not yield abundant and nutritious foliage and a copious
supply of oil-bearing seeds. When the seeds in our fruits become
atrophied, the fruit itself gains largely in size and quality. In our
poultry, a large tuft of feathers on the head is generally accompanied by a
diminished comb, and a large beard by diminished wattles. With species in
a state of nature it can hardly be maintained that the law is of universal
application; but many good observers, more especially botanists, believe in
its truth. I will not, however, here give any instances, for I see hardly
any way of distinguishing between the effects, on the one hand, of a part
being largely developed through natural selection and another and adjoining
part being reduced by the same process or by disuse, and, on the other
hand, the actual withdrawal of nutriment from one part owing to the excess
of growth in another and adjoining part.
I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more general
principle, namely, that natural selection is continually trying to
economise in every part of the organisation. If under changed conditions
of life a structure, before useful, becomes less useful, its diminution
will be favoured, for it will profit the individual not to have its
nutriment wasted in building up a useless structure. I can thus only
understand a fact with which I was much struck when examining cirripedes,
and of which many other instances could be given: namely, that when a
cirripede is parasitic within another cirripede and is thus protected, it
loses more or less completely its own shell or carapace. This is the case
with the male Ibla, and in a truly extraordinary manner with the
Proteolepas: for the carapace in all other cirripedes consists of the
three highly important anterior segments of the head enormously developed,
and furnished with great nerves and muscles; but in the parasitic and
protected Proteolepas, the whole anterior part of the head is reduced to
the merest rudiment attached to the bases of the prehensile antennae. Now
the saving of a large and complex structure, when rendered superfluous,
would be a decided advantage to each successive individual of the species;
for in the struggle for life to which every animal is exposed, each would
have a better chance of supporting itself, by less nutriment being wasted.
Thus, as I believe, natural selection will tend in the long run to reduce
any part of the organisation, as soon as it becomes, through changed
habits, superfluous, without by any means causing some other part to be
largely developed in a corresponding degree. And conversely, that natural
selection may perfectly well succeed in largely developing an organ without
requiring as a necessary compensation the reduction of some adjoining part.
MULTIPLE, RUDIMENTARY, AND LOWLY-ORGANISED STRUCTURES ARE VARIABLE.
It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both with
varieties and species, that when any part or organ is repeated many times
in the same individual (as the vertebrae in snakes, and the stamens in
polyandrous flowers) the number is variable; whereas the number of the same
part or organ, when it occurs in lesser numbers, is constant. The same
author as well as some botanists, have further remarked that multiple parts
are extremely liable to vary in structure. As "vegetative repetition," to
use Professor Owen's expression, is a sign of low organisation; the
foregoing statements accord with the common opinion of naturalists, that
beings which stand low in the scale of nature are more variable than those
which are higher. I presume that lowness here means that the several parts
of the organisation have been but little specialised for particular
functions; and as long as the same part has to perform diversified work, we
can perhaps see why it should remain variable, that is, why natural
selection should not have preserved or rejected each little deviation of
form so carefully as when the part has to serve for some one special
purpose. In the same way that a knife which has to cut all sorts of things
may be of almost any shape; whilst a tool for some particular purpose must
be of some particular shape. Natural selection, it should never be
forgotten, can act solely through and for the advantage of each being.
Rudimentary parts, as is generally admitted, are apt to be highly variable.
We shall have to recur to this subject; and I will here only add that their
variability seems to result from their uselessness, and consequently from
natural selection having had no power to check deviations in their
structure.
A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER, IN
COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY
VARIABLE.
Several years ago I was much struck by a remark to the above effect made by
Mr. Waterhouse. Professor Owen, also, seems to have come to a nearly
similar conclusion. It is hopeless to attempt to convince any one of the
truth of the above proposition without giving the long array of facts which
I have collected, and which cannot possibly be here introduced. I can only
state my conviction that it is a rule of high generality. I am aware of
several causes of error, but I hope that I have made due allowances for
them. It should be understood that the rule by no means applies to any
part, however unusually developed, unless it be unusually developed in one
species or in a few species in comparison with the same part in many
closely allied species. Thus, the wing of the bat is a most abnormal
structure in the class of mammals; but the rule would not apply here,
because the whole group of bats possesses wings; it would apply only if
some one species had wings developed in a remarkable manner in comparison
with the other species of the same genus. The rule applies very strongly
in the case of secondary sexual characters, when displayed in any unusual
manner. The term, secondary sexual characters, used by Hunter, relates to
characters which are attached to one sex, but are not directly connected
with the act of reproduction. The rule applies to males and females; but
more rarely to females, as they seldom offer remarkable secondary sexual
characters. The rule being so plainly applicable in the case of secondary
sexual characters, may be due to the great variability of these characters,
whether or not displayed in any unusual manner--of which fact I think there
can be little doubt. But that our rule is not confined to secondary sexual
characters is clearly shown in the case of hermaphrodite cirripedes; I
particularly attended to Mr. Waterhouse's remark, whilst investigating this
order, and I am fully convinced that the rule almost always holds good. I
shall, in a future work, give a list of all the more remarkable cases. I
will here give only one, as it illustrates the rule in its largest
application. The opercular valves of sessile cirripedes (rock barnacles)
are, in every sense of the word, very important structures, and they differ
extremely little even in distinct genera; but in the several species of one
genus, Pyrgoma, these valves present a marvellous amount of
diversification; the homologous valves in the different species being
sometimes wholly unlike in shape; and the amount of variation in the
individuals of the same species is so great that it is no exaggeration to
state that the varieties of the same species differ more from each other in
the characters derived from these important organs, than do the species
belonging to other distinct genera.
As with birds the individuals of the same species, inhabiting the same
country, vary extremely little, I have particularly attended to them; and
the rule certainly seems to hold good in this class. I cannot make out
that it applies to plants, and this would have seriously shaken my belief
in its truth, had not the great variability in plants made it particularly
difficult to compare their relative degrees of variability.
When we see any part or organ developed in a remarkable degree or manner in
a species, the fair presumption is that it is of high importance to that
species: nevertheless it is in this case eminently liable to variation.
Why should this be so? On the view that each species has been
independently created, with all its parts as we now see them, I can see no
explanation. But on the view that groups of species are descended from
some other species, and have been modified through natural selection, I
think we can obtain some light. First let me make some preliminary
remarks. If, in our domestic animals, any part or the whole animal be
neglected, and no selection be applied, that part (for instance, the comb
in the Dorking fowl) or the whole breed will cease to have a uniform
character: and the breed may be said to be degenerating. In rudimentary
organs, and in those which have been but little specialised for any
particular purpose, and perhaps in polymorphic groups, we see a nearly
parallel case; for in such cases natural selection either has not or cannot
come into full play, and thus the organisation is left in a fluctuating
condition. But what here more particularly concerns us is, that those
points in our domestic animals, which at the present time are undergoing
rapid change by continued selection, are also eminently liable to
variation. Look at the individuals of the same breed of the pigeon; and
see what a prodigious amount of difference there is in the beak of
tumblers, in the beak and wattle of carriers, in the carriage and tail of
fantails, etc., these being the points now mainly attended to by English
fanciers. Even in the same sub-breed, as in that of the short-faced
tumbler, it is notoriously difficult to breed nearly perfect birds, many
departing widely from the standard. There may truly be said to be a
constant struggle going on between, on the one hand, the tendency to
reversion to a less perfect state, as well as an innate tendency to new
variations, and, on the other hand, the power of steady selection to keep
the breed true. In the long run selection gains the day, and we do not
expect to fail so completely as to breed a bird as coarse as a common
tumbler pigeon from a good short-faced strain. But as long as selection is
rapidly going on, much variability in the parts undergoing modification may
always be expected.
Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other species of
the same genus, we may conclude that this part has undergone an
extraordinary amount of modification since the period when the several
species branched off from the common progenitor of the genus. This period
will seldom be remote in any extreme degree, as species rarely endure for
more than one geological period. An extraordinary amount of modification
implies an unusually large and long-continued amount of variability, which
has continually been accumulated by natural selection for the benefit of
the species. But as the variability of the extraordinarily developed part
or organ has been so great and long-continued within a period not
excessively remote, we might, as a general rule, still expect to find more
variability in such parts than in other parts of the organisation which
have remained for a much longer period nearly constant. And this, I am
convinced, is the case. That the struggle between natural selection on the
one hand, and the tendency to reversion and variability on the other hand,
will in the course of time cease; and that the most abnormally developed
organs may be made constant, I see no reason to doubt. Hence, when an
organ, however abnormal it may be, has been transmitted in approximately
the same condition to many modified descendants, as in the case of the wing
of the bat, it must have existed, according to our theory, for an immense
period in nearly the same state; and thus it has come not to be more
variable than any other structure. It is only in those cases in which the
modification has been comparatively recent and extraordinarily great that
we ought to find the GENERATIVE VARIABILITY, as it may be called, still
present in a high degree. For in this case the variability will seldom as
yet have been fixed by the continued selection of the individuals varying
in the required manner and degree, and by the continued rejection of those
tending to revert to a former and less modified condition.
SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC CHARACTERS.
The principle discussed under the last heading may be applied to our
present subject. It is notorious that specific characters are more
variable than generic. To explain by a simple example what is meant: if
in a large genus of plants some species had blue flowers and some had red,
the colour would be only a specific character, and no one would be
surprised at one of the blue species varying into red, or conversely; but
if all the species had blue flowers, the colour would become a generic
character, and its variation would be a more unusual circumstance. I have
chosen this example because the explanation which most naturalists would
advance is not here applicable, namely, that specific characters are more
variable than generic, because they are taken from parts of less
physiological importance than those commonly used for classing genera. I
believe this explanation is partly, yet only indirectly, true; I shall,
however, have to return to this point in the chapter on Classification. It
would be almost superfluous to adduce evidence in support of the statement,
that ordinary specific characters are more variable than generic; but with
respect to important characters, I have repeatedly noticed in works on
natural history, that when an author remarks with surprise that some
important organ or part, which is generally very constant throughout a
large group of species, DIFFERS considerably in closely-allied species, it
is often VARIABLE in the individuals of the same species. And this fact
shows that a character, which is generally of generic value, when it sinks
in value and becomes only of specific value, often becomes variable, though
its physiological importance may remain the same. Something of the same
kind applies to monstrosities: at least Is. Geoffroy St. Hilaire
apparently entertains no doubt, that the more an organ normally differs in
the different species of the same group, the more subject it is to
anomalies in the individuals.
On the ordinary view of each species having been independently created, why
should that part of the structure, which differs from the same part in
other independently created species of the same genus, be more variable
than those parts which are closely alike in the several species? I do not
see that any explanation can be given. But on the view that species are
only strongly marked and fixed varieties, we might expect often to find
them still continuing to vary in those parts of their structure which have
varied within a moderately recent period, and which have thus come to
differ. Or to state the case in another manner: the points in which all
the species of a genus resemble each other, and in which they differ from
allied genera, are called generic characters; and these characters may be
attributed to inheritance from a common progenitor, for it can rarely have
happened that natural selection will have modified several distinct
species, fitted to more or less widely different habits, in exactly the
same manner: and as these so-called generic characters have been inherited
from before the period when the several species first branched off from
their common progenitor, and subsequently have not varied or come to differ
in any degree, or only in a slight degree, it is not probable that they
should vary at the present day. On the other hand, the points in which
species differ from other species of the same genus are called specific
characters; and as these specific characters have varied and come to differ
since the period when the species branched off from a common progenitor, it
is probable that they should still often be in some degree variable--at
least more variable than those parts of the organisation which have for a
very long period remained constant.
SECONDARY SEXUAL CHARACTERS VARIABLE.
I think it will be admitted by naturalists, without my entering on details,
that secondary sexual characters are highly variable. It will also be
admitted that species of the same group differ from each other more widely
in their secondary sexual characters, than in other parts of their
organisation; compare, for instance, the amount of difference between the
males of gallinaceous birds, in which secondary sexual characters are
strongly displayed, with the amount of difference between the females. The
cause of the original variability of these characters is not manifest; but
we can see why they should not have been rendered as constant and uniform
as others, for they are accumulated by sexual selection, which is less
rigid in its action than ordinary selection, as it does not entail death,
but only gives fewer offspring to the less favoured males. Whatever the
cause may be of the variability of secondary sexual characters, as they are
highly variable, sexual selection will have had a wide scope for action,
and may thus have succeeded in giving to the species of the same group a
greater amount of difference in these than in other respects.
It is a remarkable fact, that the secondary differences between the two
sexes of the same species are generally displayed in the very same parts of
the organisation in which the species of the same genus differ from each
other. Of this fact I will give in illustration the first two instances
which happen to stand on my list; and as the differences in these cases are
of a very unusual nature, the relation can hardly be accidental. The same
number of joints in the tarsi is a character common to very large groups of
beetles, but in the Engidae, as Westwood has remarked, the number varies
greatly and the number likewise differs in the two sexes of the same
species. Again in the fossorial hymenoptera, the neuration of the wings is
a character of the highest importance, because common to large groups; but
in certain genera the neuration differs in the different species, and
likewise in the two sexes of the same species. Sir J. Lubbock has recently
remarked, that several minute crustaceans offer excellent illustrations of
this law. "In Pontella, for instance, the sexual characters are afforded
mainly by the anterior antennae and by the fifth pair of legs: the
specific differences also are principally given by these organs." This
relation has a clear meaning on my view: I look at all the species of the
same genus as having as certainly descended from the same progenitor, as
have the two sexes of any one species. Consequently, whatever part of the
structure of the common progenitor, or of its early descendants, became
variable; variations of this part would, it is highly probable, be taken
advantage of by natural and sexual selection, in order to fit the several
places in the economy of nature, and likewise to fit the two sexes of the
same species to each other, or to fit the males to struggle with other
males for the possession of the females.
Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which are possessed by all the species; that
the frequent extreme variability of any part which is developed in a
species in an extraordinary manner in comparison with the same part in its
congeners; and the slight degree of variability in a part, however
extraordinarily it may be developed, if it be common to a whole group of
species; that the great variability of secondary sexual characters and
their great difference in closely allied species; that secondary sexual and
ordinary specific differences are generally displayed in the same parts of
the organisation, are all principles closely connected together. All being
mainly due to the species of the same group being the descendants of a
common progenitor, from whom they have inherited much in common, to parts
which have recently and largely varied being more likely still to go on
varying than parts which have long been inherited and have not varied, to
natural selection having more or less completely, according to the lapse of
time, overmastered the tendency to reversion and to further variability, to
sexual selection being less rigid than ordinary selection, and to
variations in the same parts having been accumulated by natural and sexual
selection, and thus having been adapted for secondary sexual, and for
ordinary purposes.
DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS, SO THAT A VARIETY OF ONE
SPECIES OFTEN ASSUMES A CHARACTER PROPER TO AN ALLIED SPECIES, OR REVERTS
TO SOME OF THE CHARACTERS OF AN EARLY PROGENITOR.
These propositions will be most readily understood by looking to our
domestic races. The most distinct breeds of the pigeon, in countries
widely apart, present sub-varieties with reversed feathers on the head, and
with feathers on the feet, characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more distinct
races. The frequent presence of fourteen or even sixteen tail-feathers in
the pouter may be considered as a variation representing the normal
structure of another race, the fantail. I presume that no one will doubt
that all such analogous variations are due to the several races of the
pigeon having inherited from a common parent the same constitution and
tendency to variation, when acted on by similar unknown influences. In the
vegetable kingdom we have a case of analogous variation, in the enlarged
stems, or as commonly called roots, of the Swedish turnip and ruta-baga,
plants which several botanists rank as varieties produced by cultivation
from a common parent: if this be not so, the case will then be one of
analogous variation in two so-called distinct species; and to these a third
may be added, namely, the common turnip. According to the ordinary view of
each species having been independently created, we should have to attribute
this similarity in the enlarged stems of these three plants, not to the
vera causa of community of descent, and a consequent tendency to vary in a
like manner, but to three separate yet closely related acts of creation.
Many similar cases of analogous variation have been observed by Naudin in
the great gourd family, and by various authors in our cereals. Similar
cases occurring with insects under natural conditions have lately been
discussed with much ability by Mr. Walsh, who has grouped them under his
law of equable variability.
With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars on
the wings, white loins, a bar at the end of the tail, with the outer
feathers externally edged near their bases with white. As all these marks
are characteristic of the parent rock-pigeon, I presume that no one will
doubt that this is a case of reversion, and not of a new yet analogous
variation appearing in the several breeds. We may, I think, confidently
come to this conclusion, because, as we have seen, these coloured marks are
eminently liable to appear in the crossed offspring of two distinct and
differently coloured breeds; and in this case there is nothing in the
external conditions of life to cause the reappearance of the slaty-blue,
with the several marks, beyond the influence of the mere act of crossing on
the laws of inheritance.
No doubt it is a very surprising fact that characters should reappear after
having been lost for many, probably for hundreds of generations. But when
a breed has been crossed only once by some other breed, the offspring
occasionally show for many generations a tendency to revert in character to
the foreign breed--some say, for a dozen or even a score of generations.
After twelve generations, the proportion of blood, to use a common
expression, from one ancestor, is only 1 in 2048; and yet, as we see, it is
generally believed that a tendency to reversion is retained by this remnant
of foreign blood. In a breed which has not been crossed, but in which BOTH
parents have lost some character which their progenitor possessed, the
tendency, whether strong or weak, to reproduce the lost character might, as
was formerly remarked, for all that we can see to the contrary, be
transmitted for almost any number of generations. When a character which
has been lost in a breed, reappears after a great number of generations,
the most probable hypothesis is, not that one individual suddenly takes
after an ancestor removed by some hundred generations, but that in each
successive generation the character in question has been lying latent, and
at last, under unknown favourable conditions, is developed. With the
barb-pigeon, for instance, which very rarely produces a blue bird, it is
probable that there is a latent tendency in each generation to produce blue
plumage. The abstract improbability of such a tendency being transmitted
through a vast number of generations, is not greater than that of quite
useless or rudimentary organs being similarly transmitted. A mere tendency
to produce a rudiment is indeed sometimes thus inherited.
As all the species of the same genus are supposed to be descended from a
common progenitor, it might be expected that they would occasionally vary
in an analogous manner; so that the varieties of two or more species would
resemble each other, or that a variety of one species would resemble in
certain characters another and distinct species, this other species being,
according to our view, only a well-marked and permanent variety. But
characters exclusively due to analogous variation would probably be of an
unimportant nature, for the preservation of all functionally important
characters will have been determined through natural selection, in
accordance with the different habits of the species. It might further be
expected that the species of the same genus would occasionally exhibit
reversions to long-lost characters. As, however, we do not know the common
ancestor of any natural group, we cannot distinguish between reversionary
and analogous characters. If, for instance, we did not know that the
parent rock-pigeon was not feather-footed or turn-crowned, we could not
have told, whether such characters in our domestic breeds were reversions
or only analogous variations; but we might have inferred that the blue
colour was a case of reversion from the number of the markings, which are
correlated with this tint, and which would not probably have all appeared
together from simple variation. More especially we might have inferred
this from the blue colour and the several marks so often appearing when
differently coloured breeds are crossed. Hence, although under nature it
must generally be left doubtful, what cases are reversions to formerly
existing characters, and what are new but analogous variations, yet we
ought, on our theory, sometimes to find the varying offspring of a species
assuming characters which are already present in other members of the same
group. And this undoubtedly is the case.
The difficulty in distinguishing variable species is largely due to the
varieties mocking, as it were, other species of the same genus. A
considerable catalogue, also, could be given of forms intermediate between
two other forms, which themselves can only doubtfully be ranked as species;
and this shows, unless all these closely allied forms be considered as
independently created species, that they have in varying assumed some of
the characters of the others. But the best evidence of analogous
variations is afforded by parts or organs which are generally constant in
character, but which occasionally vary so as to resemble, in some degree,
the same part or organ in an allied species. I have collected a long list
of such cases; but here, as before, I lie under the great disadvantage of
not being able to give them. I can only repeat that such cases certainly
occur, and seem to me very remarkable.
I will, however, give one curious and complex case, not indeed as affecting
any important character, but from occurring in several species of the same
genus, partly under domestication and partly under nature. It is a case
almost certainly of reversion. The ass sometimes has very distinct
transverse bars on its legs, like those on the legs of a zebra. It has
been asserted that these are plainest in the foal, and from inquiries which
I have made, I believe this to be true. The stripe on the shoulder is
sometimes double, and is very variable in length and outline. A white ass,
but NOT an albino, has been described without either spinal or shoulder
stripe; and these stripes are sometimes very obscure, or actually quite
lost, in dark-coloured asses. The koulan of Pallas is said to have been
seen with a double shoulder-stripe. Mr. Blyth has seen a specimen of the
hemionus with a distinct shoulder-stripe, though it properly has none; and
I have been informed by Colonel Poole that foals of this species are
generally striped on the legs and faintly on the shoulder. The quagga,
though so plainly barred like a zebra over the body, is without bars on the
legs; but Dr. Gray has figured one specimen with very distinct zebra-like
bars on the hocks.
With respect to the horse, I have collected cases in England of the spinal
stripe in horses of the most distinct breeds, and of ALL colours;
transverse bars on the legs are not rare in duns, mouse-duns, and in one
instance in a chestnut; a faint shoulder-stripe may sometimes be seen in
duns, and I have seen a trace in a bay horse. My son made a careful
examination and sketch for me of a dun Belgian cart-horse with a double
stripe on each shoulder and with leg-stripes. I have myself seen a dun
Devonshire pony, and a small dun Welsh pony has been carefully described to
me, both with THREE parallel stripes on each shoulder.
In the northwest part of India the Kattywar breed of horses is so generally
striped, that, as I hear from Colonel Poole, who examined this breed for
the Indian Government, a horse without stripes is not considered as purely
bred. The spine is always striped; the legs are generally barred; and the
shoulder-stripe, which is sometimes double and sometimes treble, is common;
the side of the face, moreover, is sometimes striped. The stripes are
often plainest in the foal; and sometimes quite disappear in old horses.
Colonel Poole has seen both gray and bay Kattywar horses striped when first
foaled. I have also reason to suspect, from information given me by Mr.
W.W. Edwards, that with the English race-horse the spinal stripe is much
commoner in the foal than in the full-grown animal. I have myself recently
bred a foal from a bay mare (offspring of a Turkoman horse and a Flemish
mare) by a bay English race-horse. This foal, when a week old, was marked
on its hinder quarters and on its forehead with numerous very narrow, dark,
zebra-like bars, and its legs were feebly striped. All the stripes soon
disappeared completely. Without here entering on further details I may
state that I have collected cases of leg and shoulder stripes in horses of
very different breeds in various countries from Britain to Eastern China;
and from Norway in the north to the Malay Archipelago in the south. In all
parts of the world these stripes occur far oftenest in duns and mouse-duns;
by the term dun a large range of colour is included, from one between brown
and black to a close approach to cream colour.
I am aware that Colonel Hamilton Smith, who has written on this subject,
believes that the several breeds of the horse are descended from several
aboriginal species, one of which, the dun, was striped; and that the
above-described appearances are all due to ancient crosses with the dun
stock. But this view may be safely rejected, for it is highly improbable
that the heavy Belgian cart-horse, Welsh ponies, Norwegian cobs, the lanky
Kattywar race, etc., inhabiting the most distant parts of the world, should
have all have been crossed with one supposed aboriginal stock.
Now let us turn to the effects of crossing the several species of the horse
genus. Rollin asserts that the common mule from the ass and horse is
particularly apt to have bars on its legs; according to Mr. Gosse, in
certain parts of the United States, about nine out of ten mules have
striped legs. I once saw a mule with its legs so much striped that any one
might have thought that it was a hybrid zebra; and Mr. W.C. Martin, in his
excellent treatise on the horse, has given a figure of a similar mule. In
four coloured drawings, which I have seen, of hybrids between the ass and
zebra, the legs were much more plainly barred than the rest of the body;
and in one of them there was a double shoulder-stripe. In Lord Morton's
famous hybrid, from a chestnut mare and male quagga, the hybrid and even
the pure offspring subsequently produced from the same mare by a black
Arabian sire, were much more plainly barred across the legs than is even
the pure quagga. Lastly, and this is another most remarkable case, a
hybrid has been figured by Dr. Gray (and he informs me that he knows of a
second case) from the ass and the hemionus; and this hybrid, though the ass
only occasionally has stripes on his legs and the hemionus has none and has
not even a shoulder-stripe, nevertheless had all four legs barred, and had
three short shoulder-stripes, like those on the dun Devonshire and Welsh
ponies, and even had some zebra-like stripes on the sides of its face.
With respect to this last fact, I was so convinced that not even a stripe
of colour appears from what is commonly called chance, that I was led
solely from the occurrence of the face-stripes on this hybrid from the ass
and hemionus to ask Colonel Poole whether such face-stripes ever occurred
in the eminently striped Kattywar breed of horses, and was, as we have
seen, answered in the affirmative.
What now are we to say to these several facts? We see several distinct
species of the horse genus becoming, by simple variation, striped on the
legs like a zebra, or striped on the shoulders like an ass. In the horse
we see this tendency strong whenever a dun tint appears--a tint which
approaches to that of the general colouring of the other species of the
genus. The appearance of the stripes is not accompanied by any change of
form, or by any other new character. We see this tendency to become
striped most strongly displayed in hybrids from between several of the most
distinct species. Now observe the case of the several breeds of pigeons:
they are descended from a pigeon (including two or three sub-species or
geographical races) of a bluish colour, with certain bars and other marks;
and when any breed assumes by simple variation a bluish tint, these bars
and other marks invariably reappear; but without any other change of form
or character. When the oldest and truest breeds of various colours are
crossed, we see a strong tendency for the blue tint and bars and marks to
reappear in the mongrels. I have stated that the most probable hypothesis
to account for the reappearance of very ancient characters, is--that there
is a TENDENCY in the young of each successive generation to produce the
long-lost character, and that this tendency, from unknown causes, sometimes
prevails. And we have just seen that in several species of the horse genus
the stripes are either plainer or appear more commonly in the young than in
the old. Call the breeds of pigeons, some of which have bred true for
centuries, species; and how exactly parallel is the case with that of the
species of the horse genus! For myself, I venture confidently to look back
thousands on thousands of generations, and I see an animal striped like a
zebra, but perhaps otherwise very differently constructed, the common
parent of our domestic horse (whether or not it be descended from one or
more wild stocks) of the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created, will, I
presume, assert that each species has been created with a tendency to vary,
both under nature and under domestication, in this particular manner, so as
often to become striped like the other species of the genus; and that each
has been created with a strong tendency, when crossed with species
inhabiting distant quarters of the world, to produce hybrids resembling in
their stripes, not their own parents, but other species of the genus. To
admit this view is, as it seems to me, to reject a real for an unreal, or
at least for an unknown cause. It makes the works of God a mere mockery
and deception; I would almost as soon believe with the old and ignorant
cosmogonists, that fossil shells had never lived, but had been created in
stone so as to mock the shells now living on the sea-shore.
SUMMARY.
Our ignorance of the laws of variation is profound. Not in one case out of
a hundred can we pretend to assign any reason why this or that part has
varied. But whenever we have the means of instituting a comparison, the
same laws appear to have acted in producing the lesser differences between
varieties of the same species, and the greater differences between species
of the same genus. Changed conditions generally induce mere fluctuating
variability, but sometimes they cause direct and definite effects; and
these may become strongly marked in the course of time, though we have not
sufficient evidence on this head. Habit in producing constitutional
peculiarities, and use in strengthening, and disuse in weakening and
diminishing organs, appear in many cases to have been potent in their
effects. Homologous parts tend to vary in the same manner, and homologous
parts tend to cohere. Modifications in hard parts and in external parts
sometimes affect softer and internal parts. When one part is largely
developed, perhaps it tends to draw nourishment from the adjoining parts;
and every part of the structure which can be saved without detriment will
be saved. Changes of structure at an early age may affect parts
subsequently developed; and many cases of correlated variation, the nature
of which we are unable to understand, undoubtedly occur. Multiple parts
are variable in number and in structure, perhaps arising from such parts
not having been closely specialised for any particular function, so that
their modifications have not been closely checked by natural selection. It
follows probably from this same cause, that organic beings low in the scale
are more variable than those standing higher in the scale, and which have
their whole organisation more specialised. Rudimentary organs, from being
useless, are not regulated by natural selection, and hence are variable.
Specific characters--that is, the characters which have come to differ
since the several species of the same genus branched off from a common
parent--are more variable than generic characters, or those which have long
been inherited, and have not differed within this same period. In these
remarks we have referred to special parts or organs being still variable,
because they have recently varied and thus come to differ; but we have also
seen in the second chapter that the same principle applies to the whole
individual; for in a district where many species of a genus are found--that
is, where there has been much former variation and differentiation, or
where the manufactory of new specific forms has been actively at work--in
that district and among these species, we now find, on an average, most
varieties. Secondary sexual characters are highly variable, and such
characters differ much in the species of the same group. Variability in
the same parts of the organisation has generally been taken advantage of in
giving secondary sexual differences to the two sexes of the same species,
and specific differences to the several species of the same genus. Any
part or organ developed to an extraordinary size or in an extraordinary
manner, in comparison with the same part or organ in the allied species,
must have gone through an extraordinary amount of modification since the
genus arose; and thus we can understand why it should often still be
variable in a much higher degree than other parts; for variation is a
long-continued and slow process, and natural selection will in such cases
not as yet have had time to overcome the tendency to further variability
and to reversion to a less modified state. But when a species with an
extraordinarily developed organ has become the parent of many modified
descendants--which on our view must be a very slow process, requiring a
long lapse of time--in this case, natural selection has succeeded in giving
a fixed character to the organ, in however extraordinary a manner it may
have been developed. Species inheriting nearly the same constitution from
a common parent, and exposed to similar influences, naturally tend to
present analogous variations, or these same species may occasionally revert
to some of the characters of their ancient progenitors. Although new and
important modifications may not arise from reversion and analogous
variation, such modifications will add to the beautiful and harmonious
diversity of nature.
Whatever the cause may be of each slight difference between the offspring
and their parents--and a cause for each must exist--we have reason to
believe that it is the steady accumulation of beneficial differences which
has given rise to all the more important modifications of structure in
relation to the habits of each species.
CHAPTER VI.
DIFFICULTIES OF THE THEORY.
Difficulties of the theory of descent with modification -- Absence or
rarity of transitional varieties -- Transitions in habits of life --
Diversified habits in the same species -- Species with habits widely
different from those of their allies -- Organs of extreme perfection --
Modes of transition -- Cases of difficulty -- Natura non facit saltum --
Organs of small importance -- Organs not in all cases absolutely perfect --
The law of Unity of Type and of the Conditions of Existence embraced by the
theory of Natural Selection.
Long before the reader has arrived at this part of my work, a crowd of
difficulties will have occurred to him. Some of them are so serious that
to this day I can hardly reflect on them without being in some degree
staggered; but, to the best of my judgment, the greater number are only
apparent, and those that are real are not, I think, fatal to the theory.
These difficulties and objections may be classed under the following heads:
First, why, if species have descended from other species by fine
gradations, do we not everywhere see innumerable transitional forms? Why
is not all nature in confusion, instead of the species being, as we see
them, well defined?
Secondly, is it possible that an animal having, for instance, the structure
and habits of a bat, could have been formed by the modification of some
other animal with widely different habits and structure? Can we believe
that natural selection could produce, on the one hand, an organ of trifling
importance, such as the tail of a giraffe, which serves as a fly-flapper,
and, on the other hand, an organ so wonderful as the eye?
Thirdly, can instincts be acquired and modified through natural selection?
What shall we say to the instinct which leads the bee to make cells, and
which has practically anticipated the discoveries of profound
mathematicians?
Fourthly, how can we account for species, when crossed, being sterile and
producing sterile offspring, whereas, when varieties are crossed, their
fertility is unimpaired?
The two first heads will be here discussed; some miscellaneous objections
in the following chapter; Instinct and Hybridism in the two succeeding
chapters.
ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.
As natural selection acts solely by the preservation of profitable
modifications, each new form will tend in a fully-stocked country to take
the place of, and finally to exterminate, its own less improved parent-form
and other less-favoured forms with which it comes into competition. Thus
extinction and natural selection go hand in hand. Hence, if we look at
each species as descended from some unknown form, both the parent and all
the transitional varieties will generally have been exterminated by the
very process of the formation and perfection of the new form.
But, as by this theory innumerable transitional forms must have existed,
why do we not find them embedded in countless numbers in the crust of the
earth? It will be more convenient to discuss this question in the chapter
on the imperfection of the geological record; and I will here only state
that I believe the answer mainly lies in the record being incomparably less
perfect than is generally supposed. The crust of the earth is a vast
museum; but the natural collections have been imperfectly made, and only at
long intervals of time.
But it may be urged that when several closely allied species inhabit the
same territory, we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from north
to south over a continent, we generally meet at successive intervals with
closely allied or representative species, evidently filling nearly the same
place in the natural economy of the land. These representative species
often meet and interlock; and as the one becomes rarer and rarer, the other
becomes more and more frequent, till the one replaces the other. But if we
compare these species where they intermingle, they are generally as
absolutely distinct from each other in every detail of structure as are
specimens taken from the metropolis inhabited by each. By my theory these
allied species are descended from a common parent; and during the process
of modification, each has become adapted to the conditions of life of its
own region, and has supplanted and exterminated its original parent-form
and all the transitional varieties between its past and present states.
Hence we ought not to expect at the present time to meet with numerous
transitional varieties in each region, though they must have existed there,
and may be embedded there in a fossil condition. But in the intermediate
region, having intermediate conditions of life, why do we not now find
closely-linking intermediate varieties? This difficulty for a long time
quite confounded me. But I think it can be in large part explained.
In the first place we should be extremely cautious in inferring, because an
area is now continuous, that it has been continuous during a long period.
Geology would lead us to believe that most continents have been broken up
into islands even during the later tertiary periods; and in such islands
distinct species might have been separately formed without the possibility
of intermediate varieties existing in the intermediate zones. By changes
in the form of the land and of climate, marine areas now continuous must
often have existed within recent times in a far less continuous and uniform
condition than at present. But I will pass over this way of escaping from
the difficulty; for I believe that many perfectly defined species have been
formed on strictly continuous areas; though I do not doubt that the
formerly broken condition of areas now continuous, has played an important
part in the formation of new species, more especially with freely-crossing
and wandering animals.
In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes it is
quite remarkable how abruptly, as Alph. De Candolle has observed, a common
alpine species disappears. The same fact has been noticed by E. Forbes in
sounding the depths of the sea with the dredge. To those who look at
climate and the physical conditions of life as the all-important elements
of distribution, these facts ought to cause surprise, as climate and height
or depth graduate away insensibly. But when we bear in mind that almost
every species, even in its metropolis, would increase immensely in numbers,
were it not for other competing species; that nearly all either prey on or
serve as prey for others; in short, that each organic being is either
directly or indirectly related in the most important manner to other
organic beings--we see that the range of the inhabitants of any country by
no means exclusively depends on insensibly changing physical conditions,
but in large part on the presence of other species, on which it lives, or
by which it is destroyed, or with which it comes into competition; and as
these species are already defined objects, not blending one into another by
insensible gradations, the range of any one species, depending as it does
on the range of others, will tend to be sharply defined. Moreover, each
species on the confines of its range, where it exists in lessened numbers,
will, during fluctuations in the number of its enemies or of its prey, or
in the nature of the seasons, be extremely liable to utter extermination;
and thus its geographical range will come to be still more sharply defined.
As allied or representative species, when inhabiting a continuous area, are
generally distributed in such a manner that each has a wide range, with a
comparatively narrow neutral territory between them, in which they become
rather suddenly rarer and rarer; then, as varieties do not essentially
differ from species, the same rule will probably apply to both; and if we
take a varying species inhabiting a very large area, we shall have to adapt
two varieties to two large areas, and a third variety to a narrow
intermediate zone. The intermediate variety, consequently, will exist in
lesser numbers from inhabiting a narrow and lesser area; and practically,
as far as I can make out, this rule holds good with varieties in a state of
nature. I have met with striking instances of the rule in the case of
varieties intermediate between well-marked varieties in the genus Balanus.
And it would appear from information given me by Mr. Watson, Dr. Asa Gray,
and Mr. Wollaston, that generally, when varieties intermediate between two
other forms occur, they are much rarer numerically than the forms which
they connect. Now, if we may trust these facts and inferences, and
conclude that varieties linking two other varieties together generally have
existed in lesser numbers than the forms which they connect, then we can
understand why intermediate varieties should not endure for very long
periods: why, as a general rule, they should be exterminated and
disappear, sooner than the forms which they originally linked together.
For any form existing in lesser numbers would, as already remarked, run a
greater chance of being exterminated than one existing in large numbers;
and in this particular case the intermediate form would be eminently liable
to the inroads of closely allied forms existing on both sides of it. But
it is a far more important consideration, that during the process of
further modification, by which two varieties are supposed to be converted
and perfected into two distinct species, the two which exist in larger
numbers, from inhabiting larger areas, will have a great advantage over the
intermediate variety, which exists in smaller numbers in a narrow and
intermediate zone. For forms existing in larger numbers will have a better
chance, within any given period, of presenting further favourable
variations for natural selection to seize on, than will the rarer forms
which exist in lesser numbers. Hence, the more common forms, in the race
for life, will tend to beat and supplant the less common forms, for these
will be more slowly modified and improved. It is the same principle which,
as I believe, accounts for the common species in each country, as shown in
the second chapter, presenting on an average a greater number of
well-marked varieties than do the rarer species. I may illustrate what I
mean by supposing three varieties of sheep to be kept, one adapted to an
extensive mountainous region; a second to a comparatively narrow, hilly
tract; and a third to the wide plains at the base; and that the inhabitants
are all trying with equal steadiness and skill to improve their stocks by
selection; the chances in this case will be strongly in favour of the great
holders on the mountains or on the plains improving their breeds more
quickly than the small holders on the intermediate narrow, hilly tract; and
consequently the improved mountain or plain breed will soon take the place
of the less improved hill breed; and thus the two breeds, which originally
existed in greater numbers, will come into close contact with each other,
without the interposition of the supplanted, intermediate hill variety.
To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: first, because new varieties are very
slowly formed, for variation is a slow process, and natural selection can
do nothing until favourable individual differences or variations occur, and
until a place in the natural polity of the country can be better filled by
some modification of some one or more of its inhabitants. And such new
places will depend on slow changes of climate, or on the occasional
immigration of new inhabitants, and, probably, in a still more important
degree, on some of the old inhabitants becoming slowly modified, with the
new forms thus produced and the old ones acting and reacting on each other.
So that, in any one region and at any one time, we ought to see only a few
species presenting slight modifications of structure in some degree
permanent; and this assuredly we do see.
Secondly, areas now continuous must often have existed within the recent
period as isolated portions, in which many forms, more especially among the
classes which unite for each birth and wander much, may have separately
been rendered sufficiently distinct to rank as representative species. In
this case, intermediate varieties between the several representative
species and their common parent, must formerly have existed within each
isolated portion of the land, but these links during the process of natural
selection will have been supplanted and exterminated, so that they will no
longer be found in a living state.
Thirdly, when two or more varieties have been formed in different portions
of a strictly continuous area, intermediate varieties will, it is probable,
at first have been formed in the intermediate zones, but they will
generally have had a short duration. For these intermediate varieties
will, from reasons already assigned (namely from what we know of the actual
distribution of closely allied or representative species, and likewise of
acknowledged varieties), exist in the intermediate zones in lesser numbers
than the varieties which they tend to connect. From this cause alone the
intermediate varieties will be liable to accidental extermination; and
during the process of further modification through natural selection, they
will almost certainly be beaten and supplanted by the forms which they
connect; for these, from existing in greater numbers will, in the
aggregate, present more varieties, and thus be further improved through
natural selection and gain further advantages.
Lastly, looking not to any one time, but at all time, if my theory be true,
numberless intermediate varieties, linking closely together all the species
of the same group, must assuredly have existed; but the very process of
natural selection constantly tends, as has been so often remarked, to
exterminate the parent forms and the intermediate links. Consequently
evidence of their former existence could be found only among fossil remains,
which are preserved, as we shall attempt to show in a future chapter, in an
extremely imperfect and intermittent record.
ON THE ORIGIN AND TRANSITION OF ORGANIC BEINGS WITH PECULIAR HABITS AND
STRUCTURE.
It has been asked by the opponents of such views as I hold, how, for
instance, could a land carnivorous animal have been converted into one with
aquatic habits; for how could the animal in its transitional state have
subsisted? It would be easy to show that there now exist carnivorous
animals presenting close intermediate grades from strictly terrestrial to
aquatic habits; and as each exists by a struggle for life, it is clear that
each must be well adapted to its place in nature. Look at the Mustela
vison of North America, which has webbed feet, and which resembles an otter
in its fur, short legs, and form of tail; during summer this animal dives
for and preys on fish, but during the long winter it leaves the frozen
waters, and preys, like other polecats on mice and land animals. If a
different case had been taken, and it had been asked how an insectivorous
quadruped could possibly have been converted into a flying bat, the
question would have been far more difficult to answer. Yet I think such
difficulties have little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for, out of
the many striking cases which I have collected, I can give only one or two
instances of transitional habits and structures in allied species; and of
diversified habits, either constant or occasional, in the same species.
And it seems to me that nothing less than a long list of such cases is
sufficient to lessen the difficulty in any particular case like that of the
bat.
Look at the family of squirrels; here we have the finest gradation from
animals with their tails only slightly flattened, and from others, as Sir
J. Richardson has remarked, with the posterior part of their bodies rather
wide and with the skin on their flanks rather full, to the so-called flying
squirrels; and flying squirrels have their limbs and even the base of the
tail united by a broad expanse of skin, which serves as a parachute and
allows them to glide through the air to an astonishing distance from tree
to tree. We cannot doubt that each structure is of use to each kind of
squirrel in its own country, by enabling it to escape birds or beasts of
prey, or to collect food more quickly, or, as there is reason to believe,
to lessen the danger from occasional falls. But it does not follow from
this fact that the structure of each squirrel is the best that it is
possible to conceive under all possible conditions. Let the climate and
vegetation change, let other competing rodents or new beasts of prey
immigrate, or old ones become modified, and all analogy would lead us to
believe that some, at least, of the squirrels would decrease in numbers or
become exterminated, unless they also become modified and improved in
structure in a corresponding manner. Therefore, I can see no difficulty,
more especially under changing conditions of life, in the continued
preservation of individuals with fuller and fuller flank-membranes, each
modification being useful, each being propagated, until, by the accumulated
effects of this process of natural selection, a perfect so-called flying
squirrel was produced.
Now look at the Galeopithecus or so-called flying lemur, which was formerly
ranked among bats, but is now believed to belong to the Insectivora. An
extremely wide flank-membrane stretches from the corners of the jaw to the
tail, and includes the limbs with the elongated fingers. This flank-
membrane is furnished with an extensor muscle. Although no graduated links
of structure, fitted for gliding through the air, now connect the
Galeopithecus with the other Insectivora, yet there is no difficulty in
supposing that such links formerly existed, and that each was developed in
the same manner as with the less perfectly gliding squirrels; each grade of
structure having been useful to its possessor. Nor can I see any
insuperable difficulty in further believing it possible that the
membrane-connected fingers and fore-arm of the Galeopithecus might have
been greatly lengthened by natural selection; and this, as far as the
organs of flight are concerned, would have converted the animal into a bat.
In certain bats in which the wing-membrane extends from the top of the
shoulder to the tail and includes the hind-legs, we perhaps see traces of
an apparatus originally fitted for gliding through the air rather than for
flight.
If about a dozen genera of birds were to become extinct, who would have
ventured to surmise that birds might have existed which used their wings
solely as flappers, like the logger headed duck (Micropterus of Eyton); as
fins in the water and as front legs on the land, like the penguin; as
sails, like the ostrich; and functionally for no purpose, like the apteryx?
Yet the structure of each of these birds is good for it, under the
conditions of life to which it is exposed, for each has to live by a
struggle: but it is not necessarily the best possible under all possible
conditions. It must not be inferred from these remarks that any of the
grades of wing-structure here alluded to, which perhaps may all be the
result of disuse, indicate the steps by which birds actually acquired their
perfect power of flight; but they serve to show what diversified means of
transition are at least possible.
Seeing that a few members of such water-breathing classes as the Crustacea
and Mollusca are adapted to live on the land; and seeing that we have
flying birds and mammals, flying insects of the most diversified types, and
formerly had flying reptiles, it is conceivable that flying-fish, which now
glide far through the air, slightly rising and turning by the aid of their
fluttering fins, might have been modified into perfectly winged animals.
If this had been effected, who would have ever imagined that in an early
transitional state they had been inhabitants of the open ocean, and had
used their incipient organs of flight exclusively, so far as we know, to
escape being devoured by other fish?
When we see any structure highly perfected for any particular habit, as the
wings of a bird for flight, we should bear in mind that animals displaying
early transitional grades of the structure will seldom have survived to the
present day, for they will have been supplanted by their successors, which
were gradually rendered more perfect through natural selection.
Furthermore, we may conclude that transitional states between structures
fitted for very different habits of life will rarely have been developed at
an early period in great numbers and under many subordinate forms. Thus,
to return to our imaginary illustration of the flying-fish, it does not
seem probable that fishes capable of true flight would have been developed
under many subordinate forms, for taking prey of many kinds in many ways,
on the land and in the water, until their organs of flight had come to a
high stage of perfection, so as to have given them a decided advantage over
other animals in the battle for life. Hence the chance of discovering
species with transitional grades of structure in a fossil condition will
always be less, from their having existed in lesser numbers, than in the
case of species with fully developed structures.
I will now give two or three instances, both of diversified and of changed
habits, in the individuals of the same species. In either case it would be
easy for natural selection to adapt the structure of the animal to its
changed habits, or exclusively to one of its several habits. It is,
however, difficult to decide and immaterial for us, whether habits
generally change first and structure afterwards; or whether slight
modifications of structure lead to changed habits; both probably often
occurring almost simultaneously. Of cases of changed habits it will
suffice merely to allude to that of the many British insects which now feed
on exotic plants, or exclusively on artificial substances. Of diversified
habits innumerable instances could be given: I have often watched a tyrant
flycatcher (Saurophagus sulphuratus) in South America, hovering over one
spot and then proceeding to another, like a kestrel, and at other times
standing stationary on the margin of water, and then dashing into it like a
kingfisher at a fish. In our own country the larger titmouse (Parus major)
may be seen climbing branches, almost like a creeper; it sometimes, like a
shrike, kills small birds by blows on the head; and I have many times seen
and heard it hammering the seeds of the yew on a branch, and thus breaking
them like a nuthatch. In North America the black bear was seen by Hearne
swimming for hours with widely open mouth, thus catching, almost like a
whale, insects in the water.
As we sometimes see individuals following habits different from those
proper to their species and to the other species of the same genus, we
might expect that such individuals would occasionally give rise to new
species, having anomalous habits, and with their structure either slightly
or considerably modified from that of their type. And such instances occur
in nature. Can a more striking instance of adaptation be given than that
of a woodpecker for climbing trees and seizing insects in the chinks of the
bark? Yet in North America there are woodpeckers which feed largely on
fruit, and others with elongated wings which chase insects on the wing. On
the plains of La Plata, where hardly a tree grows, there is a woodpecker
(Colaptes campestris) which has two toes before and two behind, a long-
pointed tongue, pointed tail-feathers, sufficiently stiff to support the
bird in a vertical position on a post, but not so stiff as in the typical
wood-peckers, and a straight, strong beak. The beak, however, is not so
straight or so strong as in the typical woodpeckers but it is strong enough
to bore into wood. Hence this Colaptes, in all the essential parts of its
structure, is a woodpecker. Even in such trifling characters as the
colouring, the harsh tone of the voice, and undulatory flight, its close
blood-relationship to our common woodpecker is plainly declared; yet, as I
can assert, not only from my own observations, but from those of the
accurate Azara, in certain large districts it does not climb trees, and it
makes its nest in holes in banks! In certain other districts, however,
this same woodpecker, as Mr. Hudson states, frequents trees, and bores
holes in the trunk for its nest. I may mention as another illustration of
the varied habits of this genus, that a Mexican Colaptes has been described
by De Saussure as boring holes into hard wood in order to lay up a store of
acorns.
Petrels are the most aerial and oceanic of birds, but, in the quiet sounds
of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its
astonishing power of diving, in its manner of swimming and of flying when
made to take flight, would be mistaken by any one for an auk or a grebe;
nevertheless, it is essentially a petrel, but with many parts of its
organisation profoundly modified in relation to its new habits of life;
whereas the woodpecker of La Plata has had its structure only slightly
modified. In the case of the water-ouzel, the acutest observer, by
examining its dead body, would never have suspected its sub-aquatic habits;
yet this bird, which is allied to the thrush family, subsists by
diving,--using its wings under water and grasping stones with its feet.
All the members of the great order of Hymenopterous insects are
terrestrial, excepting the genus Proctotrupes, which Sir John Lubbock has
discovered to be aquatic in its habits; it often enters the water and dives
about by the use not of its legs but of its wings, and remains as long as
four hours beneath the surface; yet it exhibits no modification in
structure in accordance with its abnormal habits.
He who believes that each being has been created as we now see it, must
occasionally have felt surprise when he has met with an animal having
habits and structure not in agreement. What can be plainer than that the
webbed feet of ducks and geese are formed for swimming? Yet there are
upland geese with webbed feet which rarely go near the water; and no one
except Audubon, has seen the frigate-bird, which has all its four toes
webbed, alight on the surface of the ocean. On the other hand, grebes and
coots are eminently aquatic, although their toes are only bordered by
membrane. What seems plainer than that the long toes, not furnished with
membrane, of the Grallatores, are formed for walking over swamps and
floating plants. The water-hen and landrail are members of this order, yet
the first is nearly as aquatic as the coot, and the second is nearly as
terrestrial as the quail or partridge. In such cases, and many others
could be given, habits have changed without a corresponding change of
structure. The webbed feet of the upland goose may be said to have become
almost rudimentary in function, though not in structure. In the
frigate-bird, the deeply scooped membrane between the toes shows that
structure has begun to change.
He who believes in separate and innumerable acts of creation may say, that
in these cases it has pleased the Creator to cause a being of one type to
take the place of one belonging to another type; but this seems to me only
restating the fact in dignified language. He who believes in the struggle
for existence and in the principle of natural selection, will acknowledge
that every organic being is constantly endeavouring to increase in numbers;
and that if any one being varies ever so little, either in habits or
structure, and thus gains an advantage over some other inhabitant of the
same country, it will seize on the place of that inhabitant, however
different that may be from its own place. Hence it will cause him no
surprise that there should be geese and frigate-birds with webbed feet,
living on the dry land and rarely alighting on the water, that there should
be long-toed corncrakes, living in meadows instead of in swamps; that there
should be woodpeckers where hardly a tree grows; that there should be
diving thrushes and diving Hymenoptera, and petrels with the habits of
auks.
ORGANS OF EXTREME PERFECTION AND COMPLICATION.
To suppose that the eye with all its inimitable contrivances for adjusting
the focus to different distances, for admitting different amounts of light,
and for the correction of spherical and chromatic aberration, could have
been formed by natural selection, seems, I freely confess, absurd in the
highest degree. When it was first said that the sun stood still and the
world turned round, the common sense of mankind declared the doctrine
false; but the old saying of Vox populi, vox Dei, as every philosopher
knows, cannot be trusted in science. Reason tells me, that if numerous
gradations from a simple and imperfect eye to one complex and perfect can
be shown to exist, each grade being useful to its possessor, as is
certainly the case; if further, the eye ever varies and the variations be
inherited, as is likewise certainly the case; and if such variations should
be useful to any animal under changing conditions of life, then the
difficulty of believing that a perfect and complex eye could be formed by
natural selection, though insuperable by our imagination, should not be
considered as subversive of the theory. How a nerve comes to be sensitive
to light, hardly concerns us more than how life itself originated; but I
may remark that, as some of the lowest organisms in which nerves cannot be
detected, are capable of perceiving light, it does not seem impossible that
certain sensitive elements in their sarcode should become aggregated and
developed into nerves, endowed with this special sensibility.
In searching for the gradations through which an organ in any species has
been perfected, we ought to look exclusively to its lineal progenitors; but
this is scarcely ever possible, and we are forced to look to other species
and genera of the same group, that is to the collateral descendants from
the same parent-form, in order to see what gradations are possible, and for
the chance of some gradations having been transmitted in an unaltered or
little altered condition. But the state of the same organ in distinct
classes may incidentally throw light on the steps by which it has been
perfected.
The simplest organ which can be called an eye consists of an optic nerve,
surrounded by pigment-cells and covered by translucent skin, but without
any lens or other refractive body. We may, however, according to M.
Jourdain, descend even a step lower and find aggregates of pigment-cells,
apparently serving as organs of vision, without any nerves, and resting
merely on sarcodic tissue. Eyes of the above simple nature are not capable
of distinct vision, and serve only to distinguish light from darkness. In
certain star-fishes, small depressions in the layer of pigment which
surrounds the nerve are filled, as described by the author just quoted,
with transparent gelatinous matter, projecting with a convex surface, like
the cornea in the higher animals. He suggests that this serves not to form
an image, but only to concentrate the luminous rays and render their
perception more easy. In this concentration of the rays we gain the first
and by far the most important step towards the formation of a true,
picture-forming eye; for we have only to place the naked extremity of the
optic nerve, which in some of the lower animals lies deeply buried in the
body, and in some near the surface, at the right distance from the
concentrating apparatus, and an image will be formed on it.
In the great class of the Articulata, we may start from an optic nerve
simply coated with pigment, the latter sometimes forming a sort of pupil,
but destitute of lens or other optical contrivance. With insects it is now
known that the numerous facets on the cornea of their great compound eyes
form true lenses, and that the cones include curiously modified nervous
filaments. But these organs in the Articulata are so much diversified that
Muller formerly made three main classes with seven subdivisions, besides a
fourth main class of aggregated simple eyes.
When we reflect on these facts, here given much too briefly, with respect
to the wide, diversified, and graduated range of structure in the eyes of
the lower animals; and when we bear in mind how small the number of all
living forms must be in comparison with those which have become extinct,
the difficulty ceases to be very great in believing that natural selection
may have converted the simple apparatus of an optic nerve, coated with
pigment and invested by transparent membrane, into an optical instrument as
perfect as is possessed by any member of the Articulata class.
He who will go thus far, ought not to hesitate to go one step further, if
he finds on finishing this volume that large bodies of facts, otherwise
inexplicable, can be explained by the theory of modification through
natural selection; he ought to admit that a structure even as perfect as an
eagle's eye might thus be formed, although in this case he does not know
the transitional states. It has been objected that in order to modify the
eye and still preserve it as a perfect instrument, many changes would have
to be effected simultaneously, which, it is assumed, could not be done
through natural selection; but as I have attempted to show in my work on
the variation of domestic animals, it is not necessary to suppose that the
modifications were all simultaneous, if they were extremely slight and
gradual. Different kinds of modification would, also, serve for the same
general purpose: as Mr. Wallace has remarked, "If a lens has too short or
too long a focus, it may be amended either by an alteration of curvature,
or an alteration of density; if the curvature be irregular, and the rays do
not converge to a point, then any increased regularity of curvature will be
an improvement. So the contraction of the iris and the muscular movements
of the eye are neither of them essential to vision, but only improvements
which might have been added and perfected at any stage of the construction
of the instrument." Within the highest division of the animal kingdom,
namely, the Vertebrata, we can start from an eye so simple, that it
consists, as in the lancelet, of a little sack of transparent skin,
furnished with a nerve and lined with pigment, but destitute of any other
apparatus. In fishes and reptiles, as Owen has remarked, "The range of
gradation of dioptric structures is very great." It is a significant fact
that even in man, according to the high authority of Virchow, the beautiful
crystalline lens is formed in the embryo by an accumulation of epidermic
cells, lying in a sack-like fold of the skin; and the vitreous body is
formed from embryonic subcutaneous tissue. To arrive, however, at a just
conclusion regarding the formation of the eye, with all its marvellous yet
not absolutely perfect characters, it is indispensable that the reason
should conquer the imagination; but I have felt the difficulty far to
keenly to be surprised at others hesitating to extend the principle of
natural selection to so startling a length.
It is scarcely possible to avoid comparing the eye with a telescope. We
know that this instrument has been perfected by the long-continued efforts
of the highest human intellects; and we naturally infer that the eye has
been formed by a somewhat analogous process. But may not this inference be
presumptuous? Have we any right to assume that the Creator works by
intellectual powers like those of man? If we must compare the eye to an
optical instrument, we ought in imagination to take a thick layer of
transparent tissue, with spaces filled with fluid, and with a nerve
sensitive to light beneath, and then suppose every part of this layer to be
continually changing slowly in density, so as to separate into layers of
different densities and thicknesses, placed at different distances from
each other, and with the surfaces of each layer slowly changing in form.
Further we must suppose that there is a power, represented by natural
selection or the survival of the fittest, always intently watching each
slight alteration in the transparent layers; and carefully preserving each
which, under varied circumstances, in any way or degree, tends to produce a
distincter image. We must suppose each new state of the instrument to be
multiplied by the million; each to be preserved until a better is produced,
and then the old ones to be all destroyed. In living bodies, variation
will cause the slight alteration, generation will multiply them almost
infinitely, and natural selection will pick out with unerring skill each
improvement. Let this process go on for millions of years; and during each
year on millions of individuals of many kinds; and may we not believe that
a living optical instrument might thus be formed as superior to one of
glass, as the works of the Creator are to those of man?
MODES Of TRANSITION.
If it could be demonstrated that any complex organ existed, which could not
possibly have been formed by numerous, successive, slight modifications, my
theory would absolutely break down. But I can find out no such case. No
doubt many organs exist of which we do not know the transitional grades,
more especially if we look to much-isolated species, around which,
according to the theory, there has been much extinction. Or again, if we
take an organ common to all the members of a class, for in this latter case
the organ must have been originally formed at a remote period, since which
all the many members of the class have been developed; and in order to
discover the early transitional grades through which the organ has passed,
we should have to look to very ancient ancestral forms, long since become
extinct.
We should be extremely cautious in concluding that an organ could not have
been formed by transitional gradations of some kind. Numerous cases could
be given among the lower animals of the same organ performing at the same
time wholly distinct functions; thus in the larva of the dragon-fly and in
the fish Cobites the alimentary canal respires, digests, and excretes. In
the Hydra, the animal may be turned inside out, and the exterior surface
will then digest and the stomach respire. In such cases natural selection
might specialise, if any advantage were thus gained, the whole or part of
an organ, which had previously performed two functions, for one function
alone, and thus by insensible steps greatly change its nature. Many plants
are known which regularly produce at the same time differently constructed
flowers; and if such plants were to produce one kind alone, a great change
would be effected with comparative suddenness in the character of the
species. It is, however, probable that the two sorts of flowers borne by
the same plant were originally differentiated by finely graduated steps,
which may still be followed in some few cases.
Again, two distinct organs, or the same organ under two very different
forms, may simultaneously perform in the same individual the same function,
and this is an extremely important means of transition: to give one
instance--there are fish with gills or branchiae that breathe the air
dissolved in the water, at the same time that they breathe free air in
their swim-bladders, this latter organ being divided by highly vascular
partitions and having a ductus pneumaticus for the supply of air. To give
another instance from the vegetable kingdom: plants climb by three
distinct means, by spirally twining, by clasping a support with their
sensitive tendrils, and by the emission of aerial rootlets; these three
means are usually found in distinct groups, but some few species exhibit
two of the means, or even all three, combined in the same individual. In
all such cases one of the two organs might readily be modified and
perfected so as to perform all the work, being aided during the progress of
modification by the other organ; and then this other organ might be
modified for some other and quite distinct purpose, or be wholly
obliterated.
The illustration of the swim-bladder in fishes is a good one, because it
shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into one
for a widely different purpose, namely respiration. The swim-bladder has,
also, been worked in as an accessory to the auditory organs of certain
fishes. All physiologists admit that the swim-bladder is homologous, or
"ideally similar" in position and structure with the lungs of the higher
vertebrate animals: hence there is no reason to doubt that the swim-
bladder has actually been converted into lungs, or an organ used
exclusively for respiration.
According to this view it may be inferred that all vertebrate animals with
true lungs are descended by ordinary generation from an ancient and unknown
prototype which was furnished with a floating apparatus or swim-bladder.
We can thus, as I infer from Professor Owen's interesting description of
these parts, understand the strange fact that every particle of food and
drink which we swallow has to pass over the orifice of the trachea, with
some risk of falling into the lungs, notwithstanding the beautiful
contrivance by which the glottis is closed. In the higher Vertebrata the
branchiae have wholly disappeared--but in the embryo the slits on the sides
of the neck and the loop-like course of the arteries still mark their
former position. But it is conceivable that the now utterly lost branchiae
might have been gradually worked in by natural selection for some distinct
purpose: for instance, Landois has shown that the wings of insects are
developed from the trachea; it is therefore highly probable that in this
great class organs which once served for respiration have been actually
converted into organs for flight.
In considering transitions of organs, it is so important to bear in mind
the probability of conversion from one function to another, that I will
give another instance. Pedunculated cirripedes have two minute folds of
skin, called by me the ovigerous frena, which serve, through the means of a
sticky secretion, to retain the eggs until they are hatched within the
sack. These cirripedes have no branchiae, the whole surface of the body
and of the sack, together with the small frena, serving for respiration.
The Balanidae or sessile cirripedes, on the other hand, have no ovigerous
frena, the eggs lying loose at the bottom of the sack, within the
well-enclosed shell; but they have, in the same relative position with the
frena, large, much-folded membranes, which freely communicate with the
circulatory lacunae of the sack and body, and which have been considered by
all naturalists to act as branchiae. Now I think no one will dispute that
the ovigerous frena in the one family are strictly homologous with the
branchiae of the other family; indeed, they graduate into each other.
Therefore it need not be doubted that the two little folds of skin, which
originally served as ovigerous frena, but which, likewise, very slightly
aided in the act of respiration, have been gradually converted by natural
selection into branchiae, simply through an increase in their size and the
obliteration of their adhesive glands. If all pedunculated cirripedes had
become extinct, and they have suffered far more extinction than have
sessile cirripedes, who would ever have imagined that the branchiae in this
latter family had originally existed as organs for preventing the ova from
being washed out of the sack?
There is another possible mode of transition, namely, through the
acceleration or retardation of the period of reproduction. This has lately
been insisted on by Professor Cope and others in the United States. It is
now known that some animals are capable of reproduction at a very early
age, before they have acquired their perfect characters; and if this power
became thoroughly well developed in a species, it seems probable that the
adult stage of development would sooner or later be lost; and in this case,
especially if the larva differed much from the mature form, the character
of the species would be greatly changed and degraded. Again, not a few
animals, after arriving at maturity, go on changing in character during
nearly their whole lives. With mammals, for instance, the form of the
skull is often much altered with age, of which Dr. Murie has given some
striking instances with seals. Every one knows how the horns of stags
become more and more branched, and the plumes of some birds become more
finely developed, as they grow older. Professor Cope states that the teeth
of certain lizards change much in shape with advancing years. With
crustaceans not only many trivial, but some important parts assume a new
character, as recorded by Fritz Muller, after maturity. In all such cases-
-and many could be given--if the age for reproduction were retarded, the
character of the species, at least in its adult state, would be modified;
nor is it improbable that the previous and earlier stages of development
would in some cases be hurried through and finally lost. Whether species
have often or ever been modified through this comparatively sudden mode of
transition, I can form no opinion; but if this has occurred, it is probable
that the differences between the young and the mature, and between the
mature and the old, were primordially acquired by graduated steps.
SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL SELECTION.
Although we must be extremely cautious in concluding that any organ could
not have been produced by successive, small, transitional gradations, yet
undoubtedly serious cases of difficulty occur.
One of the most serious is that of neuter insects, which are often
differently constructed from either the males or fertile females; but this
case will be treated of in the next chapter. The electric organs of fishes
offer another case of special difficulty; for it is impossible to conceive
by what steps these wondrous organs have been produced. But this is not
surprising, for we do not even know of what use they are. In the gymnotus
and torpedo they no doubt serve as powerful means of defence, and perhaps
for securing prey; yet in the ray, as observed by Matteucci, an analogous
organ in the tail manifests but little electricity, even when the animal is
greatly irritated; so little that it can hardly be of any use for the above
purposes. Moreover, in the ray, besides the organ just referred to, there
is, as Dr. R. McDonnell has shown, another organ near the head, not known
to be electrical, but which appears to be the real homologue of the
electric battery in the torpedo. It is generally admitted that there
exists between these organs and ordinary muscle a close analogy, in
intimate structure, in the distribution of the nerves, and in the manner in
which they are acted on by various reagents. It should, also, be
especially observed that muscular contraction is accompanied by an
electrical discharge; and, as Dr. Radcliffe insists, "in the electrical
apparatus of the torpedo during rest, there would seem to be a charge in
every respect like that which is met with in muscle and nerve during the
rest, and the discharge of the torpedo, instead of being peculiar, may be
only another form of the discharge which attends upon the action of muscle
and motor nerve." Beyond this we cannot at present go in the way of
explanation; but as we know so little about the uses of these organs, and
as we know nothing about the habits and structure of the progenitors of the
existing electric fishes, it would be extremely bold to maintain that no
serviceable transitions are possible by which these organs might have been
gradually developed.
These organs appear at first to offer another and far more serious
difficulty; for they occur in about a dozen kinds of fish, of which several
are widely remote in their affinities. When the same organ is found in
several members of the same class, especially if in members having very
different habits of life, we may generally attribute its presence to
inheritance from a common ancestor; and its absence in some of the members
to loss through disuse or natural selection. So that, if the electric
organs had been inherited from some one ancient progenitor, we might have
expected that all electric fishes would have been specially related to each
other; but this is far from the case. Nor does geology at all lead to the
belief that most fishes formerly possessed electric organs, which their
modified descendants have now lost. But when we look at the subject more
closely, we find in the several fishes provided with electric organs, that
these are situated in different parts of the body, that they differ in
construction, as in the arrangement of the plates, and, according to
Pacini, in the process or means by which the electricity is excited--and
lastly, in being supplied with nerves proceeding from different sources,
and this is perhaps the most important of all the differences. Hence in
the several fishes furnished with electric organs, these cannot be
considered as homologous, but only as analogous in function. Consequently
there is no reason to suppose that they have been inherited from a common
progenitor; for had this been the case they would have closely resembled
each other in all respects. Thus the difficulty of an organ, apparently
the same, arising in several remotely allied species, disappears, leaving
only the lesser yet still great difficulty: namely, by what graduated
steps these organs have been developed in each separate group of fishes.
The luminous organs which occur in a few insects, belonging to widely
different families, and which are situated in different parts of the body,
offer, under our present state of ignorance, a difficulty almost exactly
parallel with that of the electric organs. Other similar cases could be
given; for instance in plants, the very curious contrivance of a mass of
pollen-grains, borne on a foot-stalk with an adhesive gland, is apparently
the same in Orchis and Asclepias, genera almost as remote as is possible
among flowering plants; but here again the parts are not homologous. In
all cases of beings, far removed from each other in the scale of
organisation, which are furnished with similar and peculiar organs, it will
be found that although the general appearance and function of the organs
may be the same, yet fundamental differences between them can always be
detected. For instance, the eyes of Cephalopods or cuttle-fish and of
vertebrate animals appear wonderfully alike; and in such widely sundered
groups no part of this resemblance can be due to inheritance from a common
progenitor. Mr. Mivart has advanced this case as one of special
difficulty, but I am unable to see the force of his argument. An organ for
vision must be formed of transparent tissue, and must include some sort of
lens for throwing an image at the back of a darkened chamber. Beyond this
superficial resemblance, there is hardly any real similarity between the
eyes of cuttle-fish and vertebrates, as may be seen by consulting Hensen's
admirable memoir on these organs in the Cephalopoda. It is impossible for
me here to enter on details, but I may specify a few of the points of
difference. The crystalline lens in the higher cuttle-fish consists of two
parts, placed one behind the other like two lenses, both having a very
different structure and disposition to what occurs in the vertebrata. The
retina is wholly different, with an actual inversion of the elemental
parts, and with a large nervous ganglion included within the membranes of
the eye. The relations of the muscles are as different as it is possible
to conceive, and so in other points. Hence it is not a little difficult to
decide how far even the same terms ought to be employed in describing the
eyes of the Cephalopoda and Vertebrata. It is, of course, open to any one
to deny that the eye in either case could have been developed through the
natural selection of successive slight variations; but if this be admitted
in the one case it is clearly possible in the other; and fundamental
differences of structure in the visual organs of two groups might have been
anticipated, in accordance with this view of their manner of formation. As
two men have sometimes independently hit on the same invention, so in the
several foregoing cases it appears that natural selection, working for the
good of each being, and taking advantage of all favourable variations, has
produced similar organs, as far as function is concerned, in distinct
organic beings, which owe none of their structure in common to inheritance
from a common progenitor.
Fritz Muller, in order to test the conclusions arrived at in this volume,
has followed out with much care a nearly similar line of argument. Several
families of crustaceans include a few species, possessing an air-breathing
apparatus and fitted to live out of the water. In two of these families,
which were more especially examined by Muller, and which are nearly related
to each other, the species agree most closely in all important characters:
namely in their sense organs, circulating systems, in the position of the
tufts of hair within their complex stomachs, and lastly in the whole
structure of the water-breathing branchiae, even to the microscopical hooks
by which they are cleansed. Hence it might have been expected that in the
few species belonging to both families which live on the land, the equally
important air-breathing apparatus would have been the same; for why should
this one apparatus, given for the same purpose, have been made to differ,
while all the other important organs were closely similar, or rather,
identical.
Fritz Muller argues that this close similarity in so many points of
structure must, in accordance with the views advanced by me, be accounted
for by inheritance from a common progenitor. But as the vast majority of
the species in the above two families, as well as most other crustaceans,
are aquatic in their habits, it is improbable in the highest degree that
their common progenitor should have been adapted for breathing air. Muller
was thus led carefully to examine the apparatus in the air-breathing
species; and he found it to differ in each in several important points, as
in the position of the orifices, in the manner in which they are opened and
closed, and in some accessory details. Now such differences are
intelligible, and might even have been expected, on the supposition that
species belonging to distinct families had slowly become adapted to live
more and more out of water, and to breathe the air. For these species,
from belonging to distinct families, would have differed to a certain
extent, and in accordance with the principle that the nature of each
variation depends on two factors, viz., the nature of the organism and that
of the surrounding conditions, their variability assuredly would not have
been exactly the same. Consequently natural selection would have had
different materials or variations to work on, in order to arrive at the
same functional result; and the structures thus acquired would almost
necessarily have differed. On the hypothesis of separate acts of creation
the whole case remains unintelligible. This line of argument seems to have
had great weight in leading Fritz Muller to accept the views maintained by
me in this volume.
Another distinguished zoologist, the late Professor Claparede, has argued
in the same manner, and has arrived at the same result. He shows that
there are parasitic mites (Acaridae), belonging to distinct sub-families
and families, which are furnished with hair-claspers. These organs must
have been independently developed, as they could not have been inherited
from a common progenitor; and in the several groups they are formed by the
modification of the fore legs, of the hind legs, of the maxillae or lips,
and of appendages on the under side of the hind part of the body.
In the foregoing cases, we see the same end gained and the same function
performed, in beings not at all or only remotely allied, by organs in
appearance, though not in development, closely similar. On the other hand,
it is a common rule throughout nature that the same end should be gained,
even sometimes in the case of closely related beings, by the most
diversified means. How differently constructed is the feathered wing of a
bird and the membrane-covered wing of a bat; and still more so the four
wings of a butterfly, the two wings of a fly, and the two wings with the
elytra of a beetle. Bivalve shells are made to open and shut, but on what
a number of patterns is the hinge constructed, from the long row of neatly
interlocking teeth in a Nucula to the simple ligament of a Mussel! Seeds
are disseminated by their minuteness, by their capsule being converted into
a light balloon-like envelope, by being embedded in pulp or flesh, formed
of the most diverse parts, and rendered nutritious, as well as
conspicuously coloured, so as to attract and be devoured by birds, by
having hooks and grapnels of many kinds and serrated awns, so as to adhere
to the fur of quadrupeds, and by being furnished with wings and plumes, as
different in shape as they are elegant in structure, so as to be wafted by
every breeze. I will give one other instance: for this subject of the
same end being gained by the most diversified means well deserves
attention. Some authors maintain that organic beings have been formed in
many ways for the sake of mere variety, almost like toys in a shop, but
such a view of nature is incredible. With plants having separated sexes,
and with those in which, though hermaphrodites, the pollen does not
spontaneously fall on the stigma, some aid is necessary for their
fertilisation. With several kinds this is effected by the pollen-grains,
which are light and incoherent, being blown by the wind through mere chance
on to the stigma; and this is the simplest plan which can well be
conceived. An almost equally simple, though very different plan occurs in
many plants in which a symmetrical flower secretes a few drops of nectar,
and is consequently visited by insects; and these carry the pollen from the
anthers to the stigma.
>From this simple stage we may pass through an inexhaustible number of
contrivances, all for the same purpose and effected in essentially the same
manner, but entailing changes in every part of the flower. The nectar may
be stored in variously shaped receptacles, with the stamens and pistils
modified in many ways, sometimes forming trap-like contrivances, and
sometimes capable of neatly adapted movements through irritability or
elasticity. From such structures we may advance till we come to such a
case of extraordinary adaptation as that lately described by Dr. Cruger in
the Coryanthes. This orchid has part of its labellum or lower lip hollowed
out into a great bucket, into which drops of almost pure water continually
fall from two secreting horns which stand above it; and when the bucket is
half-full, the water overflows by a spout on one side. The basal part of
the labellum stands over the bucket, and is itself hollowed out into a sort
of chamber with two lateral entrances; within this chamber there are
curious fleshy ridges. The most ingenious man, if he had not witnessed
what takes place, could never have imagined what purpose all these parts
serve. But Dr. Cruger saw crowds of large humble-bees visiting the
gigantic flowers of this orchid, not in order to suck nectar, but to gnaw
off the ridges within the chamber above the bucket; in doing this they
frequently pushed each other into the bucket, and their wings being thus
wetted they could not fly away, but were compelled to crawl out through the
passage formed by the spout or overflow. Dr. Cruger saw a "continual
procession" of bees thus crawling out of their involuntary bath. The
passage is narrow, and is roofed over by the column, so that a bee, in
forcing its way out, first rubs its back against the viscid stigma and then
against the viscid glands of the pollen-masses. The pollen-masses are thus
glued to the back of the bee which first happens to crawl out through the
passage of a lately expanded flower, and are thus carried away. Dr. Cruger
sent me a flower in spirits of wine, with a bee which he had killed before
it had quite crawled out, with a pollen-mass still fastened to its back.
When the bee, thus provided, flies to another flower, or to the same flower
a second time, and is pushed by its comrades into the bucket and then
crawls out by the passage, the pollen-mass necessarily comes first into
contact with the viscid stigma, and adheres to it, and the flower is
fertilised. Now at last we see the full use of every part of the flower,
of the water-secreting horns of the bucket half-full of water, which
prevents the bees from flying away, and forces them to crawl out through
the spout, and rub against the properly placed viscid pollen-masses and the
viscid stigma.
The construction of the flower in another closely allied orchid, namely,
the Catasetum, is widely different, though serving the same end; and is
equally curious. Bees visit these flowers, like those of the Coryanthes,
in order to gnaw the labellum; in doing this they inevitably touch a long,
tapering, sensitive projection, or, as I have called it, the antenna. This
antenna, when touched, transmits a sensation or vibration to a certain
membrane which is instantly ruptured; this sets free a spring by which the
pollen-mass is shot forth, like an arrow, in the right direction, and
adheres by its viscid extremity to the back of the bee. The pollen-mass of
the male plant (for the sexes are separate in this orchid) is thus carried
to the flower of the female plant, where it is brought into contact with
the stigma, which is viscid enough to break certain elastic threads, and
retain the pollen, thus effecting fertilisation.
How, it may be asked, in the foregoing and in innumerable other instances,
can we understand the graduated scale of complexity and the multifarious
means for gaining the same end. The answer no doubt is, as already
remarked, that when two forms vary, which already differ from each other in
some slight degree, the variability will not be of the same exact nature,
and consequently the results obtained through natural selection for the
same general purpose will not be the same. We should also bear in mind
that every highly developed organism has passed through many changes; and
that each modified structure tends to be inherited, so that each
modification will not readily be quite lost, but may be again and again
further altered. Hence, the structure of each part of each species, for
whatever purpose it may serve, is the sum of many inherited changes,
through which the species has passed during its successive adaptations to
changed habits and conditions of life.
Finally, then, although in many cases it is most difficult even to
conjecture by what transitions organs could have arrived at their present
state; yet, considering how small the proportion of living and known forms
is to the extinct and unknown, I have been astonished how rarely an organ
can be named, towards which no transitional grade is known to lead. It is
certainly true, that new organs appearing as if created for some special
purpose rarely or never appear in any being; as indeed is shown by that
old, but somewhat exaggerated, canon in natural history of "Natura non
facit saltum." We meet with this admission in the writings of almost every
experienced naturalist; or, as Milne Edwards has well expressed it, "Nature
is prodigal in variety, but niggard in innovation." Why, on the theory of
Creation, should there be so much variety and so little real novelty? Why
should all the parts and organs of many independent beings, each supposed
to have been separately created for its own proper place in nature, be so
commonly linked together by graduated steps? Why should not Nature take a
sudden leap from structure to structure? On the theory of natural
selection, we can clearly understand why she should not; for natural
selection acts only by taking advantage of slight successive variations;
she can never take a great and sudden leap, but must advance by the short
and sure, though slow steps.
ORGANS OF LITTLE APPARENT IMPORTANCE, AS AFFECTED BY NATURAL SELECTION.
As natural selection acts by life and death, by the survival of the
fittest, and by the destruction of the less well-fitted individuals, I have
sometimes felt great difficulty in understanding the origin or formation of
parts of little importance; almost as great, though of a very different
kind, as in the case of the most perfect and complex organs.
In the first place, we are much too ignorant in regard to the whole economy
of any one organic being to say what slight modifications would be of
importance or not. In a former chapter I have given instances of very
trifling characters, such as the down on fruit and the colour of its flesh,
the colour of the skin and hair of quadrupeds, which, from being correlated
with constitutional differences, or from determining the attacks of
insects, might assuredly be acted on by natural selection. The tail of the
giraffe looks like an artificially constructed fly-flapper; and it seems at
first incredible that this could have been adapted for its present purpose
by successive slight modifications, each better and better fitted, for so
trifling an object as to drive away flies; yet we should pause before being
too positive even in this case, for we know that the distribution and
existence of cattle and other animals in South America absolutely depend on
their power of resisting the attacks of insects: so that individuals which
could by any means defend themselves from these small enemies, would be
able to range into new pastures and thus gain a great advantage. It is not
that the larger quadrupeds are actually destroyed (except in some rare
cases) by flies, but they are incessantly harassed and their strength
reduced, so that they are more subject to disease, or not so well enabled
in a coming dearth to search for food, or to escape from beasts of prey.
Organs now of trifling importance have probably in some cases been of high
importance to an early progenitor, and, after having been slowly perfected
at a former period, have been transmitted to existing species in nearly the
same state, although now of very slight use; but any actually injurious
deviations in their structure would of course have been checked by natural
selection. Seeing how important an organ of locomotion the tail is in most
aquatic animals, its general presence and use for many purposes in so many
land animals, which in their lungs or modified swim-bladders betray their
aquatic origin, may perhaps be thus accounted for. A well-developed tail
having been formed in an aquatic animal, it might subsequently come to be
worked in for all sorts of purposes, as a fly-flapper, an organ of
prehension, or as an aid in turning, as in the case of the dog, though the
aid in this latter respect must be slight, for the hare, with hardly any
tail, can double still more quickly.
In the second place, we may easily err in attributing importance to
characters, and in believing that they have been developed through natural
selection. We must by no means overlook the effects of the definite action
of changed conditions of life, of so-called spontaneous variations, which
seem to depend in a quite subordinate degree on the nature of the
conditions, of the tendency to reversion to long-lost characters, of the
complex laws of growth, such as of correlation, comprehension, of the
pressure of one part on another, etc., and finally of sexual selection, by
which characters of use to one sex are often gained and then transmitted
more or less perfectly to the other sex, though of no use to the sex. But
structures thus indirectly gained, although at first of no advantage to a
species, may subsequently have been taken advantage of by its modified
descendants, under new conditions of life and newly acquired habits.
If green woodpeckers alone had existed, and we did not know that there were
many black and pied kinds, I dare say that we should have thought that the
green colour was a beautiful adaptation to conceal this tree-frequenting
bird from its enemies; and consequently that it was a character of
importance, and had been acquired through natural selection; as it is, the
colour is probably in chief part due to sexual selection. A trailing palm
in the Malay Archipelago climbs the loftiest trees by the aid of
exquisitely constructed hooks clustered around the ends of the branches,
and this contrivance, no doubt, is of the highest service to the plant; but
as we see nearly similar hooks on many trees which are not climbers, and
which, as there is reason to believe from the distribution of the thorn-
bearing species in Africa and South America, serve as a defence against
browsing quadrupeds, so the spikes on the palm may at first have been
developed for this object, and subsequently have been improved and taken
advantage of by the plant, as it underwent further modification and became
a climber. The naked skin on the head of a vulture is generally considered
as a direct adaptation for wallowing in putridity; and so it may be, or it
may possibly be due to the direct action of putrid matter; but we should be
very cautious in drawing any such inference, when we see that the skin on
the head of the clean-feeding male turkey is likewise naked. The sutures
in the skulls of young mammals have been advanced as a beautiful adaptation
for aiding parturition, and no doubt they facilitate, or may be
indispensable for this act; but as sutures occur in the skulls of young
birds and reptiles, which have only to escape from a broken egg, we may
infer that this structure has arisen from the laws of growth, and has been
taken advantage of in the parturition of the higher animals.
We are profoundly ignorant of the cause of each slight variation or
individual difference; and we are immediately made conscious of this by
reflecting on the differences between the breeds of our domesticated
animals in different countries, more especially in the less civilized
countries, where there has been but little methodical selection. Animals
kept by savages in different countries often have to struggle for their own
subsistence, and are exposed to a certain extent to natural selection, and
individuals with slightly different constitutions would succeed best under
different climates. With cattle susceptibility to the attacks of flies is
correlated with colour, as is the liability to be poisoned by certain
plants; so that even colour would be thus subjected to the action of
natural selection. Some observers are convinced that a damp climate
affects the growth of the hair, and that with the hair the horns are
correlated. Mountain breeds always differ from lowland breeds; and a
mountainous country would probably affect the hind limbs from exercising
them more, and possibly even the form of the pelvis; and then by the law of
homologous variation, the front limbs and the head would probably be
affected. The shape, also, of the pelvis might affect by pressure the
shape of certain parts of the young in the womb. The laborious breathing
necessary in high regions tends, as we have good reason to believe, to
increase the size of the chest; and again correlation would come into play.
The effects of lessened exercise, together with abundant food, on the whole
organisation is probably still more important, and this, as H. von
Nathusius has lately shown in his excellent Treatise, is apparently one
chief cause of the great modification which the breeds of swine have
undergone. But we are far too ignorant to speculate on the relative
importance of the several known and unknown causes of variation; and I have
made these remarks only to show that, if we are unable to account for the
characteristic differences of our several domestic breeds, which
nevertheless are generally admitted to have arisen through ordinary
generation from one or a few parent-stocks, we ought not to lay too much
stress on our ignorance of the precise cause of the slight analogous
differences between true species.
UTILITARIAN DOCTRINE, HOW FAR TRUE: BEAUTY, HOW ACQUIRED.
The foregoing remarks lead me to say a few words on the protest lately made
by some naturalists against the utilitarian doctrine that every detail of
structure has been produced for the good of its possessor. They believe
that many structures have been created for the sake of beauty, to delight
man or the Creator (but this latter point is beyond the scope of scientific
discussion), or for the sake of mere variety, a view already discussed.
Such doctrines, if true, would be absolutely fatal to my theory. I fully
admit that many structures are now of no direct use to their possessors,
and may never have been of any use to their progenitors; but this does not
prove that they were formed solely for beauty or variety. No doubt the
definite action of changed conditions, and the various causes of
modifications, lately specified, have all produced an effect, probably a
great effect, independently of any advantage thus gained. But a still more
important consideration is that the chief part of the organisation of every
living creature is due to inheritance; and consequently, though each being
assuredly is well fitted for its place in nature, many structures have now
no very close and direct relation to present habits of life. Thus, we can
hardly believe that the webbed feet of the upland goose, or of the frigate-
bird, are of special use to these birds; we cannot believe that the similar
bones in the arm of the monkey, in the fore leg of the horse, in the wing
of the bat, and in the flipper of the seal, are of special use to these
animals. We may safely attribute these structures to inheritance. But
webbed feet no doubt were as useful to the progenitor of the upland goose
and of the frigate-bird, as they now are to the most aquatic of living
birds. So we may believe that the progenitor of the seal did not possess a
flipper, but a foot with five toes fitted for walking or grasping; and we
may further venture to believe that the several bones in the limbs of the
monkey, horse and bat, were originally developed, on the principle of
utility, probably through the reduction of more numerous bones in the fin
of some ancient fish-like progenitor of the whole class. It is scarcely
possible to decide how much allowance ought to be made for such causes of
change, as the definite action of external conditions, so-called
spontaneous variations, and the complex laws of growth; but with these
important exceptions, we may conclude that the structure of every living
creature either now is, or was formerly, of some direct or indirect use to
its possessor.
With respect to the belief that organic beings have been created beautiful
for the delight of man--a belief which it has been pronounced is subversive
of my whole theory--I may first remark that the sense of beauty obviously
depends on the nature of the mind, irrespective of any real quality in the
admired object; and that the idea of what is beautiful, is not innate or
unalterable. We see this, for instance, in the men of different races
admiring an entirely different standard of beauty in their women. If
beautiful objects had been created solely for man's gratification, it ought
to be shown that before man appeared there was less beauty on the face of
the earth than since he came on the stage. Were the beautiful volute and
cone shells of the Eocene epoch, and the gracefully sculptured ammonites of
the Secondary period, created that man might ages afterwards admire them in
his cabinet? Few objects are more beautiful than the minute siliceous
cases of the diatomaceae: were these created that they might be examined
and admired under the higher powers of the microscope? The beauty in this
latter case, and in many others, is apparently wholly due to symmetry of
growth. Flowers rank among the most beautiful productions of nature; but
they have been rendered conspicuous in contrast with the green leaves, and
in consequence at the same time beautiful, so that they may be easily
observed by insects. I have come to this conclusion from finding it an
invariable rule that when a flower is fertilised by the wind it never has a
gaily-coloured corolla. Several plants habitually produce two kinds of
flowers; one kind open and coloured so as to attract insects; the other
closed, not coloured, destitute of nectar, and never visited by insects.
Hence, we may conclude that, if insects had not been developed on the face
of the earth, our plants would not have been decked with beautiful flowers,
but would have produced only such poor flowers as we see on our fir, oak,
nut and ash trees, on grasses, spinach, docks and nettles, which are all
fertilised through the agency of the wind. A similar line of argument
holds good with fruits; that a ripe strawberry or cherry is as pleasing to
the eye as to the palate--that the gaily-coloured fruit of the spindle-wood
tree and the scarlet berries of the holly are beautiful objects--will be
admitted by everyone. But this beauty serves merely as a guide to birds
and beasts, in order that the fruit may be devoured and the matured seeds
disseminated. I infer that this is the case from having as yet found no
exception to the rule that seeds are always thus disseminated when embedded
within a fruit of any kind (that is within a fleshy or pulpy envelope), if
it be coloured of any brilliant tint, or rendered conspicuous by being
white or black.
On the other hand, I willingly admit that a great number of male animals,
as all our most gorgeous birds, some fishes, reptiles, and mammals, and a
host of magnificently coloured butterflies, have been rendered beautiful
for beauty's sake. But this has been effected through sexual selection,
that is, by the more beautiful males having been continually preferred by
the females, and not for the delight of man. So it is with the music of
birds. We may infer from all this that a nearly similar taste for
beautiful colours and for musical sounds runs through a large part of the
animal kingdom. When the female is as beautifully coloured as the male,
which is not rarely the case with birds and butterflies, the cause
apparently lies in the colours acquired through sexual selection having
been transmitted to both sexes, instead of to the males alone. How the
sense of beauty in its simplest form--that is, the reception of a peculiar
kind of pleasure from certain colours, forms and sounds--was first
developed in the mind of man and of the lower animals, is a very obscure
subject. The same sort of difficulty is presented if we enquire how it is
that certain flavours and odours give pleasure, and others displeasure.
Habit in all these cases appears to have come to a certain extent into
play; but there must be some fundamental cause in the constitution of the
nervous system in each species.
Natural selection cannot possibly produce any modification in a species
exclusively for the good of another species; though throughout nature one
species incessantly takes advantage of, and profits by the structures of
others. But natural selection can and does often produce structures for
the direct injury of other animals, as we see in the fang of the adder, and
in the ovipositor of the ichneumon, by which its eggs are deposited in the
living bodies of other insects. If it could be proved that any part of the
structure of any one species had been formed for the exclusive good of
another species, it would annihilate my theory, for such could not have
been produced through natural selection. Although many statements may be
found in works on natural history to this effect, I cannot find even one
which seems to me of any weight. It is admitted that the rattlesnake has a
poison-fang for its own defence and for the destruction of its prey; but
some authors suppose that at the same time it is furnished with a rattle
for its own injury, namely, to warn its prey. I would almost as soon
believe that the cat curls the end of its tail when preparing to spring, in
order to warn the doomed mouse. It is a much more probable view that the
rattlesnake uses its rattle, the cobra expands its frill and the puff-adder
swells while hissing so loudly and harshly, in order to alarm the many
birds and beasts which are known to attack even the most venomous species.
Snakes act on the same principle which makes the hen ruffle her feathers
and expand her wings when a dog approaches her chickens. But I have not
space here to enlarge on the many ways by which animals endeavour to
frighten away their enemies.
Natural selection will never produce in a being any structure more
injurious than beneficial to that being, for natural selection acts solely
by and for the good of each. No organ will be formed, as Paley has
remarked, for the purpose of causing pain or for doing an injury to its
possessor. If a fair balance be struck between the good and evil caused by
each part, each will be found on the whole advantageous. After the lapse
of time, under changing conditions of life, if any part comes to be
injurious, it will be modified; or if it be not so, the being will become
extinct, as myriads have become extinct.
Natural selection tends only to make each organic being as perfect as, or
slightly more perfect than the other inhabitants of the same country with
which it comes into competition. And we see that this is the standard of
perfection attained under nature. The endemic productions of New Zealand,
for instance, are perfect, one compared with another; but they are now
rapidly yielding before the advancing legions of plants and animals
introduced from Europe. Natural selection will not produce absolute
perfection, nor do we always meet, as far as we can judge, with this high
standard under nature. The correction for the aberration of light is said
by Muller not to be perfect even in that most perfect organ, the human eye.
Helmholtz, whose judgment no one will dispute, after describing in the
strongest terms the wonderful powers of the human eye, adds these
remarkable words: "That which we have discovered in the way of inexactness
and imperfection in the optical machine and in the image on the retina, is
as nothing in comparison with the incongruities which we have just come
across in the domain of the sensations. One might say that nature has
taken delight in accumulating contradictions in order to remove all
foundation from the theory of a pre-existing harmony between the external
and internal worlds." If our reason leads us to admire with enthusiasm a
multitude of inimitable contrivances in nature, this same reason tells us,
though we may easily err on both sides, that some other contrivances are
less perfect. Can we consider the sting of the bee as perfect, which, when
used against many kinds of enemies, cannot be withdrawn, owing to the
backward serratures, and thus inevitably causes the death of the insect by
tearing out its viscera?
If we look at the sting of the bee, as having existed in a remote
progenitor, as a boring and serrated instrument, like that in so many
members of the same great order, and that it has since been modified but
not perfected for its present purpose, with the poison originally adapted
for some other object, such as to produce galls, since intensified, we can
perhaps understand how it is that the use of the sting should so often
cause the insect's own death: for if on the whole the power of stinging be
useful to the social community, it will fulfil all the requirements of
natural selection, though it may cause the death of some few members. If
we admire the truly wonderful power of scent by which the males of many
insects find their females, can we admire the production for this single
purpose of thousands of drones, which are utterly useless to the community
for any other purpose, and which are ultimately slaughtered by their
industrious and sterile sisters? It may be difficult, but we ought to
admire the savage instinctive hatred of the queen-bee, which urges her to
destroy the young queens, her daughters, as soon as they are born, or to
perish herself in the combat; for undoubtedly this is for the good of the
community; and maternal love or maternal hatred, though the latter
fortunately is most rare, is all the same to the inexorable principles of
natural selection. If we admire the several ingenious contrivances by
which orchids and many other plants are fertilised through insect agency,
can we consider as equally perfect the elaboration of dense clouds of
pollen by our fir-trees, so that a few granules may be wafted by chance on
to the ovules?
SUMMARY: THE LAW OF UNITY OF TYPE AND OF THE CONDITIONS OF EXISTENCE
EMBRACED BY THE THEORY OF NATURAL SELECTION.
We have in this chapter discussed some of the difficulties and objections
which may be urged against the theory. Many of them are serious; but I
think that in the discussion light has been thrown on several facts, which
on the belief of independent acts of creation are utterly obscure. We have
seen that species at any one period are not indefinitely variable, and are
not linked together by a multitude of intermediate gradations, partly
because the process of natural selection is always very slow, and at any
one time acts only on a few forms; and partly because the very process of
natural selection implies the continual supplanting and extinction of
preceding and intermediate gradations. Closely allied species, now living
on a continuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly graduate
away from one part to another. When two varieties are formed in two
districts of a continuous area, an intermediate variety will often be
formed, fitted for an intermediate zone; but from reasons assigned, the
intermediate variety will usually exist in lesser numbers than the two
forms which it connects; consequently the two latter, during the course of
further modification, from existing in greater numbers, will have a great
advantage over the less numerous intermediate variety, and will thus
generally succeed in supplanting and exterminating it.
We have seen in this chapter how cautious we should be in concluding that
the most different habits of life could not graduate into each other; that
a bat, for instance, could not have been formed by natural selection from
an animal which at first only glided through the air.
We have seen that a species under new conditions of life may change its
habits, or it may have diversified habits, with some very unlike those of
its nearest congeners. Hence we can understand, bearing in mind that each
organic being is trying to live wherever it can live, how it has arisen
that there are upland geese with webbed feet, ground woodpeckers, diving
thrushes, and petrels with the habits of auks.
Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is enough to stagger any one; yet in the case
of any organ, if we know of a long series of gradations in complexity, each
good for its possessor, then under changing conditions of life, there is no
logical impossibility in the acquirement of any conceivable degree of
perfection through natural selection. In the cases in which we know of no
intermediate or transitional states, we should be extremely cautious in
concluding that none can have existed, for the metamorphoses of many organs
show what wonderful changes in function are at least possible. For
instance, a swim-bladder has apparently been converted into an
air-breathing lung. The same organ having performed simultaneously very
different functions, and then having been in part or in whole specialised
for one function; and two distinct organs having performed at the same time
the same function, the one having been perfected whilst aided by the other,
must often have largely facilitated transitions.
We have seen that in two beings widely remote from each other in the
natural scale, organs serving for the same purpose and in external
appearance closely similar may have been separately and independently
formed; but when such organs are closely examined, essential differences in
their structure can almost always be detected; and this naturally follows
from the principle of natural selection. On the other hand, the common
rule throughout nature is infinite diversity of structure for gaining the
same end; and this again naturally follows from the same great principle.
In many cases we are far too ignorant to be enabled to assert that a part
or organ is so unimportant for the welfare of a species, that modifications
in its structure could not have been slowly accumulated by means of natural
selection. In many other cases, modifications are probably the direct
result of the laws of variation or of growth, independently of any good
having been thus gained. But even such structures have often, as we may
feel assured, been subsequently taken advantage of, and still further
modified, for the good of species under new conditions of life. We may,
also, believe that a part formerly of high importance has frequently been
retained (as the tail of an aquatic animal by its terrestrial descendants),
though it has become of such small importance that it could not, in its
present state, have been acquired by means of natural selection.
Natural selection can produce nothing in one species for the exclusive good
or injury of another; though it may well produce parts, organs, and
excretions highly useful or even indispensable, or highly injurious to
another species, but in all cases at the same time useful to the possessor.
In each well-stocked country natural selection acts through the competition
of the inhabitants and consequently leads to success in the battle for
life, only in accordance with the standard of that particular country.
Hence the inhabitants of one country, generally the smaller one, often
yield to the inhabitants of another and generally the larger country. For
in the larger country there will have existed more individuals, and more
diversified forms, and the competition will have been severer, and thus the
standard of perfection will have been rendered higher. Natural selection
will not necessarily lead to absolute perfection; nor, as far as we can
judge by our limited faculties, can absolute perfection be everywhere
predicated.
On the theory of natural selection we can clearly understand the full
meaning of that old canon in natural history, "Natura non facit saltum."
This canon, if we look to the present inhabitants alone of the world, is
not strictly correct; but if we include all those of past times, whether
known or unknown, it must on this theory be strictly true.
It is generally acknowledged that all organic beings have been formed on
two great laws--Unity of Type, and the Conditions of Existence. By unity
of type is meant that fundamental agreement in structure which we see in
organic beings of the same class, and which is quite independent of their
habits of life. On my theory, unity of type is explained by unity of
descent. The expression of conditions of existence, so often insisted on
by the illustrious Cuvier, is fully embraced by the principle of natural
selection. For natural selection acts by either now adapting the varying
parts of each being to its organic and inorganic conditions of life; or by
having adapted them during past periods of time: the adaptations being
aided in many cases by the increased use or disuse of parts, being affected
by the direct action of external conditions of life, and subjected in all
cases to the several laws of growth and variation. Hence, in fact, the law
of the Conditions of Existence is the higher law; as it includes, through
the inheritance of former variations and adaptations, that of Unity of
Type.
CHAPTER VII.
MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.
Longevity -- Modifications not necessarily simultaneous -- Modifications
apparently of no direct service -- Progressive development -- Characters of
small functional importance, the most constant -- Supposed incompetence of
natural selection to account for the incipient stages of useful structures
-- Causes which interfere with the acquisition through natural selection of
useful structures -- Gradations of structure with changed functions --
Widely different organs in members of the same class, developed from one
and the same source -- Reasons for disbelieving in great and abrupt
modifications.
I will devote this chapter to the consideration of various miscellaneous
objections which have been advanced against my views, as some of the
previous discussions may thus be made clearer; but it would be useless to
discuss all of them, as many have been made by writers who have not taken
the trouble to understand the subject. Thus a distinguished German
naturalist has asserted that the weakest part of my theory is, that I
consider all organic beings as imperfect: what I have really said is, that
all are not as perfect as they might have been in relation to their
conditions; and this is shown to be the case by so many native forms in
many quarters of the world having yielded their places to intruding
foreigners. Nor can organic beings, even if they were at any one time
perfectly adapted to their conditions of life, have remained so, when their
conditions changed, unless they themselves likewise changed; and no one
will dispute that the physical conditions of each country, as well as the
number and kinds of its inhabitants, have undergone many mutations.
A critic has lately insisted, with some parade of mathematical accuracy,
that longevity is a great advantage to all species, so that he who believes
in natural selection "must arrange his genealogical tree" in such a manner
that all the descendants have longer lives than their progenitors! Cannot
our critics conceive that a biennial plant or one of the lower animals
might range into a cold climate and perish there every winter; and yet,
owing to advantages gained through natural selection, survive from year to
year by means of its seeds or ova? Mr. E. Ray Lankester has recently
discussed this subject, and he concludes, as far as its extreme complexity
allows him to form a judgment, that longevity is generally related to the
standard of each species in the scale of organisation, as well as to the
amount of expenditure in reproduction and in general activity. And these
conditions have, it is probable, been largely determined through natural
selection.
It has been argued that, as none of the animals and plants of Egypt, of
which we know anything, have changed during the last three or four thousand
years, so probably have none in any part of the world. But, as Mr. G.H.
Lewes has remarked, this line of argument proves too much, for the ancient
domestic races figured on the Egyptian monuments, or embalmed, are closely
similar or even identical with those now living; yet all naturalists admit
that such races have been produced through the modification of their
original types. The many animals which have remained unchanged since the
commencement of the glacial period, would have been an incomparably
stronger case, for these have been exposed to great changes of climate and
have migrated over great distances; whereas, in Egypt, during the last
several thousand years, the conditions of life, as far as we know, have
remained absolutely uniform. The fact of little or no modification having
been effected since the glacial period, would have been of some avail
against those who believe in an innate and necessary law of development,
but is powerless against the doctrine of natural selection or the survival
of the fittest, which implies that when variations or individual
differences of a beneficial nature happen to arise, these will be
preserved; but this will be effected only under certain favourable
circumstances.
The celebrated palaeontologist, Bronn, at the close of his German
translation of this work, asks how, on the principle of natural selection,
can a variety live side by side with the parent species? If both have
become fitted for slightly different habits of life or conditions, they
might live together; and if we lay on one side polymorphic species, in
which the variability seems to be of a peculiar nature, and all mere
temporary variations, such as size, albinism, etc., the more permanent
varieties are generally found, as far as I can discover, inhabiting
distinct stations, such as high land or low land, dry or moist districts.
Moreover, in the case of animals which wander much about and cross freely,
their varieties seem to be generally confined to distinct regions.
Bronn also insists that distinct species never differ from each other in
single characters, but in many parts; and he asks, how it always comes that
many parts of the organisation should have been modified at the same time
through variation and natural selection? But there is no necessity for
supposing that all the parts of any being have been simultaneously
modified. The most striking modifications, excellently adapted for some
purpose, might, as was formerly remarked, be acquired by successive
variations, if slight, first in one part and then in another; and as they
would be transmitted all together, they would appear to us as if they had
been simultaneously developed. The best answer, however, to the above
objection is afforded by those domestic races which have been modified,
chiefly through man's power of selection, for some special purpose. Look
at the race and dray-horse, or at the greyhound and mastiff. Their whole
frames, and even their mental characteristics, have been modified; but if
we could trace each step in the history of their transformation--and the
latter steps can be traced--we should not see great and simultaneous
changes, but first one part and then another slightly modified and
improved. Even when selection has been applied by man to some one
character alone--of which our cultivated plants offer the best instances--
it will invariably be found that although this one part, whether it be the
flower, fruit, or leaves, has been greatly changed, almost all the other
parts have been slightly modified. This may be attributed partly to the
principle of correlated growth, and partly to so-called spontaneous
variation.
A much more serious objection has been urged by Bronn, and recently by
Broca, namely, that many characters appear to be of no service whatever to
their possessors, and therefore cannot have been influenced through natural
selection. Bronn adduces the length of the ears and tails in the different
species of hares and mice--the complex folds of enamel in the teeth of many
animals, and a multitude of analogous cases. With respect to plants, this
subject has been discussed by Nageli in an admirable essay. He admits that
natural selection has effected much, but he insists that the families of
plants differ chiefly from each other in morphological characters, which
appear to be quite unimportant for the welfare of the species. He
consequently believes in an innate tendency towards progressive and more
perfect development. He specifies the arrangement of the cells in the
tissues, and of the leaves on the axis, as cases in which natural selection
could not have acted. To these may be added the numerical divisions in the
parts of the flower, the position of the ovules, the shape of the seed,
when not of any use for dissemination, etc.
There is much force in the above objection. Nevertheless, we ought, in the
first place, to be extremely cautious in pretending to decide what
structures now are, or have formerly been, of use to each species. In the
second place, it should always be borne in mind that when one part is
modified, so will be other parts, through certain dimly seen causes, such
as an increased or diminished flow of nutriment to a part, mutual pressure,
an early developed part affecting one subsequently developed, and so forth
--as well as through other causes which lead to the many mysterious cases
of correlation, which we do not in the least understand. These agencies
may be all grouped together, for the sake of brevity, under the expression
of the laws of growth. In the third place, we have to allow for the direct
and definite action of changed conditions of life, and for so-called
spontaneous variations, in which the nature of the conditions apparently
plays a quite subordinate part. Bud-variations, such as the appearance of
a moss-rose on a common rose, or of a nectarine on a peach-tree, offer good
instances of spontaneous variations; but even in these cases, if we bear in
mind the power of a minute drop of poison in producing complex galls, we
ought not to feel too sure that the above variations are not the effect of
some local change in the nature of the sap, due to some change in the
conditions. There must be some efficient cause for each slight individual
difference, as well as for more strongly marked variations which
occasionally arise; and if the unknown cause were to act persistently, it
is almost certain that all the individuals of the species would be
similarly modified.
In the earlier editions of this work I underrated, as it now seems
probable, the frequency and importance of modifications due to spontaneous
variability. But it is impossible to attribute to this cause the
innumerable structures which are so well adapted to the habits of life of
each species. I can no more believe in this than that the well-adapted
form of a race-horse or greyhound, which before the principle of selection
by man was well understood, excited so much surprise in the minds of the
older naturalists, can thus be explained.
It may be worth while to illustrate some of the foregoing remarks. With
respect to the assumed inutility of various parts and organs, it is hardly
necessary to observe that even in the higher and best-known animals many
structures exist, which are so highly developed that no one doubts that
they are of importance, yet their use has not been, or has only recently
been, ascertained. As Bronn gives the length of the ears and tail in the
several species of mice as instances, though trifling ones, of differences
in structure which can be of no special use, I may mention that, according
to Dr. Schobl, the external ears of the common mouse are supplied in an
extraordinary manner with nerves, so that they no doubt serve as tactile
organs; hence the length of the ears can hardly be quite unimportant. We
shall, also, presently see that the tail is a highly useful prehensile
organ to some of the species; and its use would be much influence by its
length.
With respect to plants, to which on account of Nageli's essay I shall
confine myself in the following remarks, it will be admitted that the
flowers of the orchids present a multitude of curious structures, which a
few years ago would have been considered as mere morphological differences
without any special function; but they are now known to be of the highest
importance for the fertilisation of the species through the aid of insects,
and have probably been gained through natural selection. No one until
lately would have imagined that in dimorphic and trimorphic plants the
different lengths of the stamens and pistils, and their arrangement, could
have been of any service, but now we know this to be the case.
In certain whole groups of plants the ovules stand erect, and in others
they are suspended; and within the same ovarium of some few plants, one
ovule holds the former and a second ovule the latter position. These
positions seem at first purely morphological, or of no physiological
signification; but Dr. Hooker informs me that within the same ovarium the
upper ovules alone in some cases, and in others the lower ones alone are
fertilised; and he suggests that this probably depends on the direction in
which the pollen-tubes enter the ovarium. If so, the position of the
ovules, even when one is erect and the other suspended within the same
ovarium, would follow the selection of any slight deviations in position
which favoured their fertilisation, and the production of seed.
Several plants belonging to distinct orders habitually produce flowers of
two kinds--the one open, of the ordinary structure, the other closed and
imperfect. These two kinds of flowers sometimes differ wonderfully in
structure, yet may be seen to graduate into each other on the same plant.
The ordinary and open flowers can be intercrossed; and the benefits which
certainly are derived from this process are thus secured. The closed and
imperfect flowers are, however, manifestly of high importance, as they
yield with the utmost safety a large stock of seed, with the expenditure of
wonderfully little pollen. The two kinds of flowers often differ much, as
just stated, in structure. The petals in the imperfect flowers almost
always consist of mere rudiments, and the pollen-grains are reduced in
diameter. In Ononis columnae five of the alternate stamens are
rudimentary; and in some species of Viola three stamens are in this state,
two retaining their proper function, but being of very small size. In six
out of thirty of the closed flowers in an Indian violet (name unknown, for
the plants have never produced with me perfect flowers), the sepals are
reduced from the normal number of five to three. In one section of the
Malpighiaceae the closed flowers, according to A. de Jussieu, are still
further modified, for the five stamens which stand opposite to the sepals
are all aborted, a sixth stamen standing opposite to a petal being alone
developed; and this stamen is not present in the ordinary flowers of this
species; the style is aborted; and the ovaria are reduced from three to
two. Now although natural selection may well have had the power to prevent
some of the flowers from expanding, and to reduce the amount of pollen,
when rendered by the closure of the flowers superfluous, yet hardly any of
the above special modifications can have been thus determined, but must
have followed from the laws of growth, including the functional inactivity
of parts, during the progress of the reduction of the pollen and the
closure of the flowers.
It is so necessary to appreciate the important effects of the laws of
growth, that I will give some additional cases of another kind, namely of
differences in the same part or organ, due to differences in relative
position on the same plant. In the Spanish chestnut, and in certain fir-
trees, the angles of divergence of the leaves differ, according to Schacht,
in the nearly horizontal and in the upright branches. In the common rue
and some other plants, one flower, usually the central or terminal one,
opens first, and has five sepals and petals, and five divisions to the
ovarium; while all the other flowers on the plant are tetramerous. In the
British Adoxa the uppermost flower generally has two calyx-lobes with the
other organs tetramerous, while the surrounding flowers generally have
three calyx-lobes with the other organs pentamerous. In many Compositae
and Umbelliferae (and in some other plants) the circumferential flowers
have their corollas much more developed than those of the centre; and this
seems often connected with the abortion of the reproductive organs. It is
a more curious fact, previously referred to, that the achenes or seeds of
the circumference and centre sometimes differ greatly in form, colour and
other characters. In Carthamus and some other Compositae the central
achenes alone are furnished with a pappus; and in Hyoseris the same head
yields achenes of three different forms. In certain Umbelliferae the
exterior seeds, according to Tausch, are orthospermous, and the central one
coelospermous, and this is a character which was considered by De Candolle
to be in other species of the highest systematic importance. Professor
Braun mentions a Fumariaceous genus, in which the flowers in the lower part
of the spike bear oval, ribbed, one-seeded nutlets; and in the upper part
of the spike, lanceolate, two-valved and two-seeded siliques. In these
several cases, with the exception of that of the well-developed ray-
florets, which are of service in making the flowers conspicuous to insects,
natural selection cannot, as far as we can judge, have come into play, or
only in a quite subordinate manner. All these modifications follow from
the relative position and inter-action of the parts; and it can hardly be
doubted that if all the flowers and leaves on the same plant had been
subjected to the same external and internal condition, as are the flowers
and leaves in certain positions, all would have been modified in the same
manner.
In numerous other cases we find modifications of structure, which are
considered by botanists to be generally of a highly important nature,
affecting only some of the flowers on the same plant, or occurring on
distinct plants, which grow close together under the same conditions. As
these variations seem of no special use to the plants, they cannot have
been influenced by natural selection. Of their cause we are quite
ignorant; we cannot even attribute them, as in the last class of cases, to
any proximate agency, such as relative position. I will give only a few
instances. It is so common to observe on the same plant, flowers
indifferently tetramerous, pentamerous, etc., that I need not give
examples; but as numerical variations are comparatively rare when the parts
are few, I may mention that, according to De Candolle, the flowers of
Papaver bracteatum offer either two sepals with four petals (which is the
common type with poppies), or three sepals with six petals. The manner in
which the petals are folded in the bud is in most groups a very constant
morphological character; but Professor Asa Gray states that with some
species of Mimulus, the aestivation is almost as frequently that of the
Rhinanthideae as of the Antirrhinideae, to which latter tribe the genus
belongs. Aug. St. Hilaire gives the following cases: the genus
Zanthoxylon belongs to a division of the Rutaceae with a single ovary, but
in some species flowers may be found on the same plant, and even in the
same panicle, with either one or two ovaries. In Helianthemum the capsule
has been described as unilocular or tri-locular; and in H. mutabile, "Une
lame PLUS OU MOINS LARGE, s'etend entre le pericarpe et le placenta." In
the flowers of Saponaria officinalis Dr. Masters has observed instances of
both marginal and free central placentation. Lastly, St. Hilaire found
towards the southern extreme of the range of Gomphia oleaeformis two forms
which he did not at first doubt were distinct species, but he subsequently
saw them growing on the same bush; and he then adds, "Voila donc dans un
meme individu des loges et un style qui se rattachent tantot a un axe
verticale et tantot a un gynobase."
We thus see that with plants many morphological changes may be attributed
to the laws of growth and the inter-action of parts, independently of
natural selection. But with respect to Nageli's doctrine of an innate
tendency towards perfection or progressive development, can it be said in
the case of these strongly pronounced variations, that the plants have been
caught in the act of progressing towards a higher state of development? On
the contrary, I should infer from the mere fact of the parts in question
differing or varying greatly on the same plant, that such modifications
were of extremely small importance to the plants themselves, of whatever
importance they may generally be to us for our classifications. The
acquisition of a useless part can hardly be said to raise an organism in
the natural scale; and in the case of the imperfect, closed flowers, above
described, if any new principle has to be invoked, it must be one of
retrogression rather than of progression; and so it must be with many
parasitic and degraded animals. We are ignorant of the exciting cause of
the above specified modifications; but if the unknown cause were to act
almost uniformly for a length of time, we may infer that the result would
be almost uniform; and in this case all the individuals of the species
would be modified in the same manner.
>From the fact of the above characters being unimportant for the welfare of
the species, any slight variations which occurred in them would not have
been accumulated and augmented through natural selection. A structure
which has been developed through long-continued selection, when it ceases
to be of service to a species, generally becomes variable, as we see with
rudimentary organs; for it will no longer be regulated by this same power
of selection. But when, from the nature of the organism and of the
conditions, modifications have been induced which are unimportant for the
welfare of the species, they may be, and apparently often have been,
transmitted in nearly the same state to numerous, otherwise modified,
descendants. It cannot have been of much importance to the greater number
of mammals, birds, or reptiles, whether they were clothed with hair,
feathers or scales; yet hair has been transmitted to almost all mammals,
feathers to all birds, and scales to all true reptiles. A structure,
whatever it may be, which is common to many allied forms, is ranked by us
as of high systematic importance, and consequently is often assumed to be
of high vital importance to the species. Thus, as I am inclined to
believe, morphological differences, which we consider as important--such as
the arrangement of the leaves, the divisions of the flower or of the
ovarium, the position of the ovules, etc., first appeared in many cases as
fluctuating variations, which sooner or later became constant through the
nature of the organism and of the surrounding conditions, as well as
through the intercrossing of distinct individuals, but not through natural
selection; for as these morphological characters do not affect the welfare
of the species, any slight deviations in them could not have been governed
or accumulated through this latter agency. It is a strange result which we
thus arrive at, namely, that characters of slight vital importance to the
species, are the most important to the systematist; but, as we shall
hereafter see when we treat of the genetic principle of classification,
this is by no means so paradoxical as it may at first appear.
Although we have no good evidence of the existence in organic beings of an
innate tendency towards progressive development, yet this necessarily
follows, as I have attempted to show in the fourth chapter, through the
continued action of natural selection. For the best definition which has
ever been given of a high standard of organisation, is the degree to which
the parts have been specialised or differentiated; and natural selection
tends towards this end, inasmuch as the parts are thus enabled to perform
their functions more efficiently.
A distinguished zoologist, Mr. St. George Mivart, has recently collected
all the objections which have ever been advanced by myself and others
against the theory of natural selection, as propounded by Mr. Wallace and
myself, and has illustrated them with admirable art and force. When thus
marshalled, they make a formidable array; and as it forms no part of Mr.
Mivart's plan to give the various facts and considerations opposed to his
conclusions, no slight effort of reason and memory is left to the reader,
who may wish to weigh the evidence on both sides. When discussing special
cases, Mr. Mivart passes over the effects of the increased use and disuse
of parts, which I have always maintained to be highly important, and have
treated in my "Variation under Domestication" at greater length than, as I
believe, any other writer. He likewise often assumes that I attribute
nothing to variation, independently of natural selection, whereas in the
work just referred to I have collected a greater number of well-established
cases than can be found in any other work known to me. My judgment may not
be trustworthy, but after reading with care Mr. Mivart's book, and
comparing each section with what I have said on the same head, I never
before felt so strongly convinced of the general truth of the conclusions
here arrived at, subject, of course, in so intricate a subject, to much
partial error.
All Mr. Mivart's objections will be, or have been, considered in the
present volume. The one new point which appears to have struck many
readers is, "That natural selection is incompetent to account for the
incipient stages of useful structures." This subject is intimately
connected with that of the gradation of the characters, often accompanied
by a change of function, for instance, the conversion of a swim-bladder
into lungs, points which were discussed in the last chapter under two
headings. Nevertheless, I will here consider in some detail several of the
cases advanced by Mr. Mivart, selecting those which are the most
illustrative, as want of space prevents me from considering all.
The giraffe, by its lofty stature, much elongated neck, fore legs, head and
tongue, has its whole frame beautifully adapted for browsing on the higher
branches of trees. It can thus obtain food beyond the reach of the other
Ungulata or hoofed animals inhabiting the same country; and this must be a
great advantage to it during dearths. The Niata cattle in South America
show us how small a difference in structure may make, during such periods,
a great difference in preserving an animal's life. These cattle can browse
as well as others on grass, but from the projection of the lower jaw they
cannot, during the often recurrent droughts, browse on the twigs of trees,
reeds, etc., to which food the common cattle and horses are then driven; so
that at these times the Niatas perish, if not fed by their owners. Before
coming to Mr. Mivart's objections, it may be well to explain once again how
natural selection will act in all ordinary cases. Man has modified some of
his animals, without necessarily having attended to special points of
structure, by simply preserving and breeding from the fleetest individuals,
as with the race-horse and greyhound, or as with the game-cock, by breeding
from the victorious birds. So under nature with the nascent giraffe, the
individuals which were the highest browsers and were able during dearths to
reach even an inch or two above the others, will often have been preserved;
for they will have roamed over the whole country in search of food. That
the individuals of the same species often differ slightly in the relative
lengths of all their parts may be seen in many works of natural history, in
which careful measurements are given. These slight proportional
differences, due to the laws of growth and variation, are not of the
slightest use or importance to most species. But it will have been
otherwise with the nascent giraffe, considering its probable habits of
life; for those individuals which had some one part or several parts of
their bodies rather more elongated than usual, would generally have
survived. These will have intercrossed and left offspring, either
inheriting the same bodily peculiarities, or with a tendency to vary again
in the same manner; while the individuals less favoured in the same
respects will have been the most liable to perish.
We here see that there is no need to separate single pairs, as man does,
when he methodically improves a breed: natural selection will preserve and
thus separate all the superior individuals, allowing them freely to
intercross, and will destroy all the inferior individuals. By this process
long-continued, which exactly corresponds with what I have called
unconscious selection by man, combined, no doubt, in a most important
manner with the inherited effects of the increased use of parts, it seems
to me almost certain that an ordinary hoofed quadruped might be converted
into a giraffe.
To this conclusion Mr. Mivart brings forward two objections. One is that
the increased size of the body would obviously require an increased supply
of food, and he considers it as "very problematical whether the
disadvantages thence arising would not, in times of scarcity, more than
counterbalance the advantages." But as the giraffe does actually exist in
large numbers in Africa, and as some of the largest antelopes in the world,
taller than an ox, abound there, why should we doubt that, as far as size
is concerned, intermediate gradations could formerly have existed there,
subjected as now to severe dearths. Assuredly the being able to reach, at
each stage of increased size, to a supply of food, left untouched by the
other hoofed quadrupeds of the country, would have been of some advantage
to the nascent giraffe. Nor must we overlook the fact, that increased bulk
would act as a protection against almost all beasts of prey excepting the
lion; and against this animal, its tall neck--and the taller the better--
would, as Mr. Chauncey Wright has remarked, serve as a watch-tower. It is
from this cause, as Sir S. Baker remarks, that no animal is more difficult
to stalk than the giraffe. This animal also uses its long neck as a means
of offence or defence, by violently swinging its head armed with stump-like
horns. The preservation of each species can rarely be determined by any
one advantage, but by the union of all, great and small.
Mr. Mivart then asks (and this is his second objection), if natural
selection be so potent, and if high browsing be so great an advantage, why
has not any other hoofed quadruped acquired a long neck and lofty stature,
besides the giraffe, and, in a lesser degree, the camel, guanaco and
macrauchenia? Or, again, why has not any member of the group acquired a
long proboscis? With respect to South Africa, which was formerly inhabited
by numerous herds of the giraffe, the answer is not difficult, and can best
be given by an illustration. In every meadow in England, in which trees
grow, we see the lower branches trimmed or planed to an exact level by the
browsing of the horses or cattle; and what advantage would it be, for
instance, to sheep, if kept there, to acquire slightly longer necks? In
every district some one kind of animal will almost certainly be able to
browse higher than the others; and it is almost equally certain that this
one kind alone could have its neck elongated for this purpose, through
natural selection and the effects of increased use. In South Africa the
competition for browsing on the higher branches of the acacias and other
trees must be between giraffe and giraffe, and not with the other ungulate
animals.
Why, in other quarters of the world, various animals belonging to this same
order have not acquired either an elongated neck or a proboscis, cannot be
distinctly answered; but it is as unreasonable to expect a distinct answer
to such a question as why some event in the history of mankind did not
occur in one country while it did in another. We are ignorant with respect
to the conditions which determine the numbers and range of each species,
and we cannot even conjecture what changes of structure would be favourable
to its increase in some new country. We can, however, see in a general
manner that various causes might have interfered with the development of a
long neck or proboscis. To reach the foliage at a considerable height
(without climbing, for which hoofed animals are singularly ill-constructed)
implies greatly increased bulk of body; and we know that some areas support
singularly few large quadrupeds, for instance South America, though it is
so luxuriant, while South Africa abounds with them to an unparalleled
degree. Why this should be so we do not know; nor why the later tertiary
periods should have been much more favourable for their existence than the
present time. Whatever the causes may have been, we can see that certain
districts and times would have been much more favourable than others for
the development of so large a quadruped as the giraffe.
In order that an animal should acquire some structure specially and largely
developed, it is almost indispensable that several other parts should be
modified and coadapted. Although every part of the body varies slightly,
it does not follow that the necessary parts should always vary in the right
direction and to the right degree. With the different species of our
domesticated animals we know that the parts vary in a different manner and
degree, and that some species are much more variable than others. Even if
the fitting variations did arise, it does not follow that natural selection
would be able to act on them and produce a structure which apparently would
be beneficial to the species. For instance, if the number of individuals
existing in a country is determined chiefly through destruction by beasts
of prey--by external or internal parasites, etc.--as seems often to be the
case, then natural selection will be able to do little, or will be greatly
retarded, in modifying any particular structure for obtaining food.
Lastly, natural selection is a slow process, and the same favourable
conditions must long endure in order that any marked effect should thus be
produced. Except by assigning such general and vague reasons, we cannot
explain why, in many quarters of the world, hoofed quadrupeds have not
acquired much elongated necks or other means for browsing on the higher
branches of trees.
Objections of the same nature as the foregoing have been advanced by many
writers. In each case various causes, besides the general ones just
indicated, have probably interfered with the acquisition through natural
selection of structures, which it is thought would be beneficial to certain
species. One writer asks, why has not the ostrich acquired the power of
flight? But a moment's reflection will show what an enormous supply of
food would be necessary to give to this bird of the desert force to move
its huge body through the air. Oceanic islands are inhabited by bats and
seals, but by no terrestrial mammals; yet as some of these bats are
peculiar species, they must have long inhabited their present homes.
Therefore Sir C. Lyell asks, and assigns certain reasons in answer, why
have not seals and bats given birth on such islands to forms fitted to live
on the land? But seals would necessarily be first converted into
terrestrial carnivorous animals of considerable size, and bats into
terrestrial insectivorous animals; for the former there would be no prey;
for the bats ground-insects would serve as food, but these would already be
largely preyed on by the reptiles or birds, which first colonise and abound
on most oceanic islands. Gradations of structure, with each stage
beneficial to a changing species, will be favoured only under certain
peculiar conditions. A strictly terrestrial animal, by occasionally
hunting for food in shallow water, then in streams or lakes, might at last
be converted into an animal so thoroughly aquatic as to brave the open
ocean. But seals would not find on oceanic islands the conditions
favourable to their gradual reconversion into a terrestrial form. Bats, as
formerly shown, probably acquired their wings by at first gliding through
the air from tree to tree, like the so-called flying squirrels, for the
sake of escaping from their enemies, or for avoiding falls; but when the
power of true flight had once been acquired, it would never be reconverted
back, at least for the above purposes, into the less efficient power of
gliding through the air. Bats, might, indeed, like many birds, have had
their wings greatly reduced in size, or completely lost, through disuse;
but in this case it would be necessary that they should first have acquired
the power of running quickly on the ground, by the aid of their hind legs
alone, so as to compete with birds or other ground animals; and for such a
change a bat seems singularly ill-fitted. These conjectural remarks have
been made merely to show that a transition of structure, with each step
beneficial, is a highly complex affair; and that there is nothing strange
in a transition not having occurred in any particular case.
Lastly, more than one writer has asked why have some animals had their
mental powers more highly developed than others, as such development would
be advantageous to all? Why have not apes acquired the intellectual powers
of man? Various causes could be assigned; but as they are conjectural, and
their relative probability cannot be weighed, it would be useless to give
them. A definite answer to the latter question ought not to be expected,
seeing that no one can solve the simpler problem, why, of two races of
savages, one has risen higher in the scale of civilisation than the other;
and this apparently implies increased brain power.
We will return to Mr. Mivart's other objections. Insects often resemble
for the sake of protection various objects, such as green or decayed
leaves, dead twigs, bits of lichen, flowers, spines, excrement of birds,
and living insects; but to this latter point I shall hereafter recur. The
resemblance is often wonderfully close, and is not confined to colour, but
extends to form, and even to the manner in which the insects hold
themselves. The caterpillars which project motionless like dead twigs from
the bushes on which they feed, offer an excellent instance of a resemblance
of this kind. The cases of the imitation of such objects as the excrement
of birds, are rare and exceptional. On this head, Mr. Mivart remarks, "As,
according to Mr. Darwin's theory, there is a constant tendency to
indefinite variation, and as the minute incipient variations will be in ALL
DIRECTIONS, they must tend to neutralize each other, and at first to form
such unstable modifications that it is difficult, if not impossible, to see
how such indefinite oscillations of infinitesimal beginnings can ever build
up a sufficiently appreciable resemblance to a leaf, bamboo, or other
object, for natural selection to seize upon and perpetuate."
But in all the foregoing cases the insects in their original state no doubt
presented some rude and accidental resemblance to an object commonly found
in the stations frequented by them. Nor is this at all improbable,
considering the almost infinite number of surrounding objects and the
diversity in form and colour of the hosts of insects which exist. As some
rude resemblance is necessary for the first start, we can understand how it
is that the larger and higher animals do not (with the exception, as far as
I know, of one fish) resemble for the sake of protection special objects,
but only the surface which commonly surrounds them, and this chiefly in
colour. Assuming that an insect originally happened to resemble in some
degree a dead twig or a decayed leaf, and that it varied slightly in many
ways, then all the variations which rendered the insect at all more like
any such object, and thus favoured its escape, would be preserved, while
other variations would be neglected and ultimately lost; or, if they
rendered the insect at all less like the imitated object, they would be
eliminated. There would indeed be force in Mr. Mivart's objection, if we
were to attempt to account for the above resemblances, independently of
natural selection, through mere fluctuating variability; but as the case
stands there is none.
Nor can I see any force in Mr. Mivart's difficulty with respect to "the
last touches of perfection in the mimicry;" as in the case given by Mr.
Wallace, of a walking-stick insect (Ceroxylus laceratus), which resembles
"a stick grown over by a creeping moss or jungermannia." So close was this
resemblance, that a native Dyak maintained that the foliaceous excrescences
were really moss. Insects are preyed on by birds and other enemies whose
sight is probably sharper than ours, and every grade in resemblance which
aided an insect to escape notice or detection, would tend towards its
preservation; and the more perfect the resemblance so much the better for
the insect. Considering the nature of the differences between the species
in the group which includes the above Ceroxylus, there is nothing
improbable in this insect having varied in the irregularities on its
surface, and in these having become more or less green-coloured; for in
every group the characters which differ in the several species are the most
apt to vary, while the generic characters, or those common to all the
species, are the most constant.
The Greenland whale is one of the most wonderful animals in the world, and
the baleen, or whalebone, one of its greatest peculiarities. The baleen
consists of a row, on each side of the upper jaw, of about 300 plates or
laminae, which stand close together transversely to the longer axis of the
mouth. Within the main row there are some subsidiary rows. The
extremities and inner margins of all the plates are frayed into stiff
bristles, which clothe the whole gigantic palate, and serve to strain or
sift the water, and thus to secure the minute prey on which these great
animals subsist. The middle and longest lamina in the Greenland whale is
ten, twelve, or even fifteen feet in length; but in the different species
of Cetaceans there are gradations in length; the middle lamina being in one
species, according to Scoresby, four feet, in another three, in another
eighteen inches, and in the Balaenoptera rostrata only about nine inches in
length. The quality of the whalebone also differs in the different
species.
With respect to the baleen, Mr. Mivart remarks that if it "had once
attained such a size and development as to be at all useful, then its
preservation and augmentation within serviceable limits would be promoted
by natural selection alone. But how to obtain the beginning of such useful
development?" In answer, it may be asked, why should not the early
progenitors of the whales with baleen have possessed a mouth constructed
something like the lamellated beak of a duck? Ducks, like whales, subsist
by sifting the mud and water; and the family has sometimes been called
Criblatores, or sifters. I hope that I may not be misconstrued into saying
that the progenitors of whales did actually possess mouths lamellated like
the beak of a duck. I wish only to show that this is not incredible, and
that the immense plates of baleen in the Greenland whale might have been
developed from such lamellae by finely graduated steps, each of service to
its possessor.
The beak of a shoveller-duck (Spatula clypeata) is a more beautiful and
complex structure than the mouth of a whale. The upper mandible is
furnished on each side (in the specimen examined by me) with a row or comb
formed of 188 thin, elastic lamellae, obliquely bevelled so as to be
pointed, and placed transversely to the longer axis of the mouth. They
arise from the palate, and are attached by flexible membrane to the sides
of the mandible. Those standing towards the middle are the longest, being
about one-third of an inch in length, and they project fourteen one-
hundredths of an inch beneath the edge. At their bases there is a short
subsidiary row of obliquely transverse lamellae. In these several respects
they resemble the plates of baleen in the mouth of a whale. But towards
the extremity of the beak they differ much, as they project inward, instead
of straight downward. The entire head of the shoveller, though
incomparably less bulky, is about one-eighteenth of the length of the head
of a moderately large Balaenoptera rostrata, in which species the baleen is
only nine inches long; so that if we were to make the head of the shoveller
as long as that of the Balaenoptera, the lamellae would be six inches in
length, that is, two-thirds of the length of the baleen in this species of
whale. The lower mandible of the shoveller-duck is furnished with lamellae
of equal length with these above, but finer; and in being thus furnished it
differs conspicuously from the lower jaw of a whale, which is destitute of
baleen. On the other hand, the extremities of these lower lamellae are
frayed into fine bristly points, so that they thus curiously resemble the
plates of baleen. In the genus Prion, a member of the distinct family of
the Petrels, the upper mandible alone is furnished with lamellae, which are
well developed and project beneath the margin; so that the beak of this
bird resembles in this respect the mouth of a whale.
>From the highly developed structure of the shoveller's beak we may proceed
(as I have learned from information and specimens sent to me by Mr.
Salvin), without any great break, as far as fitness for sifting is
concerned, through the beak of the Merganetta armata, and in some respects
through that of the Aix sponsa, to the beak of the common duck. In this
latter species the lamellae are much coarser than in the shoveller, and are
firmly attached to the sides of the mandible; they are only about fifty in
number on each side, and do not project at all beneath the margin. They
are square-topped, and are edged with translucent, hardish tissue, as if
for crushing food. The edges of the lower mandible are crossed by numerous
fine ridges, which project very little. Although the beak is thus very
inferior as a sifter to that of a shoveller, yet this bird, as every one
knows, constantly uses it for this purpose. There are other species, as I
hear from Mr. Salvin, in which the lamellae are considerably less developed
than in the common duck; but I do not know whether they use their beaks for
sifting the water.
Turning to another group of the same family. In the Egyptian goose
(Chenalopex) the beak closely resembles that of the common duck; but the
lamellae are not so numerous, nor so distinct from each other, nor do they
project so much inward; yet this goose, as I am informed by Mr. E.
Bartlett, "uses its bill like a duck by throwing the water out at the
corners." Its chief food, however, is grass, which it crops like the
common goose. In this latter bird the lamellae of the upper mandible are
much coarser than in the common duck, almost confluent, about twenty-seven
in number on each side, and terminating upward in teeth-like knobs. The
palate is also covered with hard rounded knobs. The edges of the lower
mandible are serrated with teeth much more prominent, coarser and sharper
than in the duck. The common goose does not sift the water, but uses its
beak exclusively for tearing or cutting herbage, for which purpose it is so
well fitted that it can crop grass closer than almost any other animal.
There are other species of geese, as I hear from Mr. Bartlett, in which the
lamellae are less developed than in the common goose.
We thus see that a member of the duck family, with a beak constructed like
that of a common goose and adapted solely for grazing, or even a member
with a beak having less well-developed lamellae, might be converted by
small changes into a species like the Egyptian goose--this into one like
the common duck--and, lastly, into one like the shoveller, provided with a
beak almost exclusively adapted for sifting the water; for this bird could
hardly use any part of its beak, except the hooked tip, for seizing or
tearing solid food. The beak of a goose, as I may add, might also be
converted by small changes into one provided with prominent, recurved
teeth, like those of the Merganser (a member of the same family), serving
for the widely different purpose of securing live fish.
Returning to the whales. The Hyperoodon bidens is destitute of true teeth
in an efficient condition, but its palate is roughened, according to
Lacepede, with small unequal, hard points of horn. There is, therefore,
nothing improbable in supposing that some early Cetacean form was provided
with similar points of horn on the palate, but rather more regularly
placed, and which, like the knobs on the beak of the goose, aided it in
seizing or tearing its food. If so, it will hardly be denied that the
points might have been converted through variation and natural selection
into lamellae as well-developed as those of the Egyptian goose, in which
case they would have been used both for seizing objects and for sifting the
water; then into lamellae like those of the domestic duck; and so onward,
until they became as well constructed as those of the shoveller, in which
case they would have served exclusively as a sifting apparatus. From this
stage, in which the lamellae would be two-thirds of the length of the
plates of baleen in the Balaenoptera rostrata, gradations, which may be
observed in still-existing Cetaceans, lead us onward to the enormous plates
of baleen in the Greenland whale. Nor is there the least reason to doubt
that each step in this scale might have been as serviceable to certain
ancient Cetaceans, with the functions of the parts slowly changing during
the progress of development, as are the gradations in the beaks of the
different existing members of the duck-family. We should bear in mind that
each species of duck is subjected to a severe struggle for existence, and
that the structure of every part of its frame must be well adapted to its
conditions of life.
The Pleuronectidae, or Flat-fish, are remarkable for their asymmetrical
bodies. They rest on one side--in the greater number of species on the
left, but in some on the right side; and occasionally reversed adult
specimens occur. The lower, or resting-surface, resembles at first sight
the ventral surface of an ordinary fish; it is of a white colour, less
developed in many ways than the upper side, with the lateral fins often of
smaller size. But the eyes offer the most remarkable peculiarity; for they
are both placed on the upper side of the head. During early youth,
however, they stand opposite to each other, and the whole body is then
symmetrical, with both sides equally coloured. Soon the eye proper to the
lower side begins to glide slowly round the head to the upper side; but
does not pass right through the skull, as was formerly thought to be the
case. It is obvious that unless the lower eye did thus travel round, it
could not be used by the fish while lying in its habitual position on one
side. The lower eye would, also, have been liable to be abraded by the
sandy bottom. That the Pleuronectidae are admirably adapted by their
flattened and asymmetrical structure for their habits of life, is manifest
from several species, such as soles, flounders, etc., being extremely
common. The chief advantages thus gained seem to be protection from their
enemies, and facility for feeding on the ground. The different members,
however, of the family present, as Schiodte remarks, "a long series of
forms exhibiting a gradual transition from Hippoglossus pinguis, which does
not in any considerable degree alter the shape in which it leaves the ovum,
to the soles, which are entirely thrown to one side."
Mr. Mivart has taken up this case, and remarks that a sudden spontaneous
transformation in the position of the eyes is hardly conceivable, in which
I quite agree with him. He then adds: "If the transit was gradual, then
how such transit of one eye a minute fraction of the journey towards the
other side of the head could benefit the individual is, indeed, far from
clear. It seems, even, that such an incipient transformation must rather
have been injurious." But he might have found an answer to this objection
in the excellent observations published in 1867 by Malm. The
Pleuronectidae, while very young and still symmetrical, with their eyes
standing on opposite sides of the head, cannot long retain a vertical
position, owing to the excessive depth of their bodies, the small size of
their lateral fins, and to their being destitute of a swim-bladder. Hence,
soon growing tired, they fall to the bottom on one side. While thus at
rest they often twist, as Malm observed, the lower eye upward, to see above
them; and they do this so vigorously that the eye is pressed hard against
the upper part of the orbit. The forehead between the eyes consequently
becomes, as could be plainly seen, temporarily contracted in breadth. On
one occasion Malm saw a young fish raise and depress the lower eye through
an angular distance of about seventy degrees.
We should remember that the skull at this early age is cartilaginous and
flexible, so that it readily yields to muscular action. It is also known
with the higher animals, even after early youth, that the skull yields and
is altered in shape, if the skin or muscles be permanently contracted
through disease or some accident. With long-eared rabbits, if one ear
flops forward and downward, its weight drags forward all the bones of the
skull on the same side, of which I have given a figure. Malm states that
the newly-hatched young of perches, salmon, and several other symmetrical
fishes, have the habit of occasionally resting on one side at the bottom;
and he has observed that they often then strain their lower eyes so as to
look upward; and their skulls are thus rendered rather crooked. These
fishes, however, are soon able to hold themselves in a vertical position,
and no permanent effect is thus produced. With the Pleuronectidae, on the
other hand, the older they grow the more habitually they rest on one side,
owing to the increasing flatness of their bodies, and a permanent effect is
thus produced on the form of the head, and on the position of the eyes.
Judging from analogy, the tendency to distortion would no doubt be
increased through the principle of inheritance. Schiodte believes, in
opposition to some other naturalists, that the Pleuronectidae are not quite
symmetrical even in the embryo; and if this be so, we could understand how
it is that certain species, while young, habitually fall over and rest on
the left side, and other species on the right side. Malm adds, in
confirmation of the above view, that the adult Trachypterus arcticus, which
is not a member of the Pleuronectidae, rests on its left side at the
bottom, and swims diagonally through the water; and in this fish, the two
sides of the head are said to be somewhat dissimilar. Our great authority
on Fishes, Dr. Gunther, concludes his abstract of Malm's paper, by
remarking that "the author gives a very simple explanation of the abnormal
condition of the Pleuronectoids."
We thus see that the first stages of the transit of the eye from one side
of the head to the other, which Mr. Mivart considers would be injurious,
may be attributed to the habit, no doubt beneficial to the individual and
to the species, of endeavouring to look upward with both eyes, while
resting on one side at the bottom. We may also attribute to the inherited
effects of use the fact of the mouth in several kinds of flat-fish being
bent towards the lower surface, with the jaw bones stronger and more
effective on this, the eyeless side of the head, than on the other, for the
sake, as Dr. Traquair supposes, of feeding with ease on the ground.
Disuse, on the other hand, will account for the less developed condition of
the whole inferior half of the body, including the lateral fins; though
Yarrel thinks that the reduced size of these fins is advantageous to the
fish, as "there is so much less room for their action than with the larger
fins above." Perhaps the lesser number of teeth in the proportion of four
to seven in the upper halves of the two jaws of the plaice, to twenty-five
to thirty in the lower halves, may likewise be accounted for by disuse.
>From the colourless state of the ventral surface of most fishes and of many
other animals, we may reasonably suppose that the absence of colour in
flat-fish on the side, whether it be the right or left, which is under-
most, is due to the exclusion of light. But it cannot be supposed that the
peculiar speckled appearance of the upper side of the sole, so like the
sandy bed of the sea, or the power in some species, as recently shown by
Pouchet, of changing their colour in accordance with the surrounding
surface, or the presence of bony tubercles on the upper side of the turbot,
are due to the action of the light. Here natural selection has probably
come into play, as well as in adapting the general shape of the body of
these fishes, and many other peculiarities, to their habits of life. We
should keep in mind, as I have before insisted, that the inherited effects
of the increased use of parts, and perhaps of their disuse, will be
strengthened by natural selection. For all spontaneous variations in the
right direction will thus be preserved; as will those individuals which
inherit in the highest degree the effects of the increased and beneficial
use of any part. How much to attribute in each particular case to the
effects of use, and how much to natural selection, it seems impossible to
decide.
I may give another instance of a structure which apparently owes its origin
exclusively to use or habit. The extremity of the tail in some American
monkeys has been converted into a wonderfully perfect prehensile organ, and
serves as a fifth hand. A reviewer, who agrees with Mr. Mivart in every
detail, remarks on this structure: "It is impossible to believe that in
any number of ages the first slight incipient tendency to grasp could
preserve the lives of the individuals possessing it, or favour their chance
of having and of rearing offspring." But there is no necessity for any
such belief. Habit, and this almost implies that some benefit great or
small is thus derived, would in all probability suffice for the work.
Brehm saw the young of an African monkey (Cercopithecus) clinging to the
under surface of their mother by their hands, and at the same time they
hooked their little tails round that of their mother. Professor Henslow
kept in confinement some harvest mice (Mus messorius) which do not possess
a structurally prehensive tail; but he frequently observed that they curled
their tails round the branches of a bush placed in the cage, and thus aided
themselves in climbing. I have received an analogous account from Dr.
Gunther, who has seen a mouse thus suspend itself. If the harvest mouse
had been more strictly arboreal, it would perhaps have had its tail
rendered structurally prehensile, as is the case with some members of the
same order. Why Cercopithecus, considering its habits while young, has not
become thus provided, it would be difficult to say. It is, however,
possible that the long tail of this monkey may be of more service to it as
a balancing organ in making its prodigious leaps, than as a prehensile
organ.
The mammary glands are common to the whole class of mammals, and are
indispensable for their existence; they must, therefore, have been
developed at an extremely remote period, and we can know nothing positively
about their manner of development. Mr. Mivart asks: "Is it conceivable
that the young of any animal was ever saved from destruction by
accidentally sucking a drop of scarcely nutritious fluid from an
accidentally hypertrophied cutaneous gland of its mother? And even if one
was so, what chance was there of the perpetuation of such a variation?"
But the case is not here put fairly. It is admitted by most evolutionists
that mammals are descended from a marsupial form; and if so, the mammary
glands will have been at first developed within the marsupial sack. In the
case of the fish (Hippocampus) the eggs are hatched, and the young are
reared for a time, within a sack of this nature; and an American
naturalist, Mr. Lockwood, believes from what he has seen of the development
of the young, that they are nourished by a secretion from the cutaneous
glands of the sack. Now, with the early progenitors of mammals, almost
before they deserved to be thus designated, is it not at least possible
that the young might have been similarly nourished? And in this case, the
individuals which secreted a fluid, in some degree or manner the most
nutritious, so as to partake of the nature of milk, would in the long run
have reared a larger number of well-nourished offspring, than would the
individuals which secreted a poorer fluid; and thus the cutaneous glands,
which are the homologues of the mammary glands, would have been improved or
rendered more effective. It accords with the widely extended principle of
specialisation, that the glands over a certain space of the sack should
have become more highly developed than the remainder; and they would then
have formed a breast, but at first without a nipple, as we see in the
Ornithorhyncus, at the base of the mammalian series. Through what agency
the glands over a certain space became more highly specialised than the
others, I will not pretend to decide, whether in part through compensation
of growth, the effects of use, or of natural selection.
The development of the mammary glands would have been of no service, and
could not have been affected through natural selection, unless the young at
the same time were able to partake of the secretion. There is no greater
difficulty in understanding how young mammals have instinctively learned to
suck the breast, than in understanding how unhatched chickens have learned
to break the egg-shell by tapping against it with their specially adapted
beaks; or how a few hours after leaving the shell they have learned to pick
up grains of food. In such cases the most probable solution seems to be,
that the habit was at first acquired by practice at a more advanced age,
and afterwards transmitted to the offspring at an earlier age. But the
young kangaroo is said not to suck, only to cling to the nipple of its
mother, who has the power of injecting milk into the mouth of her helpless,
half-formed offspring. On this head Mr. Mivart remarks: "Did no special
provision exist, the young one must infallibly be choked by the intrusion
of the milk into the wind-pipe. But there IS a special provision. The
larynx is so elongated that it rises up into the posterior end of the nasal
passage, and is thus enabled to give free entrance to the air for the
lungs, while the milk passes harmlessly on each side of this elongated
larynx, and so safely attains the gullet behind it." Mr. Mivart then asks
how did natural selection remove in the adult kangaroo (and in most other
mammals, on the assumption that they are descended from a marsupial form),
"this at least perfectly innocent and harmless structure?" It may be
suggested in answer that the voice, which is certainly of high importance
to many animals, could hardly have been used with full force as long as the
larynx entered the nasal passage; and Professor Flower has suggested to me
that this structure would have greatly interfered with an animal swallowing
solid food.
We will now turn for a short space to the lower divisions of the animal
kingdom. The Echinodermata (star-fishes, sea-urchins, etc.) are furnished
with remarkable organs, called pedicellariae, which consist, when well
developed, of a tridactyle forceps--that is, of one formed of three
serrated arms, neatly fitting together and placed on the summit of a
flexible stem, moved by muscles. These forceps can seize firmly hold of
any object; and Alexander Agassiz has seen an Echinus or sea-urchin rapidly
passing particles of excrement from forceps to forceps down certain lines
of its body, in order that its shell should not be fouled. But there is no
doubt that besides removing dirt of all kinds, they subserve other
functions; and one of these apparently is defence.
With respect to these organs, Mr. Mivart, as on so many previous occasions,
asks: "What would be the utility of the FIRST RUDIMENTARY BEGINNINGS of
such structures, and how could such insipient buddings have ever preserved
the life of a single Echinus?" He adds, "not even the SUDDEN development
of the snapping action would have been beneficial without the freely
movable stalk, nor could the latter have been efficient without the
snapping jaws, yet no minute, nearly indefinite variations could
simultaneously evolve these complex co-ordinations of structure; to deny
this seems to do no less than to affirm a startling paradox." Paradoxical
as this may appear to Mr. Mivart, tridactyle forcepses, immovably fixed at
the base, but capable of a snapping action, certainly exist on some star-
fishes; and this is intelligible if they serve, at least in part, as a
means of defence. Mr. Agassiz, to whose great kindness I am indebted for
much information on the subject, informs me that there are other star-
fishes, in which one of the three arms of the forceps is reduced to a
support for the other two; and again, other genera in which the third arm
is completely lost. In Echinoneus, the shell is described by M. Perrier as
bearing two kinds of pedicellariae, one resembling those of Echinus, and
the other those of Spatangus; and such cases are always interesting as
affording the means of apparently sudden transitions, through the abortion
of one of the two states of an organ.
With respect to the steps by which these curious organs have been evolved,
Mr. Agassiz infers from his own researches and those of Mr. Muller, that
both in star-fishes and sea-urchins the pedicellariae must undoubtedly be
looked at as modified spines. This may be inferred from their manner of
development in the individual, as well as from a long and perfect series of
gradations in different species and genera, from simple granules to
ordinary spines, to perfect tridactyle pedicellariae. The gradation
extends even to the manner in which ordinary spines and the pedicellariae,
with their supporting calcareous rods, are articulated to the shell. In
certain genera of star-fishes, "the very combinations needed to show that
the pedicellariae are only modified branching spines" may be found. Thus
we have fixed spines, with three equi-distant, serrated, movable branches,
articulated to near their bases; and higher up, on the same spine, three
other movable branches. Now when the latter arise from the summit of a
spine they form, in fact, a rude tridactyle pedicellariae, and such may be
seen on the same spine together with the three lower branches. In this
case the identity in nature between the arms of the pedicellariae and the
movable branches of a spine, is unmistakable. It is generally admitted
that the ordinary spines serve as a protection; and if so, there can be no
reason to doubt that those furnished with serrated and movable branches
likewise serve for the same purpose; and they would thus serve still more
effectively as soon as by meeting together they acted as a prehensile or
snapping apparatus. Thus every gradation, from an ordinary fixed spine to
a fixed pedicellariae, would be of service.
In certain genera of star-fishes these organs, instead of being fixed or
borne on an immovable support, are placed on the summit of a flexible and
muscular, though short, stem; and in this case they probably subserve some
additional function besides defence. In the sea-urchins the steps can be
followed by which a fixed spine becomes articulated to the shell, and is
thus rendered movable. I wish I had space here to give a fuller abstract
of Mr. Agassiz's interesting observations on the development of the
pedicellariae. All possible gradations, as he adds, may likewise be found
between the pedicellariae of the star-fishes and the hooks of the
Ophiurians, another group of the Echinodermata; and again between the
pedicellariae of sea-urchins and the anchors of the Holothuriae, also
belonging to the same great class.
Certain compound animals, or zoophytes, as they have been termed, namely
the Polyzoa, are provided with curious organs called avicularia. These
differ much in structure in the different species. In their most perfect
condition they curiously resemble the head and beak of a vulture in
miniature, seated on a neck and capable of movement, as is likewise the
lower jaw or mandible. In one species observed by me, all the avicularia
on the same branch often moved simultaneously backwards and forwards, with
the lower jaw widely open, through an angle of about 90 degrees, in the
course of five seconds; and their movement caused the whole polyzoary to
tremble. When the jaws are touched with a needle they seize it so firmly
that the branch can thus be shaken.
Mr. Mivart adduces this case, chiefly on account of the supposed difficulty
of organs, namely the avicularia of the Polyzoa and the pedicellariae of
the Echinodermata, which he considers as "essentially similar," having been
developed through natural selection in widely distinct divisions of the
animal kingdom. But, as far as structure is concerned, I can see no
similarity between tridactyle pedicellariae and avicularia. The latter
resembles somewhat more closely the chelae or pincers of Crustaceans; and
Mr. Mivart might have adduced with equal appropriateness this resemblance
as a special difficulty, or even their resemblance to the head and beak of
a bird. The avicularia are believed by Mr. Busk, Dr. Smitt and Dr.
Nitsche--naturalists who have carefully studied this group--to be
homologous with the zooids and their cells which compose the zoophyte, the
movable lip or lid of the cell corresponding with the lower and movable
mandible of the avicularium. Mr. Busk, however, does not know of any
gradations now existing between a zooid and an avicularium. It is
therefore impossible to conjecture by what serviceable gradations the one
could have been converted into the other, but it by no means follows from
this that such gradations have not existed.
As the chelae of Crustaceans resemble in some degree the avicularia of
Polyzoa, both serving as pincers, it may be worth while to show that with
the former a long series of serviceable gradations still exists. In the
first and simplest stage, the terminal segment of a limb shuts down either
on the square summit of the broad penultimate segment, or against one whole
side, and is thus enabled to catch hold of an object, but the limb still
serves as an organ of locomotion. We next find one corner of the broad
penultimate segment slightly prominent, sometimes furnished with irregular
teeth, and against these the terminal segment shuts down. By an increase
in the size of this projection, with its shape, as well as that of the
terminal segment, slightly modified and improved, the pincers are rendered
more and more perfect, until we have at last an instrument as efficient as
the chelae of a lobster. And all these gradations can be actually traced.
Besides the avicularia, the polyzoa possess curious organs called
vibracula. These generally consist of long bristles, capable of movement
and easily excited. In one species examined by me the vibracula were
slightly curved and serrated along the outer margin, and all of them on the
same polyzoary often moved simultaneously; so that, acting like long oars,
they swept a branch rapidly across the object-glass of my microscope. When
a branch was placed on its face, the vibracula became entangled, and they
made violent efforts to free themselves. They are supposed to serve as a
defence, and may be seen, as Mr. Busk remarks, "to sweep slowly and
carefully over the surface of the polyzoary, removing what might be noxious
to the delicate inhabitants of the cells when their tentacula are
protruded." The avicularia, like the vibracula, probably serve for
defence, but they also catch and kill small living animals, which, it is
believed, are afterwards swept by the currents within reach of the
tentacula of the zooids. Some species are provided with avicularia and
vibracula, some with avicularia alone and a few with vibracula alone.
It is not easy to imagine two objects more widely different in appearance
than a bristle or vibraculum, and an avicularium like the head of a bird;
yet they are almost certainly homologous and have been developed from the
same common source, namely a zooid with its cell. Hence, we can understand
how it is that these organs graduate in some cases, as I am informed by Mr.
Busk, into each other. Thus, with the avicularia of several species of
Lepralia, the movable mandible is so much produced and is so like a bristle
that the presence of the upper or fixed beak alone serves to determine its
avicularian nature. The vibracula may have been directly developed from
the lips of the cells, without having passed through the avicularian stage;
but it seems more probable that they have passed through this stage, as
during the early stages of the transformation, the other parts of the cell,
with the included zooid, could hardly have disappeared at once. In many
cases the vibracula have a grooved support at the base, which seems to
represent the fixed beak; though this support in some species is quite
absent. This view of the development of the vibracula, if trustworthy, is
interesting; for supposing that all the species provided with avicularia
had become extinct, no one with the most vivid imagination would ever have
thought that the vibracula had originally existed as part of an organ,
resembling a bird's head, or an irregular box or hood. It is interesting
to see two such widely different organs developed from a common origin; and
as the movable lip of the cell serves as a protection to the zooid, there
is no difficulty in believing that all the gradations, by which the lip
became converted first into the lower mandible of an avicularium, and then
into an elongated bristle, likewise served as a protection in different
ways and under different circumstances.
In the vegetable kingdom Mr. Mivart only alludes to two cases, namely the
structure of the flowers of orchids, and the movements of climbing plants.
With respect to the former, he says: "The explanation of their ORIGIN is
deemed thoroughly unsatisfactory--utterly insufficient to explain the
incipient, infinitesimal beginnings of structures which are of utility only
when they are considerably developed." As I have fully treated this
subject in another work, I will here give only a few details on one alone
of the most striking peculiarities of the flowers of orchids, namely, their
pollinia. A pollinium, when highly developed, consists of a mass of
pollen-grains, affixed to an elastic foot-stalk or caudicle, and this to a
little mass of extremely viscid matter. The pollinia are by this means
transported by insects from one flower to the stigma of another. In some
orchids there is no caudicle to the pollen-masses, and the grains are
merely tied together by fine threads; but as these are not confined to
orchids, they need not here be considered; yet I may mention that at the
base of the orchidaceous series, in Cypripedium, we can see how the threads
were probably first developed. In other orchids the threads cohere at one
end of the pollen-masses; and this forms the first or nascent trace of a
caudicle. That this is the origin of the caudicle, even when of
considerable length and highly developed, we have good evidence in the
aborted pollen-grains which can sometimes be detected embedded within the
central and solid parts.
With respect to the second chief peculiarity, namely, the little mass of
viscid matter attached to the end of the caudicle, a long series of
gradations can be specified, each of plain service to the plant. In most
flowers belonging to other orders the stigma secretes a little viscid
matter. Now, in certain orchids similar viscid matter is secreted, but in
much larger quantities by one alone of the three stigmas; and this stigma,
perhaps in consequence of the copious secretion, is rendered sterile. When
an insect visits a flower of this kind, it rubs off some of the viscid
matter, and thus at the same time drags away some of the pollen-grains.
>From this simple condition, which differs but little from that of a
multitude of common flowers, there are endless gradations--to species in
which the pollen-mass terminates in a very short, free caudicle--to others
in which the caudicle becomes firmly attached to the viscid matter, with
the sterile stigma itself much modified. In this latter case we have a
pollinium in its most highly developed and perfect condition. He who will
carefully examine the flowers of orchids for himself will not deny the
existence of the above series of gradations--from a mass of pollen-grains
merely tied together by threads, with the stigma differing but little from
that of the ordinary flowers, to a highly complex pollinium, admirably
adapted for transportal by insects; nor will he deny that all the
gradations in the several species are admirably adapted in relation to the
general structure of each flower for its fertilisation by different
insects. In this, and in almost every other case, the enquiry may be
pushed further backwards; and it may be asked how did the stigma of an
ordinary flower become viscid, but as we do not know the full history of
any one group of beings, it is as useless to ask, as it is hopeless to
attempt answering, such questions.
We will now turn to climbing plants. These can be arranged in a long
series, from those which simply twine round a support, to those which I
have called leaf-climbers, and to those provided with tendrils. In these
two latter classes the stems have generally, but not always, lost the power
of twining, though they retain the power of revolving, which the tendrils
likewise possess. The gradations from leaf-climbers to tendril bearers are
wonderfully close, and certain plants may be differently placed in either
class. But in ascending the series from simple twiners to leaf-climbers,
an important quality is added, namely sensitiveness to a touch, by which
means the foot-stalks of the leaves or flowers, or these modified and
converted into tendrils, are excited to bend round and clasp the touching
object. He who will read my memoir on these plants will, I think, admit
that all the many gradations in function and structure between simple
twiners and tendril-bearers are in each case beneficial in a high degree to
the species. For instance, it is clearly a great advantage to a twining
plant to become a leaf-climber; and it is probable that every twiner which
possessed leaves with long foot-stalks would have been developed into a
leaf-climber, if the foot-stalks had possessed in any slight degree the
requisite sensitiveness to a touch.
As twining is the simplest means of ascending a support, and forms the
basis of our series, it may naturally be asked how did plants acquire this
power in an incipient degree, afterwards to be improved and increased
through natural selection. The power of twining depends, firstly, on the
stems while young being extremely flexible (but this is a character common
to many plants which are not climbers); and, secondly, on their continually
bending to all points of the compass, one after the other in succession, in
the same order. By this movement the stems are inclined to all sides, and
are made to move round and round. As soon as the lower part of a stem
strikes against any object and is stopped, the upper part still goes on
bending and revolving, and thus necessarily twines round and up the
support. The revolving movement ceases after the early growth of each
shoot. As in many widely separated families of plants, single species and
single genera possess the power of revolving, and have thus become twiners,
they must have independently acquired it, and cannot have inherited it from
a common progenitor. Hence, I was led to predict that some slight tendency
to a movement of this kind would be found to be far from uncommon with
plants which did not climb; and that this had afforded the basis for
natural selection to work on and improve. When I made this prediction, I
knew of only one imperfect case, namely, of the young flower-peduncles of a
Maurandia which revolved slightly and irregularly, like the stems of
twining plants, but without making any use of this habit. Soon afterwards
Fritz Muller discovered that the young stems of an Alisma and of a Linum--
plants which do not climb and are widely separated in the natural system--
revolved plainly, though irregularly, and he states that he has reason to
suspect that this occurs with some other plants. These slight movements
appear to be of no service to the plants in question; anyhow, they are not
of the least use in the way of climbing, which is the point that concerns
us. Nevertheless we can see that if the stems of these plants had been
flexible, and if under the conditions to which they are exposed it had
profited them to ascend to a height, then the habit of slightly and
irregularly revolving might have been increased and utilised through
natural selection, until they had become converted into well-developed
twining species.
With respect to the sensitiveness of the foot-stalks of the leaves and
flowers, and of tendrils, nearly the same remarks are applicable as in the
case of the revolving movements of twining plants. As a vast number of
species, belonging to widely distinct groups, are endowed with this kind of
sensitiveness, it ought to be found in a nascent condition in many plants
which have not become climbers. This is the case: I observed that the
young flower-peduncles of the above Maurandia curved themselves a little
towards the side which was touched. Morren found in several species of
Oxalis that the leaves and their foot-stalks moved, especially after
exposure to a hot sun, when they were gently and repeatedly touched, or
when the plant was shaken. I repeated these observations on some other
species of Oxalis with the same result; in some of them the movement was
distinct, but was best seen in the young leaves; in others it was extremely
slight. It is a more important fact that according to the high authority
of Hofmeister, the young shoots and leaves of all plants move after being
shaken; and with climbing plants it is, as we know, only during the early
stages of growth that the foot-stalks and tendrils are sensitive.
It is scarcely possible that the above slight movements, due to a touch or
shake, in the young and growing organs of plants, can be of any functional
importance to them. But plants possess, in obedience to various stimuli,
powers of movement, which are of manifest importance to them; for instance,
towards and more rarely from the light--in opposition to, and more rarely
in the direction of, the attraction of gravity. When the nerves and
muscles of an animal are excited by galvanism or by the absorption of
strychnine, the consequent movements may be called an incidental result,
for the nerves and muscles have not been rendered specially sensitive to
these stimuli. So with plants it appears that, from having the power of
movement in obedience to certain stimuli, they are excited in an incidental
manner by a touch, or by being shaken. Hence there is no great difficulty
in admitting that in the case of leaf-climbers and tendril-bearers, it is
this tendency which has been taken advantage of and increased through
natural selection. It is, however, probable, from reasons which I have
assigned in my memoir, that this will have occurred only with plants which
had already acquired the power of revolving, and had thus become twiners.
I have already endeavoured to explain how plants became twiners, namely, by
the increase of a tendency to slight and irregular revolving movements,
which were at first of no use to them; this movement, as well as that due
to a touch or shake, being the incidental result of the power of moving,
gained for other and beneficial purposes. Whether, during the gradual
development of climbing plants, natural selection has been aided by the
inherited effects of use, I will not pretend to decide; but we know that
certain periodical movements, for instance the so-called sleep of plants,
are governed by habit.
I have now considered enough, perhaps more than enough, of the cases,
selected with care by a skilful naturalist, to prove that natural selection
is incompetent to account for the incipient stages of useful structures;
and I have shown, as I hope, that there is no great difficulty on this
head. A good opportunity has thus been afforded for enlarging a little on
gradations of structure, often associated with strange functions--an
important subject, which was not treated at sufficient length in the former
editions of this work. I will now briefly recapitulate the foregoing
cases.
With the giraffe, the continued preservation of the individuals of some
extinct high-reaching ruminant, which had the longest necks, legs, etc.,
and could browse a little above the average height, and the continued
destruction of those which could not browse so high, would have sufficed
for the production of this remarkable quadruped; but the prolonged use of
all the parts, together with inheritance, will have aided in an important
manner in their co-ordination. With the many insects which imitate various
objects, there is no improbability in the belief that an accidental
resemblance to some common object was in each case the foundation for the
work of natural selection, since perfected through the occasional
preservation of slight variations which made the resemblance at all closer;
and this will have been carried on as long as the insect continued to vary,
and as long as a more and more perfect resemblance led to its escape from
sharp-sighted enemies. In certain species of whales there is a tendency to
the formation of irregular little points of horn on the palate; and it
seems to be quite within the scope of natural selection to preserve all
favourable variations, until the points were converted, first into
lamellated knobs or teeth, like those on the beak of a goose--then into
short lamellae, like those of the domestic ducks--and then into lamellae,
as perfect as those of the shoveller-duck--and finally into the gigantic
plates of baleen, as in the mouth of the Greenland whale. In the family of
the ducks, the lamellae are first used as teeth, then partly as teeth and
partly as a sifting apparatus, and at last almost exclusively for this
latter purpose.
With such structures as the above lamellae of horn or whalebone, habit or
use can have done little or nothing, as far as we can judge, towards their
development. On the other hand, the transportal of the lower eye of a
flat-fish to the upper side of the head, and the formation of a prehensile
tail, may be attributed almost wholly to continued use, together with
inheritance. With respect to the mammae of the higher animals, the most
probable conjecture is that primordially the cutaneous glands over the
whole surface of a marsupial sack secreted a nutritious fluid; and that
these glands were improved in function through natural selection, and
concentrated into a confined area, in which case they would have formed a
mamma. There is no more difficulty in understanding how the branched
spines of some ancient Echinoderm, which served as a defence, became
developed through natural selection into tridactyle pedicellariae, than in
understanding the development of the pincers of crustaceans, through
slight, serviceable modifications in the ultimate and penultimate segments
of a limb, which was at first used solely for locomotion. In the
avicularia and vibracula of the Polyzoa we have organs widely different in
appearance developed from the same source; and with the vibracula we can
understand how the successive gradations might have been of service. With
the pollinia of orchids, the threads which originally served to tie
together the pollen-grains, can be traced cohering into caudicles; and the
steps can likewise be followed by which viscid matter, such as that
secreted by the stigmas of ordinary flowers, and still subserving nearly
but not quite the same purpose, became attached to the free ends of the
caudicles--all these gradations being of manifest benefit to the plants in
question. With respect to climbing plants, I need not repeat what has been
so lately said.
It has often been asked, if natural selection be so potent, why has not
this or that structure been gained by certain species, to which it would
apparently have been advantageous? But it is unreasonable to expect a
precise answer to such questions, considering our ignorance of the past
history of each species, and of the conditions which at the present day
determine its numbers and range. In most cases only general reasons, but
in some few cases special reasons, can be assigned. Thus to adapt a
species to new habits of life, many co-ordinated modifications are almost
indispensable, and it may often have happened that the requisite parts did
not vary in the right manner or to the right degree. Many species must
have been prevented from increasing in numbers through destructive
agencies, which stood in no relation to certain structures, which we
imagine would have been gained through natural selection from appearing to
us advantageous to the species. In this case, as the struggle for life did
not depend on such structures, they could not have been acquired through
natural selection. In many cases complex and long-enduring conditions,
often of a peculiar nature, are necessary for the development of a
structure; and the requisite conditions may seldom have concurred. The
belief that any given structure, which we think, often erroneously, would
have been beneficial to a species, would have been gained under all
circumstances through natural selection, is opposed to what we can
understand of its manner of action. Mr. Mivart does not deny that natural
selection has effected something; but he considers it as "demonstrably
insufficient" to account for the phenomena which I explain by its agency.
His chief arguments have now been considered, and the others will hereafter
be considered. They seem to me to partake little of the character of
demonstration, and to have little weight in comparison with those in favour
of the power of natural selection, aided by the other agencies often
specified. I am bound to add, that some of the facts and arguments here
used by me, have been advanced for the same purpose in an able article
lately published in the "Medico-Chirurgical Review."
At the present day almost all naturalists admit evolution under some form.
Mr. Mivart believes that species change through "an internal force or
tendency," about which it is not pretended that anything is known. That
species have a capacity for change will be admitted by all evolutionists;
but there is no need, as it seems to me, to invoke any internal force
beyond the tendency to ordinary variability, which through the aid of
selection, by man has given rise to many well-adapted domestic races, and
which, through the aid of natural selection, would equally well give rise
by graduated steps to natural races or species. The final result will
generally have been, as already explained, an advance, but in some few
cases a retrogression, in organisation.
Mr. Mivart is further inclined to believe, and some naturalists agree with
him, that new species manifest themselves "with suddenness and by
modifications appearing at once." For instance, he supposes that the
differences between the extinct three-toed Hipparion and the horse arose
suddenly. He thinks it difficult to believe that the wing of a bird "was
developed in any other way than by a comparatively sudden modification of a
marked and important kind;" and apparently he would extend the same view to
the wings of bats and pterodactyles. This conclusion, which implies great
breaks or discontinuity in the series, appears to me improbable in the
highest degree.
Everyone who believes in slow and gradual evolution, will of course admit
that specific changes may have been as abrupt and as great as any single
variation which we meet with under nature, or even under domestication.
But as species are more variable when domesticated or cultivated than under
their natural conditions, it is not probable that such great and abrupt
variations have often occurred under nature, as are known occasionally to
arise under domestication. Of these latter variations several may be
attributed to reversion; and the characters which thus reappear were, it is
probable, in many cases at first gained in a gradual manner. A still
greater number must be called monstrosities, such as six-fingered men,
porcupine men, Ancon sheep, Niata cattle, etc.; and as they are widely
different in character from natural species, they throw very little light
on our subject. Excluding such cases of abrupt variations, the few which
remain would at best constitute, if found in a state of nature, doubtful
species, closely related to their parental types.
My reasons for doubting whether natural species have changed as abruptly as
have occasionally domestic races, and for entirely disbelieving that they
have changed in the wonderful manner indicated by Mr. Mivart, are as
follows. According to our experience, abrupt and strongly marked
variations occur in our domesticated productions, singly and at rather long
intervals of time. If such occurred under nature, they would be liable, as
formerly explained, to be lost by accidental causes of destruction and by
subsequent intercrossing; and so it is known to be under domestication,
unless abrupt variations of this kind are specially preserved and separated
by the care of man. Hence, in order that a new species should suddenly
appear in the manner supposed by Mr. Mivart, it is almost necessary to
believe, in opposition to all analogy, that several wonderfully changed
individuals appeared simultaneously within the same district. This
difficulty, as in the case of unconscious selection by man, is avoided on
the theory of gradual evolution, through the preservation of a large number
of individuals, which varied more or less in any favourable direction, and
of the destruction of a large number which varied in an opposite manner.
That many species have been evolved in an extremely gradual manner, there
can hardly be a doubt. The species and even the genera of many large
natural families are so closely allied together that it is difficult to
distinguish not a few of them. On every continent, in proceeding from
north to south, from lowland to upland, etc., we meet with a host of
closely related or representative species; as we likewise do on certain
distinct continents, which we have reason to believe were formerly
connected. But in making these and the following remarks, I am compelled
to allude to subjects hereafter to be discussed. Look at the many outlying
islands round a continent, and see how many of their inhabitants can be
raised only to the rank of doubtful species. So it is if we look to past
times, and compare the species which have just passed away with those still
living within the same areas; or if we compare the fossil species embedded
in the sub-stages of the same geological formation. It is indeed manifest
that multitudes of species are related in the closest manner to other
species that still exist, or have lately existed; and it will hardly be
maintained that such species have been developed in an abrupt or sudden
manner. Nor should it be forgotten, when we look to the special parts of
allied species, instead of to distinct species, that numerous and
wonderfully fine gradations can be traced, connecting together widely
different structures.
Many large groups of facts are intelligible only on the principle that
species have been evolved by very small steps. For instance, the fact that
the species included in the larger genera are more closely related to each
other, and present a greater number of varieties than do the species in the
smaller genera. The former are also grouped in little clusters, like
varieties round species; and they present other analogies with varieties,
as was shown in our second chapter. On this same principle we can
understand how it is that specific characters are more variable than
generic characters; and how the parts which are developed in an
extraordinary degree or manner are more variable than other parts of the
same species. Many analogous facts, all pointing in the same direction,
could be added.
Although very many species have almost certainly been produced by steps not
greater than those separating fine varieties; yet it may be maintained that
some have been developed in a different and abrupt manner. Such an
admission, however, ought not to be made without strong evidence being
assigned. The vague and in some respects false analogies, as they have
been shown to be by Mr. Chauncey Wright, which have been advanced in favour
of this view, such as the sudden crystallisation of inorganic substances,
or the falling of a facetted spheroid from one facet to another, hardly
deserve consideration. One class of facts, however, namely, the sudden
appearance of new and distinct forms of life in our geological formations
supports at first sight the belief in abrupt development. But the value of
this evidence depends entirely on the perfection of the geological record,
in relation to periods remote in the history of the world. If the record
is as fragmentary as many geologists strenuously assert, there is nothing
strange in new forms appearing as if suddenly developed.
Unless we admit transformations as prodigious as those advocated by Mr.
Mivart, such as the sudden development of the wings of birds or bats, or
the sudden conversion of a Hipparion into a horse, hardly any light is
thrown by the belief in abrupt modifications on the deficiency of
connecting links in our geological formations. But against the belief in
such abrupt changes, embryology enters a strong protest. It is notorious
that the wings of birds and bats, and the legs of horses or other
quadrupeds, are undistinguishable at an early embryonic period, and that
they become differentiated by insensibly fine steps. Embryological
resemblances of all kinds can be accounted for, as we shall hereafter see,
by the progenitors of our existing species having varied after early youth,
and having transmitted their newly-acquired characters to their offspring,
at a corresponding age. The embryo is thus left almost unaffected, and
serves as a record of the past condition of the species. Hence it is that
existing species during the early stages of their development so often
resemble ancient and extinct forms belonging to the same class. On this
view of the meaning of embryological resemblances, and indeed on any view,
it is incredible that an animal should have undergone such momentous and
abrupt transformations as those above indicated, and yet should not bear
even a trace in its embryonic condition of any sudden modification, every
detail in its structure being developed by insensibly fine steps.
He who believes that some ancient form was transformed suddenly through an
internal force or tendency into, for instance, one furnished with wings,
will be almost compelled to assume, in opposition to all analogy, that many
individuals varied simultaneously. It cannot be denied that such abrupt
and great changes of structure are widely different from those which most
species apparently have undergone. He will further be compelled to believe
that many structures beautifully adapted to all the other parts of the same
creature and to the surrounding conditions, have been suddenly produced;
and of such complex and wonderful co-adaptations, he will not be able to
assign a shadow of an explanation. He will be forced to admit that these
great and sudden transformations have left no trace of their action on the
embryo. To admit all this is, as it seems to me, to enter into the realms
of miracle, and to leave those of science.
CHAPTER VIII.
INSTINCT.
Instincts comparable with habits, but different in their origin --
Instincts graduated -- Aphides and ants -- Instincts variable -- Domestic
instincts, their origin -- Natural instincts of the cuckoo, molothrus,
ostrich, and parasitic bees -- Slave-making ants -- Hive-bee, its
cell-making instinct -- Changes of instinct and structure not necessarily
simultaneous -- Difficulties of the theory of the Natural Selection of
instincts -- Neuter or sterile insects -- Summary.
Many instincts are so wonderful that their development will probably appear
to the reader a difficulty sufficient to overthrow my whole theory. I may
here premise, that I have nothing to do with the origin of the mental
powers, any more than I have with that of life itself. We are concerned
only with the diversities of instinct and of the other mental faculties in
animals of the same class.
I will not attempt any definition of instinct. It would be easy to show
that several distinct mental actions are commonly embraced by this term;
but every one understands what is meant, when it is said that instinct
impels the cuckoo to migrate and to lay her eggs in other birds' nests. An
action, which we ourselves require experience to enable us to perform, when
performed by an animal, more especially by a very young one, without
experience, and when performed by many individuals in the same way, without
their knowing for what purpose it is performed, is usually said to be
instinctive. But I could show that none of these characters are universal.
A little dose of judgment or reason, as Pierre Huber expresses it, often
comes into play, even with animals low in the scale of nature.
Frederick Cuvier and several of the older metaphysicians have compared
instinct with habit. This comparison gives, I think, an accurate notion of
the frame of mind under which an instinctive action is performed, but not
necessarily of its origin. How unconsciously many habitual actions are
performed, indeed not rarely in direct opposition to our conscious will!
yet they may be modified by the will or reason. Habits easily become
associated with other habits, with certain periods of time and states of
the body. When once acquired, they often remain constant throughout life.
Several other points of resemblance between instincts and habits could be
pointed out. As in repeating a well-known song, so in instincts, one
action follows another by a sort of rhythm; if a person be interrupted in a
song, or in repeating anything by rote, he is generally forced to go back
to recover the habitual train of thought: so P. Huber found it was with a
caterpillar, which makes a very complicated hammock; for if he took a
caterpillar which had completed its hammock up to, say, the sixth stage of
construction, and put it into a hammock completed up only to the third
stage, the caterpillar simply re-performed the fourth, fifth, and sixth
stages of construction. If, however, a caterpillar were taken out of a
hammock made up, for instance, to the third stage, and were put into one
finished up to the sixth stage, so that much of its work was already done
for it, far from deriving any benefit from this, it was much embarrassed,
and, in order to complete its hammock, seemed forced to start from the
third stage, where it had left off, and thus tried to complete the already
finished work.
If we suppose any habitual action to become inherited--and it can be shown
that this does sometimes happen--then the resemblance between what
originally was a habit and an instinct becomes so close as not to be
distinguished. If Mozart, instead of playing the pianoforte at three years
old with wonderfully little practice, had played a tune with no practice at
all, be might truly be said to have done so instinctively. But it would be
a serious error to suppose that the greater number of instincts have been
acquired by habit in one generation, and then transmitted by inheritance to
succeeding generations. It can be clearly shown that the most wonderful
instincts with which we are acquainted, namely, those of the hive-bee and
of many ants, could not possibly have been acquired by habit.
It will be universally admitted that instincts are as important as
corporeal structures for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a
species; and if it can be shown that instincts do vary ever so little, then
I can see no difficulty in natural selection preserving and continually
accumulating variations of instinct to any extent that was profitable. It
is thus, as I believe, that all the most complex and wonderful instincts
have originated. As modifications of corporeal structure arise from, and
are increased by, use or habit, and are diminished or lost by disuse, so I
do not doubt it has been with instincts. But I believe that the effects of
habit are in many cases of subordinate importance to the effects of the
natural selection of what may be called spontaneous variations of
instincts;--that is of variations produced by the same unknown causes which
produce slight deviations of bodily structure.
No complex instinct can possibly be produced through natural selection,
except by the slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures, we
ought to find in nature, not the actual transitional gradations by which
each complex instinct has been acquired--for these could be found only in
the lineal ancestors of each species--but we ought to find in the
collateral lines of descent some evidence of such gradations; or we ought
at least to be able to show that gradations of some kind are possible; and
this we certainly can do. I have been surprised to find, making allowance
for the instincts of animals having been but little observed, except in
Europe and North America, and for no instinct being known among extinct
species, how very generally gradations, leading to the most complex
instincts, can be discovered. Changes of instinct may sometimes be
facilitated by the same species having different instincts at different
periods of life, or at different seasons of the year, or when placed under
different circumstances, etc.; in which case either the one or the other
instinct might be preserved by natural selection. And such instances of
diversity of instinct in the same species can be shown to occur in nature.
Again, as in the case of corporeal structure, and conformably to my theory,
the instinct of each species is good for itself, but has never, as far as
we can judge, been produced for the exclusive good of others. One of the
strongest instances of an animal apparently performing an action for the
sole good of another, with which I am acquainted, is that of aphides
voluntarily yielding, as was first observed by Huber, their sweet excretion
to ants: that they do so voluntarily, the following facts show. I removed
all the ants from a group of about a dozen aphides on a dock-plant, and
prevented their attendance during several hours. After this interval, I
felt sure that the aphides would want to excrete. I watched them for some
time through a lens, but not one excreted; I then tickled and stroked them
with a hair in the same manner, as well as I could, as the ants do with
their antennae; but not one excreted. Afterwards, I allowed an ant to
visit them, and it immediately seemed, by its eager way of running about to
be well aware what a rich flock it had discovered; it then began to play
with its antennae on the abdomen first of one aphis and then of another;
and each, as soon as it felt the antennae, immediately lifted up its
abdomen and excreted a limpid drop of sweet juice, which was eagerly
devoured by the ant. Even the quite young aphides behaved in this manner,
showing that the action was instinctive, and not the result of experience.
It is certain, from the observations of Huber, that the aphides show no
dislike to the ants: if the latter be not present they are at last
compelled to eject their excretion. But as the excretion is extremely
viscid, it is no doubt a convenience to the aphides to have it removed;
therefore probably they do not excrete solely for the good of the ants.
Although there is no evidence that any animal performs an action for the
exclusive good of another species, yet each tries to take advantage of the
instincts of others, as each takes advantage of the weaker bodily structure
of other species. So again certain instincts cannot be considered as
absolutely perfect; but as details on this and other such points are not
indispensable, they may be here passed over.
As some degree of variation in instincts under a state of nature, and the
inheritance of such variations, are indispensable for the action of natural
selection, as many instances as possible ought to be given; but want of
space prevents me. I can only assert that instincts certainly do vary--for
instance, the migratory instinct, both in extent and direction, and in its
total loss. So it is with the nests of birds, which vary partly in
dependence on the situations chosen, and on the nature and temperature of
the country inhabited, but often from causes wholly unknown to us. Audubon
has given several remarkable cases of differences in the nests of the same
species in the northern and southern United States. Why, it has been
asked, if instinct be variable, has it not granted to the bee "the ability
to use some other material when wax was deficient?" But what other natural
material could bees use? They will work, as I have seen, with wax hardened
with vermilion or softened with lard. Andrew Knight observed that his
bees, instead of laboriously collecting propolis, used a cement of wax and
turpentine, with which he had covered decorticated trees. It has lately
been shown that bees, instead of searching for pollen, will gladly use a
very different substance, namely, oatmeal. Fear of any particular enemy is
certainly an instinctive quality, as may be seen in nestling birds, though
it is strengthened by experience, and by the sight of fear of the same
enemy in other animals. The fear of man is slowly acquired, as I have
elsewhere shown, by the various animals which inhabit desert islands; and
we see an instance of this, even in England, in the greater wildness of all
our large birds in comparison with our small birds; for the large birds
have been most persecuted by man. We may safely attribute the greater
wildness of our large birds to this cause; for in uninhabited islands large
birds are not more fearful than small; and the magpie, so wary in England,
is tame in Norway, as is the hooded crow in Egypt.
That the mental qualities of animals of the same kind, born in a state of
nature, vary much, could be shown by many facts. Several cases could also
be adduced of occasional and strange habits in wild animals, which, if
advantageous to the species, might have given rise, through natural
selection, to new instincts. But I am well aware that these general
statements, without the facts in detail, can produce but a feeble effect on
the reader's mind. I can only repeat my assurance, that I do not speak
without good evidence.
INHERITED CHANGES OF HABIT OR INSTINCT IN DOMESTICATED ANIMALS.
The possibility, or even probability, of inherited variations of instinct
in a state of nature will be strengthened by briefly considering a few
cases under domestication. We shall thus be enabled to see the part which
habit and the selection of so-called spontaneous variations have played in
modifying the mental qualities of our domestic animals. It is notorious
how much domestic animals vary in their mental qualities. With cats, for
instance, one naturally takes to catching rats, and another mice, and these
tendencies are known to be inherited. One cat, according to Mr. St. John,
always brought home game birds, another hares or rabbits, and another
hunted on marshy ground and almost nightly caught woodcocks or snipes. A
number of curious and authentic instances could be given of various shades
of disposition and taste, and likewise of the oddest tricks, associated
with certain frames of mind or periods of time. But let us look to the
familiar case of the breeds of dogs: it cannot be doubted that young
pointers (I have myself seen striking instances) will sometimes point and
even back other dogs the very first time that they are taken out;
retrieving is certainly in some degree inherited by retrievers; and a
tendency to run round, instead of at, a flock of sheep, by shepherd-dogs.
I cannot see that these actions, performed without experience by the young,
and in nearly the same manner by each individual, performed with eager
delight by each breed, and without the end being known--for the young
pointer can no more know that he points to aid his master, than the white
butterfly knows why she lays her eggs on the leaf of the cabbage--I cannot
see that these actions differ essentially from true instincts. If we were
to behold one kind of wolf, when young and without any training, as soon as
it scented its prey, stand motionless like a statue, and then slowly crawl
forward with a peculiar gait; and another kind of wolf rushing round,
instead of at, a herd of deer, and driving them to a distant point, we
should assuredly call these actions instinctive. Domestic instincts, as
they may be called, are certainly far less fixed than natural instincts;
but they have been acted on by far less rigorous selection, and have been
transmitted for an incomparably shorter period, under less fixed conditions
of life.
How strongly these domestic instincts, habits, and dispositions are
inherited, and how curiously they become mingled, is well shown when
different breeds of dogs are crossed. Thus it is known that a cross with a
bull-dog has affected for many generations the courage and obstinacy of
greyhounds; and a cross with a greyhound has given to a whole family of
shepherd-dogs a tendency to hunt hares. These domestic instincts, when
thus tested by crossing, resemble natural instincts, which in a like manner
become curiously blended together, and for a long period exhibit traces of
the instincts of either parent: for example, Le Roy describes a dog, whose
great-grandfather was a wolf, and this dog showed a trace of its wild
parentage only in one way, by not coming in a straight line to his master,
when called.
Domestic instincts are sometimes spoken of as actions which have become
inherited solely from long-continued and compulsory habit, but this is not
true. No one would ever have thought of teaching, or probably could have
taught, the tumbler-pigeon to tumble--an action which, as I have witnessed,
is performed by young birds, that have never seen a pigeon tumble. We may
believe that some one pigeon showed a slight tendency to this strange
habit, and that the long-continued selection of the best individuals in
successive generations made tumblers what they now are; and near Glasgow
there are house-tumblers, as I hear from Mr. Brent, which cannot fly
eighteen inches high without going head over heels. It may be doubted
whether any one would have thought of training a dog to point, had not some
one dog naturally shown a tendency in this line; and this is known
occasionally to happen, as I once saw, in a pure terrier: the act of
pointing is probably, as many have thought, only the exaggerated pause of
an animal preparing to spring on its prey. When the first tendency to
point was once displayed, methodical selection and the inherited effects of
compulsory training in each successive generation would soon complete the
work; and unconscious selection is still in progress, as each man tries to
procure, without intending to improve the breed, dogs which stand and hunt
best. On the other hand, habit alone in some cases has sufficed; hardly
any animal is more difficult to tame than the young of the wild rabbit;
scarcely any animal is tamer than the young of the tame rabbit; but I can
hardly suppose that domestic rabbits have often been selected for tameness
alone; so that we must attribute at least the greater part of the inherited
change from extreme wildness to extreme tameness, to habit and
long-continued close confinement.
Natural instincts are lost under domestication: a remarkable instance of
this is seen in those breeds of fowls which very rarely or never become
"broody," that is, never wish to sit on their eggs. Familiarity alone
prevents our seeing how largely and how permanently the minds of our
domestic animals have been modified. It is scarcely possible to doubt that
the love of man has become instinctive in the dog. All wolves, foxes,
jackals and species of the cat genus, when kept tame, are most eager to
attack poultry, sheep and pigs; and this tendency has been found incurable
in dogs which have been brought home as puppies from countries such as
Tierra del Fuego and Australia, where the savages do not keep these
domestic animals. How rarely, on the other hand, do our civilised dogs,
even when quite young, require to be taught not to attack poultry, sheep,
and pigs! No doubt they occasionally do make an attack, and are then
beaten; and if not cured, they are destroyed; so that habit and some degree
of selection have probably concurred in civilising by inheritance our dogs.
On the other hand, young chickens have lost wholly by habit, that fear of
the dog and cat which no doubt was originally instinctive in them, for I am
informed by Captain Hutton that the young chickens of the parent stock, the
Gallus bankiva, when reared in India under a hen, are at first excessively
wild. So it is with young pheasants reared in England under a hen. It is
not that chickens have lost all fear, but fear only of dogs and cats, for
if the hen gives the danger chuckle they will run (more especially young
turkeys) from under her and conceal themselves in the surrounding grass or
thickets; and this is evidently done for the instinctive purpose of
allowing, as we see in wild ground-birds, their mother to fly away. But
this instinct retained by our chickens has become useless under
domestication, for the mother-hen has almost lost by disuse the power of
flight.
Hence, we may conclude that under domestication instincts have been
acquired and natural instincts have been lost, partly by habit and partly
by man selecting and accumulating, during successive generations, peculiar
mental habits and actions, which at first appeared from what we must in our
ignorance call an accident. In some cases compulsory habit alone has
sufficed to produce inherited mental changes; in other cases compulsory
habit has done nothing, and all has been the result of selection, pursued
both methodically and unconsciously; but in most cases habit and selection
have probably concurred.
SPECIAL INSTINCTS.
We shall, perhaps, best understand how instincts in a state of nature have
become modified by selection by considering a few cases. I will select
only three, namely, the instinct which leads the cuckoo to lay her eggs in
other birds' nests; the slave-making instinct of certain ants; and the
cell-making power of the hive-bee: these two latter instincts have
generally and justly been ranked by naturalists as the most wonderful of
all known instincts.
INSTINCTS OF THE CUCKOO.
It is supposed by some naturalists that the more immediate cause of the
instinct of the cuckoo is that she lays her eggs, not daily, but at
intervals of two or three days; so that, if she were to make her own nest
and sit on her own eggs, those first laid would have to be left for some
time unincubated or there would be eggs and young birds of different ages
in the same nest. If this were the case the process of laying and hatching
might be inconveniently long, more especially as she migrates at a very
early period; and the first hatched young would probably have to be fed by
the male alone. But the American cuckoo is in this predicament, for she
makes her own nest and has eggs and young successively hatched, all at the
same time. It has been both asserted and denied that the American cuckoo
occasionally lays her eggs in other birds' nests; but I have lately heard
from Dr. Merrill, of Iowa, that he once found in Illinois a young cuckoo,
together with a young jay in the nest of a blue jay (Garrulus cristatus);
and as both were nearly full feathered, there could be no mistake in their
identification. I could also give several instances of various birds which
have been known occasionally to lay their eggs in other birds' nests. Now
let us suppose that the ancient progenitor of our European cuckoo had the
habits of the American cuckoo, and that she occasionally laid an egg in
another bird's nest. If the old bird profited by this occasional habit
through being enabled to emigrate earlier or through any other cause; or if
the young were made more vigorous by advantage being taken of the mistaken
instinct of another species than when reared by their own mother,
encumbered as she could hardly fail to be by having eggs and young of
different ages at the same time, then the old birds or the fostered young
would gain an advantage. And analogy would lead us to believe that the
young thus reared would be apt to follow by inheritance the occasional and
aberrant habit of their mother, and in their turn would be apt to lay their
eggs in other birds' nests, and thus be more successful in rearing their
young. By a continued process of this nature, I believe that the strange
instinct of our cuckoo has been generated. It has, also recently been
ascertained on sufficient evidence, by Adolf Muller, that the cuckoo
occasionally lays her eggs on the bare ground, sits on them and feeds her
young. This rare event is probably a case of reversion to the long-lost,
aboriginal instinct of nidification.
It has been objected that I have not noticed other related instincts and
adaptations of structure in the cuckoo, which are spoken of as necessarily
co-ordinated. But in all cases, speculation on an instinct known to us
only in a single species, is useless, for we have hitherto had no facts to
guide us. Until recently the instincts of the European and of the non-
parasitic American cuckoo alone were known; now, owing to Mr. Ramsay's
observations, we have learned something about three Australian species,
which lay their eggs in other birds' nests. The chief points to be
referred to are three: first, that the common cuckoo, with rare
exceptions, lays only one egg in a nest, so that the large and voracious
young bird receives ample food. Secondly, that the eggs are remarkably
small, not exceeding those of the skylark--a bird about one-fourth as large
as the cuckoo. That the small size of the egg is a real case of adaptation
we may infer from the fact of the mon-parasitic American cuckoo laying
full-sized eggs. Thirdly, that the young cuckoo, soon after birth, has the
instinct, the strength and a properly shaped back for ejecting its foster-
brothers, which then perish from cold and hunger. This has been boldly
called a beneficent arrangement, in order that the young cuckoo may get
sufficient food, and that its foster-brothers may perish before they had
acquired much feeling!
Turning now to the Australian species: though these birds generally lay
only one egg in a nest, it is not rare to find two and even three eggs in
the same nest. In the bronze cuckoo the eggs vary greatly in size, from
eight to ten lines in length. Now, if it had been of an advantage to this
species to have laid eggs even smaller than those now laid, so as to have
deceived certain foster-parents, or, as is more probable, to have been
hatched within a shorter period (for it is asserted that there is a
relation between the size of eggs and the period of their incubation), then
there is no difficulty in believing that a race or species might have been
formed which would have laid smaller and smaller eggs; for these would have
been more safely hatched and reared. Mr. Ramsay remarks that two of the
Australian cuckoos, when they lay their eggs in an open nest, manifest a
decided preference for nests containing eggs similar in colour to their
own. The European species apparently manifests some tendency towards a
similar instinct, but not rarely departs from it, as is shown by her laying
her dull and pale-coloured eggs in the nest of the hedge-warbler with
bright greenish-blue eggs. Had our cuckoo invariably displayed the above
instinct, it would assuredly have been added to those which it is assumed
must all have been acquired together. The eggs of the Australian bronze
cuckoo vary, according to Mr. Ramsay, to an extraordinary degree in colour;
so that in this respect, as well as in size, natural selection might have
secured and fixed any advantageous variation.
In the case of the European cuckoo, the offspring of the foster-parents are
commonly ejected from the nest within three days after the cuckoo is
hatched; and as the latter at this age is in a most helpless condition, Mr.
Gould was formerly inclined to believe that the act of ejection was
performed by the foster-parents themselves. But he has now received a
trustworthy account of a young cuckoo which was actually seen, while still
blind and not able even to hold up its own head, in the act of ejecting its
foster-brothers. One of these was replaced in the nest by the observer,
and was again thrown out. With respect to the means by which this strange
and odious instinct was acquired, if it were of great importance for the
young cuckoo, as is probably the case, to receive as much food as possible
soon after birth, I can see no special difficulty in its having gradually
acquired, during successive generations, the blind desire, the strength,
and structure necessary for the work of ejection; for those cuckoos which
had such habits and structure best developed would be the most securely
reared. The first step towards the acquisition of the proper instinct
might have been mere unintentional restlessness on the part of the young
bird, when somewhat advanced in age and strength; the habit having been
afterwards improved, and transmitted to an earlier age. I can see no more
difficulty in this than in the unhatched young of other birds acquiring the
instinct to break through their own shells; or than in young snakes
acquiring in their upper jaws, as Owen has remarked, a transitory sharp
tooth for cutting through the tough egg-shell. For if each part is liable
to individual variations at all ages, and the variations tend to be
inherited at a corresponding or earlier age--propositions which cannot be
disputed--then the instincts and structure of the young could be slowly
modified as surely as those of the adult; and both cases must stand or fall
together with the whole theory of natural selection.
Some species of Molothrus, a widely distinct genus of American birds,
allied to our starlings, have parasitic habits like those of the cuckoo;
and the species present an interesting gradation in the perfection of their
instincts. The sexes of Molothrus badius are stated by an excellent
observer, Mr. Hudson, sometimes to live promiscuously together in flocks,
and sometimes to pair. They either build a nest of their own or seize on
one belonging to some other bird, occasionally throwing out the nestlings
of the stranger. They either lay their eggs in the nest thus appropriated,
or oddly enough build one for themselves on the top of it. They usually
sit on their own eggs and rear their own young; but Mr. Hudson says it is
probable that they are occasionally parasitic, for he has seen the young of
this species following old birds of a distinct kind and clamouring to be
fed by them. The parasitic habits of another species of Molothrus, the M.
bonariensis, are much more highly developed than those of the last, but are
still far from perfect. This bird, as far as it is known, invariably lays
its eggs in the nests of strangers; but it is remarkable that several
together sometimes commence to build an irregular untidy nest of their own,
placed in singular ill-adapted situations, as on the leaves of a large
thistle. They never, however, as far as Mr. Hudson has ascertained,
complete a nest for themselves. They often lay so many eggs--from fifteen
to twenty--in the same foster-nest, that few or none can possibly be
hatched. They have, moreover, the extraordinary habit of pecking holes in
the eggs, whether of their own species or of their foster parents, which
they find in the appropriated nests. They drop also many eggs on the bare
ground, which are thus wasted. A third species, the M. pecoris of North
America, has acquired instincts as perfect as those of the cuckoo, for it
never lays more than one egg in a foster-nest, so that the young bird is
securely reared. Mr. Hudson is a strong disbeliever in evolution, but he
appears to have been so much struck by the imperfect instincts of the
Molothrus bonariensis that he quotes my words, and asks, "Must we consider
these habits, not as especially endowed or created instincts, but as small
consequences of one general law, namely, transition?"
Various birds, as has already been remarked, occasionally lay their eggs in
the nests of other birds. This habit is not very uncommon with the
Gallinaceae, and throws some light on the singular instinct of the ostrich.
In this family several hen birds unite and lay first a few eggs in one nest
and then in another; and these are hatched by the males. This instinct may
probably be accounted for by the fact of the hens laying a large number of
eggs, but, as with the cuckoo, at intervals of two or three days. The
instinct, however, of the American ostrich, as in the case of the Molothrus
bonariensis, has not as yet been perfected; for a surprising number of eggs
lie strewed over the plains, so that in one day's hunting I picked up no
less than twenty lost and wasted eggs.
Many bees are parasitic, and regularly lay their eggs in the nests of other
kinds of bees. This case is more remarkable than that of the cuckoo; for
these bees have not only had their instincts but their structure modified
in accordance with their parasitic habits; for they do not possess the
pollen-collecting apparatus which would have been indispensable if they had
stored up food for their own young. Some species of Sphegidae (wasp-like
insects) are likewise parasitic; and M. Fabre has lately shown good reason
for believing that, although the Tachytes nigra generally makes its own
burrow and stores it with paralysed prey for its own larvae, yet that, when
this insect finds a burrow already made and stored by another sphex, it
takes advantage of the prize, and becomes for the occasion parasitic. In
this case, as with that of the Molothrus or cuckoo, I can see no difficulty
in natural selection making an occasional habit permanent, if of advantage
to the species, and if the insect whose nest and stored food are
feloniously appropriated, be not thus exterminated.
SLAVE-MAKING INSTINCT.
This remarkable instinct was first discovered in the Formica (Polyerges)
rufescens by Pierre Huber, a better observer even than his celebrated
father. This ant is absolutely dependent on its slaves; without their aid,
the species would certainly become extinct in a single year. The males and
fertile females do no work of any kind, and the workers or sterile females,
though most energetic and courageous in capturing slaves, do no other work.
They are incapable of making their own nests, or of feeding their own
larvae. When the old nest is found inconvenient, and they have to migrate,
it is the slaves which determine the migration, and actually carry their
masters in their jaws. So utterly helpless are the masters, that when
Huber shut up thirty of them without a slave, but with plenty of the food
which they like best, and with their larvae and pupae to stimulate them to
work, they did nothing; they could not even feed themselves, and many
perished of hunger. Huber then introduced a single slave (F. fusca), and
she instantly set to work, fed and saved the survivors; made some cells and
tended the larvae, and put all to rights. What can be more extraordinary
than these well-ascertained facts? If we had not known of any other
slave-making ant, it would have been hopeless to speculate how so wonderful
an instinct could have been perfected.
Another species, Formica sanguinea, was likewise first discovered by P.
Huber to be a slave-making ant. This species is found in the southern
parts of England, and its habits have been attended to by Mr. F. Smith, of
the British Museum, to whom I am much indebted for information on this and
other subjects. Although fully trusting to the statements of Huber and Mr.
Smith, I tried to approach the subject in a sceptical frame of mind, as any
one may well be excused for doubting the existence of so extraordinary an
instinct as that of making slaves. Hence, I will give the observations
which I made in some little detail. I opened fourteen nests of F.
sanguinea, and found a few slaves in all. Males and fertile females of the
slave-species (F. fusca) are found only in their own proper communities,
and have never been observed in the nests of F. sanguinea. The slaves are
black and not above half the size of their red masters, so that the
contrast in their appearance is great. When the nest is slightly
disturbed, the slaves occasionally come out, and like their masters are
much agitated and defend the nest: when the nest is much disturbed, and
the larvae and pupae are exposed, the slaves work energetically together
with their masters in carrying them away to a place of safety. Hence, it
is clear that the slaves feel quite at home. During the months of June and
July, on three successive years, I watched for many hours several nests in
Surrey and Sussex, and never saw a slave either leave or enter a nest. As,
during these months, the slaves are very few in number, I thought that they
might behave differently when more numerous; but Mr. Smith informs me that
he has watched the nests at various hours during May, June and August, both
in Surrey and Hampshire, and has never seen the slaves, though present in
large numbers in August, either leave or enter the nest. Hence, he
considers them as strictly household slaves. The masters, on the other
hand, may be constantly seen bringing in materials for the nest, and food
of all kinds. During the year 1860, however, in the month of July, I came
across a community with an unusually large stock of slaves, and I observed
a few slaves mingled with their masters leaving the nest, and marching
along the same road to a tall Scotch-fir tree, twenty-five yards distant,
which they ascended together, probably in search of aphides or cocci.
According to Huber, who had ample opportunities for observation, the slaves
in Switzerland habitually work with their masters in making the nest, and
they alone open and close the doors in the morning and evening; and, as
Huber expressly states, their principal office is to search for aphides.
This difference in the usual habits of the masters and slaves in the two
countries, probably depends merely on the slaves being captured in greater
numbers in Switzerland than in England.
One day I fortunately witnessed a migration of F. sanguinea from one nest
to another, and it was a most interesting spectacle to behold the masters
carefully carrying their slaves in their jaws instead of being carried by
them, as in the case of F. rufescens. Another day my attention was struck
by about a score of the slave-makers haunting the same spot, and evidently
not in search of food; they approached and were vigorously repulsed by an
independent community of the slave species (F. fusca); sometimes as many as
three of these ants clinging to the legs of the slave-making F. sanguinea.
The latter ruthlessly killed their small opponents and carried their dead
bodies as food to their nest, twenty-nine yards distant; but they were
prevented from getting any pupae to rear as slaves. I then dug up a small
parcel of the pupae of F. fusca from another nest, and put them down on a
bare spot near the place of combat; they were eagerly seized and carried
off by the tyrants, who perhaps fancied that, after all, they had been
victorious in their late combat.
At the same time I laid on the same place a small parcel of the pupae of
another species, F. flava, with a few of these little yellow ants still
clinging to the fragments of their nest. This species is sometimes, though
rarely, made into slaves, as has been described by Mr. Smith. Although so
small a species, it is very courageous, and I have seen it ferociously
attack other ants. In one instance I found to my surprise an independent
community of F. flava under a stone beneath a nest of the slave-making F.
sanguinea; and when I had accidentally disturbed both nests, the little
ants attacked their big neighbours with surprising courage. Now I was
curious to ascertain whether F. sanguinea could distinguish the pupae of F.
fusca, which they habitually make into slaves, from those of the little and
furious F. flava, which they rarely capture, and it was evident that they
did at once distinguish them; for we have seen that they eagerly and
instantly seized the pupae of F. fusca, whereas they were much terrified
when they came across the pupae, or even the earth from the nest, of F.
flava, and quickly ran away; but in about a quarter of an hour, shortly
after all the little yellow ants had crawled away, they took heart and
carried off the pupae.
One evening I visited another community of F. sanguinea, and found a number
of these ants returning home and entering their nests, carrying the dead
bodies of F. fusca (showing that it was not a migration) and numerous
pupae. I traced a long file of ants burdened with booty, for about forty
yards back, to a very thick clump of heath, whence I saw the last
individual of F. sanguinea emerge, carrying a pupa; but I was not able to
find the desolated nest in the thick heath. The nest, however, must have
been close at hand, for two or three individuals of F. fusca were rushing
about in the greatest agitation, and one was perched motionless with its
own pupa in its mouth on the top of a spray of heath, an image of despair
over its ravaged home.
Such are the facts, though they did not need confirmation by me, in regard
to the wonderful instinct of making slaves. Let it be observed what a
contrast the instinctive habits of F. sanguinea present with those of the
continental F. rufescens. The latter does not build its own nest, does not
determine its own migrations, does not collect food for itself or its
young, and cannot even feed itself: it is absolutely dependent on its
numerous slaves. Formica sanguinea, on the other hand, possesses much
fewer slaves, and in the early part of the summer extremely few. The
masters determine when and where a new nest shall be formed, and when they
migrate, the masters carry the slaves. Both in Switzerland and England the
slaves seem to have the exclusive care of the larvae, and the masters alone
go on slave-making expeditions. In Switzerland the slaves and masters work
together, making and bringing materials for the nest: both, but chiefly
the slaves, tend and milk as it may be called, their aphides; and thus both
collect food for the community. In England the masters alone usually leave
the nest to collect building materials and food for themselves, their
slaves and larvae. So that the masters in this country receive much less
service from their slaves than they do in Switzerland.
By what steps the instinct of F. sanguinea originated I will not pretend to
conjecture. But as ants which are not slave-makers, will, as I have seen,
carry off pupae of other species, if scattered near their nests, it is
possible that such pupae originally stored as food might become developed;
and the foreign ants thus unintentionally reared would then follow their
proper instincts, and do what work they could. If their presence proved
useful to the species which had seized them--if it were more advantageous
to this species, to capture workers than to procreate them--the habit of
collecting pupae, originally for food, might by natural selection be
strengthened and rendered permanent for the very different purpose of
raising slaves. When the instinct was once acquired, if carried out to a
much less extent even than in our British F. sanguinea, which, as we have
seen, is less aided by its slaves than the same species in Switzerland,
natural selection might increase and modify the instinct--always supposing
each modification to be of use to the species--until an ant was formed as
abjectly dependent on its slaves as is the Formica rufescens.
CELL-MAKING INSTINCT OF THE HIVE-BEE.
I will not here enter on minute details on this subject, but will merely
give an outline of the conclusions at which I have arrived. He must be a
dull man who can examine the exquisite structure of a comb, so beautifully
adapted to its end, without enthusiastic admiration. We hear from
mathematicians that bees have practically solved a recondite problem, and
have made their cells of the proper shape to hold the greatest possible
amount of honey, with the least possible consumption of precious wax in
their construction. It has been remarked that a skilful workman, with
fitting tools and measures, would find it very difficult to make cells of
wax of the true form, though this is effected by a crowd of bees working in
a dark hive. Granting whatever instincts you please, it seems at first
quite inconceivable how they can make all the necessary angles and planes,
or even perceive when they are correctly made. But the difficulty is not
nearly so great as at first appears: all this beautiful work can be shown,
I think, to follow from a few simple instincts.
I was led to investigate this subject by Mr. Waterhouse, who has shown that
the form of the cell stands in close relation to the presence of adjoining
cells; and the following view may, perhaps, be considered only as a
modification of his theory. Let us look to the great principle of
gradation, and see whether Nature does not reveal to us her method of work.
At one end of a short series we have humble-bees, which use their old
cocoons to hold honey, sometimes adding to them short tubes of wax, and
likewise making separate and very irregular rounded cells of wax. At the
other end of the series we have the cells of the hive-bee, placed in a
double layer: each cell, as is well known, is an hexagonal prism, with the
basal edges of its six sides bevelled so as to join an inverted pyramid, of
three rhombs. These rhombs have certain angles, and the three which form
the pyramidal base of a single cell on one side of the comb, enter into the
composition of the bases of three adjoining cells on the opposite side. In
the series between the extreme perfection of the cells of the hive-bee and
the simplicity of those of the humble-bee, we have the cells of the Mexican
Melipona domestica, carefully described and figured by Pierre Huber. The
Melipona itself is intermediate in structure between the hive and humble
bee, but more nearly related to the latter: it forms a nearly regular
waxen comb of cylindrical cells, in which the young are hatched, and, in
addition, some large cells of wax for holding honey. These latter cells
are nearly spherical and of nearly equal sizes, and are aggregated into an
irregular mass. But the important point to notice is, that these cells are
always made at that degree of nearness to each other that they would have
intersected or broken into each other if the spheres had been completed;
but this is never permitted, the bees building perfectly flat walls of wax
between the spheres which thus tend to intersect. Hence, each cell
consists of an outer spherical portion, and of two, three, or more flat
surfaces, according as the cell adjoins two, three or more other cells.
When one cell rests on three other cells, which, from the spheres being
nearly of the same size, is very frequently and necessarily the case, the
three flat surfaces are united into a pyramid; and this pyramid, as Huber
has remarked, is manifestly a gross imitation of the three-sided pyramidal
base of the cell of the hive-bee. As in the cells of the hive-bee, so
here, the three plane surfaces in any one cell necessarily enter into the
construction of three adjoining cells. It is obvious that the Melipona
saves wax, and what is more important, labour, by this manner of building;
for the flat walls between the adjoining cells are not double, but are of
the same thickness as the outer spherical portions, and yet each flat
portion forms a part of two cells.
Reflecting on this case, it occurred to me that if the Melipona had made
its spheres at some given distance from each other, and had made them of
equal sizes and had arranged them symmetrically in a double layer, the
resulting structure would have been as perfect as the comb of the hive-bee.
Accordingly I wrote to Professor Miller, of Cambridge, and this geometer
has kindly read over the following statement, drawn up from his
information, and tells me that it is strictly correct:-
If a number of equal spheres be described with their centres placed in two
parallel layers; with the centre of each sphere at the distance of radius x
sqrt(2) or radius x 1.41421 (or at some lesser distance), from the centres
of the six surrounding spheres in the same layer; and at the same distance
from the centres of the adjoining spheres in the other and parallel layer;
then, if planes of intersection between the several spheres in both layers
be formed, there will result a double layer of hexagonal prisms united
together by pyramidal bases formed of three rhombs; and the rhombs and the
sides of the hexagonal prisms will have every angle identically the same
with the best measurements which have been made of the cells of the
hive-bee. But I hear from Professor Wyman, who has made numerous careful
measurements, that the accuracy of the workmanship of the bee has been
greatly exaggerated; so much so, that whatever the typical form of the cell
may be, it is rarely, if ever, realised.
Hence we may safely conclude that, if we could slightly modify the
instincts already possessed by the Melipona, and in themselves not very
wonderful, this bee would make a structure as wonderfully perfect as that
of the hive-bee. We must suppose the Melipona to have the power of forming
her cells truly spherical, and of equal sizes; and this would not be very
surprising, seeing that she already does so to a certain extent, and seeing
what perfectly cylindrical burrows many insects make in wood, apparently by
turning round on a fixed point. We must suppose the Melipona to arrange
her cells in level layers, as she already does her cylindrical cells; and
we must further suppose, and this is the greatest difficulty, that she can
somehow judge accurately at what distance to stand from her
fellow-labourers when several are making their spheres; but she is already
so far enabled to judge of distance, that she always describes her spheres
so as to intersect to a certain extent; and then she unites the points of
intersection by perfectly flat surfaces. By such modifications of
instincts which in themselves are not very wonderful--hardly more wonderful
than those which guide a bird to make its nest--I believe that the hive-bee
has acquired, through natural selection, her inimitable architectural
powers.
But this theory can be tested by experiment. Following the example of Mr.
Tegetmeier, I separated two combs, and put between them a long, thick,
rectangular strip of wax: the bees instantly began to excavate minute
circular pits in it; and as they deepened these little pits, they made them
wider and wider until they were converted into shallow basins, appearing to
the eye perfectly true or parts of a sphere, and of about the diameter of a
cell. It was most interesting to observe that, wherever several bees had
begun to excavate these basins near together, they had begun their work at
such a distance from each other that by the time the basins had acquired
the above stated width (i.e. about the width of an ordinary cell), and were
in depth about one sixth of the diameter of the sphere of which they formed
a part, the rims of the basins intersected or broke into each other. As
soon as this occurred, the bees ceased to excavate, and began to build up
flat walls of wax on the lines of intersection between the basins, so that
each hexagonal prism was built upon the scalloped edge of a smooth basin,
instead of on the straight edges of a three-sided pyramid as in the case of
ordinary cells.
I then put into the hive, instead of a thick, rectangular piece of wax, a
thin and narrow, knife-edged ridge, coloured with vermilion. The bees
instantly began on both sides to excavate little basins near to each other,
in the same way as before; but the ridge of wax was so thin, that the
bottoms of the basins, if they had been excavated to the same depth as in
the former experiment, would have broken into each other from the opposite
sides. The bees, however, did not suffer this to happen, and they stopped
their excavations in due time; so that the basins, as soon as they had been
a little deepened, came to have flat bases; and these flat bases, formed by
thin little plates of the vermilion wax left ungnawed, were situated, as
far as the eye could judge, exactly along the planes of imaginary
intersection between the basins on the opposite side of the ridge of wax.
In some parts, only small portions, in other parts, large portions of a
rhombic plate were thus left between the opposed basins, but the work, from
the unnatural state of things, had not been neatly performed. The bees
must have worked at very nearly the same rate in circularly gnawing away
and deepening the basins on both sides of the ridge of vermilion wax, in
order to have thus succeeded in leaving flat plates between the basins, by
stopping work at the planes of intersection.
Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip of wax,
perceiving when they have gnawed the wax away to the proper thinness, and
then stopping their work. In ordinary combs it has appeared to me that the
bees do not always succeed in working at exactly the same rate from the
opposite sides; for I have noticed half-completed rhombs at the base of a
just-commenced cell, which were slightly concave on one side, where I
suppose that the bees had excavated too quickly, and convex on the opposed
side where the bees had worked less quickly. In one well-marked instance,
I put the comb back into the hive, and allowed the bees to go on working
for a short time, and again examined the cell, and I found that the rhombic
plate had been completed, and had become PERFECTLY FLAT: it was absolutely
impossible, from the extreme thinness of the little plate, that they could
have effected this by gnawing away the convex side; and I suspect that the
bees in such cases stand in the opposed cells and push and bend the ductile
and warm wax (which as I have tried is easily done) into its proper
intermediate plane, and thus flatten it.
>From the experiment of the ridge of vermilion wax we can see that, if the
bees were to build for themselves a thin wall of wax, they could make their
cells of the proper shape, by standing at the proper distance from each
other, by excavating at the same rate, and by endeavouring to make equal
spherical hollows, but never allowing the spheres to break into each other.
Now bees, as may be clearly seen by examining the edge of a growing comb,
do make a rough, circumferential wall or rim all round the comb; and they
gnaw this away from the opposite sides, always working circularly as they
deepen each cell. They do not make the whole three-sided pyramidal base of
any one cell at the same time, but only that one rhombic plate which stands
on the extreme growing margin, or the two plates, as the case may be; and
they never complete the upper edges of the rhombic plates, until the
hexagonal walls are commenced. Some of these statements differ from those
made by the justly celebrated elder Huber, but I am convinced of their
accuracy; and if I had space, I could show that they are conformable with
my theory.
Huber's statement, that the very first cell is excavated out of a little
parallel-sided wall of wax, is not, as far as I have seen, strictly
correct; the first commencement having always been a little hood of wax;
but I will not here enter on details. We see how important a part
excavation plays in the construction of the cells; but it would be a great
error to suppose that the bees cannot build up a rough wall of wax in the
proper position--that is, along the plane of intersection between two
adjoining spheres. I have several specimens showing clearly that they can
do this. Even in the rude circumferential rim or wall of wax round a
growing comb, flexures may sometimes be observed, corresponding in position
to the planes of the rhombic basal plates of future cells. But the rough
wall of wax has in every case to be finished off, by being largely gnawed
away on both sides. The manner in which the bees build is curious; they
always make the first rough wall from ten to twenty times thicker than the
excessively thin finished wall of the cell, which will ultimately be left.
We shall understand how they work, by supposing masons first to pile up a
broad ridge of cement, and then to begin cutting it away equally on both
sides near the ground, till a smooth, very thin wall is left in the middle;
the masons always piling up the cut-away cement, and adding fresh cement on
the summit of the ridge. We shall thus have a thin wall steadily growing
upward but always crowned by a gigantic coping. From all the cells, both
those just commenced and those completed, being thus crowned by a strong
coping of wax, the bees can cluster and crawl over the comb without
injuring the delicate hexagonal walls. These walls, as Professor Miller
has kindly ascertained for me, vary greatly in thickness; being, on an
average of twelve measurements made near the border of the comb, 1/352 of
an inch in thickness; whereas the basal rhomboidal plates are thicker,
nearly in the proportion of three to two, having a mean thickness, from
twenty-one measurements, of 1/229 of an inch. By the above singular manner
of building, strength is continually given to the comb, with the utmost
ultimate economy of wax.
It seems at first to add to the difficulty of understanding how the cells
are made, that a multitude of bees all work together; one bee after working
a short time at one cell going to another, so that, as Huber has stated, a
score of individuals work even at the commencement of the first cell. I
was able practically to show this fact, by covering the edges of the
hexagonal walls of a single cell, or the extreme margin of the
circumferential rim of a growing comb, with an extremely thin layer of
melted vermilion wax; and I invariably found that the colour was most
delicately diffused by the bees--as delicately as a painter could have done
it with his brush--by atoms of the coloured wax having been taken from the
spot on which it had been placed, and worked into the growing edges of the
cells all round. The work of construction seems to be a sort of balance
struck between many bees, all instinctively standing at the same relative
distance from each other, all trying to sweep equal spheres, and then
building up, or leaving ungnawed, the planes of intersection between these
spheres. It was really curious to note in cases of difficulty, as when two
pieces of comb met at an angle, how often the bees would pull down and
rebuild in different ways the same cell, sometimes recurring to a shape
which they had at first rejected.
When bees have a place on which they can stand in their proper positions
for working--for instance, on a slip of wood, placed directly under the
middle of a comb growing downwards, so that the comb has to be built over
one face of the slip--in this case the bees can lay the foundations of one
wall of a new hexagon, in its strictly proper place, projecting beyond the
other completed cells. It suffices that the bees should be enabled to
stand at their proper relative distances from each other and from the walls
of the last completed cells, and then, by striking imaginary spheres, they
can build up a wall intermediate between two adjoining spheres; but, as far
as I have seen, they never gnaw away and finish off the angles of a cell
till a large part both of that cell and of the adjoining cells has been
built. This capacity in bees of laying down under certain circumstances a
rough wall in its proper place between two just-commenced cells, is
important, as it bears on a fact, which seems at first subversive of the
foregoing theory; namely, that the cells on the extreme margin of
wasp-combs are sometimes strictly hexagonal; but I have not space here to
enter on this subject. Nor does there seem to me any great difficulty in a
single insect (as in the case of a queen-wasp) making hexagonal cells, if
she were to work alternately on the inside and outside of two or three
cells commenced at the same time, always standing at the proper relative
distance from the parts of the cells just begun, sweeping spheres or
cylinders, and building up intermediate planes.
As natural selection acts only by the accumulation of slight modifications
of structure or instinct, each profitable to the individual under its
conditions of life, it may reasonably be asked, how a long and graduated
succession of modified architectural instincts, all tending towards the
present perfect plan of construction, could have profited the progenitors
of the hive-bee? I think the answer is not difficult: cells constructed
like those of the bee or the wasp gain in strength, and save much in labour
and space, and in the materials of which they are constructed. With
respect to the formation of wax, it is known that bees are often hard
pressed to get sufficient nectar; and I am informed by Mr. Tegetmeier that
it has been experimentally proved that from twelve to fifteen pounds of dry
sugar are consumed by a hive of bees for the secretion of a pound of wax;
so that a prodigious quantity of fluid nectar must be collected and
consumed by the bees in a hive for the secretion of the wax necessary for
the construction of their combs. Moreover, many bees have to remain idle
for many days during the process of secretion. A large store of honey is
indispensable to support a large stock of bees during the winter; and the
security of the hive is known mainly to depend on a large number of bees
being supported. Hence the saving of wax by largely saving honey, and the
time consumed in collecting the honey, must be an important element of
success any family of bees. Of course the success of the species may be
dependent on the number of its enemies, or parasites, or on quite distinct
causes, and so be altogether independent of the quantity of honey which the
bees can collect. But let us suppose that this latter circumstance
determined, as it probably often has determined, whether a bee allied to
our humble-bees could exist in large numbers in any country; and let us
further suppose that the community lived through the winter, and
consequently required a store of honey: there can in this case be no doubt
that it would be an advantage to our imaginary humble-bee if a slight
modification of her instincts led her to make her waxen cells near
together, so as to intersect a little; for a wall in common even to two
adjoining cells would save some little labour and wax. Hence, it would
continually be more and more advantageous to our humble-bees, if they were
to make their cells more and more regular, nearer together, and aggregated
into a mass, like the cells of the Melipona; for in this case a large part
of the bounding surface of each cell would serve to bound the adjoining
cells, and much labour and wax would be saved. Again, from the same cause,
it would be advantageous to the Melipona, if she were to make her cells
closer together, and more regular in every way than at present; for then,
as we have seen, the spherical surfaces would wholly disappear and be
replaced by plane surfaces; and the Melipona would make a comb as perfect
as that of the hive-bee. Beyond this stage of perfection in architecture,
natural selection could not lead; for the comb of the hive-bee, as far as
we can see, is absolutely perfect in economising labour and wax.
Thus, as I believe, the most wonderful of all known instincts, that of the
hive-bee, can be explained by natural selection having taken advantage of
numerous, successive, slight modifications of simpler instincts; natural
selection having, by slow degrees, more and more perfectly led the bees to
sweep equal spheres at a given distance from each other in a double layer,
and to build up and excavate the wax along the planes of intersection. The
bees, of course, no more knowing that they swept their spheres at one
particular distance from each other, than they know what are the several
angles of the hexagonal prisms and of the basal rhombic plates; the motive
power of the process of natural selection having been the construction of
cells of due strength and of the proper size and shape for the larvae, this
being effected with the greatest possible economy of labour and wax; that
individual swarm which thus made the best cells with least labour, and
least waste of honey in the secretion of wax, having succeeded best, and
having transmitted their newly-acquired economical instincts to new swarms,
which in their turn will have had the best chance of succeeding in the
struggle for existence.
OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED TO INSTINCTS:
NEUTER AND STERILE INSECTS.
It has been objected to the foregoing view of the origin of instincts that
"the variations of structure and of instinct must have been simultaneous
and accurately adjusted to each other, as a modification in the one without
an immediate corresponding change in the other would have been fatal." The
force of this objection rests entirely on the assumption that the changes
in the instincts and structure are abrupt. To take as an illustration the
case of the larger titmouse, (Parus major) alluded to in a previous
chapter; this bird often holds the seeds of the yew between its feet on a
branch, and hammers with its beak till it gets at the kernel. Now what
special difficulty would there be in natural selection preserving all the
slight individual variations in the shape of the beak, which were better
and better adapted to break open the seeds, until a beak was formed, as
well constructed for this purpose as that of the nuthatch, at the same time
that habit, or compulsion, or spontaneous variations of taste, led the bird
to become more and more of a seed-eater? In this case the beak is supposed
to be slowly modified by natural selection, subsequently to, but in
accordance with, slowly changing habits or taste; but let the feet of the
titmouse vary and grow larger from correlation with the beak, or from any
other unknown cause, and it is not improbable that such larger feet would
lead the bird to climb more and more until it acquired the remarkable
climbing instinct and power of the nuthatch. In this case a gradual change
of structure is supposed to lead to changed instinctive habits. To take
one more case: few instincts are more remarkable than that which leads the
swift of the Eastern Islands to make its nest wholly of inspissated saliva.
Some birds build their nests of mud, believed to be moistened with saliva;
and one of the swifts of North America makes its nest (as I have seen) of
sticks agglutinated with saliva, and even with flakes of this substance.
Is it then very improbable that the natural selection of individual swifts,
which secreted more and more saliva, should at last produce a species with
instincts leading it to neglect other materials and to make its nest
exclusively of inspissated saliva? And so in other cases. It must,
however, be admitted that in many instances we cannot conjecture whether it
was instinct or structure which first varied.
No doubt many instincts of very difficult explanation could be opposed to
the theory of natural selection--cases, in which we cannot see how an
instinct could have originated; cases, in which no intermediate gradations
are known to exist; cases of instincts of such trifling importance, that
they could hardly have been acted on by natural selection; cases of
instincts almost identically the same in animals so remote in the scale of
nature that we cannot account for their similarity by inheritance from a
common progenitor, and consequently must believe that they were
independently acquired through natural selection. I will not here enter on
these several cases, but will confine myself to one special difficulty,
which at first appeared to me insuperable, and actually fatal to the whole
theory. I allude to the neuters or sterile females in insect communities:
for these neuters often differ widely in instinct and in structure from
both the males and fertile females, and yet, from being sterile, they
cannot propagate their kind.
The subject well deserves to be discussed at great length, but I will here
take only a single case, that of working or sterile ants. How the workers
have been rendered sterile is a difficulty; but not much greater than that
of any other striking modification of structure; for it can be shown that
some insects and other articulate animals in a state of nature occasionally
become sterile; and if such insects had been social, and it had been
profitable to the community that a number should have been annually born
capable of work, but incapable of procreation, I can see no especial
difficulty in this having been effected through natural selection. But I
must pass over this preliminary difficulty. The great difficulty lies in
the working ants differing widely from both the males and the fertile
females in structure, as in the shape of the thorax, and in being destitute
of wings and sometimes of eyes, and in instinct. As far as instinct alone
is concerned, the wonderful difference in this respect between the workers
and the perfect females would have been better exemplified by the hive-bee.
If a working ant or other neuter insect had been an ordinary animal, I
should have unhesitatingly assumed that all its characters had been slowly
acquired through natural selection; namely, by individuals having been born
with slight profitable modifications, which were inherited by the
offspring, and that these again varied and again were selected, and so
onwards. But with the working ant we have an insect differing greatly from
its parents, yet absolutely sterile; so that it could never have
transmitted successively acquired modifications of structure or instinct to
its progeny. It may well be asked how it is possible to reconcile this
case with the theory of natural selection?
First, let it be remembered that we have innumerable instances, both in our
domestic productions and in those in a state of nature, of all sorts of
differences of inherited structure which are correlated with certain ages
and with either sex. We have differences correlated not only with one sex,
but with that short period when the reproductive system is active, as in
the nuptial plumage of many birds, and in the hooked jaws of the male
salmon. We have even slight differences in the horns of different breeds
of cattle in relation to an artificially imperfect state of the male sex;
for oxen of certain breeds have longer horns than the oxen of other breeds,
relatively to the length of the horns in both the bulls and cows of these
same breeds. Hence, I can see no great difficulty in any character
becoming correlated with the sterile condition of certain members of insect
communities; the difficulty lies in understanding how such correlated
modifications of structure could have been slowly accumulated by natural
selection.
This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be applied to
the family, as well as to the individual, and may thus gain the desired
end. Breeders of cattle wish the flesh and fat to be well marbled
together. An animal thus characterized has been slaughtered, but the
breeder has gone with confidence to the same stock and has succeeded. Such
faith may be placed in the power of selection that a breed of cattle,
always yielding oxen with extraordinarily long horns, could, it is
probable, be formed by carefully watching which individual bulls and cows,
when matched, produced oxen with the longest horns; and yet no one ox would
ever have propagated its kind. Here is a better and real illustration:
According to M. Verlot, some varieties of the double annual stock, from
having been long and carefully selected to the right degree, always produce
a large proportion of seedlings bearing double and quite sterile flowers,
but they likewise yield some single and fertile plants. These latter, by
which alone the variety can be propagated, may be compared with the fertile
male and female ants, and the double sterile plants with the neuters of the
same community. As with the varieties of the stock, so with social
insects, selection has been applied to the family, and not to the
individual, for the sake of gaining a serviceable end. Hence, we may
conclude that slight modifications of structure or of instinct, correlated
with the sterile condition of certain members of the community, have proved
advantageous; consequently the fertile males and females have flourished,
and transmitted to their fertile offspring a tendency to produce sterile
members with the same modifications. This process must have been repeated
many times, until that prodigious amount of difference between the fertile
and sterile females of the same species has been produced which we see in
many social insects.
But we have not as yet touched on the acme of the difficulty; namely, the
fact that the neuters of several ants differ, not only from the fertile
females and males, but from each other, sometimes to an almost incredible
degree, and are thus divided into two or even three castes. The castes,
moreover, do not generally graduate into each other, but are perfectly well
defined; being as distinct from each other as are any two species of the
same genus, or rather as any two genera of the same family. Thus, in
Eciton, there are working and soldier neuters, with jaws and instincts
extraordinarily different: in Cryptocerus, the workers of one caste alone
carry a wonderful sort of shield on their heads, the use of which is quite
unknown: in the Mexican Myrmecocystus, the workers of one caste never
leave the nest; they are fed by the workers of another caste, and they have
an enormously developed abdomen which secretes a sort of honey, supplying
the place of that excreted by the aphides, or the domestic cattle as they
may be called, which our European ants guard and imprison.
It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful and
well-established facts at once annihilate the theory. In the simpler case
of neuter insects all of one caste, which, as I believe, have been rendered
different from the fertile males and females through natural selection, we
may conclude from the analogy of ordinary variations, that the successive,
slight, profitable modifications did not first arise in all the neuters in
the same nest, but in some few alone; and that by the survival of the
communities with females which produced most neuters having the
advantageous modification, all the neuters ultimately came to be thus
characterized. According to this view we ought occasionally to find in the
same nest neuter-insects, presenting gradations of structure; and this we
do find, even not rarely, considering how few neuter-insects out of Europe
have been carefully examined. Mr. F. Smith has shown that the neuters of
several British ants differ surprisingly from each other in size and
sometimes in colour; and that the extreme forms can be linked together by
individuals taken out of the same nest: I have myself compared perfect
gradations of this kind. It sometimes happens that the larger or the
smaller sized workers are the most numerous; or that both large and small
are numerous, while those of an intermediate size are scanty in numbers.
Formica flava has larger and smaller workers, with some few of intermediate
size; and, in this species, as Mr. F. Smith has observed, the larger
workers have simple eyes (ocelli), which, though small, can be plainly
distinguished, whereas the smaller workers have their ocelli rudimentary.
Having carefully dissected several specimens of these workers, I can affirm
that the eyes are far more rudimentary in the smaller workers than can be
accounted for merely by their proportionately lesser size; and I fully
believe, though I dare not assert so positively, that the workers of
intermediate size have their ocelli in an exactly intermediate condition.
So that here we have two bodies of sterile workers in the same nest,
differing not only in size, but in their organs of vision, yet connected by
some few members in an intermediate condition. I may digress by adding,
that if the smaller workers had been the most useful to the community, and
those males and females had been continually selected, which produced more
and more of the smaller workers, until all the workers were in this
condition; we should then have had a species of ant with neuters in nearly
the same condition as those of Myrmica. For the workers of Myrmica have
not even rudiments of ocelli, though the male and female ants of this genus
have well-developed ocelli.
I may give one other case: so confidently did I expect occasionally to
find gradations of important structures between the different castes of
neuters in the same species, that I gladly availed myself of Mr. F. Smith's
offer of numerous specimens from the same nest of the driver ant (Anomma)
of West Africa. The reader will perhaps best appreciate the amount of
difference in these workers by my giving, not the actual measurements, but
a strictly accurate illustration: the difference was the same as if we
were to see a set of workmen building a house, of whom many were five feet
four inches high, and many sixteen feet high; but we must in addition
suppose that the larger workmen had heads four instead of three times as
big as those of the smaller men, and jaws nearly five times as big. The
jaws, moreover, of the working ants of the several sizes differed
wonderfully in shape, and in the form and number of the teeth. But the
important fact for us is that, though the workers can be grouped into
castes of different sizes, yet they graduate insensibly into each other, as
does the widely-different structure of their jaws. I speak confidently on
this latter point, as Sir J. Lubbock made drawings for me, with the camera
lucida, of the jaws which I dissected from the workers of the several
sizes. Mr. Bates, in his interesting "Naturalist on the Amazons," has
described analogous cases.
With these facts before me, I believe that natural selection, by acting on
the fertile ants or parents, could form a species which should regularly
produce neuters, all of large size with one form of jaw, or all of small
size with widely different jaws; or lastly, and this is the greatest
difficulty, one set of workers of one size and structure, and
simultaneously another set of workers of a different size and structure; a
graduated series having first been formed, as in the case of the driver
ant, and then the extreme forms having been produced in greater and greater
numbers, through the survival of the parents which generated them, until
none with an intermediate structure were produced.
An analogous explanation has been given by Mr. Wallace, of the equally
complex case, of certain Malayan butterflies regularly appearing under two
or even three distinct female forms; and by Fritz Muller, of certain
Brazilian crustaceans likewise appearing under two widely distinct male
forms. But this subject need not here be discussed.
I have now explained how, I believe, the wonderful fact of two distinctly
defined castes of sterile workers existing in the same nest, both widely
different from each other and from their parents, has originated. We can
see how useful their production may have been to a social community of
ants, on the same principle that the division of labour is useful to
civilised man. Ants, however, work by inherited instincts and by inherited
organs or tools, while man works by acquired knowledge and manufactured
instruments. But I must confess, that, with all my faith in natural
selection, I should never have anticipated that this principle could have
been efficient in so high a degree, had not the case of these neuter
insects led me to this conclusion. I have, therefore, discussed this case,
at some little but wholly insufficient length, in order to show the power
of natural selection, and likewise because this is by far the most serious
special difficulty which my theory has encountered. The case, also, is
very interesting, as it proves that with animals, as with plants, any
amount of modification may be effected by the accumulation of numerous,
slight, spontaneous variations, which are in any way profitable, without
exercise or habit having been brought into play. For peculiar habits,
confined to the workers of sterile females, however long they might be
followed, could not possibly affect the males and fertile females, which
alone leave descendants. I am surprised that no one has advanced this
demonstrative case of neuter insects, against the well-known doctrine of
inherited habit, as advanced by Lamarck.
SUMMARY.
I have endeavoured in this chapter briefly to show that the mental
qualities of our domestic animals vary, and that the variations are
inherited. Still more briefly I have attempted to show that instincts vary
slightly in a state of nature. No one will dispute that instincts are of
the highest importance to each animal. Therefore, there is no real
difficulty, under changing conditions of life, in natural selection
accumulating to any extent slight modifications of instinct which are in
any way useful. In many cases habit or use and disuse have probably come
into play. I do not pretend that the facts given in this chapter
strengthen in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it. On the other hand,
the fact that instincts are not always absolutely perfect and are liable to
mistakes; that no instinct can be shown to have been produced for the good
of other animals, though animals take advantage of the instincts of others;
that the canon in natural history, of "Natura non facit saltum," is
applicable to instincts as well as to corporeal structure, and is plainly
explicable on the foregoing views, but is otherwise inexplicable--all tend
to corroborate the theory of natural selection.
This theory is also strengthened by some few other facts in regard to
instincts; as by that common case of closely allied, but distinct, species,
when inhabiting distant parts of the world and living under considerably
different conditions of life, yet often retaining nearly the same
instincts. For instance, we can understand, on the principle of
inheritance, how it is that the thrush of tropical South America lines its
nest with mud, in the same peculiar manner as does our British thrush; how
it is that the Hornbills of Africa and India have the same extraordinary
instinct of plastering up and imprisoning the females in a hole in a tree,
with only a small hole left in the plaster through which the males feed
them and their young when hatched; how it is that the male wrens
(Troglodytes) of North America, build "cock-nests," to roost in, like the
males of our Kitty-wrens,--a habit wholly unlike that of any other known
bird. Finally, it may not be a logical deduction, but to my imagination it
is far more satisfactory to look at such instincts as the young cuckoo
ejecting its foster-brothers, ants making slaves, the larvae of
ichneumonidae feeding within the live bodies of caterpillars, not as
specially endowed or created instincts, but as small consequences of one
general law leading to the advancement of all organic beings--namely,
multiply, vary, let the strongest live and the weakest die.
CHAPTER IX.
HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids --
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication -- Laws governing the sterility of
hybrids -- Sterility not a special endowment, but incidental on other
differences, not accumulated by natural selection -- Causes of the
sterility of first crosses and of hybrids -- Parallelism between the
effects of changed conditions of life and of crossing -- Dimorphism and
trimorphism -- Fertility of varieties when crossed and of their mongrel
offspring not universal -- Hybrids and mongrels compared independently of
their fertility -- Summary.
The view commonly entertained by naturalists is that species, when
intercrossed, have been specially endowed with sterility, in order to
prevent their confusion. This view certainly seems at first highly
probable, for species living together could hardly have been kept distinct
had they been capable of freely crossing. The subject is in many ways
important for us, more especially as the sterility of species when first
crossed, and that of their hybrid offspring, cannot have been acquired, as
I shall show, by the preservation of successive profitable degrees of
sterility. It is an incidental result of differences in the reproductive
systems of the parent-species.
In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded; namely, the
sterility of species when first crossed, and the sterility of the hybrids
produced from them.
Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no offspring.
Hybrids, on the other hand, have their reproductive organs functionally
impotent, as may be clearly seen in the state of the male element in both
plants and animals; though the formative organs themselves are perfect in
structure, as far as the microscope reveals. In the first case the two
sexual elements which go to form the embryo are perfect; in the second case
they are either not at all developed, or are imperfectly developed. This
distinction is important, when the cause of the sterility, which is common
to the two cases, has to be considered. The distinction probably has been
slurred over, owing to the sterility in both cases being looked on as a
special endowment, beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed to be
descended from common parents, when crossed, and likewise the fertility of
their mongrel offspring, is, with reference to my theory, of equal
importance with the sterility of species; for it seems to make a broad and
clear distinction between varieties and species.
DEGREES OF STERILITY.
First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of
those two conscientious and admirable observers, Kolreuter and Gartner, who
almost devoted their lives to this subject, without being deeply impressed
with the high generality of some degree of sterility. Kolreuter makes the
rule universal; but then he cuts the knot, for in ten cases in which he
found two forms, considered by most authors as distinct species, quite
fertile together, he unhesitatingly ranks them as varieties. Gartner,
also, makes the rule equally universal; and he disputes the entire
fertility of Kolreuter's ten cases. But in these and in many other cases,
Gartner is obliged carefully to count the seeds, in order to show that
there is any degree of sterility. He always compares the maximum number of
seeds produced by two species when first crossed, and the maximum produced
by their hybrid offspring, with the average number produced by both pure
parent-species in a state of nature. But causes of serious error here
intervene: a plant, to be hybridised, must be castrated, and, what is
often more important, must be secluded in order to prevent pollen being
brought to it by insects from other plants. Nearly all the plants
experimented on by Gartner were potted, and were kept in a chamber in his
house. That these processes are often injurious to the fertility of a
plant cannot be doubted; for Gartner gives in his table about a score of
cases of plants which he castrated, and artificially fertilised with their
own pollen, and (excluding all cases such as the Leguminosae, in which
there is an acknowledged difficulty in the manipulation) half of these
twenty plants had their fertility in some degree impaired. Moreover, as
Gartner repeatedly crossed some forms, such as the common red and blue
pimpernels (Anagallis arvensis and coerulea), which the best botanists rank
as varieties, and found them absolutely sterile, we may doubt whether many
species are really so sterile, when intercrossed, as he believed.
It is certain, on the one hand, that the sterility of various species when
crossed is so different in degree and graduates away so insensibly, and, on
the other hand, that the fertility of pure species is so easily affected by
various circumstances, that for all practical purposes it is most difficult
to say where perfect fertility ends and sterility begins. I think no
better evidence of this can be required than that the two most experienced
observers who have ever lived, namely Kolreuter and Gartner, arrived at
diametrically opposite conclusions in regard to some of the very same
forms. It is also most instructive to compare--but I have not space here
to enter on details--the evidence advanced by our best botanists on the
question whether certain doubtful forms should be ranked as species or
varieties, with the evidence from fertility adduced by different
hybridisers, or by the same observer from experiments made during different
years. It can thus be shown that neither sterility nor fertility affords
any certain distinction between species and varieties. The evidence from
this source graduates away, and is doubtful in the same degree as is the
evidence derived from other constitutional and structural differences.
In regard to the sterility of hybrids in successive generations; though
Gartner was enabled to rear some hybrids, carefully guarding them from a
cross with either pure parent, for six or seven, and in one case for ten
generations, yet he asserts positively that their fertility never
increases, but generally decreases greatly and suddenly. With respect to
this decrease, it may first be noticed that when any deviation in structure
or constitution is common to both parents, this is often transmitted in an
augmented degree to the offspring; and both sexual elements in hybrid
plants are already affected in some degree. But I believe that their
fertility has been diminished in nearly all these cases by an independent
cause, namely, by too close interbreeding. I have made so many experiments
and collected so many facts, showing on the one hand that an occasional
cross with a distinct individual or variety increases the vigour and
fertility of the offspring, and on the other hand that very close
interbreeding lessens their vigour and fertility, that I cannot doubt the
correctness of this conclusion. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits of insects
must be carefully prevented during the flowering season: hence hybrids, if
left to themselves, will generally be fertilised during each generation by
pollen from the same flower; and this would probably be injurious to their
fertility, already lessened by their hybrid origin. I am strengthened in
this conviction by a remarkable statement repeatedly made by Gartner,
namely, that if even the less fertile hybrids be artificially fertilised
with hybrid pollen of the same kind, their fertility, notwithstanding the
frequent ill effects from manipulation, sometimes decidedly increases, and
goes on increasing. Now, in the process of artificial fertilisation,
pollen is as often taken by chance (as I know from my own experience) from
the anthers of another flower, as from the anthers of the flower itself
which is to be fertilised; so that a cross between two flowers, though
probably often on the same plant, would be thus effected. Moreover,
whenever complicated experiments are in progress, so careful an observer as
Gartner would have castrated his hybrids, and this would have insured in
each generation a cross with pollen from a distinct flower, either from the
same plant or from another plant of the same hybrid nature. And thus, the
strange fact of an increase of fertility in the successive generations of
ARTIFICIALLY FERTILISED hybrids, in contrast with those spontaneously self-
fertilised, may, as I believe, be accounted for by too close interbreeding
having been avoided.
Now let us turn to the results arrived at by a third most experienced
hybridiser, namely, the Hon. and Rev. W. Herbert. He is as emphatic in his
conclusion that some hybrids are perfectly fertile--as fertile as the pure
parent-species--as are Kolreuter and Gartner that some degree of sterility
between distinct species is a universal law of nature. He experimented on
some of the very same species as did Gartner. The difference in their
results may, I think, be in part accounted for by Herbert's great
horticultural skill, and by his having hot-houses at his command. Of his
many important statements I will here give only a single one as an example,
namely, that "every ovule in a pod of Crinum capense fertilised by C.
revolutum produced a plant, which I never saw to occur in a case of its
natural fecundation." So that here we have perfect, or even more than
commonly perfect fertility, in a first cross between two distinct species.
This case of the Crinum leads me to refer to a singular fact, namely, that
individual plants of certain species of Lobelia, Verbascum and Passiflora,
can easily be fertilised by the pollen from a distinct species, but not by
pollen from the same plant, though this pollen can be proved to be
perfectly sound by fertilising other plants or species. In the genus
Hippeastrum, in Corydalis as shown by Professor Hildebrand, in various
orchids as shown by Mr. Scott and Fritz Muller, all the individuals are in
this peculiar condition. So that with some species, certain abnormal
individuals, and in other species all the individuals, can actually be
hybridised much more readily than they can be fertilised by pollen from the
same individual plant! To give one instance, a bulb of Hippeastrum aulicum
produced four flowers; three were fertilised by Herbert with their own
pollen, and the fourth was subsequently fertilised by the pollen of a
compound hybrid descended from three distinct species: the result was that
"the ovaries of the three first flowers soon ceased to grow, and after a
few days perished entirely, whereas the pod impregnated by the pollen of
the hybrid made vigorous growth and rapid progress to maturity, and bore
good seed, which vegetated freely." Mr. Herbert tried similar experiments
during many years, and always with the same result. These cases serve to
show on what slight and mysterious causes the lesser or greater fertility
of a species sometimes depends.
The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, etc., have been crossed, yet many of these hybrids
seed freely. For instance, Herbert asserts that a hybrid from Calceolaria
integrifolia and plantaginea, species most widely dissimilar in general
habit, "reproduces itself as perfectly as if it had been a natural species
from the mountains of Chile." I have taken some pains to ascertain the
degree of fertility of some of the complex crosses of Rhododendrons, and I
am assured that many of them are perfectly fertile. Mr. C. Noble, for
instance, informs me that he raises stocks for grafting from a hybrid
between Rhod. ponticum and catawbiense, and that this hybrid "seeds as
freely as it is possible to imagine." Had hybrids, when fairly treated,
always gone on decreasing in fertility in each successive generation, as
Gartner believed to be the case, the fact would have been notorious to
nurserymen. Horticulturists raise large beds of the same hybrid, and such
alone are fairly treated, for by insect agency the several individuals are
allowed to cross freely with each other, and the injurious influence of
close interbreeding is thus prevented. Any one may readily convince
himself of the efficiency of insect agency by examining the flowers of the
more sterile kinds of hybrid Rhododendrons, which produce no pollen, for he
will find on their stigmas plenty of pollen brought from other flowers.
In regard to animals, much fewer experiments have been carefully tried than
with plants. If our systematic arrangements can be trusted, that is, if
the genera of animals are as distinct from each other as are the genera of
plants, then we may infer that animals more widely distinct in the scale of
nature can be crossed more easily than in the case of plants; but the
hybrids themselves are, I think, more sterile. It should, however, be
borne in mind that, owing to few animals breeding freely under confinement,
few experiments have been fairly tried: for instance, the canary-bird has
been crossed with nine distinct species of finches, but, as not one of
these breeds freely in confinement, we have no right to expect that the
first crosses between them and the canary, or that their hybrids, should be
perfectly fertile. Again, with respect to the fertility in successive
generations of the more fertile hybrid animals, I hardly know of an
instance in which two families of the same hybrid have been raised at the
same time from different parents, so as to avoid the ill effects of close
interbreeding. On the contrary, brothers and sisters have usually been
crossed in each successive generation, in opposition to the constantly
repeated admonition of every breeder. And in this case, it is not at all
surprising that the inherent sterility in the hybrids should have gone on
increasing.
Although I know of hardly any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have reason to believe that the hybrids
from Cervulus vaginalis and Reevesii, and from Phasianus colchicus with P.
torquatus, are perfectly fertile. M. Quatrefages states that the hybrids
from two moths (Bombyx cynthia and arrindia) were proved in Paris to be
fertile inter se for eight generations. It has lately been asserted that
two such distinct species as the hare and rabbit, when they can be got to
breed together, produce offspring, which are highly fertile when crossed
with one of the parent-species. The hybrids from the common and Chinese
geese (A. cygnoides), species which are so different that they are
generally ranked in distinct genera, have often bred in this country with
either pure parent, and in one single instance they have bred inter se.
This was effected by Mr. Eyton, who raised two hybrids from the same
parents, but from different hatches; and from these two birds he raised no
less than eight hybrids (grandchildren of the pure geese) from one nest.
In India, however, these cross-bred geese must be far more fertile; for I
am assured by two eminently capable judges, namely Mr. Blyth and Captain
Hutton, that whole flocks of these crossed geese are kept in various parts
of the country; and as they are kept for profit, where neither pure
parent-species exists, they must certainly be highly or perfectly fertile.
With our domesticated animals, the various races when crossed together are
quite fertile; yet in many cases they are descended from two or more wild
species. From this fact we must conclude either that the aboriginal
parent-species at first produced perfectly fertile hybrids, or that the
hybrids subsequently reared under domestication became quite fertile. This
latter alternative, which was first propounded by Pallas, seems by far the
most probable, and can, indeed, hardly be doubted. It is, for instance,
almost certain that our dogs are descended from several wild stocks; yet,
with perhaps the exception of certain indigenous domestic dogs of South
America, all are quite fertile together; but analogy makes me greatly
doubt, whether the several aboriginal species would at first have freely
bred together and have produced quite fertile hybrids. So again I have
lately acquired decisive evidence that the crossed offspring from the
Indian humped and common cattle are inter se perfectly fertile; and from
the observations by Rutimeyer on their important osteological differences,
as well as from those by Mr. Blyth on their differences in habits, voice,
constitution, etc., these two forms must be regarded as good and distinct
species. The same remarks may be extended to the two chief races of the
pig. We must, therefore, either give up the belief of the universal
sterility of species when crossed; or we must look at this sterility in
animals, not as an indelible characteristic, but as one capable of being
removed by domestication.
Finally, considering all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility, both
in first crosses and in hybrids, is an extremely general result; but that
it cannot, under our present state of knowledge, be considered as
absolutely universal.
LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
We will now consider a little more in detail the laws governing the
sterility of first crosses and of hybrids. Our chief object will be to see
whether or not these laws indicate that species have been specially endowed
with this quality, in order to prevent their crossing and blending together
in utter confusion. The following conclusions are drawn up chiefly from
Gartner's admirable work on the hybridisation of plants. I have taken much
pains to ascertain how far they apply to animals, and, considering how
scanty our knowledge is in regard to hybrid animals, I have been surprised
to find how generally the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility, both of first
crosses and of hybrids, graduates from zero to perfect fertility. It is
surprising in how many curious ways this gradation can be shown; but only
the barest outline of the facts can here be given. When pollen from a
plant of one family is placed on the stigma of a plant of a distinct
family, it exerts no more influence than so much inorganic dust. From this
absolute zero of fertility, the pollen of different species applied to the
stigma of some one species of the same genus, yields a perfect gradation in
the number of seeds produced, up to nearly complete or even quite complete
fertility; and, as we have seen, in certain abnormal cases, even to an
excess of fertility, beyond that which the plant's own pollen produces. So
in hybrids themselves, there are some which never have produced, and
probably never would produce, even with the pollen of the pure parents, a
single fertile seed: but in some of these cases a first trace of fertility
may be detected, by the pollen of one of the pure parent-species causing
the flower of the hybrid to wither earlier than it otherwise would have
done; and the early withering of the flower is well known to be a sign of
incipient fertilisation. From this extreme degree of sterility we have
self-fertilised hybrids producing a greater and greater number of seeds up
to perfect fertility.
The hybrids raised from two species which are very difficult to cross, and
which rarely produce any offspring, are generally very sterile; but the
parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced--two classes of facts which are
generally confounded together--is by no means strict. There are many
cases, in which two pure species, as in the genus Verbascum, can be united
with unusual facility, and produce numerous hybrid offspring, yet these
hybrids are remarkably sterile. On the other hand, there are species which
can be crossed very rarely, or with extreme difficulty, but the hybrids,
when at last produced, are very fertile. Even within the limits of the
same genus, for instance in Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is that of pure species. But the
fertility of first crosses is likewise innately variable; for it is not
always the same in degree when the same two species are crossed under the
same circumstances; it depends in part upon the constitution of the
individuals which happen to have been chosen for the experiment. So it is
with hybrids, for their degree of fertility is often found to differ
greatly in the several individuals raised from seed out of the same capsule
and exposed to the same conditions.
By the term systematic affinity is meant, the general resemblance between
species in structure and constitution. Now the fertility of first crosses,
and of the hybrids produced from them, is largely governed by their
systematic affinity. This is clearly shown by hybrids never having been
raised between species ranked by systematists in distinct families; and on
the other hand, by very closely allied species generally uniting with
facility. But the correspondence between systematic affinity and the
facility of crossing is by no means strict. A multitude of cases could be
given of very closely allied species which will not unite, or only with
extreme difficulty; and on the other hand of very distinct species which
unite with the utmost facility. In the same family there may be a genus,
as Dianthus, in which very many species can most readily be crossed; and
another genus, as Silene, in which the most persevering efforts have failed
to produce between extremely close species a single hybrid. Even within
the limits of the same genus, we meet with this same difference; for
instance, the many species of Nicotiana have been more largely crossed than
the species of almost any other genus; but Gartner found that N. acuminata,
which is not a particularly distinct species, obstinately failed to
fertilise, or to be fertilised, by no less than eight other species of
Nicotiana. Many analogous facts could be given.
No one has been able to point out what kind or what amount of difference,
in any recognisable character, is sufficient to prevent two species
crossing. It can be shown that plants most widely different in habit and
general appearance, and having strongly marked differences in every part of
the flower, even in the pollen, in the fruit, and in the cotyledons, can be
crossed. Annual and perennial plants, deciduous and evergreen trees,
plants inhabiting different stations and fitted for extremely different
climates, can often be crossed with ease.
By a reciprocal cross between two species, I mean the case, for instance,
of a female-ass being first crossed by a stallion, and then a mare by a
male-ass: these two species may then be said to have been reciprocally
crossed. There is often the widest possible difference in the facility of
making reciprocal crosses. Such cases are highly important, for they prove
that the capacity in any two species to cross is often completely
independent of their systematic affinity, that is of any difference in
their structure or constitution, excepting in their reproductive systems.
The diversity of the result in reciprocal crosses between the same two
species was long ago observed by Kolreuter. To give an instance:
Mirabilis jalapa can easily be fertilised by the pollen of M. longiflora,
and the hybrids thus produced are sufficiently fertile; but Kolreuter tried
more than two hundred times, during eight following years, to fertilise
reciprocally M. longiflora with the pollen of M. jalapa, and utterly
failed. Several other equally striking cases could be given. Thuret has
observed the same fact with certain sea-weeds or Fuci. Gartner, moreover,
found that this difference of facility in making reciprocal crosses is
extremely common in a lesser degree. He has observed it even between
closely related forms (as Matthiola annua and glabra) which many botanists
rank only as varieties. It is also a remarkable fact that hybrids raised
from reciprocal crosses, though of course compounded of the very same two
species, the one species having first been used as the father and then as
the mother, though they rarely differ in external characters, yet generally
differ in fertility in a small, and occasionally in a high degree.
Several other singular rules could be given from Gartner: for instance,
some species have a remarkable power of crossing with other species; other
species of the same genus have a remarkable power of impressing their
likeness on their hybrid offspring; but these two powers do not at all
necessarily go together. There are certain hybrids which, instead of
having, as is usual, an intermediate character between their two parents,
always closely resemble one of them; and such hybrids, though externally so
like one of their pure parent-species, are with rare exceptions extremely
sterile. So again among hybrids which are usually intermediate in
structure between their parents, exceptional and abnormal individuals
sometimes are born, which closely resemble one of their pure parents; and
these hybrids are almost always utterly sterile, even when the other
hybrids raised from seed from the same capsule have a considerable degree
of fertility. These facts show how completely the fertility of a hybrid
may be independent of its external resemblance to either pure parent.
Considering the several rules now given, which govern the fertility of
first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess; that their fertility, besides being eminently
susceptible to favourable and unfavourable conditions, is innately
variable; that it is by no means always the same in degree in the first
cross and in the hybrids produced from this cross; that the fertility of
hybrids is not related to the degree in which they resemble in external
appearance either parent; and lastly, that the facility of making a first
cross between any two species is not always governed by their systematic
affinity or degree of resemblance to each other. This latter statement is
clearly proved by the difference in the result of reciprocal crosses
between the same two species, for, according as the one species or the
other is used as the father or the mother, there is generally some
difference, and occasionally the widest possible difference, in the
facility of effecting an union. The hybrids, moreover, produced from
reciprocal crosses often differ in fertility.
Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded in
nature? I think not. For why should the sterility be so extremely
different in degree, when various species are crossed, all of which we must
suppose it would be equally important to keep from blending together? Why
should the degree of sterility be innately variable in the individuals of
the same species? Why should some species cross with facility and yet
produce very sterile hybrids; and other species cross with extreme
difficulty, and yet produce fairly fertile hybrids? Why should there often
be so great a difference in the result of a reciprocal cross between the
same two species? Why, it may even be asked, has the production of hybrids
been permitted? To grant to species the special power of producing
hybrids, and then to stop their further propagation by different degrees of
sterility, not strictly related to the facility of the first union between
their parents, seems a strange arrangement.
The foregoing rules and facts, on the other hand, appear to me clearly to
indicate that the sterility, both of first crosses and of hybrids, is
simply incidental or dependent on unknown differences in their reproductive
systems; the differences being of so peculiar and limited a nature, that,
in reciprocal crosses between the same two species, the male sexual element
of the one will often freely act on the female sexual element of the other,
but not in a reversed direction. It will be advisable to explain a little
more fully, by an example, what I mean by sterility being incidental on
other differences, and not a specially endowed quality. As the capacity of
one plant to be grafted or budded on another is unimportant for their
welfare in a state of nature, I presume that no one will suppose that this
capacity is a SPECIALLY endowed quality, but will admit that it is
incidental on differences in the laws of growth of the two plants. We can
sometimes see the reason why one tree will not take on another from
differences in their rate of growth, in the hardness of their wood, in the
period of the flow or nature of their sap, etc.; but in a multitude of
cases we can assign no reason whatever. Great diversity in the size of two
plants, one being woody and the other herbaceous, one being evergreen and
the other deciduous, and adaptation to widely different climates, does not
always prevent the two grafting together. As in hybridisation, so with
grafting, the capacity is limited by systematic affinity, for no one has
been able to graft together trees belonging to quite distinct families;
and, on the other hand, closely allied species and varieties of the same
species, can usually, but not invariably, be grafted with ease. But this
capacity, as in hybridisation, is by no means absolutely governed by
systematic affinity. Although many distinct genera within the same family
have been grafted together, in other cases species of the same genus will
not take on each other. The pear can be grafted far more readily on the
quince, which is ranked as a distinct genus, than on the apple, which is a
member of the same genus. Even different varieties of the pear take with
different degrees of facility on the quince; so do different varieties of
the apricot and peach on certain varieties of the plum.
As Gartner found that there was sometimes an innate difference in different
INDIVIDUALS of the same two species in crossing; so Sagaret believes this
to be the case with different individuals of the same two species in being
grafted together. As in reciprocal crosses, the facility of effecting an
union is often very far from equal, so it sometimes is in grafting. The
common gooseberry, for instance, cannot be grafted on the currant, whereas
the currant will take, though with difficulty, on the gooseberry.
We have seen that the sterility of hybrids which have their reproductive
organs in an imperfect condition, is a different case from the difficulty
of uniting two pure species, which have their reproductive organs perfect;
yet these two distinct classes of cases run to a large extent parallel.
Something analogous occurs in grafting; for Thouin found that three species
of Robinia, which seeded freely on their own roots, and which could be
grafted with no great difficulty on a fourth species, when thus grafted
were rendered barren. On the other hand, certain species of Sorbus, when
grafted on other species, yielded twice as much fruit as when on their own
roots. We are reminded by this latter fact of the extraordinary cases of
Hippeastrum, Passiflora, etc., which seed much more freely when fertilised
with the pollen of a distinct species than when fertilised with pollen from
the same plant.
We thus see that, although there is a clear and great difference between
the mere adhesion of grafted stocks and the union of the male and female
elements in the act of reproduction, yet that there is a rude degree of
parallelism in the results of grafting and of crossing distinct species.
And as we must look at the curious and complex laws governing the facility
with which trees can be grafted on each other as incidental on unknown
differences in their vegetative systems, so I believe that the still more
complex laws governing the facility of first crosses are incidental on
unknown differences in their reproductive systems. These differences in
both cases follow, to a certain extent, as might have been expected,
systematic affinity, by which term every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The
facts by no means seem to indicate that the greater or lesser difficulty of
either grafting or crossing various species has been a special endowment;
although in the case of crossing, the difficulty is as important for the
endurance and stability of specific forms as in the case of grafting it is
unimportant for their welfare.
ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
At one time it appeared to me probable, as it has to others, that the
sterility of first crosses and of hybrids might have been slowly acquired
through the natural selection of slightly lessened degrees of fertility,
which, like any other variation, spontaneously appeared in certain
individuals of one variety when crossed with those of another variety. For
it would clearly be advantageous to two varieties or incipient species if
they could be kept from blending, on the same principle that, when man is
selecting at the same time two varieties, it is necessary that he should
keep them separate. In the first place, it may be remarked that species
inhabiting distinct regions are often sterile when crossed; now it could
clearly have been of no advantage to such separated species to have been
rendered mutually sterile, and consequently this could not have been
effected through natural selection; but it may perhaps be argued, that, if
a species was rendered sterile with some one compatriot, sterility with
other species would follow as a necessary contingency. In the second
place, it is almost as much opposed to the theory of natural selection as
to that of special creation, that in reciprocal crosses the male element of
one form should have been rendered utterly impotent on a second form, while
at the same time the male element of this second form is enabled freely to
fertilise the first form; for this peculiar state of the reproductive
system could hardly have been advantageous to either species.
In considering the probability of natural selection having come into
action, in rendering species mutually sterile, the greatest difficulty will
be found to lie in the existence of many graduated steps, from slightly
lessened fertility to absolute sterility. It may be admitted that it would
profit an incipient species, if it were rendered in some slight degree
sterile when crossed with its parent form or with some other variety; for
thus fewer bastardised and deteriorated offspring would be produced to
commingle their blood with the new species in process of formation. But he
who will take the trouble to reflect on the steps by which this first
degree of sterility could be increased through natural selection to that
high degree which is common with so many species, and which is universal
with species which have been differentiated to a generic or family rank,
will find the subject extraordinarily complex. After mature reflection, it
seems to me that this could not have been effected through natural
selection. Take the case of any two species which, when crossed, produced
few and sterile offspring; now, what is there which could favour the
survival of those individuals which happened to be endowed in a slightly
higher degree with mutual infertility, and which thus approached by one
small step towards absolute sterility? Yet an advance of this kind, if the
theory of natural selection be brought to bear, must have incessantly
occurred with many species, for a multitude are mutually quite barren.
With sterile neuter insects we have reason to believe that modifications in
their structure and fertility have been slowly accumulated by natural
selection, from an advantage having been thus indirectly given to the
community to which they belonged over other communities of the same
species; but an individual animal not belonging to a social community, if
rendered slightly sterile when crossed with some other variety, would not
thus itself gain any advantage or indirectly give any advantage to the
other individuals of the same variety, thus leading to their preservation.
But it would be superfluous to discuss this question in detail: for with
plants we have conclusive evidence that the sterility of crossed species
must be due to some principle, quite independent of natural selection.
Both Gartner and Kolreuter have proved that in genera including numerous
species, a series can be formed from species which when crossed yield fewer
and fewer seeds, to species which never produce a single seed, but yet are
affected by the pollen of certain other species, for the germen swells. It
is here manifestly impossible to select the more sterile individuals, which
have already ceased to yield seeds; so that this acme of sterility, when
the germen alone is effected, cannot have been gained through selection;
and from the laws governing the various grades of sterility being so
uniform throughout the animal and vegetable kingdoms, we may infer that the
cause, whatever it may be, is the same or nearly the same in all cases.
We will now look a little closer at the probable nature of the differences
between species which induce sterility in first crosses and in hybrids. In
the case of first crosses, the greater or less difficulty in effecting a
union and in obtaining offspring apparently depends on several distinct
causes. There must sometimes be a physical impossibility in the male
element reaching the ovule, as would be the case with a plant having a
pistil too long for the pollen-tubes to reach the ovarium. It has also
been observed that when the pollen of one species is placed on the stigma
of a distantly allied species, though the pollen-tubes protrude, they do
not penetrate the stigmatic surface. Again, the male element may reach the
female element, but be incapable of causing an embryo to be developed, as
seems to have been the case with some of Thuret's experiments on Fuci. No
explanation can be given of these facts, any more than why certain trees
cannot be grafted on others. Lastly, an embryo may be developed, and then
perish at an early period. This latter alternative has not been
sufficiently attended to; but I believe, from observations communicated to
me by Mr. Hewitt, who has had great experience in hybridising pheasants and
fowls, that the early death of the embryo is a very frequent cause of
sterility in first crosses. Mr. Salter has recently given the results of
an examination of about 500 eggs produced from various crosses between
three species of Gallus and their hybrids; the majority of these eggs had
been fertilised; and in the majority of the fertilised eggs, the embryos
had either been partially developed and had then perished, or had become
nearly mature, but the young chickens had been unable to break through the
shell. Of the chickens which were born, more than four-fifths died within
the first few days, or at latest weeks, "without any obvious cause,
apparently from mere inability to live;" so that from the 500 eggs only
twelve chickens were reared. With plants, hybridized embryos probably
often perish in a like manner; at least it is known that hybrids raised
from very distinct species are sometimes weak and dwarfed, and perish at an
early age; of which fact Max Wichura has recently given some striking cases
with hybrid willows. It may be here worth noticing that in some cases of
parthenogenesis, the embryos within the eggs of silk moths which had not
been fertilised, pass through their early stages of development and then
perish like the embryos produced by a cross between distinct species.
Until becoming acquainted with these facts, I was unwilling to believe in
the frequent early death of hybrid embryos; for hybrids, when once born,
are generally healthy and long-lived, as we see in the case of the common
mule. Hybrids, however, are differently circumstanced before and after
birth: when born and living in a country where their two parents live,
they are generally placed under suitable conditions of life. But a hybrid
partakes of only half of the nature and constitution of its mother; it may
therefore, before birth, as long as it is nourished within its mother's
womb, or within the egg or seed produced by the mother, be exposed to
conditions in some degree unsuitable, and consequently be liable to perish
at an early period; more especially as all very young beings are eminently
sensitive to injurious or unnatural conditions of life. But after all, the
cause more probably lies in some imperfection in the original act of
impregnation, causing the embryo to be imperfectly developed, rather than
in the conditions to which it is subsequently exposed.
In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is somewhat different. I have more than
once alluded to a large body of facts showing that, when animals and plants
are removed from their natural conditions, they are extremely liable to
have their reproductive systems seriously affected. This, in fact, is the
great bar to the domestication of animals. Between the sterility thus
superinduced and that of hybrids, there are many points of similarity. In
both cases the sterility is independent of general health, and is often
accompanied by excess of size or great luxuriance. In both cases the
sterility occurs in various degrees; in both, the male element is the most
liable to be affected; but sometimes the female more than the male. In
both, the tendency goes to a certain extent with systematic affinity, for
whole groups of animals and plants are rendered impotent by the same
unnatural conditions; and whole groups of species tend to produce sterile
hybrids. On the other hand, one species in a group will sometimes resist
great changes of conditions with unimpaired fertility; and certain species
in a group will produce unusually fertile hybrids. No one can tell till he
tries, whether any particular animal will breed under confinement, or any
exotic plant seed freely under culture; nor can he tell till he tries,
whether any two species of a genus will produce more or less sterile
hybrids. Lastly, when organic beings are placed during several generations
under conditions not natural to them, they are extremely liable to vary,
which seems to be partly due to their reproductive systems having been
specially affected, though in a lesser degree than when sterility ensues.
So it is with hybrids, for their offspring in successive generations are
eminently liable to vary, as every experimentalist has observed.
Thus we see that when organic beings are placed under new and unnatural
conditions, and when hybrids are produced by the unnatural crossing of two
species, the reproductive system, independently of the general state of
health, is affected in a very similar manner. In the one case, the
conditions of life have been disturbed, though often in so slight a degree
as to be inappreciable by us; in the other case, or that of hybrids, the
external conditions have remained the same, but the organisation has been
disturbed by two distinct structures and constitutions, including of course
the reproductive systems, having been blended into one. For it is scarcely
possible that two organisations should be compounded into one, without some
disturbance occurring in the development, or periodical action, or mutual
relations of the different parts and organs one to another or to the
conditions of life. When hybrids are able to breed inter se, they transmit
to their offspring from generation to generation the same compounded
organisation, and hence we need not be surprised that their sterility,
though in some degree variable, does not diminish; it is even apt to
increase, this being generally the result, as before explained, of too
close interbreeding. The above view of the sterility of hybrids being
caused by two constitutions being compounded into one has been strongly
maintained by Max Wichura.
It must, however, be owned that we cannot understand, on the above or any
other view, several facts with respect to the sterility of hybrids; for
instance, the unequal fertility of hybrids produced from reciprocal
crosses; or the increased sterility in those hybrids which occasionally and
exceptionally resemble closely either pure parent. Nor do I pretend that
the foregoing remarks go to the root of the matter: no explanation is
offered why an organism, when placed under unnatural conditions, is
rendered sterile. All that I have attempted to show is, that in two cases,
in some respects allied, sterility is the common result--in the one case
from the conditions of life having been disturbed, in the other case from
the organisation having been disturbed by two organisations being
compounded into one.
A similar parallelism holds good with an allied yet very different class of
facts. It is an old and almost universal belief, founded on a considerable
body of evidence, which I have elsewhere given, that slight changes in the
conditions of life are beneficial to all living things. We see this acted
on by farmers and gardeners in their frequent exchanges of seed, tubers,
etc., from one soil or climate to another, and back again. During the
convalescence of animals, great benefit is derived from almost any change
in their habits of life. Again, both with plants and animals, there is the
clearest evidence that a cross between individuals of the same species,
which differ to a certain extent, gives vigour and fertility to the
offspring; and that close interbreeding continued during several
generations between the nearest relations, if these be kept under the same
conditions of life, almost always leads to decreased size, weakness, or
sterility.
Hence it seems that, on the one hand, slight changes in the conditions of
life benefit all organic beings, and on the other hand, that slight
crosses, that is, crosses between the males and females of the same
species, which have been subjected to slightly different conditions, or
which have slightly varied, give vigour and fertility to the offspring.
But, as we have seen, organic beings long habituated to certain uniform
conditions under a state of nature, when subjected, as under confinement,
to a considerable change in their conditions, very frequently are rendered
more or less sterile; and we know that a cross between two forms that have
become widely or specifically different, produce hybrids which are almost
always in some degree sterile. I am fully persuaded that this double
parallelism is by no means an accident or an illusion. He who is able to
explain why the elephant, and a multitude of other animals, are incapable
of breeding when kept under only partial confinement in their native
country, will be able to explain the primary cause of hybrids being so
generally sterile. He will at the same time be able to explain how it is
that the races of some of our domesticated animals, which have often been
subjected to new and not uniform conditions, are quite fertile together,
although they are descended from distinct species, which would probably
have been sterile if aboriginally crossed. The above two parallel series
of facts seem to be connected together by some common but unknown bond,
which is essentially related to the principle of life; this principle,
according to Mr. Herbert Spencer, being that life depends on, or consists
in, the incessant action and reaction of various forces, which, as
throughout nature, are always tending towards an equilibrium; and when this
tendency is slightly disturbed by any change, the vital forces gain in
power.
RECIPROCAL DIMORPHISM AND TRIMORPHISM.
This subject may be here briefly discussed, and will be found to throw some
light on hybridism. Several plants belonging to distinct orders present
two forms, which exist in about equal numbers and which differ in no
respect except in their reproductive organs; one form having a long pistil
with short stamens, the other a short pistil with long stamens; the two
having differently sized pollen-grains. With trimorphic plants there are
three forms likewise differing in the lengths of their pistils and stamens,
in the size and colour of the pollen-grains, and in some other respects;
and as in each of the three forms there are two sets of stamens, the three
forms possess altogether six sets of stamens and three kinds of pistils.
These organs are so proportioned in length to each other that half the
stamens in two of the forms stand on a level with the stigma of the third
form. Now I have shown, and the result has been confirmed by other
observers, that in order to obtain full fertility with these plants, it is
necessary that the stigma of the one form should be fertilised by pollen
taken from the stamens of corresponding height in another form. So that
with dimorphic species two unions, which may be called legitimate, are
fully fertile; and two, which may be called illegitimate, are more or less
infertile. With trimorphic species six unions are legitimate, or fully
fertile, and twelve are illegitimate, or more or less infertile.
The infertility which may be observed in various dimorphic and trimorphic
plants, when they are illegitimately fertilised, that is by pollen taken
from stamens not corresponding in height with the pistil, differs much in
degree, up to absolute and utter sterility; just in the same manner as
occurs in crossing distinct species. As the degree of sterility in the
latter case depends in an eminent degree on the conditions of life being
more or less favourable, so I have found it with illegitimate unions. It
is well known that if pollen of a distinct species be placed on the stigma
of a flower, and its own pollen be afterwards, even after a considerable
interval of time, placed on the same stigma, its action is so strongly
prepotent that it generally annihilates the effect of the foreign pollen;
so it is with the pollen of the several forms of the same species, for
legitimate pollen is strongly prepotent over illegitimate pollen, when both
are placed on the same stigma. I ascertained this by fertilising several
flowers, first illegitimately, and twenty-four hours afterwards
legitimately, with pollen taken from a peculiarly coloured variety, and all
the seedlings were similarly coloured; this shows that the legitimate
pollen, though applied twenty-four hours subsequently, had wholly destroyed
or prevented the action of the previously applied illegitimate pollen.
Again, as in making reciprocal crosses between the same two species, there
is occasionally a great difference in the result, so the same thing occurs
with trimorphic plants; for instance, the mid-styled form of Lythrum
salicaria was illegitimately fertilised with the greatest ease by pollen
from the longer stamens of the short-styled form, and yielded many seeds;
but the latter form did not yield a single seed when fertilised by the
longer stamens of the mid-styled form.
In all these respects, and in others which might be added, the forms of the
same undoubted species, when illegitimately united, behave in exactly the
same manner as do two distinct species when crossed. This led me carefully
to observe during four years many seedlings, raised from several
illegitimate unions. The chief result is that these illegitimate plants,
as they may be called, are not fully fertile. It is possible to raise from
dimorphic species, both long-styled and short-styled illegitimate plants,
and from trimorphic plants all three illegitimate forms. These can then be
properly united in a legitimate manner. When this is done, there is no
apparent reason why they should not yield as many seeds as did their
parents when legitimately fertilised. But such is not the case. They are
all infertile, in various degrees; some being so utterly and incurably
sterile that they did not yield during four seasons a single seed or even
seed-capsule. The sterility of these illegitimate plants, when united with
each other in a legitimate manner, may be strictly compared with that of
hybrids when crossed inter se. If, on the other hand, a hybrid is crossed
with either pure parent-species, the sterility is usually much lessened:
and so it is when an illegitimate plant is fertilised by a legitimate
plant. In the same manner as the sterility of hybrids does not always run
parallel with the difficulty of making the first cross between the two
parent-species, so that sterility of certain illegitimate plants was
unusually great, while the sterility of the union from which they were
derived was by no means great. With hybrids raised from the same seed-
capsule the degree of sterility is innately variable, so it is in a marked
manner with illegitimate plants. Lastly, many hybrids are profuse and
persistent flowerers, while other and more sterile hybrids produce few
flowers, and are weak, miserable dwarfs; exactly similar cases occur with
the illegitimate offspring of various dimorphic and trimorphic plants.
Altogether there is the closest identity in character and behaviour between
illegitimate plants and hybrids. It is hardly an exaggeration to maintain
that illegitimate plants are hybrids, produced within the limits of the
same species by the improper union of certain forms, while ordinary hybrids
are produced from an improper union between so-called distinct species. We
have also already seen that there is the closest similarity in all respects
between first illegitimate unions and first crosses between distinct
species. This will perhaps be made more fully apparent by an illustration;
we may suppose that a botanist found two well-marked varieties (and such
occur) of the long-styled form of the trimorphic Lythrum salicaria, and
that he determined to try by crossing whether they were specifically
distinct. He would find that they yielded only about one-fifth of the
proper number of seed, and that they behaved in all the other above
specified respects as if they had been two distinct species. But to make
the case sure, he would raise plants from his supposed hybridised seed, and
he would find that the seedlings were miserably dwarfed and utterly
sterile, and that they behaved in all other respects like ordinary hybrids.
He might then maintain that he had actually proved, in accordance with the
common view, that his two varieties were as good and as distinct species as
any in the world; but he would be completely mistaken.
The facts now given on dimorphic and trimorphic plants are important,
because they show us, first, that the physiological test of lessened
fertility, both in first crosses and in hybrids, is no safe criterion of
specific distinction; secondly, because we may conclude that there is some
unknown bond which connects the infertility of illegitimate unions with
that of their illegitimate offspring, and we are led to extend the same
view to first crosses and hybrids; thirdly, because we find, and this seems
to me of especial importance, that two or three forms of the same species
may exist and may differ in no respect whatever, either in structure or in
constitution, relatively to external conditions, and yet be sterile when
united in certain ways. For we must remember that it is the union of the
sexual elements of individuals of the same form, for instance, of two long-
styled forms, which results in sterility; while it is the union of the
sexual elements proper to two distinct forms which is fertile. Hence the
case appears at first sight exactly the reverse of what occurs, in the
ordinary unions of the individuals of the same species and with crosses
between distinct species. It is, however, doubtful whether this is really
so; but I will not enlarge on this obscure subject.
We may, however, infer as probable from the consideration of dimorphic and
trimorphic plants, that the sterility of distinct species when crossed and
of their hybrid progeny, depends exclusively on the nature of their sexual
elements, and not on any difference in their structure or general
constitution. We are also led to this same conclusion by considering
reciprocal crosses, in which the male of one species cannot be united, or
can be united with great difficulty, with the female of a second species,
while the converse cross can be effected with perfect facility. That
excellent observer, Gartner, likewise concluded that species when crossed
are sterile owing to differences confined to their reproductive systems.
FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING, NOT
UNIVERSAL.
It may be urged as an overwhelming argument that there must be some
essential distinction between species and varieties inasmuch as the latter,
however much they may differ from each other in external appearance, cross
with perfect facility, and yield perfectly fertile offspring. With some
exceptions, presently to be given, I fully admit that this is the rule.
But the subject is surrounded by difficulties, for, looking to varieties
produced under nature, if two forms hitherto reputed to be varieties be
found in any degree sterile together, they are at once ranked by most
naturalists as species. For instance, the blue and red pimpernel, which
are considered by most botanists as varieties, are said by Gartner to be
quite sterile when crossed, and he consequently ranks them as undoubted
species. If we thus argue in a circle, the fertility of all varieties
produced under nature will assuredly have to be granted.
If we turn to varieties, produced, or supposed to have been produced, under
domestication, we are still involved in some doubt. For when it is stated,
for instance, that certain South American indigenous domestic dogs do not
readily unite with European dogs, the explanation which will occur to
everyone, and probably the true one, is that they are descended from
aboriginally distinct species. Nevertheless the perfect fertility of so
many domestic races, differing widely from each other in appearance, for
instance, those of the pigeon, or of the cabbage, is a remarkable fact;
more especially when we reflect how many species there are, which, though
resembling each other most closely, are utterly sterile when intercrossed.
Several considerations, however, render the fertility of domestic varieties
less remarkable. In the first place, it may be observed that the amount of
external difference between two species is no sure guide to their degree of
mutual sterility, so that similar differences in the case of varieties
would be no sure guide. It is certain that with species the cause lies
exclusively in differences in their sexual constitution. Now the varying
conditions to which domesticated animals and cultivated plants have been
subjected, have had so little tendency towards modifying the reproductive
system in a manner leading to mutual sterility, that we have good grounds
for admitting the directly opposite doctrine of Pallas, namely, that such
conditions generally eliminate this tendency; so that the domesticated
descendants of species, which in their natural state probably would have
been in some degree sterile when crossed, become perfectly fertile
together. With plants, so far is cultivation from giving a tendency
towards sterility between distinct species, that in several well-
authenticated cases already alluded to, certain plants have been affected
in an opposite manner, for they have become self-impotent, while still
retaining the capacity of fertilising, and being fertilised by, other
species. If the Pallasian doctrine of the elimination of sterility through
long-continued domestication be admitted, and it can hardly be rejected, it
becomes in the highest degree improbable that similar conditions long-
continued should likewise induce this tendency; though in certain cases,
with species having a peculiar constitution, sterility might occasionally
be thus caused. Thus, as I believe, we can understand why, with
domesticated animals, varieties have not been produced which are mutually
sterile; and why with plants only a few such cases, immediately to be
given, have been observed.
The real difficulty in our present subject is not, as it appears to me, why
domestic varieties have not become mutually infertile when crossed, but why
this has so generally occurred with natural varieties, as soon as they have
been permanently modified in a sufficient degree to take rank as species.
We are far from precisely knowing the cause; nor is this surprising, seeing
how profoundly ignorant we are in regard to the normal and abnormal action
of the reproductive system. But we can see that species, owing to their
struggle for existence with numerous competitors, will have been exposed
during long periods of time to more uniform conditions, than have domestic
varieties; and this may well make a wide difference in the result. For we
know how commonly wild animals and plants, when taken from their natural
conditions and subjected to captivity, are rendered sterile; and the
reproductive functions of organic beings which have always lived under
natural conditions would probably in like manner be eminently sensitive to
the influence of an unnatural cross. Domesticated productions, on the
other hand, which, as shown by the mere fact of their domestication, were
not originally highly sensitive to changes in their conditions of life, and
which can now generally resist with undiminished fertility repeated changes
of conditions, might be expected to produce varieties, which would be
little liable to have their reproductive powers injuriously affected by the
act of crossing with other varieties which had originated in a like manner.
I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it is impossible to resist the
evidence of the existence of a certain amount of sterility in the few
following cases, which I will briefly abstract. The evidence is at least
as good as that from which we believe in the sterility of a multitude of
species. The evidence is also derived from hostile witnesses, who in all
other cases consider fertility and sterility as safe criterions of specific
distinction. Gartner kept, during several years, a dwarf kind of maize
with yellow seeds, and a tall variety with red seeds growing near each
other in his garden; and although these plants have separated sexes, they
never naturally crossed. He then fertilised thirteen flowers of the one
kind with pollen of the other; but only a single head produced any seed,
and this one head produced only five grains. Manipulation in this case
could not have been injurious, as the plants have separated sexes. No one,
I believe, has suspected that these varieties of maize are distinct
species; and it is important to notice that the hybrid plants thus raised
were themselves PERFECTLY fertile; so that even Gartner did not venture to
consider the two varieties as specifically distinct.
Girou de Buzareingues crossed three varieties of gourd, which like the
maize has separated sexes, and he asserts that their mutual fertilisation
is by so much the less easy as their differences are greater. How far
these experiments may be trusted, I know not; but the forms experimented on
are ranked by Sagaret, who mainly founds his classification by the test of
infertility, as varieties, and Naudin has come to the same conclusion.
The following case is far more remarkable, and seems at first incredible;
but it is the result of an astonishing number of experiments made during
many years on nine species of Verbascum, by so good an observer and so
hostile a witness as Gartner: namely, that the yellow and white varieties
when crossed produce less seed than the similarly coloured varieties of the
same species. Moreover, he asserts that, when yellow and white varieties
of one species are crossed with yellow and white varieties of a DISTINCT
species, more seed is produced by the crosses between the similarly
coloured flowers, than between those which are differently coloured. Mr.
Scott also has experimented on the species and varieties of Verbascum; and
although unable to confirm Gartner's results on the crossing of the
distinct species, he finds that the dissimilarly coloured varieties of the
same species yield fewer seeds, in the proportion of eighty-six to 100,
than the similarly coloured varieties. Yet these varieties differ in no
respect, except in the colour of their flowers; and one variety can
sometimes be raised from the seed of another.
Kolreuter, whose accuracy has been confirmed by every subsequent observer,
has proved the remarkable fact that one particular variety of the common
tobacco was more fertile than the other varieties, when crossed with a
widely distinct species. He experimented on five forms which are commonly
reputed to be varieties, and which he tested by the severest trial, namely,
by reciprocal crosses, and he found their mongrel offspring perfectly
fertile. But one of these five varieties, when used either as the father
or mother, and crossed with the Nicotiana glutinosa, always yielded hybrids
not so sterile as those which were produced from the four other varieties
when crossed with N. glutinosa. Hence the reproductive system of this one
variety must have been in some manner and in some degree modified.
>From these facts it can no longer be maintained that varieties when crossed
are invariably quite fertile. From the great difficulty of ascertaining
the infertility of varieties in a state of nature, for a supposed variety,
if proved to be infertile in any degree, would almost universally be ranked
as a species; from man attending only to external characters in his
domestic varieties, and from such varieties not having been exposed for
very long periods to uniform conditions of life; from these several
considerations we may conclude that fertility does not constitute a
fundamental distinction between varieties and species when crossed. The
general sterility of crossed species may safely be looked at, not as a
special acquirement or endowment, but as incidental on changes of an
unknown nature in their sexual elements.
HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY.
Independently of the question of fertility, the offspring of species and of
varieties when crossed may be compared in several other respects. Gartner,
whose strong wish it was to draw a distinct line between species and
varieties, could find very few, and, as it seems to me, quite unimportant
differences between the so-called hybrid offspring of species, and the
so-called mongrel offspring of varieties. And, on the other hand, they
agree most closely in many important respects.
I shall here discuss this subject with extreme brevity. The most important
distinction is, that in the first generation mongrels are more variable
than hybrids; but Gartner admits that hybrids from species which have long
been cultivated are often variable in the first generation; and I have
myself seen striking instances of this fact. Gartner further admits that
hybrids between very closely allied species are more variable than those
from very distinct species; and this shows that the difference in the
degree of variability graduates away. When mongrels and the more fertile
hybrids are propagated for several generations, an extreme amount of
variability in the offspring in both cases is notorious; but some few
instances of both hybrids and mongrels long retaining a uniform character
could be given. The variability, however, in the successive generations of
mongrels is, perhaps, greater than in hybrids.
This greater variability in mongrels than in hybrids does not seem at all
surprising. For the parents of mongrels are varieties, and mostly domestic
varieties (very few experiments having been tried on natural varieties),
and this implies that there has been recent variability; which would often
continue and would augment that arising from the act of crossing. The
slight variability of hybrids in the first generation, in contrast with
that in the succeeding generations, is a curious fact and deserves
attention. For it bears on the view which I have taken of one of the
causes of ordinary variability; namely, that the reproductive system, from
being eminently sensitive to changed conditions of life, fails under these
circumstances to perform its proper function of producing offspring closely
similar in all respects to the parent-form. Now, hybrids in the first
generation are descended from species (excluding those long cultivated)
which have not had their reproductive systems in any way affected, and they
are not variable; but hybrids themselves have their reproductive systems
seriously affected, and their descendants are highly variable.
But to return to our comparison of mongrels and hybrids: Gartner states
that mongrels are more liable than hybrids to revert to either parent form;
but this, if it be true, is certainly only a difference in degree.
Moreover, Gartner expressly states that the hybrids from long cultivated
plants are more subject to reversion than hybrids from species in their
natural state; and this probably explains the singular difference in the
results arrived at by different observers. Thus Max Wichura doubts whether
hybrids ever revert to their parent forms, and he experimented on
uncultivated species of willows, while Naudin, on the other hand, insists
in the strongest terms on the almost universal tendency to reversion in
hybrids, and he experimented chiefly on cultivated plants. Gartner further
states that when any two species, although most closely allied to each
other, are crossed with a third species, the hybrids are widely different
from each other; whereas if two very distinct varieties of one species are
crossed with another species, the hybrids do not differ much. But this
conclusion, as far as I can make out, is founded on a single experiment;
and seems directly opposed to the results of several experiments made by
Kolreuter.
Such alone are the unimportant differences which Gartner is able to point
out between hybrid and mongrel plants. On the other hand, the degrees and
kinds of resemblance in mongrels and in hybrids to their respective
parents, more especially in hybrids produced from nearly related species,
follow, according to Gartner the same laws. When two species are crossed,
one has sometimes a prepotent power of impressing its likeness on the
hybrid. So I believe it to be with varieties of plants; and with animals,
one variety certainly often has this prepotent power over another variety.
Hybrid plants produced from a reciprocal cross generally resemble each
other closely, and so it is with mongrel plants from a reciprocal cross.
Both hybrids and mongrels can be reduced to either pure parent form, by
repeated crosses in successive generations with either parent.
These several remarks are apparently applicable to animals; but the subject
is here much complicated, partly owing to the existence of secondary sexual
characters; but more especially owing to prepotency in transmitting
likeness running more strongly in one sex than in the other, both when one
species is crossed with another and when one variety is crossed with
another variety. For instance, I think those authors are right who
maintain that the ass has a prepotent power over the horse, so that both
the mule and the hinny resemble more closely the ass than the horse; but
that the prepotency runs more strongly in the male than in the female ass,
so that the mule, which is an offspring of the male ass and mare, is more
like an ass than is the hinny, which is the offspring of the female-ass and
stallion.
Much stress has been laid by some authors on the supposed fact, that it is
only with mongrels that the offspring are not intermediate in character,
but closely resemble one of their parents; but this does sometimes occur
with hybrids, yet I grant much less frequently than with mongrels. Looking
to the cases which I have collected of cross-bred animals closely
resembling one parent, the resemblances seem chiefly confined to characters
almost monstrous in their nature, and which have suddenly appeared--such as
albinism, melanism, deficiency of tail or horns, or additional fingers and
toes; and do not relate to characters which have been slowly acquired
through selection. A tendency to sudden reversions to the perfect
character of either parent would, also, be much more likely to occur with
mongrels, which are descended from varieties often suddenly produced and
semi-monstrous in character, than with hybrids, which are descended from
species slowly and naturally produced. On the whole, I entirely agree with
Dr. Prosper Lucas, who, after arranging an enormous body of facts with
respect to animals, comes to the conclusion that the laws of resemblance of
the child to its parents are the same, whether the two parents differ
little or much from each other, namely, in the union of individuals of the
same variety, or of different varieties, or of distinct species.
Independently of the question of fertility and sterility, in all other
respects there seems to be a general and close similarity in the offspring
of crossed species, and of crossed varieties. If we look at species as
having been specially created, and at varieties as having been produced by
secondary laws, this similarity would be an astonishing fact. But it
harmonises perfectly with the view that there is no essential distinction
between species and varieties.
SUMMARY OF CHAPTER.
First crosses between forms, sufficiently distinct to be ranked as species,
and their hybrids, are very generally, but not universally, sterile. The
sterility is of all degrees, and is often so slight that the most careful
experimentalists have arrived at diametrically opposite conclusions in
ranking forms by this test. The sterility is innately variable in
individuals of the same species, and is eminently susceptible to action of
favourable and unfavourable conditions. The degree of sterility does not
strictly follow systematic affinity, but is governed by several curious and
complex laws. It is generally different, and sometimes widely different in
reciprocal crosses between the same two species. It is not always equal in
degree in a first cross and in the hybrids produced from this cross.
In the same manner as in grafting trees, the capacity in one species or
variety to take on another, is incidental on differences, generally of an
unknown nature, in their vegetative systems, so in crossing, the greater or
less facility of one species to unite with another is incidental on unknown
differences in their reproductive systems. There is no more reason to
think that species have been specially endowed with various degrees of
sterility to prevent their crossing and blending in nature, than to think
that trees have been specially endowed with various and somewhat analogous
degrees of difficulty in being grafted together in order to prevent their
inarching in our forests.
The sterility of first crosses and of their hybrid progeny has not been
acquired through natural selection. In the case of first crosses it seems
to depend on several circumstances; in some instances in chief part on the
early death of the embryo. In the case of hybrids, it apparently depends
on their whole organisation having been disturbed by being compounded from
two distinct forms; the sterility being closely allied to that which so
frequently affects pure species, when exposed to new and unnatural
conditions of life. He who will explain these latter cases will be able to
explain the sterility of hybrids. This view is strongly supported by a
parallelism of another kind: namely, that, firstly, slight changes in the
conditions of life add to the vigour and fertility of all organic beings;
and secondly, that the crossing of forms, which have been exposed to
slightly different conditions of life, or which have varied, favours the
size, vigour and fertility of their offspring. The facts given on the
sterility of the illegitimate unions of dimorphic and trimorphic plants and
of their illegitimate progeny, perhaps render it probable that some unknown
bond in all cases connects the degree of fertility of first unions with
that of their offspring. The consideration of these facts on dimorphism,
as well as of the results of reciprocal crosses, clearly leads to the
conclusion that the primary cause of the sterility of crossed species is
confined to differences in their sexual elements. But why, in the case of
distinct species, the sexual elements should so generally have become more
or less modified, leading to their mutual infertility, we do not know; but
it seems to stand in some close relation to species having been exposed for
long periods of time to nearly uniform conditions of life.
It is not surprising that the difficulty in crossing any two species, and
the sterility of their hybrid offspring, should in most cases correspond,
even if due to distinct causes: for both depend on the amount of
difference between the species which are crossed. Nor is it surprising
that the facility of effecting a first cross, and the fertility of the
hybrids thus produced, and the capacity of being grafted together--though
this latter capacity evidently depends on widely different
circumstances--should all run, to a certain extent, parallel with the
systematic affinity of the forms subjected to experiment; for systematic
affinity includes resemblances of all kinds.
First crosses between forms known to be varieties, or sufficiently alike to
be considered as varieties, and their mongrel offspring, are very
generally, but not, as is so often stated, invariably fertile. Nor is this
almost universal and perfect fertility surprising, when it is remembered
how liable we are to argue in a circle with respect to varieties in a state
of nature; and when we remember that the greater number of varieties have
been produced under domestication by the selection of mere external
differences, and that they have not been long exposed to uniform conditions
of life. It should also be especially kept in mind, that long-continued
domestication tends to eliminate sterility, and is therefore little likely
to induce this same quality. Independently of the question of fertility,
in all other respects there is the closest general resemblance between
hybrids and mongrels, in their variability, in their power of absorbing
each other by repeated crosses, and in their inheritance of characters from
both parent-forms. Finally, then, although we are as ignorant of the
precise cause of the sterility of first crosses and of hybrids as we are
why animals and plants removed from their natural conditions become
sterile, yet the facts given in this chapter do not seem to me opposed to
the belief that species aboriginally existed as varieties.
CHAPTER X.
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day -- On the
nature of extinct intermediate varieties; on their number -- On the lapse
of time, as inferred from the rate of denudation and of deposition number
-- On the lapse of time as estimated by years -- On the poorness of our
palaeontological collections -- On the intermittence of geological
formations -- On the denudation of granitic areas -- On the absence of
intermediate varieties in any one formation -- On the sudden appearance of
groups of species -- On their sudden appearance in the lowest known
fossiliferous strata -- Antiquity of the habitable earth.
In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely, the distinctness of specific forms
and their not being blended together by innumerable transitional links, is
a very obvious difficulty. I assigned reasons why such links do not
commonly occur at the present day under the circumstances apparently most
favourable for their presence, namely, on an extensive and continuous area
with graduated physical conditions. I endeavoured to show, that the life
of each species depends in a more important manner on the presence of other
already defined organic forms, than on climate, and, therefore, that the
really governing conditions of life do not graduate away quite insensibly
like heat or moisture. I endeavoured, also, to show that intermediate
varieties, from existing in lesser numbers than the forms which they
connect, will generally be beaten out and exterminated during the course of
further modification and improvement. The main cause, however, of
innumerable intermediate links not now occurring everywhere throughout
nature depends, on the very process of natural selection, through which new
varieties continually take the places of and supplant their parent-forms.
But just in proportion as this process of extermination has acted on an
enormous scale, so must the number of intermediate varieties, which have
formerly existed, be truly enormous. Why then is not every geological
formation and every stratum full of such intermediate links? Geology
assuredly does not reveal any such finely graduated organic chain; and
this, perhaps, is the most obvious and serious objection which can be urged
against my theory. The explanation lies, as I believe, in the extreme
imperfection of the geological record.
In the first place, it should always be borne in mind what sort of
intermediate forms must, on the theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid picturing to
myself forms DIRECTLY intermediate between them. But this is a wholly
false view; we should always look for forms intermediate between each
species and a common but unknown progenitor; and the progenitor will
generally have differed in some respects from all its modified descendants.
To give a simple illustration: the fantail and pouter pigeons are both
descended from the rock-pigeon; if we possessed all the intermediate
varieties which have ever existed, we should have an extremely close series
between both and the rock-pigeon; but we should have no varieties directly
intermediate between the fantail and pouter; none, for instance, combining
a tail somewhat expanded with a crop somewhat enlarged, the characteristic
features of these two breeds. These two breeds, moreover, have become so
much modified, that, if we had no historical or indirect evidence regarding
their origin, it would not have been possible to have determined from a
mere comparison of their structure with that of the rock-pigeon, C. livia,
whether they had descended from this species or from some other allied
species, such as C. oenas.
So with natural species, if we look to forms very distinct, for instance to
the horse and tapir, we have no reason to suppose that links directly
intermediate between them ever existed, but between each and an unknown
common parent. The common parent will have had in its whole organisation
much general resemblance to the tapir and to the horse; but in some points
of structure may have differed considerably from both, even perhaps more
than they differ from each other. Hence, in all such cases, we should be
unable to recognise the parent-form of any two or more species, even if we
closely compared the structure of the parent with that of its modified
descendants, unless at the same time we had a nearly perfect chain of the
intermediate links.
It is just possible, by the theory, that one of two living forms might have
descended from the other; for instance, a horse from a tapir; and in this
case DIRECT intermediate links will have existed between them. But such a
case would imply that one form had remained for a very long period
unaltered, whilst its descendants had undergone a vast amount of change;
and the principle of competition between organism and organism, between
child and parent, will render this a very rare event; for in all cases the
new and improved forms of life tend to supplant the old and unimproved
forms.
By the theory of natural selection all living species have been connected
with the parent-species of each genus, by differences not greater than we
see between the natural and domestic varieties of the same species at the
present day; and these parent-species, now generally extinct, have in their
turn been similarly connected with more ancient forms; and so on backwards,
always converging to the common ancestor of each great class. So that the
number of intermediate and transitional links, between all living and
extinct species, must have been inconceivably great. But assuredly, if
this theory be true, such have lived upon the earth.
ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF DEPOSITION AND EXTENT OF
DENUDATION.
Independently of our not finding fossil remains of such infinitely numerous
connecting links, it may be objected that time cannot have sufficed for so
great an amount of organic change, all changes having been effected slowly.
It is hardly possible for me to recall to the reader who is not a practical
geologist, the facts leading the mind feebly to comprehend the lapse of
time. He who can read Sir Charles Lyell's grand work on the Principles of
Geology, which the future historian will recognise as having produced a
revolution in natural science, and yet does not admit how vast have been
the past periods of time, may at once close this volume. Not that it
suffices to study the Principles of Geology, or to read special treatises
by different observers on separate formations, and to mark how each author
attempts to give an inadequate idea of the duration of each formation, or
even of each stratum. We can best gain some idea of past time by knowing
the agencies at work; and learning how deeply the surface of the land has
been denuded, and how much sediment has been deposited. As Lyell has well
remarked, the extent and thickness of our sedimentary formations are the
result and the measure of the denudation which the earth's crust has
elsewhere undergone. Therefore a man should examine for himself the great
piles of superimposed strata, and watch the rivulets bringing down mud, and
the waves wearing away the sea-cliffs, in order to comprehend something
about the duration of past time, the monuments of which we see all around
us.
It is good to wander along the coast, when formed of moderately hard rocks,
and mark the process of degradation. The tides in most cases reach the
cliffs only for a short time twice a day, and the waves eat into them only
when they are charged with sand or pebbles; for there is good evidence that
pure water effects nothing in wearing away rock. At last the base of the
cliff is undermined, huge fragments fall down, and these remaining fixed,
have to be worn away atom by atom, until after being reduced in size they
can be rolled about by the waves, and then they are more quickly ground
into pebbles, sand, or mud. But how often do we see along the bases of
retreating cliffs rounded boulders, all thickly clothed by marine
productions, showing how little they are abraded and how seldom they are
rolled about! Moreover, if we follow for a few miles any line of rocky
cliff, which is undergoing degradation, we find that it is only here and
there, along a short length or round a promontory, that the cliffs are at
the present time suffering. The appearance of the surface and the
vegetation show that elsewhere years have elapsed since the waters washed
their base.
We have, however, recently learned from the observations of Ramsay, in the
van of many excellent observers--of Jukes, Geikie, Croll and others, that
subaerial degradation is a much more important agency than coast-action, or
the power of the waves. The whole surface of the land is exposed to the
chemical action of the air and of the rainwater, with its dissolved
carbonic acid, and in colder countries to frost; the disintegrated matter
is carried down even gentle slopes during heavy rain, and to a greater
extent than might be supposed, especially in arid districts, by the wind;
it is then transported by the streams and rivers, which, when rapid deepen
their channels, and triturate the fragments. On a rainy day, even in a
gently undulating country, we see the effects of subaerial degradation in
the muddy rills which flow down every slope. Messrs. Ramsay and Whitaker
have shown, and the observation is a most striking one, that the great
lines of escarpment in the Wealden district and those ranging across
England, which formerly were looked at as ancient sea-coasts, cannot have
been thus formed, for each line is composed of one and the same formation,
while our sea-cliffs are everywhere formed by the intersection of various
formations. This being the case, we are compelled to admit that the
escarpments owe their origin in chief part to the rocks of which they are
composed, having resisted subaerial denudation better than the surrounding
surface; this surface consequently has been gradually lowered, with the
lines of harder rock left projecting. Nothing impresses the mind with the
vast duration of time, according to our ideas of time, more forcibly than
the conviction thus gained that subaerial agencies, which apparently have
so little power, and which seem to work so slowly, have produced great
results.
When thus impressed with the slow rate at which the land is worn away
through subaerial and littoral action, it is good, in order to appreciate
the past duration of time, to consider, on the one hand, the masses of rock
which have been removed over many extensive areas, and on the other hand
the thickness of our sedimentary formations. I remember having been much
struck when viewing volcanic islands, which have been worn by the waves and
pared all round into perpendicular cliffs of one or two thousand feet in
height; for the gentle slope of the lava-streams, due to their formerly
liquid state, showed at a glance how far the hard, rocky beds had once
extended into the open ocean. The same story is told still more plainly by
faults--those great cracks along which the strata have been upheaved on one
side, or thrown down on the other, to the height or depth of thousands of
feet; for since the crust cracked, and it makes no great difference whether
the upheaval was sudden, or, as most geologists now believe, was slow and
effected by many starts, the surface of the land has been so completely
planed down that no trace of these vast dislocations is externally visible.
The Craven fault, for instance, extends for upward of thirty miles, and
along this line the vertical displacement of the strata varies from 600 to
3,000 feet. Professor Ramsay has published an account of a downthrow in
Anglesea of 2,300 feet; and he informs me that he fully believes that there
is one in Merionethshire of 12,000 feet; yet in these cases there is
nothing on the surface of the land to show such prodigious movements; the
pile of rocks on either side of the crack having been smoothly swept away.
On the other hand, in all parts of the world the piles of sedimentary
strata are of wonderful thickness. In the Cordillera, I estimated one mass
of conglomerate at ten thousand feet; and although conglomerates have
probably been accumulated at a quicker rate than finer sediments, yet from
being formed of worn and rounded pebbles, each of which bears the stamp of
time, they are good to show how slowly the mass must have been heaped
together. Professor Ramsay has given me the maximum thickness, from actual
measurement in most cases, of the successive formations in DIFFERENT parts
of Great Britain; and this is the result:
Feet
Palaeozoic strata (not including igneous beds)..57,154
Secondary strata................................13,190
Tertiary strata..................................2,240
--making altogether 72,584 feet; that is, very nearly thirteen and
three-quarters British miles. Some of these formations, which are
represented in England by thin beds, are thousands of feet in thickness on
the Continent. Moreover, between each successive formation we have, in the
opinion of most geologists, blank periods of enormous length. So that the
lofty pile of sedimentary rocks in Britain gives but an inadequate idea of
the time which has elapsed during their accumulation. The consideration of
these various facts impresses the mind almost in the same manner as does
the vain endeavour to grapple with the idea of eternity.
Nevertheless this impression is partly false. Mr. Croll, in an interesting
paper, remarks that we do not err "in forming too great a conception of the
length of geological periods," but in estimating them by years. When
geologists look at large and complicated phenomena, and then at the figures
representing several million years, the two produce a totally different
effect on the mind, and the figures are at once pronounced too small. In
regard to subaerial denudation, Mr. Croll shows, by calculating the known
amount of sediment annually brought down by certain rivers, relatively to
their areas of drainage, that 1,000 feet of solid rock, as it became
gradually disintegrated, would thus be removed from the mean level of the
whole area in the course of six million years. This seems an astonishing
result, and some considerations lead to the suspicion that it may be too
large, but if halved or quartered it is still very surprising. Few of us,
however, know what a million really means: Mr. Croll gives the following
illustration: Take a narrow strip of paper, eighty-three feet four inches
in length, and stretch it along the wall of a large hall; then mark off at
one end the tenth of an inch. This tenth of an inch will represent one
hundred years, and the entire strip a million years. But let it be borne
in mind, in relation to the subject of this work, what a hundred years
implies, represented as it is by a measure utterly insignificant in a hall
of the above dimensions. Several eminent breeders, during a single
lifetime, have so largely modified some of the higher animals, which
propagate their kind much more slowly than most of the lower animals, that
they have formed what well deserves to be called a new sub-breed. Few men
have attended with due care to any one strain for more than half a century,
so that a hundred years represents the work of two breeders in succession.
It is not to be supposed that species in a state of nature ever change so
quickly as domestic animals under the guidance of methodical selection.
The comparison would be in every way fairer with the effects which follow
from unconscious selection, that is, the preservation of the most useful or
beautiful animals, with no intention of modifying the breed; but by this
process of unconscious selection, various breeds have been sensibly changed
in the course of two or three centuries.
Species, however, probably change much more slowly, and within the same
country only a few change at the same time. This slowness follows from all
the inhabitants of the same country being already so well adapted to each
other, that new places in the polity of nature do not occur until after
long intervals, due to the occurrence of physical changes of some kind, or
through the immigration of new forms. Moreover, variations or individual
differences of the right nature, by which some of the inhabitants might be
better fitted to their new places under the altered circumstance, would not
always occur at once. Unfortunately we have no means of determining,
according to the standard of years, how long a period it takes to modify a
species; but to the subject of time we must return.
ON THE POORNESS OF PALAEONTOLOGICAL COLLECTIONS.
Now let us turn to our richest museums, and what a paltry display we
behold! That our collections are imperfect is admitted by every one. The
remark of that admirable palaeontologist, Edward Forbes, should never be
forgotten, namely, that very many fossil species are known and named from
single and often broken specimens, or from a few specimens collected on
some one spot. Only a small portion of the surface of the earth has been
geologically explored, and no part with sufficient care, as the important
discoveries made every year in Europe prove. No organism wholly soft can
be preserved. Shells and bones decay and disappear when left on the bottom
of the sea, where sediment is not accumulating. We probably take a quite
erroneous view, when we assume that sediment is being deposited over nearly
the whole bed of the sea, at a rate sufficiently quick to embed and
preserve fossil remains. Throughout an enormously large proportion of the
ocean, the bright blue tint of the water bespeaks its purity. The many
cases on record of a formation conformably covered, after an immense
interval of time, by another and later formation, without the underlying
bed having suffered in the interval any wear and tear, seem explicable only
on the view of the bottom of the sea not rarely lying for ages in an
unaltered condition. The remains which do become embedded, if in sand or
gravel, will, when the beds are upraised, generally be dissolved by the
percolation of rain water charged with carbonic acid. Some of the many
kinds of animals which live on the beach between high and low water mark
seem to be rarely preserved. For instance, the several species of the
Chthamalinae (a sub-family of sessile cirripedes) coat the rocks all over
the world in infinite numbers: they are all strictly littoral, with the
exception of a single Mediterranean species, which inhabits deep water and
this has been found fossil in Sicily, whereas not one other species has
hitherto been found in any tertiary formation: yet it is known that the
genus Chthamalus existed during the Chalk period. Lastly, many great
deposits, requiring a vast length of time for their accumulation, are
entirely destitute of organic remains, without our being able to assign any
reason: one of the most striking instances is that of the Flysch
formation, which consists of shale and sandstone, several thousand,
occasionally even six thousand feet in thickness, and extending for at
least 300 miles from Vienna to Switzerland; and although this great mass
has been most carefully searched, no fossils, except a few vegetable
remains, have been found.
With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that our
evidence is fragmentary in an extreme degree. For instance, until recently
not a land-shell was known belonging to either of these vast periods, with
the exception of one species discovered by Sir C. Lyell and Dr. Dawson in
the carboniferous strata of North America; but now land-shells have been
found in the lias. In regard to mammiferous remains, a glance at the
historical table published in Lyell's Manual, will bring home the truth,
how accidental and rare is their preservation, far better than pages of
detail. Nor is their rarity surprising, when we remember how large a
proportion of the bones of tertiary mammals have been discovered either in
caves or in lacustrine deposits; and that not a cave or true lacustrine bed
is known belonging to the age of our secondary or palaeozoic formations.
But the imperfection in the geological record largely results from another
and more important cause than any of the foregoing; namely, from the
several formations being separated from each other by wide intervals of
time. This doctrine has been emphatically admitted by many geologists and
palaeontologists, who, like E. Forbes, entirely disbelieve in the change of
species. When we see the formations tabulated in written works, or when we
follow them in nature, it is difficult to avoid believing that they are
closely consecutive. But we know, for instance, from Sir R. Murchison's
great work on Russia, what wide gaps there are in that country between the
superimposed formations; so it is in North America, and in many other parts
of the world. The most skilful geologist, if his attention had been
confined exclusively to these large territories, would never have suspected
that during the periods which were blank and barren in his own country,
great piles of sediment, charged with new and peculiar forms of life, had
elsewhere been accumulated. And if, in every separate territory, hardly
any idea can be formed of the length of time which has elapsed between the
consecutive formations, we may infer that this could nowhere be
ascertained. The frequent and great changes in the mineralogical
composition of consecutive formations, generally implying great changes in
the geography of the surrounding lands, whence the sediment was derived,
accord with the belief of vast intervals of time having elapsed between
each formation.
We can, I think, see why the geological formations of each region are
almost invariably intermittent; that is, have not followed each other in
close sequence. Scarcely any fact struck me more when examining many
hundred miles of the South American coasts, which have been upraised
several hundred feet within the recent period, than the absence of any
recent deposits sufficiently extensive to last for even a short geological
period. Along the whole west coast, which is inhabited by a peculiar
marine fauna, tertiary beds are so poorly developed that no record of
several successive and peculiar marine faunas will probably be preserved to
a distant age. A little reflection will explain why, along the rising
coast of the western side of South America, no extensive formations with
recent or tertiary remains can anywhere be found, though the supply of
sediment must for ages have been great, from the enormous degradation of
the coast rocks and from the muddy streams entering the sea. The
explanation, no doubt, is that the littoral and sub-littoral deposits are
continually worn away, as soon as they are brought up by the slow and
gradual rising of the land within the grinding action of the coast-waves.
We may, I think, conclude that sediment must be accumulated in extremely
thick, solid, or extensive masses, in order to withstand the incessant
action of the waves, when first upraised and during subsequent oscillations
of level, as well as the subsequent subaerial degradation. Such thick and
extensive accumulations of sediment may be formed in two ways; either in
profound depths of the sea, in which case the bottom will not be inhabited
by so many and such varied forms of life as the more shallow seas; and the
mass when upraised will give an imperfect record of the organisms which
existed in the neighbourhood during the period of its accumulation. Or
sediment may be deposited to any thickness and extent over a shallow
bottom, if it continue slowly to subside. In this latter case, as long as
the rate of subsidence and supply of sediment nearly balance each other,
the sea will remain shallow and favourable for many and varied forms, and
thus a rich fossiliferous formation, thick enough, when upraised, to resist
a large amount of denudation, may be formed.
I am convinced that nearly all our ancient formations, which are throughout
the greater part of their thickness RICH IN FOSSILS, have thus been formed
during subsidence. Since publishing my views on this subject in 1845, I
have watched the progress of geology, and have been surprised to note how
author after author, in treating of this or that great formation, has come
to the conclusion that it was accumulated during subsidence. I may add,
that the only ancient tertiary formation on the west coast of South
America, which has been bulky enough to resist such degradation as it has
as yet suffered, but which will hardly last to a distant geological age,
was deposited during a downward oscillation of level, and thus gained
considerable thickness.
All geological facts tell us plainly that each area has undergone numerous
slow oscillations of level, and apparently these oscillations have affected
wide spaces. Consequently, formations rich in fossils and sufficiently
thick and extensive to resist subsequent degradation, will have been formed
over wide spaces during periods of subsidence, but only where the supply of
sediment was sufficient to keep the sea shallow and to embed and preserve
the remains before they had time to decay. On the other hand, as long as
the bed of the sea remained stationary, THICK deposits cannot have been
accumulated in the shallow parts, which are the most favourable to life.
Still less can this have happened during the alternate periods of
elevation; or, to speak more accurately, the beds which were then
accumulated will generally have been destroyed by being upraised and
brought within the limits of the coast-action.
These remarks apply chiefly to littoral and sublittoral deposits. In the
case of an extensive and shallow sea, such as that within a large part of
the Malay Archipelago, where the depth varies from thirty or forty to sixty
fathoms, a widely extended formation might be formed during a period of
elevation, and yet not suffer excessively from denudation during its slow
upheaval; but the thickness of the formation could not be great, for owing
to the elevatory movement it would be less than the depth in which it was
formed; nor would the deposit be much consolidated, nor be capped by
overlying formations, so that it would run a good chance of being worn away
by atmospheric degradation and by the action of the sea during subsequent
oscillations of level. It has, however, been suggested by Mr. Hopkins,
that if one part of the area, after rising and before being denuded,
subsided, the deposit formed during the rising movement, though not thick,
might afterwards become protected by fresh accumulations, and thus be
preserved for a long period.
Mr. Hopkins also expresses his belief that sedimentary beds of considerable
horizontal extent have rarely been completely destroyed. But all
geologists, excepting the few who believe that our present metamorphic
schists and plutonic rocks once formed the primordial nucleus of the globe,
will admit that these latter rocks have been stripped of their covering to
an enormous extent. For it is scarcely possible that such rocks could have
been solidified and crystallised while uncovered; but if the metamorphic
action occurred at profound depths of the ocean, the former protecting
mantle of rock may not have been very thick. Admitting then that gneiss,
mica-schist, granite, diorite, etc., were once necessarily covered up, how
can we account for the naked and extensive areas of such rocks in many
parts of the world, except on the belief that they have subsequently been
completely denuded of all overlying strata? That such extensive areas do
exist cannot be doubted: the granitic region of Parime is described by
Humboldt as being at least nineteen times as large as Switzerland. South
of the Amazon, Boue colours an area composed of rocks of this nature as
equal to that of Spain, France, Italy, part of Germany, and the British
Islands, all conjoined. This region has not been carefully explored, but
from the concurrent testimony of travellers, the granitic area is very
large: thus Von Eschwege gives a detailed section of these rocks,
stretching from Rio de Janeiro for 260 geographical miles inland in a
straight line; and I travelled for 150 miles in another direction, and saw
nothing but granitic rocks. Numerous specimens, collected along the whole
coast, from near Rio de Janeiro to the mouth of the Plata, a distance of
1,100 geographical miles, were examined by me, and they all belonged to
this class. Inland, along the whole northern bank of the Plata, I saw,
besides modern tertiary beds, only one small patch of slightly
metamorphosed rock, which alone could have formed a part of the original
capping of the granitic series. Turning to a well-known region, namely, to
the United States and Canada, as shown in Professor H.D. Rogers' beautiful
map, I have estimated the areas by cutting out and weighing the paper, and
I find that the metamorphic (excluding the "semi-metamorphic") and granite
rocks exceed, in the proportion of 19 to 12.5, the whole of the newer
Palaeozoic formations. In many regions the metamorphic and granite rocks
would be found much more widely extended than they appear to be, if all the
sedimentary beds were removed which rest unconformably on them, and which
could not have formed part of the original mantle under which they were
crystallised. Hence, it is probable that in some parts of the world whole
formations have been completely denuded, with not a wreck left behind.
One remark is here worth a passing notice. During periods of elevation the
area of the land and of the adjoining shoal parts of the sea will be
increased and new stations will often be formed--all circumstances
favourable, as previously explained, for the formation of new varieties and
species; but during such periods there will generally be a blank in the
geological record. On the other hand, during subsidence, the inhabited
area and number of inhabitants will decrease (excepting on the shores of a
continent when first broken up into an archipelago), and consequently
during subsidence, though there will be much extinction, few new varieties
or species will be formed; and it is during these very periods of
subsidence that the deposits which are richest in fossils have been
accumulated.
ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES IN ANY SINGLE FORMATION.
>From these several considerations it cannot be doubted that the geological
record, viewed as a whole, is extremely imperfect; but if we confine our
attention to any one formation, it becomes much more difficult to
understand why we do not therein find closely graduated varieties between
the allied species which lived at its commencement and at its close.
Several cases are on record of the same species presenting varieties in the
upper and lower parts of the same formation. Thus Trautschold gives a
number of instances with Ammonites, and Hilgendorf has described a most
curious case of ten graduated forms of Planorbis multiformis in the
successive beds of a fresh-water formation in Switzerland. Although each
formation has indisputably required a vast number of years for its
deposition, several reasons can be given why each should not commonly
include a graduated series of links between the species which lived at its
commencement and close, but I cannot assign due proportional weight to the
following considerations.
Although each formation may mark a very long lapse of years, each probably
is short compared with the period requisite to change one species into
another. I am aware that two palaeontologists, whose opinions are worthy
of much deference, namely Bronn and Woodward, have concluded that the
average duration of each formation is twice or thrice as long as the
average duration of specific forms. But insuperable difficulties, as it
seems to me, prevent us from coming to any just conclusion on this head.
When we see a species first appearing in the middle of any formation, it
would be rash in the extreme to infer that it had not elsewhere previously
existed. So again, when we find a species disappearing before the last
layers have been deposited, it would be equally rash to suppose that it
then became extinct. We forget how small the area of Europe is compared
with the rest of the world; nor have the several stages of the same
formation throughout Europe been correlated with perfect accuracy.
We may safely infer that with marine animals of all kinds there has been a
large amount of migration due to climatal and other changes; and when we
see a species first appearing in any formation, the probability is that it
only then first immigrated into that area. It is well known, for instance,
that several species appear somewhat earlier in the palaeozoic beds of
North America than in those of Europe; time having apparently been required
for their migration from the American to the European seas. In examining
the latest deposits, in various quarters of the world, it has everywhere
been noted, that some few still existing species are common in the deposit,
but have become extinct in the immediately surrounding sea; or, conversely,
that some are now abundant in the neighbouring sea, but are rare or absent
in this particular deposit. It is an excellent lesson to reflect on the
ascertained amount of migration of the inhabitants of Europe during the
glacial epoch, which forms only a part of one whole geological period; and
likewise to reflect on the changes of level, on the extreme change of
climate, and on the great lapse of time, all included within this same
glacial period. Yet it may be doubted whether, in any quarter of the
world, sedimentary deposits, INCLUDING FOSSIL REMAINS, have gone on
accumulating within the same area during the whole of this period. It is
not, for instance, probable that sediment was deposited during the whole of
the glacial period near the mouth of the Mississippi, within that limit of
depth at which marine animals can best flourish: for we know that great
geographical changes occurred in other parts of America during this space
of time. When such beds as were deposited in shallow water near the mouth
of the Mississippi during some part of the glacial period shall have been
upraised, organic remains will probably first appear and disappear at
different levels, owing to the migrations of species and to geographical
changes. And in the distant future, a geologist, examining these beds,
would be tempted to conclude that the average duration of life of the
embedded fossils had been less than that of the glacial period, instead of
having been really far greater, that is, extending from before the glacial
epoch to the present day.
In order to get a perfect gradation between two forms in the upper and
lower parts of the same formation, the deposit must have gone on
continuously accumulating during a long period, sufficient for the slow
process of modification; hence, the deposit must be a very thick one; and
the species undergoing change must have lived in the same district
throughout the whole time. But we have seen that a thick formation,
fossiliferous throughout its entire thickness, can accumulate only during a
period of subsidence; and to keep the depth approximately the same, which
is necessary that the same marine species may live on the same space, the
supply of sediment must nearly counterbalance the amount of subsidence.
But this same movement of subsidence will tend to submerge the area whence
the sediment is derived, and thus diminish the supply, whilst the downward
movement continues. In fact, this nearly exact balancing between the
supply of sediment and the amount of subsidence is probably a rare
contingency; for it has been observed by more than one palaeontologist that
very thick deposits are usually barren of organic remains, except near
their upper or lower limits.
It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation composed
of beds of widely different mineralogical composition, we may reasonably
suspect that the process of deposition has been more or less interrupted.
Nor will the closest inspection of a formation give us any idea of the
length of time which its deposition may have consumed. Many instances
could be given of beds, only a few feet in thickness, representing
formations which are elsewhere thousands of feet in thickness, and which
must have required an enormous period for their accumulation; yet no one
ignorant of this fact would have even suspected the vast lapse of time
represented by the thinner formation. Many cases could be given of the
lower beds of a formation having been upraised, denuded, submerged, and
then re-covered by the upper beds of the same formation--facts, showing
what wide, yet easily overlooked, intervals have occurred in its
accumulation. In other cases we have the plainest evidence in great
fossilised trees, still standing upright as they grew, of many long
intervals of time and changes of level during the process of deposition,
which would not have been suspected, had not the trees been preserved:
thus Sir C. Lyell and Dr. Dawson found carboniferous beds 1,400 feet thick
in Nova Scotia, with ancient root-bearing strata, one above the other, at
no less than sixty-eight different levels. Hence, when the same species
occurs at the bottom, middle, and top of a formation, the probability is
that it has not lived on the same spot during the whole period of
deposition, but has disappeared and reappeared, perhaps many times, during
the same geological period. Consequently if it were to undergo a
considerable amount of modification during the deposition of any one
geological formation, a section would not include all the fine intermediate
gradations which must on our theory have existed, but abrupt, though
perhaps slight, changes of form.
It is all-important to remember that naturalists have no golden rule by
which to distinguish species and varieties; they grant some little
variability to each species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank both as species,
unless they are enabled to connect them together by the closest
intermediate gradations; and this, from the reasons just assigned, we can
seldom hope to effect in any one geological section. Supposing B and C to
be two species, and a third, A, to be found in an older and underlying bed;
even if A were strictly intermediate between B and C, it would simply be
ranked as a third and distinct species, unless at the same time it could be
closely connected by intermediate varieties with either one or both forms.
Nor should it be forgotten, as before explained, that A might be the actual
progenitor of B and C, and yet would not necessarily be strictly
intermediate between them in all respects. So that we might obtain the
parent-species and its several modified descendants from the lower and
upper beds of the same formation, and unless we obtained numerous
transitional gradations, we should not recognise their blood-relationship,
and should consequently rank them as distinct species.
It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the very
fine species of D'Orbigny and others into the rank of varieties; and on
this view we do find the kind of evidence of change which on the theory we
ought to find. Look again at the later tertiary deposits, which include
many shells believed by the majority of naturalists to be identical with
existing species; but some excellent naturalists, as Agassiz and Pictet,
maintain that all these tertiary species are specifically distinct, though
the distinction is admitted to be very slight; so that here, unless we
believe that these eminent naturalists have been misled by their
imaginations, and that these late tertiary species really present no
difference whatever from their living representatives, or unless we admit,
in opposition to the judgment of most naturalists, that these tertiary
species are all truly distinct from the recent, we have evidence of the
frequent occurrence of slight modifications of the kind required. If we
look to rather wider intervals of time, namely, to distinct but consecutive
stages of the same great formation, we find that the embedded fossils,
though universally ranked as specifically different, yet are far more
closely related to each other than are the species found in more widely
separated formations; so that here again we have undoubted evidence of
change in the direction required by the theory; but to this latter subject
I shall return in the following chapter.
With animals and plants that propagate rapidly and do not wander much,
there is reason to suspect, as we have formerly seen, that their varieties
are generally at first local; and that such local varieties do not spread
widely and supplant their parent-form until they have been modified and
perfected in some considerable degree. According to this view, the chance
of discovering in a formation in any one country all the early stages of
transition between any two forms, is small, for the successive changes are
supposed to have been local or confined to some one spot. Most marine
animals have a wide range; and we have seen that with plants it is those
which have the widest range, that oftenest present varieties, so that, with
shells and other marine animals, it is probable that those which had the
widest range, far exceeding the limits of the known geological formations
in Europe, have oftenest given rise, first to local varieties and
ultimately to new species; and this again would greatly lessen the chance
of our being able to trace the stages of transition in any one geological
formation.
It is a more important consideration, leading to the same result, as lately
insisted on by Dr. Falconer, namely, that the period during which each
species underwent modification, though long as measured by years, was
probably short in comparison with that during which it remained without
undergoing any change.
It should not be forgotten, that at the present day, with perfect specimens
for examination, two forms can seldom be connected by intermediate
varieties, and thus proved to be the same species, until many specimens are
collected from many places; and with fossil species this can rarely be
done. We shall, perhaps, best perceive the improbability of our being
enabled to connect species by numerous, fine, intermediate, fossil links,
by asking ourselves whether, for instance, geologists at some future period
will be able to prove that our different breeds of cattle, sheep, horses,
and dogs are descended from a single stock or from several aboriginal
stocks; or, again, whether certain sea-shells inhabiting the shores of
North America, which are ranked by some conchologists as distinct species
from their European representatives, and by other conchologists as only
varieties, are really varieties, or are, as it is called, specifically
distinct. This could be effected by the future geologist only by his
discovering in a fossil state numerous intermediate gradations; and such
success is improbable in the highest degree.
It has been asserted over and over again, by writers who believe in the
immutability of species, that geology yields no linking forms. This
assertion, as we shall see in the next chapter, is certainly erroneous. As
Sir J. Lubbock has remarked, "Every species is a link between other allied
forms." If we take a genus having a score of species, recent and extinct,
and destroy four-fifths of them, no one doubts that the remainder will
stand much more distinct from each other. If the extreme forms in the
genus happen to have been thus destroyed, the genus itself will stand more
distinct from other allied genera. What geological research has not
revealed, is the former existence of infinitely numerous gradations, as
fine as existing varieties, connecting together nearly all existing and
extinct species. But this ought not to be expected; yet this has been
repeatedly advanced as a most serious objection against my views.
It may be worth while to sum up the foregoing remarks on the causes of the
imperfection of the geological record under an imaginary illustration. The
Malay Archipelago is about the size of Europe from the North Cape to the
Mediterranean, and from Britain to Russia; and therefore equals all the
geological formations which have been examined with any accuracy, excepting
those of the United States of America. I fully agree with Mr. Godwin-
Austen, that the present condition of the Malay Archipelago, with its
numerous large islands separated by wide and shallow seas, probably
represents the former state of Europe, while most of our formations were
accumulating. The Malay Archipelago is one of the richest regions in
organic beings; yet if all the species were to be collected which have ever
lived there, how imperfectly would they represent the natural history of
the world!
But we have every reason to believe that the terrestrial productions of the
archipelago would be preserved in an extremely imperfect manner in the
formations which we suppose to be there accumulating. Not many of the
strictly littoral animals, or of those which lived on naked submarine
rocks, would be embedded; and those embedded in gravel or sand would not
endure to a distant epoch. Wherever sediment did not accumulate on the bed
of the sea, or where it did not accumulate at a sufficient rate to protect
organic bodies from decay, no remains could be preserved.
Formations rich in fossils of many kinds, and of thickness sufficient to
last to an age as distant in futurity as the secondary formations lie in
the past, would generally be formed in the archipelago only during periods
of subsidence. These periods of subsidence would be separated from each
other by immense intervals of time, during which the area would be either
stationary or rising; whilst rising, the fossiliferous formations on the
steeper shores would be destroyed, almost as soon as accumulated, by the
incessant coast-action, as we now see on the shores of South America. Even
throughout the extensive and shallow seas within the archipelago,
sedimentary beds could hardly be accumulated of great thickness during the
periods of elevation, or become capped and protected by subsequent
deposits, so as to have a good chance of enduring to a very distant future.
During the periods of subsidence, there would probably be much extinction
of life; during the periods of elevation, there would be much variation,
but the geological record would then be less perfect.
It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with a
contemporaneous accumulation of sediment, would EXCEED the average duration
of the same specific forms; and these contingencies are indispensable for
the preservation of all the transitional gradations between any two or more
species. If such gradations were not all fully preserved, transitional
varieties would merely appear as so many new, though closely allied
species. It is also probable that each great period of subsidence would be
interrupted by oscillations of level, and that slight climatical changes
would intervene during such lengthy periods; and in these cases the
inhabitants of the archipelago would migrate, and no closely consecutive
record of their modifications could be preserved in any one formation.
Very many of the marine inhabitants of the archipelago now range thousands
of miles beyond its confines; and analogy plainly leads to the belief that
it would be chiefly these far-ranging species, though only some of them,
which would oftenest produce new varieties; and the varieties would at
first be local or confined to one place, but if possessed of any decided
advantage, or when further modified and improved, they would slowly spread
and supplant their parent-forms. When such varieties returned to their
ancient homes, as they would differ from their former state in a nearly
uniform, though perhaps extremely slight degree, and as they would be found
embedded in slightly different sub-stages of the same formation, they
would, according to the principles followed by many palaeontologists, be
ranked as new and distinct species.
If then there be some degree of truth in these remarks, we have no right to
expect to find, in our geological formations, an infinite number of those
fine transitional forms, which, on our theory, have connected all the past
and present species of the same group into one long and branching chain of
life. We ought only to look for a few links, and such assuredly we do
find--some more distantly, some more closely, related to each other; and
these links, let them be ever so close, if found in different stages of the
same formation, would, by many palaeontologists, be ranked as distinct
species. But I do not pretend that I should ever have suspected how poor
was the record in the best preserved geological sections, had not the
absence of innumerable transitional links between the species which lived
at the commencement and close of each formation, pressed so hardly on my
theory.
ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED SPECIES.
The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palaeontologists--for
instance, by Agassiz, Pictet, and Sedgwick, as a fatal objection to the
belief in the transmutation of species. If numerous species, belonging to
the same genera or families, have really started into life at once, the
fact would be fatal to the theory of evolution through natural selection.
For the development by this means of a group of forms, all of which are
descended from some one progenitor, must have been an extremely slow
process; and the progenitors must have lived long before their modified
descendants. But we continually overrate the perfection of the geological
record, and falsely infer, because certain genera or families have not been
found beneath a certain stage, that they did not exist before that stage.
In all cases positive palaeontological evidence may be implicitly trusted;
negative evidence is worthless, as experience has so often shown. We
continually forget how large the world is, compared with the area over
which our geological formations have been carefully examined; we forget
that groups of species may elsewhere have long existed, and have slowly
multiplied, before they invaded the ancient archipelagoes of Europe and the
United States. We do not make due allowance for the enormous intervals of
time which have elapsed between our consecutive formations, longer perhaps
in many cases than the time required for the accumulation of each
formation. These intervals will have given time for the multiplication of
species from some one parent-form: and in the succeeding formation, such
groups or species will appear as if suddenly created.
I may here recall a remark formerly made, namely, that it might require a
long succession of ages to adapt an organism to some new and peculiar line
of life, for instance, to fly through the air; and consequently that the
transitional forms would often long remain confined to some one region; but
that, when this adaptation had once been effected, and a few species had
thus acquired a great advantage over other organisms, a comparatively short
time would be necessary to produce many divergent forms, which would spread
rapidly and widely throughout the world. Professor Pictet, in his
excellent Review of this work, in commenting on early transitional forms,
and taking birds as an illustration, cannot see how the successive
modifications of the anterior limbs of a supposed prototype could possibly
have been of any advantage. But look at the penguins of the Southern
Ocean; have not these birds their front limbs in this precise intermediate
state of "neither true arms nor true wings?" Yet these birds hold their
place victoriously in the battle for life; for they exist in infinite
numbers and of many kinds. I do not suppose that we here see the real
transitional grades through which the wings of birds have passed; but what
special difficulty is there in believing that it might profit the modified
descendants of the penguin, first to become enabled to flap along the
surface of the sea like the logger-headed duck, and ultimately to rise from
its surface and glide through the air?
I will now give a few examples to illustrate the foregoing remarks, and to
show how liable we are to error in supposing that whole groups of species
have suddenly been produced. Even in so short an interval as that between
the first and second editions of Pictet's great work on Palaeontology,
published in 1844-46 and in 1853-57, the conclusions on the first
appearance and disappearance of several groups of animals have been
considerably modified; and a third edition would require still further
changes. I may recall the well-known fact that in geological treatises,
published not many years ago, mammals were always spoken of as having
abruptly come in at the commencement of the tertiary series. And now one
of the richest known accumulations of fossil mammals belongs to the middle
of the secondary series; and true mammals have been discovered in the new
red sandstone at nearly the commencement of this great series. Cuvier used
to urge that no monkey occurred in any tertiary stratum; but now extinct
species have been discovered in India, South America and in Europe, as far
back as the miocene stage. Had it not been for the rare accident of the
preservation of footsteps in the new red sandstone of the United States,
who would have ventured to suppose that no less than at least thirty
different bird-like animals, some of gigantic size, existed during that
period? Not a fragment of bone has been discovered in these beds. Not
long ago, palaeontologists maintained that the whole class of birds came
suddenly into existence during the eocene period; but now we know, on the
authority of Professor Owen, that a bird certainly lived during the
deposition of the upper greensand; and still more recently, that strange
bird, the Archeopteryx, with a long lizard-like tail, bearing a pair of
feathers on each joint, and with its wings furnished with two free claws,
has been discovered in the oolitic slates of Solenhofen. Hardly any recent
discovery shows more forcibly than this how little we as yet know of the
former inhabitants of the world.
I may give another instance, which, from having passed under my own eyes
has much struck me. In a memoir on Fossil Sessile Cirripedes, I stated
that, from the large number of existing and extinct tertiary species; from
the extraordinary abundance of the individuals of many species all over the
world, from the Arctic regions to the equator, inhabiting various zones of
depths, from the upper tidal limits to fifty fathoms; from the perfect
manner in which specimens are preserved in the oldest tertiary beds; from
the ease with which even a fragment of a valve can be recognised; from all
these circumstances, I inferred that, had sessile cirripedes existed during
the secondary periods, they would certainly have been preserved and
discovered; and as not one species had then been discovered in beds of this
age, I concluded that this great group had been suddenly developed at the
commencement of the tertiary series. This was a sore trouble to me,
adding, as I then thought, one more instance of the abrupt appearance of a
great group of species. But my work had hardly been published, when a
skilful palaeontologist, M. Bosquet, sent me a drawing of a perfect
specimen of an unmistakable sessile cirripede, which he had himself
extracted from the chalk of Belgium. And, as if to make the case as
striking as possible, this cirripede was a Chthamalus, a very common,
large, and ubiquitous genus, of which not one species has as yet been found
even in any tertiary stratum. Still more recently, a Pyrgoma, a member of
a distinct subfamily of sessile cirripedes, has been discovered by Mr.
Woodward in the upper chalk; so that we now have abundant evidence of the
existence of this group of animals during the secondary period.
The case most frequently insisted on by palaeontologists of the apparently
sudden appearance of a whole group of species, is that of the teleostean
fishes, low down, according to Agassiz, in the Chalk period. This group
includes the large majority of existing species. But certain Jurassic and
Triassic forms are now commonly admitted to be teleostean; and even some
palaeozoic forms have thus been classed by one high authority. If the
teleosteans had really appeared suddenly in the northern hemisphere at the
commencement of the chalk formation, the fact would have been highly
remarkable; but it would not have formed an insuperable difficulty, unless
it could likewise have been shown that at the same period the species were
suddenly and simultaneously developed in other quarters of the world. It
is almost superfluous to remark that hardly any fossil-fish are known from
south of the equator; and by running through Pictet's Palaeontology it will
be seen that very few species are known from several formations in Europe.
Some few families of fish now have a confined range; the teleostean fishes
might formerly have had a similarly confined range, and after having been
largely developed in some one sea, have spread widely. Nor have we any
right to suppose that the seas of the world have always been so freely open
from south to north as they are at present. Even at this day, if the Malay
Archipelago were converted into land, the tropical parts of the Indian
Ocean would form a large and perfectly enclosed basin, in which any great
group of marine animals might be multiplied; and here they would remain
confined, until some of the species became adapted to a cooler climate, and
were enabled to double the southern capes of Africa or Australia, and thus
reach other and distant seas.
>From these considerations, from our ignorance of the geology of other
countries beyond the confines of Europe and the United States, and from the
revolution in our palaeontological knowledge effected by the discoveries of
the last dozen years, it seems to me to be about as rash to dogmatize on
the succession of organic forms throughout the world, as it would be for a
naturalist to land for five minutes on a barren point in Australia, and
then to discuss the number and range of its productions.
ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST KNOWN
FOSSILIFEROUS STRATA.
There is another and allied difficulty, which is much more serious. I
allude to the manner in which species belonging to several of the main
divisions of the animal kingdom suddenly appear in the lowest known
fossiliferous rocks. Most of the arguments which have convinced me that
all the existing species of the same group are descended from a single
progenitor, apply with equal force to the earliest known species. For
instance, it cannot be doubted that all the Cambrian and Silurian
trilobites are descended from some one crustacean, which must have lived
long before the Cambrian age, and which probably differed greatly from any
known animal. Some of the most ancient animals, as the Nautilus, Lingula,
etc., do not differ much from living species; and it cannot on our theory
be supposed, that these old species were the progenitors of all the species
belonging to the same groups which have subsequently appeared, for they are
not in any degree intermediate in character.
Consequently, if the theory be true, it is indisputable that before the
lowest Cambrian stratum was deposited long periods elapsed, as long as, or
probably far longer than, the whole interval from the Cambrian age to the
present day; and that during these vast periods the world swarmed with
living creatures. Here we encounter a formidable objection; for it seems
doubtful whether the earth, in a fit state for the habitation of living
creatures, has lasted long enough. Sir W. Thompson concludes that the
consolidation of the crust can hardly have occurred less than twenty or
more than four hundred million years ago, but probably not less than
ninety-eight or more than two hundred million years. These very wide
limits show how doubtful the data are; and other elements may have
hereafter to be introduced into the problem. Mr. Croll estimates that
about sixty million years have elapsed since the Cambrian period, but this,
judging from the small amount of organic change since the commencement of
the Glacial epoch, appears a very short time for the many and great
mutations of life, which have certainly occurred since the Cambrian
formation; and the previous one hundred and forty million years can hardly
be considered as sufficient for the development of the varied forms of life
which already existed during the Cambrian period. It is, however,
probable, as Sir William Thompson insists, that the world at a very early
period was subjected to more rapid and violent changes in its physical
conditions than those now occurring; and such changes would have tended to
induce changes at a corresponding rate in the organisms which then existed.
To the question why we do not find rich fossiliferous deposits belonging to
these assumed earliest periods prior to the Cambrian system, I can give no
satisfactory answer. Several eminent geologists, with Sir R. Murchison at
their head, were until recently convinced that we beheld in the organic
remains of the lowest Silurian stratum the first dawn of life. Other
highly competent judges, as Lyell and E. Forbes, have disputed this
conclusion. We should not forget that only a small portion of the world is
known with accuracy. Not very long ago M. Barrande added another and lower
stage, abounding with new and peculiar species, beneath the then known
Silurian system; and now, still lower down in the Lower Cambrian formation,
Mr Hicks has found South Wales beds rich in trilobites, and containing
various molluscs and annelids. The presence of phosphatic nodules and
bituminous matter, even in some of the lowest azotic rocks, probably
indicates life at these periods; and the existence of the Eozoon in the
Laurentian formation of Canada is generally admitted. There are three
great series of strata beneath the Silurian system in Canada, in the lowest
of which the Eozoon is found. Sir W. Logan states that their "united
thickness may possibly far surpass that of all the succeeding rocks, from
the base of the palaeozoic series to the present time. We are thus carried
back to a period so remote, that the appearance of the so-called primordial
fauna (of Barrande) may by some be considered as a comparatively modern
event." The Eozoon belongs to the most lowly organised of all classes of
animals, but is highly organised for its class; it existed in countless
numbers, and, as Dr. Dawson has remarked, certainly preyed on other minute
organic beings, which must have lived in great numbers. Thus the words,
which I wrote in 1859, about the existence of living beings long before the
Cambrian period, and which are almost the same with those since used by Sir
W. Logan, have proved true. Nevertheless, the difficulty of assigning any
good reason for the absence of vast piles of strata rich in fossils beneath
the Cambrian system is very great. It does not seem probable that the most
ancient beds have been quite worn away by denudation, or that their fossils
have been wholly obliterated by metamorphic action, for if this had been
the case we should have found only small remnants of the formations next
succeeding them in age, and these would always have existed in a partially
metamorphosed condition. But the descriptions which we possess of the
Silurian deposits over immense territories in Russia and in North America,
do not support the view that the older a formation is the more invariably
it has suffered extreme denudation and metamorphism.
The case at present must remain inexplicable; and may be truly urged as a
valid argument against the views here entertained. To show that it may
hereafter receive some explanation, I will give the following hypothesis.
>From the nature of the organic remains which do not appear to have
inhabited profound depths, in the several formations of Europe and of the
United States; and from the amount of sediment, miles in thickness, of
which the formations are composed, we may infer that from first to last
large islands or tracts of land, whence the sediment was derived, occurred
in the neighbourhood of the now existing continents of Europe and North
America. This same view has since been maintained by Agassiz and others.
But we do not know what was the state of things in the intervals between
the several successive formations; whether Europe and the United States
during these intervals existed as dry land, or as a submarine surface near
land, on which sediment was not deposited, or as the bed of an open and
unfathomable sea.
Looking to the existing oceans, which are thrice as extensive as the land,
we see them studded with many islands; but hardly one truly oceanic island
(with the exception of New Zealand, if this can be called a truly oceanic
island) is as yet known to afford even a remnant of any palaeozoic or
secondary formation. Hence, we may perhaps infer, that during the
palaeozoic and secondary periods, neither continents nor continental
islands existed where our oceans now extend; for had they existed,
palaeozoic and secondary formations would in all probability have been
accumulated from sediment derived from their wear and tear; and would have
been at least partially upheaved by the oscillations of level, which must
have intervened during these enormously long periods. If, then, we may
infer anything from these facts, we may infer that, where our oceans now
extend, oceans have extended from the remotest period of which we have any
record; and on the other hand, that where continents now exist, large
tracts of land have existed, subjected, no doubt, to great oscillations of
level, since the Cambrian period. The coloured map appended to my volume
on Coral Reefs, led me to conclude that the great oceans are still mainly
areas of subsidence, the great archipelagoes still areas of oscillations of
level, and the continents areas of elevation. But we have no reason to
assume that things have thus remained from the beginning of the world. Our
continents seem to have been formed by a preponderance, during many
oscillations of level, of the force of elevation. But may not the areas of
preponderant movement have changed in the lapse of ages? At a period long
antecedent to the Cambrian epoch, continents may have existed where oceans
are now spread out, and clear and open oceans may have existed where our
continents now stand. Nor should we be justified in assuming that if, for
instance, the bed of the Pacific Ocean were now converted into a continent
we should there find sedimentary formations, in recognisable condition,
older than the Cambrian strata, supposing such to have been formerly
deposited; for it might well happen that strata which had subsided some
miles nearer to the centre of the earth, and which had been pressed on by
an enormous weight of superincumbent water, might have undergone far more
metamorphic action than strata which have always remained nearer to the
surface. The immense areas in some parts of the world, for instance in
South America, of naked metamorphic rocks, which must have been heated
under great pressure, have always seemed to me to require some special
explanation; and we may perhaps believe that we see in these large areas
the many formations long anterior to the Cambrian epoch in a completely
metamorphosed and denuded condition.
The several difficulties here discussed, namely, that, though we find in
our geological formations many links between the species which now exist
and which formerly existed, we do not find infinitely numerous fine
transitional forms closely joining them all together. The sudden manner in
which several groups of species first appear in our European formations,
the almost entire absence, as at present known, of formations rich in
fossils beneath the Cambrian strata, are all undoubtedly of the most
serious nature. We see this in the fact that the most eminent
palaeontologists, namely, Cuvier, Agassiz, Barrande, Pictet, Falconer, E.
Forbes, etc., and all our greatest geologists, as Lyell, Murchison,
Sedgwick, etc., have unanimously, often vehemently, maintained the
immutability of species. But Sir Charles Lyell now gives the support of
his high authority to the opposite side, and most geologists and
palaeontologists are much shaken in their former belief. Those who believe
that the geological record is in any degree perfect, will undoubtedly at
once reject my theory. For my part, following out Lyell's metaphor, I look
at the geological record as a history of the world imperfectly kept and
written in a changing dialect. Of this history we possess the last volume
alone, relating only to two or three countries. Of this volume, only here
and there a short chapter has been preserved, and of each page, only here
and there a few lines. Each word of the slowly-changing language, more or
less different in the successive chapters, may represent the forms of life,
which are entombed in our consecutive formations, and which falsely appear
to have been abruptly introduced. On this view the difficulties above
discussed are greatly diminished or even disappear.
CHAPTER XI.
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
On the slow and successive appearance of new species -- On their different
rates of change -- Species once lost do not reappear -- Groups of species
follow the same general rules in their appearance and disappearance as do
single species -- On extinction -- On simultaneous changes in the forms of
life throughout the world -- On the affinities of extinct species to each
other and to living species -- On the state of development of ancient forms
-- On the succession of the same types within the same areas -- Summary of
preceding and present chapters.
Let us now see whether the several facts and laws relating to the
geological succession of organic beings accord best with the common view of
the immutability of species, or with that of their slow and gradual
modification, through variation and natural selection.
New species have appeared very slowly, one after another, both on the land
and in the waters. Lyell has shown that it is hardly possible to resist
the evidence on this head in the case of the several tertiary stages; and
every year tends to fill up the blanks between the stages, and to make the
proportion between the lost and existing forms more gradual. In some of
the most recent beds, though undoubtedly of high antiquity if measured by
years, only one or two species are extinct, and only one or two are new,
having appeared there for the first time, either locally, or, as far as we
know, on the face of the earth. The secondary formations are more broken;
but, as Bronn has remarked, neither the appearance nor disappearance of the
many species embedded in each formation has been simultaneous.
Species belonging to different genera and classes have not changed at the
same rate, or in the same degree. In the older tertiary beds a few living
shells may still be found in the midst of a multitude of extinct forms.
Falconer has given a striking instance of a similar fact, for an existing
crocodile is associated with many lost mammals and reptiles in the
sub-Himalayan deposits. The Silurian Lingula differs but little from the
living species of this genus; whereas most of the other Silurian Molluscs
and all the Crustaceans have changed greatly. The productions of the land
seem to have changed at a quicker rate than those of the sea, of which a
striking instance has been observed in Switzerland. There is some reason
to believe that organisms high in the scale, change more quickly than those
that are low: though there are exceptions to this rule. The amount of
organic change, as Pictet has remarked, is not the same in each successive
so-called formation. Yet if we compare any but the most closely related
formations, all the species will be found to have undergone some change.
When a species has once disappeared from the face of the earth, we have no
reason to believe that the same identical form ever reappears. The
strongest apparent exception to this latter rule is that of the so-called
"colonies" of M. Barrande, which intrude for a period in the midst of an
older formation, and then allow the pre-existing fauna to reappear; but
Lyell's explanation, namely, that it is a case of temporary migration from
a distinct geographical province, seems satisfactory.
These several facts accord well with our theory, which includes no fixed
law of development, causing all the inhabitants of an area to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be slow, and will generally affect only a few species at
the same time; for the variability of each species is independent of that
of all others. Whether such variations or individual differences as may
arise will be accumulated through natural selection in a greater or less
degree, thus causing a greater or less amount of permanent modification,
will depend on many complex contingencies--on the variations being of a
beneficial nature, on the freedom of intercrossing, on the slowly changing
physical conditions of the country, on the immigration of new colonists,
and on the nature of the other inhabitants with which the varying species
come into competition. Hence it is by no means surprising that one species
should retain the same identical form much longer than others; or, if
changing, should change in a less degree. We find similar relations
between the existing inhabitants of distinct countries; for instance, the
land-shells and coleopterous insects of Madeira have come to differ
considerably from their nearest allies on the continent of Europe, whereas
the marine shells and birds have remained unaltered. We can perhaps
understand the apparently quicker rate of change in terrestrial and in more
highly organised productions compared with marine and lower productions, by
the more complex relations of the higher beings to their organic and
inorganic conditions of life, as explained in a former chapter. When many
of the inhabitants of any area have become modified and improved, we can
understand, on the principle of competition, and from the all-important
relations of organism to organism in the struggle for life, that any form
which did not become in some degree modified and improved, would be liable
to extermination. Hence, we see why all the species in the same region do
at last, if we look to long enough intervals of time, become modified; for
otherwise they would become extinct.
In members of the same class the average amount of change, during long and
equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of enduring formations, rich in fossils, depends on great
masses of sediment being deposited on subsiding areas, our formations have
been almost necessarily accumulated at wide and irregularly intermittent
intervals of time; consequently the amount of organic change exhibited by
the fossils embedded in consecutive formations is not equal. Each
formation, on this view, does not mark a new and complete act of creation,
but only an occasional scene, taken almost at hazard, in an ever slowly
changing drama.
We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and inorganic,
should recur. For though the offspring of one species might be adapted
(and no doubt this has occurred in innumerable instances) to fill the place
of another species in the economy of nature, and thus supplant it; yet the
two forms--the old and the new--would not be identically the same; for both
would almost certainly inherit different characters from their distinct
progenitors; and organisms already differing would vary in a different
manner. For instance, it is possible, if all our fantail-pigeons were
destroyed, that fanciers might make a new breed hardly distinguishable from
the present breed; but if the parent rock-pigeon were likewise destroyed,
and under nature we have every reason to believe that parent forms are
generally supplanted and exterminated by their improved offspring, it is
incredible that a fantail, identical with the existing breed, could be
raised from any other species of pigeon, or even from any other well
established race of the domestic pigeon, for the successive variations
would almost certainly be in some degree different, and the newly-formed
variety would probably inherit from its progenitor some characteristic
differences.
Groups of species, that is, genera and families, follow the same general
rules in their appearance and disappearance as do single species, changing
more or less quickly, and in a greater or lesser degree. A group, when it
has once disappeared, never reappears; that is, its existence, as long as
it lasts, is continuous. I am aware that there are some apparent
exceptions to this rule, but the exceptions are surprisingly few, so few
that E. Forbes, Pictet, and Woodward (though all strongly opposed to such
views as I maintain) admit its truth; and the rule strictly accords with
the theory. For all the species of the same group, however long it may
have lasted, are the modified descendants one from the other, and all from
a common progenitor. In the genus Lingula, for instance, the species which
have successively appeared at all ages must have been connected by an
unbroken series of generations, from the lowest Silurian stratum to the
present day.
We have seen in the last chapter that whole groups of species sometimes
falsely appear to have been abruptly developed; and I have attempted to
give an explanation of this fact, which if true would be fatal to my views.
But such cases are certainly exceptional; the general rule being a gradual
increase in number, until the group reaches its maximum, and then, sooner
or later, a gradual decrease. If the number of the species included within
a genus, or the number of the genera within a family, be represented by a
vertical line of varying thickness, ascending through the successive
geological formations, in which the species are found, the line will
sometimes falsely appear to begin at its lower end, not in a sharp point,
but abruptly; it then gradually thickens upwards, often keeping of equal
thickness for a space, and ultimately thins out in the upper beds, marking
the decrease and final extinction of the species. This gradual increase in
number of the species of a group is strictly conformable with the theory;
for the species of the same genus, and the genera of the same family, can
increase only slowly and progressively; the process of modification and the
production of a number of allied forms necessarily being a slow and gradual
process, one species first giving rise to two or three varieties, these
being slowly converted into species, which in their turn produce by equally
slow steps other varieties and species, and so on, like the branching of a
great tree from a single stem, till the group becomes large.
ON EXTINCTION.
We have as yet only spokesn incidentally of the disappearance of species
and of groups of species. On the theory of natural selection, the
extinction of old forms and the production of new and improved forms are
intimately connected together. The old notion of all the inhabitants of
the earth having been swept away by catastrophes at successive periods is
very generally given up, even by those geologists, as Elie de Beaumont,
Murchison, Barrande, etc., whose general views would naturally lead them to
this conclusion. On the contrary, we have every reason to believe, from
the study of the tertiary formations, that species and groups of species
gradually disappear, one after another, first from one spot, then from
another, and finally from the world. In some few cases, however, as by the
breaking of an isthmus and the consequent irruption of a multitude of new
inhabitants into an adjoining sea, or by the final subsidence of an island,
the process of extinction may have been rapid. Both single species and
whole groups of species last for very unequal periods; some groups, as we
have seen, have endured from the earliest known dawn of life to the present
day; some have disappeared before the close of the palaeozoic period. No
fixed law seems to determine the length of time during which any single
species or any single genus endures. There is reason to believe that the
extinction of a whole group of species is generally a slower process than
their production: if their appearance and disappearance be represented, as
before, by a vertical line of varying thickness the line is found to taper
more gradually at its upper end, which marks the progress of extermination,
than at its lower end, which marks the first appearance and the early
increase in number of the species. In some cases, however, the
extermination of whole groups, as of ammonites, towards the close of the
secondary period, has been wonderfully sudden.
The extinction of species has been involved in the most gratuitous mystery.
Some authors have even supposed that, as the individual has a definite
length of life, so have species a definite duration. No one can have
marvelled more than I have done at the extinction of species. When I found
in La Plata the tooth of a horse embedded with the remains of Mastodon,
Megatherium, Toxodon and other extinct monsters, which all co-existed with
still living shells at a very late geological period, I was filled with
astonishment; for, seeing that the horse, since its introduction by the
Spaniards into South America, has run wild over the whole country and has
increased in numbers at an unparalleled rate, I asked myself what could so
recently have exterminated the former horse under conditions of life
apparently so favourable. But my astonishment was groundless. Professor
Owen soon perceived that the tooth, though so like that of the existing
horse, belonged to an extinct species. Had this horse been still living,
but in some degree rare, no naturalist would have felt the least surprise
at its rarity; for rarity is the attribute of a vast number of species of
all classes, in all countries. If we ask ourselves why this or that
species is rare, we answer that something is unfavourable in its conditions
of life; but what that something is, we can hardly ever tell. On the
supposition of the fossil horse still existing as a rare species, we might
have felt certain, from the analogy of all other mammals, even of the
slow-breeding elephant, and from the history of the naturalisation of the
domestic horse in South America, that under more favourable conditions it
would in a very few years have stocked the whole continent. But we could
not have told what the unfavourable conditions were which checked its
increase, whether some one or several contingencies, and at what period of
the horse's life, and in what degree they severally acted. If the
conditions had gone on, however slowly, becoming less and less favourable,
we assuredly should not have perceived the fact, yet the fossil horse would
certainly have become rarer and rarer, and finally extinct--its place being
seized on by some more successful competitor.
It is most difficult always to remember that the increase of every living
creature is constantly being checked by unperceived hostile agencies; and
that these same unperceived agencies are amply sufficient to cause rarity,
and finally extinction. So little is this subject understood, that I have
heard surprise repeatedly expressed at such great monsters as the Mastodon
and the more ancient Dinosaurians having become extinct; as if mere bodily
strength gave victory in the battle of life. Mere size, on the contrary,
would in some cases determine, as has been remarked by Owen, quicker
extermination, from the greater amount of requisite food. Before man
inhabited India or Africa, some cause must have checked the continued
increase of the existing elephant. A highly capable judge, Dr. Falconer,
believes that it is chiefly insects which, from incessantly harassing and
weakening the elephant in India, check its increase; and this was Bruce's
conclusion with respect to the African elephant in Abyssinia. It is
certain that insects and blood-sucking bats determine the existence of the
larger naturalised quadrupeds in several parts of South America.
We see in many cases in the more recent tertiary formations that rarity
precedes extinction; and we know that this has been the progress of events
with those animals which have been exterminated, either locally or wholly,
through man's agency. I may repeat what I published in 1845, namely, that
to admit that species generally become rare before they become extinct--to
feel no surprise at the rarity of a species, and yet to marvel greatly when
the species ceases to exist, is much the same as to admit that sickness in
the individual is the forerunner of death--to feel no surprise at sickness,
but, when the sick man dies, to wonder and to suspect that he died by some
deed of violence.
The theory of natural selection is grounded on the belief that each new
variety and ultimately each new species, is produced and maintained by
having some advantage over those with which it comes into competition; and
the consequent extinction of less-favoured forms almost inevitably follows.
It is the same with our domestic productions: when a new and slightly
improved variety has been raised, it at first supplants the less improved
varieties in the same neighbourhood; when much improved it is transported
far and near, like our short-horn cattle, and takes the place of other
breeds in other countries. Thus the appearance of new forms and the
disappearance of old forms, both those naturally and artificially produced,
are bound together. In flourishing groups, the number of new specific
forms which have been produced within a given time has at some periods
probably been greater than the number of the old specific forms which have
been exterminated; but we know that species have not gone on indefinitely
increasing, at least during the later geological epochs, so that, looking
to later times, we may believe that the production of new forms has caused
the extinction of about the same number of old forms.
The competition will generally be most severe, as formerly explained and
illustrated by examples, between the forms which are most like each other
in all respects. Hence the improved and modified descendants of a species
will generally cause the extermination of the parent-species; and if many
new forms have been developed from any one species, the nearest allies of
that species, i.e. the species of the same genus, will be the most liable
to extermination. Thus, as I believe, a number of new species descended
from one species, that is a new genus, comes to supplant an old genus,
belonging to the same family. But it must often have happened that a new
species belonging to some one group has seized on the place occupied by a
species belonging to a distinct group, and thus have caused its
extermination. If many allied forms be developed from the successful
intruder, many will have to yield their places; and it will generally be
the allied forms, which will suffer from some inherited inferiority in
common. But whether it be species belonging to the same or to a distinct
class, which have yielded their places to other modified and improved
species, a few of the sufferers may often be preserved for a long time,
from being fitted to some peculiar line of life, or from inhabiting some
distant and isolated station, where they will have escaped severe
competition. For instance, some species of Trigonia, a great genus of
shells in the secondary formations, survive in the Australian seas; and a
few members of the great and almost extinct group of Ganoid fishes still
inhabit our fresh waters. Therefore, the utter extinction of a group is
generally, as we have seen, a slower process than its production.
With respect to the apparently sudden extermination of whole families or
orders, as of Trilobites at the close of the palaeozoic period, and of
Ammonites at the close of the secondary period, we must remember what has
been already said on the probable wide intervals of time between our
consecutive formations; and in these intervals there may have been much
slow extermination. Moreover, when, by sudden immigration or by unusually
rapid development, many species of a new group have taken possession of an
area, many of the older species will have been exterminated in a
correspondingly rapid manner; and the forms which thus yield their places
will commonly be allied, for they will partake of the same inferiority in
common.
Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct accords well with the theory of natural
selection. We need not marvel at extinction; if we must marvel, let it be
at our presumption in imagining for a moment that we understand the many
complex contingencies on which the existence of each species depends. If
we forget for an instant that each species tends to increase inordinately,
and that some check is always in action, yet seldom perceived by us, the
whole economy of nature will be utterly obscured. Whenever we can
precisely say why this species is more abundant in individuals than that;
why this species and not another can be naturalised in a given country;
then, and not until then, we may justly feel surprise why we cannot account
for the extinction of any particular species or group of species.
ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THE WORLD.
Scarcely any palaeontological discovery is more striking than the fact that
the forms of life change almost simultaneously throughout the world. Thus
our European Chalk formation can be recognised in many distant regions,
under the most different climates, where not a fragment of the mineral
chalk itself can be found; namely, in North America, in equatorial South
America, in Tierra del Fuego, at the Cape of Good Hope, and in the
peninsula of India. For at these distant points, the organic remains in
certain beds present an unmistakable resemblance to those of the Chalk. It
is not that the same species are met with; for in some cases not one
species is identically the same, but they belong to the same families,
genera, and sections of genera, and sometimes are similarly characterised
in such trifling points as mere superficial sculpture. Moreover, other
forms, which are not found in the Chalk of Europe, but which occur in the
formations either above or below, occur in the same order at these distant
points of the world. In the several successive palaeozoic formations of
Russia, Western Europe and North America, a similar parallelism in the
forms of life has been observed by several authors; so it is, according to
Lyell, with the European and North American tertiary deposits. Even if the
few fossil species which are common to the Old and New Worlds were kept
wholly out of view, the general parallelism in the successive forms of
life, in the palaeozoic and tertiary stages, would still be manifest, and
the several formations could be easily correlated.
These observations, however, relate to the marine inhabitants of the world:
we have not sufficient data to judge whether the productions of the land
and of fresh water at distant points change in the same parallel manner.
We may doubt whether they have thus changed: if the Megatherium, Mylodon,
Macrauchenia, and Toxodon had been brought to Europe from La Plata, without
any information in regard to their geological position, no one would have
suspected that they had co-existed with sea-shells all still living; but as
these anomalous monsters co-existed with the Mastodon and Horse, it might
at least have been inferred that they had lived during one of the later
tertiary stages.
When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same year, or even to the same century, or even
that it has a very strict geological sense; for if all the marine animals
now living in Europe, and all those that lived in Europe during the
pleistocene period (a very remote period as measured by years, including
the whole glacial epoch) were compared with those now existing in South
America or in Australia, the most skilful naturalist would hardly be able
to say whether the present or the pleistocene inhabitants of Europe
resembled most closely those of the southern hemisphere. So, again,
several highly competent observers maintain that the existing productions
of the United States are more closely related to those which lived in
Europe during certain late tertiary stages, than to the present inhabitants
of Europe; and if this be so, it is evident that fossiliferous beds now
deposited on the shores of North America would hereafter be liable to be
classed with somewhat older European beds. Nevertheless, looking to a
remotely future epoch, there can be little doubt that all the more modern
MARINE formations, namely, the upper pliocene, the pleistocene and strictly
modern beds of Europe, North and South America, and Australia, from
containing fossil remains in some degree allied, and from not including
those forms which are found only in the older underlying deposits, would be
correctly ranked as simultaneous in a geological sense.
The fact of the forms of life changing simultaneously in the above large
sense, at distant parts of the world, has greatly struck those admirable
observers, MM. de Verneuil and d'Archiac. After referring to the
parallelism of the palaeozoic forms of life in various parts of Europe,
they add, "If struck by this strange sequence, we turn our attention to
North America, and there discover a series of analogous phenomena, it will
appear certain that all these modifications of species, their extinction,
and the introduction of new ones, cannot be owing to mere changes in marine
currents or other causes more or less local and temporary, but depend on
general laws which govern the whole animal kingdom." M. Barrande has made
forcible remarks to precisely the same effect. It is, indeed, quite futile
to look to changes of currents, climate, or other physical conditions, as
the cause of these great mutations in the forms of life throughout the
world, under the most different climates. We must, as Barrande has
remarked, look to some special law. We shall see this more clearly when we
treat of the present distribution of organic beings, and find how slight is
the relation between the physical conditions of various countries and the
nature of their inhabitants.
This great fact of the parallel succession of the forms of life throughout
the world, is explicable on the theory of natural selection. New species
are formed by having some advantage over older forms; and the forms, which
are already dominant, or have some advantage over the other forms in their
own country, give birth to the greatest number of new varieties or
incipient species. We have distinct evidence on this head, in the plants
which are dominant, that is, which are commonest and most widely diffused,
producing the greatest number of new varieties. It is also natural that
the dominant, varying and far-spreading species, which have already
invaded, to a certain extent, the territories of other species, should be
those which would have the best chance of spreading still further, and of
giving rise in new countries to other new varieties and species. The
process of diffusion would often be very slow, depending on climatal and
geographical changes, on strange accidents, and on the gradual
acclimatization of new species to the various climates through which they
might have to pass, but in the course of time the dominant forms would
generally succeed in spreading and would ultimately prevail. The diffusion
would, it is probable, be slower with the terrestrial inhabitants of
distinct continents than with the marine inhabitants of the continuous sea.
We might therefore expect to find, as we do find, a less strict degree of
parallelism in the succession of the productions of the land than with
those of the sea.
Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the world,
accords well with the principle of new species having been formed by
dominant species spreading widely and varying; the new species thus
produced being themselves dominant, owing to their having had some
advantage over their already dominant parents, as well as over other
species; and again spreading, varying, and producing new forms. The old
forms which are beaten and which yield their places to the new and
victorious forms, will generally be allied in groups, from inheriting some
inferiority in common; and, therefore, as new and improved groups spread
throughout the world, old groups disappear from the world; and the
succession of forms everywhere tends to correspond both in their first
appearance and final disappearance.
There is one other remark connected with this subject worth making. I have
given my reasons for believing that most of our great formations, rich in
fossils, were deposited during periods of subsidence; and that blank
intervals of vast duration, as far as fossils are concerned, occurred
during the periods when the bed of the sea was either stationary or rising,
and likewise when sediment was not thrown down quickly enough to embed and
preserve organic remains. During these long and blank intervals I suppose
that the inhabitants of each region underwent a considerable amount of
modification and extinction, and that there was much migration from other
parts of the world. As we have reason to believe that large areas are
affected by the same movement, it is probable that strictly contemporaneous
formations have often been accumulated over very wide spaces in the same
quarter of the world; but we are very far from having any right to conclude
that this has invariably been the case, and that large areas have
invariably been affected by the same movements. When two formations have
been deposited in two regions during nearly, but not exactly, the same
period, we should find in both, from the causes explained in the foregoing
paragraphs, the same general succession in the forms of life; but the
species would not exactly correspond; for there will have been a little
more time in the one region than in the other for modification, extinction,
and immigration.
I suspect that cases of this nature occur in Europe. Mr. Prestwich, in his
admirable Memoirs on the eocene deposits of England and France, is able to
draw a close general parallelism between the successive stages in the two
countries; but when he compares certain stages in England with those in
France, although he finds in both a curious accordance in the numbers of
the species belonging to the same genera, yet the species themselves differ
in a manner very difficult to account for considering the proximity of the
two areas, unless, indeed, it be assumed that an isthmus separated two seas
inhabited by distinct, but contemporaneous faunas. Lyell has made similar
observations on some of the later tertiary formations. Barrande, also,
shows that there is a striking general parallelism in the successive
Silurian deposits of Bohemia and Scandinavia; nevertheless he finds a
surprising amount of difference in the species. If the several formations
in these regions have not been deposited during the same exact periods--a
formation in one region often corresponding with a blank interval in the
other--and if in both regions the species have gone on slowly changing
during the accumulation of the several formations and during the long
intervals of time between them; in this case the several formations in the
two regions could be arranged in the same order, in accordance with the
general succession of the forms of life, and the order would falsely appear
to be strictly parallel; nevertheless the species would not all be the same
in the apparently corresponding stages in the two regions.
ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, AND TO LIVING FORMS.
Let us now look to the mutual affinities of extinct and living species.
All fall into a few grand classes; and this fact is at once explained on
the principle of descent. The more ancient any form is, the more, as a
general rule, it differs from living forms. But, as Buckland long ago
remarked, extinct species can all be classed either in still existing
groups, or between them. That the extinct forms of life help to fill up
the intervals between existing genera, families, and orders, is certainly
true; but as this statement has often been ignored or even denied, it may
be well to make some remarks on this subject, and to give some instances.
If we confine our attention either to the living or to the extinct species
of the same class, the series is far less perfect than if we combine both
into one general system. In the writings of Professor Owen we continually
meet with the expression of generalised forms, as applied to extinct
animals; and in the writings of Agassiz, of prophetic or synthetic types;
and these terms imply that such forms are, in fact, intermediate or
connecting links. Another distinguished palaeontologist, M. Gaudry, has
shown in the most striking manner that many of the fossil mammals
discovered by him in Attica serve to break down the intervals between
existing genera. Cuvier ranked the Ruminants and Pachyderms as two of the
most distinct orders of mammals; but so many fossil links have been
disentombed that Owen has had to alter the whole classification, and has
placed certain Pachyderms in the same sub-order with ruminants; for
example, he dissolves by gradations the apparently wide interval between
the pig and the camel. The Ungulata or hoofed quadrupeds are now divided
into the even-toed or odd-toed divisions; but the Macrauchenia of South
America connects to a certain extent these two grand divisions. No one
will deny that the Hipparion is intermediate between the existing horse and
certain other ungulate forms. What a wonderful connecting link in the
chain of mammals is the Typotherium from South America, as the name given
to it by Professor Gervais expresses, and which cannot be placed in any
existing order. The Sirenia form a very distinct group of the mammals, and
one of the most remarkable peculiarities in existing dugong and lamentin is
the entire absence of hind limbs, without even a rudiment being left; but
the extinct Halitherium had, according to Professor Flower, an ossified
thigh-bone "articulated to a well-defined acetabulum in the pelvis," and it
thus makes some approach to ordinary hoofed quadrupeds, to which the
Sirenia are in other respects allied. The cetaceans or whales are widely
different from all other mammals, but the tertiary Zeuglodon and Squalodon,
which have been placed by some naturalists in an order by themselves, are
considered by Professor Huxley to be undoubtedly cetaceans, "and to
constitute connecting links with the aquatic carnivora."
Even the wide interval between birds and reptiles has been shown by the
naturalist just quoted to be partially bridged over in the most unexpected
manner, on the one hand, by the ostrich and extinct Archeopteryx, and on
the other hand by the Compsognathus, one of the Dinosaurians--that group
which includes the most gigantic of all terrestrial reptiles. Turning to
the Invertebrata, Barrande asserts, a higher authority could not be named,
that he is every day taught that, although palaeozoic animals can certainly
be classed under existing groups, yet that at this ancient period the
groups were not so distinctly separated from each other as they now are.
Some writers have objected to any extinct species, or group of species,
being considered as intermediate between any two living species, or groups
of species. If by this term it is meant that an extinct form is directly
intermediate in all its characters between two living forms or groups, the
objection is probably valid. But in a natural classification many fossil
species certainly stand between living species, and some extinct genera
between living genera, even between genera belonging to distinct families.
The most common case, especially with respect to very distinct groups, such
as fish and reptiles, seems to be that, supposing them to be distinguished
at the present day by a score of characters, the ancient members are
separated by a somewhat lesser number of characters, so that the two groups
formerly made a somewhat nearer approach to each other than they now do.
It is a common belief that the more ancient a form is, by so much the more
it tends to connect by some of its characters groups now widely separated
from each other. This remark no doubt must be restricted to those groups
which have undergone much change in the course of geological ages; and it
would be difficult to prove the truth of the proposition, for every now and
then even a living animal, as the Lepidosiren, is discovered having
affinities directed towards very distinct groups. Yet if we compare the
older Reptiles and Batrachians, the older Fish, the older Cephalopods, and
the eocene Mammals, with the recent members of the same classes, we must
admit that there is truth in the remark.
Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat complex, I
must request the reader to turn to the diagram in the fourth chapter. We
may suppose that the numbered letters in italics represent genera, and the
dotted lines diverging from them the species in each genus. The diagram is
much too simple, too few genera and too few species being given, but this
is unimportant for us. The horizontal lines may represent successive
geological formations, and all the forms beneath the uppermost line may be
considered as extinct. The three existing genera, a14, q14, p14, will form
a small family; b14 and f14, a closely allied family or subfamily; and o14,
i14, m14, a third family. These three families, together with the many
extinct genera on the several lines of descent diverging from the parent
form (A) will form an order; for all will have inherited something in
common from their ancient progenitor. On the principle of the continued
tendency to divergence of character, which was formerly illustrated by this
diagram, the more recent any form is the more it will generally differ from
its ancient progenitor. Hence, we can understand the rule that the most
ancient fossils differ most from existing forms. We must not, however,
assume that divergence of character is a necessary contingency; it depends
solely on the descendants from a species being thus enabled to seize on
many and different places in the economy of nature. Therefore it is quite
possible, as we have seen in the case of some Silurian forms, that a
species might go on being slightly modified in relation to its slightly
altered conditions of life, and yet retain throughout a vast period the
same general characteristics. This is represented in the diagram by the
letter F14.
All the many forms, extinct and recent, descended from (A), make, as before
remarked, one order; and this order, from the continued effects of
extinction and divergence of character, has become divided into several
sub-families and families, some of which are supposed to have perished at
different periods, and some to have endured to the present day.
By looking at the diagram we can see that if many of the extinct forms
supposed to be embedded in the successive formations, were discovered at
several points low down in the series, the three existing families on the
uppermost line would be rendered less distinct from each other. If, for
instance, the genera a1, a5, a10, f8, m3, m6, m9, were disinterred, these
three families would be so closely linked together that they probably would
have to be united into one great family, in nearly the same manner as has
occurred with ruminants and certain pachyderms. Yet he who objected to
consider as intermediate the extinct genera, which thus link together the
living genera of three families, would be partly justified, for they are
intermediate, not directly, but only by a long and circuitous course
through many widely different forms. If many extinct forms were to be
discovered above one of the middle horizontal lines or geological
formations--for instance, above No. VI.--but none from beneath this line,
then only two of the families (those on the left hand a14, etc., and b14,
etc.) would have to be united into one; and there would remain two families
which would be less distinct from each other than they were before the
discovery of the fossils. So again, if the three families formed of eight
genera (a14 to m14), on the uppermost line, be supposed to differ from each
other by half-a-dozen important characters, then the families which existed
at a period marked VI would certainly have differed from each other by a
less number of characters; for they would at this early stage of descent
have diverged in a less degree from their common progenitor. Thus it comes
that ancient and extinct genera are often in a greater or less degree
intermediate in character between their modified descendants, or between
their collateral relations.
Under nature the process will be far more complicated than is represented
in the diagram; for the groups will have been more numerous; they will have
endured for extremely unequal lengths of time, and will have been modified
in various degrees. As we possess only the last volume of the geological
record, and that in a very broken condition, we have no right to expect,
except in rare cases, to fill up the wide intervals in the natural system,
and thus to unite distinct families or orders. All that we have a right to
expect is, that those groups which have, within known geological periods,
undergone much modification, should in the older formations make some
slight approach to each other; so that the older members should differ less
from each other in some of their characters than do the existing members of
the same groups; and this by the concurrent evidence of our best
palaeontologists is frequently the case.
Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each other
and to living forms, are explained in a satisfactory manner. And they are
wholly inexplicable on any other view.
On this same theory, it is evident that the fauna during any one great
period in the earth's history will be intermediate in general character
between that which preceded and that which succeeded it. Thus the species
which lived at the sixth great stage of descent in the diagram are the
modified offspring of those which lived at the fifth stage, and are the
parents of those which became still more modified at the seventh stage;
hence they could hardly fail to be nearly intermediate in character between
the forms of life above and below. We must, however, allow for the entire
extinction of some preceding forms, and in any one region for the
immigration of new forms from other regions, and for a large amount of
modification during the long and blank intervals between the successive
formations. Subject to these allowances, the fauna of each geological
period undoubtedly is intermediate in character, between the preceding and
succeeding faunas. I need give only one instance, namely, the manner in
which the fossils of the Devonian system, when this system was first
discovered, were at once recognised by palaeontologists as intermediate in
character between those of the overlying carboniferous and underlying
Silurian systems. But each fauna is not necessarily exactly intermediate,
as unequal intervals of time have elapsed between consecutive formations.
It is no real objection to the truth of the statement that the fauna of
each period as a whole is nearly intermediate in character between the
preceding and succeeding faunas, that certain genera offer exceptions to
the rule. For instance, the species of mastodons and elephants, when
arranged by Dr. Falconer in two series--in the first place according to
their mutual affinities, and in the second place according to their periods
of existence--do not accord in arrangement. The species extreme in
character are not the oldest or the most recent; nor are those which are
intermediate in character, intermediate in age. But supposing for an
instant, in this and other such cases, that the record of the first
appearance and disappearance of the species was complete, which is far from
the case, we have no reason to believe that forms successively produced
necessarily endure for corresponding lengths of time. A very ancient form
may occasionally have lasted much longer than a form elsewhere subsequently
produced, especially in the case of terrestrial productions inhabiting
separated districts. To compare small things with great; if the principal
living and extinct races of the domestic pigeon were arranged in serial
affinity, this arrangement would not closely accord with the order in time
of their production, and even less with the order of their disappearance;
for the parent rock-pigeon still lives; and many varieties between the
rock-pigeon and the carrier have become extinct; and carriers which are
extreme in the important character of length of beak originated earlier
than short-beaked tumblers, which are at the opposite end of the series in
this respect.
Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is the
fact, insisted on by all palaeontologists, that fossils from two
consecutive formations are far more closely related to each other, than are
the fossils from two remote formations. Pictet gives as a well-known
instance, the general resemblance of the organic remains from the several
stages of the Chalk formation, though the species are distinct in each
stage. This fact alone, from its generality, seems to have shaken
Professor Pictet in his belief in the immutability of species. He who is
acquainted with the distribution of existing species over the globe, will
not attempt to account for the close resemblance of distinct species in
closely consecutive formations, by the physical conditions of the ancient
areas having remained nearly the same. Let it be remembered that the forms
of life, at least those inhabiting the sea, have changed almost
simultaneously throughout the world, and therefore under the most different
climates and conditions. Consider the prodigious vicissitudes of climate
during the pleistocene period, which includes the whole glacial epoch, and
note how little the specific forms of the inhabitants of the sea have been
affected.
On the theory of descent, the full meaning of the fossil remains from
closely consecutive formations, being closely related, though ranked as
distinct species, is obvious. As the accumulation of each formation has
often been interrupted, and as long blank intervals have intervened between
successive formations, we ought not to expect to find, as I attempted to
show in the last chapter, in any one or in any two formations, all the
intermediate varieties between the species which appeared at the
commencement and close of these periods: but we ought to find after
intervals, very long as measured by years, but only moderately long as
measured geologically, closely allied forms, or, as they have been called
by some authors, representative species; and these assuredly we do find.
We find, in short, such evidence of the slow and scarcely sensible
mutations of specific forms, as we have the right to expect.
ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED WITH LIVING FORMS.
We have seen in the fourth chapter that the degree of differentiation and
specialisation of the parts in organic beings, when arrived at maturity, is
the best standard, as yet suggested, of their degree of perfection or
highness. We have also seen that, as the specialisation of parts is an
advantage to each being, so natural selection will tend to render the
organisation of each being more specialised and perfect, and in this sense
higher; not but that it may leave many creatures with simple and unimproved
structures fitted for simple conditions of life, and in some cases will
even degrade or simplify the organisation, yet leaving such degraded beings
better fitted for their new walks of life. In another and more general
manner, new species become superior to their predecessors; for they have to
beat in the struggle for life all the older forms, with which they come
into close competition. We may therefore conclude that if under a nearly
similar climate the eocene inhabitants of the world could be put into
competition with the existing inhabitants, the former would be beaten and
exterminated by the latter, as would the secondary by the eocene, and the
palaeozoic by the secondary forms. So that by this fundamental test of
victory in the battle for life, as well as by the standard of the
specialisation of organs, modern forms ought, on the theory of natural
selection, to stand higher than ancient forms. Is this the case? A large
majority of palaeontologists would answer in the affirmative; and it seems
that this answer must be admitted as true, though difficult of proof.
It is no valid objection to this conclusion, that certain Brachiopods have
been but slightly modified from an extremely remote geological epoch; and
that certain land and fresh-water shells have remained nearly the same,
from the time when, as far as is known, they first appeared. It is not an
insuperable difficulty that Foraminifera have not, as insisted on by Dr.
Carpenter, progressed in organisation since even the Laurentian epoch; for
some organisms would have to remain fitted for simple conditions of life,
and what could be better fitted for this end than these lowly organised
Protozoa? Such objections as the above would be fatal to my view, if it
included advance in organisation as a necessary contingent. They would
likewise be fatal, if the above Foraminifera, for instance, could be proved
to have first come into existence during the Laurentian epoch, or the above
Brachiopods during the Cambrian formation; for in this case, there would
not have been time sufficient for the development of these organisms up to
the standard which they had then reached. When advanced up to any given
point, there is no necessity, on the theory of natural selection, for their
further continued process; though they will, during each successive age,
have to be slightly modified, so as to hold their places in relation to
slight changes in their conditions. The foregoing objections hinge on the
question whether we really know how old the world is, and at what period
the various forms of life first appeared; and this may well be disputed.
The problem whether organisation on the whole has advanced is in many ways
excessively intricate. The geological record, at all times imperfect, does
not extend far enough back to show with unmistakable clearness that within
the known history of the world organisation has largely advanced. Even at
the present day, looking to members of the same class, naturalists are not
unanimous which forms ought to be ranked as highest: thus, some look at
the selaceans or sharks, from their approach in some important points of
structure to reptiles, as the highest fish; others look at the teleosteans
as the highest. The ganoids stand intermediate between the selaceans and
teleosteans; the latter at the present day are largely preponderant in
number; but formerly selaceans and ganoids alone existed; and in this case,
according to the standard of highness chosen, so will it be said that
fishes have advanced or retrograded in organisation. To attempt to compare
members of distinct types in the scale of highness seems hopeless; who will
decide whether a cuttle-fish be higher than a bee--that insect which the
great Von Baer believed to be "in fact more highly organised than a fish,
although upon another type?" In the complex struggle for life it is quite
credible that crustaceans, not very high in their own class, might beat
cephalopods, the highest molluscs; and such crustaceans, though not highly
developed, would stand very high in the scale of invertebrate animals, if
judged by the most decisive of all trials--the law of battle. Beside these
inherent difficulties in deciding which forms are the most advanced in
organisation, we ought not solely to compare the highest members of a class
at any two periods--though undoubtedly this is one and perhaps the most
important element in striking a balance--but we ought to compare all the
members, high and low, at two periods. At an ancient epoch the highest and
lowest molluscoidal animals, namely, cephalopods and brachiopods, swarmed
in numbers; at the present time both groups are greatly reduced, while
others, intermediate in organisation, have largely increased; consequently
some naturalists maintain that molluscs were formerly more highly developed
than at present; but a stronger case can be made out on the opposite side,
by considering the vast reduction of brachiopods, and the fact that our
existing cephalopods, though few in number, are more highly organised than
their ancient representatives. We ought also to compare the relative
proportional numbers, at any two periods, of the high and low classes
throughout the world: if, for instance, at the present day fifty thousand
kinds of vertebrate animals exist, and if we knew that at some former
period only ten thousand kinds existed, we ought to look at this increase
in number in the highest class, which implies a great displacement of lower
forms, as a decided advance in the organisation of the world. We thus see
how hopelessly difficult it is to compare with perfect fairness, under such
extremely complex relations, the standard of organisation of the
imperfectly-known faunas of successive periods.
We shall appreciate this difficulty more clearly by looking to certain
existing faunas and floras. From the extraordinary manner in which
European productions have recently spread over New Zealand, and have seized
on places which must have been previously occupied by the indigenes, we
must believe, that if all the animals and plants of Great Britain were set
free in New Zealand, a multitude of British forms would in the course of
time become thoroughly naturalized there, and would exterminate many of the
natives. On the other hand, from the fact that hardly a single inhabitant
of the southern hemisphere has become wild in any part of Europe, we may
well doubt whether, if all the productions of New Zealand were set free in
Great Britain, any considerable number would be enabled to seize on places
now occupied by our native plants and animals. Under this point of view,
the productions of Great Britain stand much higher in the scale than those
of New Zealand. Yet the most skilful naturalist, from an examination of
the species of the two countries, could not have foreseen this result.
Agassiz and several other highly competent judges insist that ancient
animals resemble to a certain extent the embryos of recent animals
belonging to the same classes; and that the geological succession of
extinct forms is nearly parallel with the embryological development of
existing forms. This view accords admirably well with our theory. In a
future chapter I shall attempt to show that the adult differs from its
embryo, owing to variations having supervened at a not early age, and
having been inherited at a corresponding age. This process, whilst it
leaves the embryo almost unaltered, continually adds, in the course of
successive generations, more and more difference to the adult. Thus the
embryo comes to be left as a sort of picture, preserved by nature, of the
former and less modified condition of the species. This view may be true,
and yet may never be capable of proof. Seeing, for instance, that the
oldest known mammals, reptiles, and fishes strictly belong to their proper
classes, though some of these old forms are in a slight degree less
distinct from each other than are the typical members of the same groups at
the present day, it would be vain to look for animals having the common
embryological character of the Vertebrata, until beds rich in fossils are
discovered far beneath the lowest Cambrian strata--a discovery of which the
chance is small.
ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE LATER
TERTIARY PERIODS.
Mr. Clift many years ago showed that the fossil mammals from the Australian
caves were closely allied to the living marsupials of that continent. In
South America, a similar relationship is manifest, even to an uneducated
eye, in the gigantic pieces of armour, like those of the armadillo, found
in several parts of La Plata; and Professor Owen has shown in the most
striking manner that most of the fossil mammals, buried there in such
numbers, are related to South American types. This relationship is even
more clearly seen in the wonderful collection of fossil bones made by MM.
Lund and Clausen in the caves of Brazil. I was so much impressed with
these facts that I strongly insisted, in 1839 and 1845, on this "law of the
succession of types,"--on "this wonderful relationship in the same
continent between the dead and the living." Professor Owen has
subsequently extended the same generalisation to the mammals of the Old
World. We see the same law in this author's restorations of the extinct
and gigantic birds of New Zealand. We see it also in the birds of the
caves of Brazil. Mr. Woodward has shown that the same law holds good with
sea-shells, but, from the wide distribution of most molluscs, it is not
well displayed by them. Other cases could be added, as the relation
between the extinct and living land-shells of Madeira; and between the
extinct and living brackish water-shells of the Aralo-Caspian Sea.
Now, what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man who, after comparing
the present climate of Australia and of parts of South America, under the
same latitude, would attempt to account, on the one hand through dissimilar
physical conditions, for the dissimilarity of the inhabitants of these two
continents; and, on the other hand through similarity of conditions, for
the uniformity of the same types in each continent during the later
tertiary periods. Nor can it be pretended that it is an immutable law that
marsupials should have been chiefly or solely produced in Australia; or
that Edentata and other American types should have been solely produced in
South America. For we know that Europe in ancient times was peopled by
numerous marsupials; and I have shown in the publications above alluded to,
that in America the law of distribution of terrestrial mammals was formerly
different from what it now is. North America formerly partook strongly of
the present character of the southern half of the continent; and the
southern half was formerly more closely allied, than it is at present, to
the northern half. In a similar manner we know, from Falconer and
Cautley's discoveries, that Northern India was formerly more closely
related in its mammals to Africa than it is at the present time. Analogous
facts could be given in relation to the distribution of marine animals.
On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the same
areas, is at once explained; for the inhabitants of each quarter of the
world will obviously tend to leave in that quarter, during the next
succeeding period of time, closely allied though in some degree modified
descendants. If the inhabitants of one continent formerly differed greatly
from those of another continent, so will their modified descendants still
differ in nearly the same manner and degree. But after very long intervals
of time, and after great geographical changes, permitting much
intermigration, the feebler will yield to the more dominant forms, and
there will be nothing immutable in the distribution of organic beings.
It may be asked in ridicule whether I suppose that the megatherium and
other allied huge monsters, which formerly lived in South America, have
left behind them the sloth, armadillo, and anteater, as their degenerate
descendants. This cannot for an instant be admitted. These huge animals
have become wholly extinct, and have left no progeny. But in the caves of
Brazil there are many extinct species which are closely allied in size and
in all other characters to the species still living in South America; and
some of these fossils may have been the actual progenitors of the living
species. It must not be forgotten that, on our theory, all the species of
the same genus are the descendants of some one species; so that, if six
genera, each having eight species, be found in one geological formation,
and in a succeeding formation there be six other allied or representative
genera, each with the same number of species, then we may conclude that
generally only one species of each of the older genera has left modified
descendants, which constitute the new genera containing the several
species; the other seven species of each old genus having died out and left
no progeny. Or, and this will be a far commoner case, two or three species
in two or three alone of the six older genera will be the parents of the
new genera: the other species and the other old genera having become
utterly extinct. In failing orders, with the genera and species decreasing
in numbers as is the case with the Edentata of South America, still fewer
genera and species will leave modified blood-descendants.
SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.
I have attempted to show that the geological record is extremely imperfect;
that only a small portion of the globe has been geologically explored with
care; that only certain classes of organic beings have been largely
preserved in a fossil state; that the number both of specimens and of
species, preserved in our museums, is absolutely as nothing compared with
the number of generations which must have passed away even during a single
formation; that, owing to subsidence being almost necessary for the
accumulation of deposits rich in fossil species of many kinds, and thick
enough to outlast future degradation, great intervals of time must have
elapsed between most of our successive formations; that there has probably
been more extinction during the periods of subsidence, and more variation
during the periods of elevation, and during the latter the record will have
been least perfectly kept; that each single formation has not been
continuously deposited; that the duration of each formation is probably
short compared with the average duration of specific forms; that migration
has played an important part in the first appearance of new forms in any
one area and formation; that widely ranging species are those which have
varied most frequently, and have oftenest given rise to new species; that
varieties have at first been local; and lastly, although each species must
have passed through numerous transitional stages, it is probable that the
periods, during which each underwent modification, though many and long as
measured by years, have been short in comparison with the periods during
which each remained in an unchanged condition. These causes, taken
conjointly, will to a large extent explain why--though we do find many
links--we do not find interminable varieties, connecting together all
extinct and existing forms by the finest graduated steps. It should also
be constantly borne in mind that any linking variety between two forms,
which might be found, would be ranked, unless the whole chain could be
perfectly restored, as a new and distinct species; for it is not pretended
that we have any sure criterion by which species and varieties can be
discriminated.
He who rejects this view of the imperfection of the geological record, will
rightly reject the whole theory. For he may ask in vain where are the
numberless transitional links which must formerly have connected the
closely allied or representative species, found in the successive stages of
the same great formation? He may disbelieve in the immense intervals of
time which must have elapsed between our consecutive formations; he may
overlook how important a part migration has played, when the formations of
any one great region, as those of Europe, are considered; he may urge the
apparent, but often falsely apparent, sudden coming in of whole groups of
species. He may ask where are the remains of those infinitely numerous
organisms which must have existed long before the Cambrian system was
deposited? We now know that at least one animal did then exist; but I can
answer this last question only by supposing that where our oceans now
extend they have extended for an enormous period, and where our oscillating
continents now stand they have stood since the commencement of the Cambrian
system; but that, long before that epoch, the world presented a widely
different aspect; and that the older continents, formed of formations older
than any known to us, exist now only as remnants in a metamorphosed
condition, or lie still buried under the ocean.
Passing from these difficulties, the other great leading facts in
palaeontology agree admirably with the theory of descent with modification
through variation and natural selection. We can thus understand how it is
that new species come in slowly and successively; how species of different
classes do not necessarily change together, or at the same rate, or in the
same degree; yet in the long run that all undergo modification to some
extent. The extinction of old forms is the almost inevitable consequence
of the production of new forms. We can understand why, when a species has
once disappeared, it never reappears. Groups of species increase in
numbers slowly, and endure for unequal periods of time; for the process of
modification is necessarily slow, and depends on many complex
contingencies. The dominant species belonging to large and dominant groups
tend to leave many modified descendants, which form new sub-groups and
groups. As these are formed, the species of the less vigorous groups, from
their inferiority inherited from a common progenitor, tend to become
extinct together, and to leave no modified offspring on the face of the
earth. But the utter extinction of a whole group of species has sometimes
been a slow process, from the survival of a few descendants, lingering in
protected and isolated situations. When a group has once wholly
disappeared, it does not reappear; for the link of generation has been
broken.
We can understand how it is that dominant forms which spread widely and
yield the greatest number of varieties tend to people the world with
allied, but modified, descendants; and these will generally succeed in
displacing the groups which are their inferiors in the struggle for
existence. Hence, after long intervals of time, the productions of the
world appear to have changed simultaneously.
We can understand how it is that all the forms of life, ancient and recent,
make together a few grand classes. We can understand, from the continued
tendency to divergence of character, why the more ancient a form is, the
more it generally differs from those now living. Why ancient and extinct
forms often tend to fill up gaps between existing forms, sometimes blending
two groups, previously classed as distinct into one; but more commonly
bringing them only a little closer together. The more ancient a form is,
the more often it stands in some degree intermediate between groups now
distinct; for the more ancient a form is, the more nearly it will be
related to, and consequently resemble, the common progenitor of groups,
since become widely divergent. Extinct forms are seldom directly
intermediate between existing forms; but are intermediate only by a long
and circuitous course through other extinct and different forms. We can
clearly see why the organic remains of closely consecutive formations are
closely allied; for they are closely linked together by generation. We can
clearly see why the remains of an intermediate formation are intermediate
in character.
The inhabitants of the world at each successive period in its history have
beaten their predecessors in the race for life, and are, in so far, higher
in the scale, and their structure has generally become more specialised;
and this may account for the common belief held by so many
palaeontologists, that organisation on the whole has progressed. Extinct
and ancient animals resemble to a certain extent the embryos of the more
recent animals belonging to the same classes, and this wonderful fact
receives a simple explanation according to our views. The succession of
the same types of structure within the same areas during the later
geological periods ceases to be mysterious, and is intelligible on the
principle of inheritance.
If, then, the geological record be as imperfect as many believe, and it may
at least be asserted that the record cannot be proved to be much more
perfect, the main objections to the theory of natural selection are greatly
diminished or disappear. On the other hand, all the chief laws of
palaeontology plainly proclaim, as it seems to me, that species have been
produced by ordinary generation: old forms having been supplanted by new
and improved forms of life, the products of variation and the survival of
the fittest.
CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in physical
conditions -- Importance of barriers -- Affinity of the productions of the
same continent -- Centres of creation -- Means of dispersal by changes of
climate and of the level of the land, and by occasional means -- Dispersal
during the Glacial period -- Alternate Glacial periods in the North and
South.
In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions can
be wholly accounted for by climatal and other physical conditions. Of
late, almost every author who has studied the subject has come to this
conclusion. The case of America alone would almost suffice to prove its
truth; for if we exclude the arctic and northern temperate parts, all
authors agree that one of the most fundamental divisions in geographical
distribution is that between the New and Old Worlds; yet if we travel over
the vast American continent, from the central parts of the United States to
its extreme southern point, we meet with the most diversified conditions;
humid districts, arid deserts, lofty mountains, grassy plains, forests,
marshes, lakes and great rivers, under almost every temperature. There is
hardly a climate or condition in the Old World which cannot be paralleled
in the New--at least so closely as the same species generally require. No
doubt small areas can be pointed out in the Old World hotter than any in
the New World; but these are not inhabited by a fauna different from that
of the surrounding districts; for it is rare to find a group of organisms
confined to a small area, of which the conditions are peculiar in only a
slight degree. Notwithstanding this general parallelism in the conditions
of Old and New Worlds, how widely different are their living productions!
In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes 25
and 35 degrees, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or, again, we may compare the productions
of South America south of latitude 35 degrees with those north of 25
degrees, which consequently are separated by a space of ten degrees of
latitude, and are exposed to considerably different conditions; yet they
are incomparably more closely related to each other than they are to the
productions of Australia or Africa under nearly the same climate.
Analogous facts could be given with respect to the inhabitants of the sea.
A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions of
various regions. We see this in the great difference in nearly all the
terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a slightly
different climate, there might have been free migration for the northern
temperate forms, as there now is for the strictly arctic productions. We
see the same fact in the great difference between the inhabitants of
Australia, Africa, and South America under the same latitude; for these
countries are almost as much isolated from each other as is possible. On
each continent, also, we see the same fact; for on the opposite sides of
lofty and continuous mountain-ranges, and of great deserts and even of
large rivers, we find different productions; though as mountain chains,
deserts, etc., are not as impassable, or likely to have endured so long, as
the oceans separating continents, the differences are very inferior in
degree to those characteristic of distinct continents.
Turning to the sea, we find the same law. The marine inhabitants of the
eastern and western shores of South America are very distinct, with
extremely few shells, crustacea, or echinodermata in common; but Dr.
Gunther has recently shown that about thirty per cent of the fishes are the
same on the opposite sides of the isthmus of Panama; and this fact has led
naturalists to believe that the isthmus was formerly open. Westward of the
shores of America, a wide space of open ocean extends, with not an island
as a halting-place for emigrants; here we have a barrier of another kind,
and as soon as this is passed we meet in the eastern islands of the Pacific
with another and totally distinct fauna. So that three marine faunas range
northward and southward in parallel lines not far from each other, under
corresponding climate; but from being separated from each other by
impassable barriers, either of land or open sea, they are almost wholly
distinct. On the other hand, proceeding still further westward from the
eastern islands of the tropical parts of the Pacific, we encounter no
impassable barriers, and we have innumerable islands as halting-places, or
continuous coasts, until, after travelling over a hemisphere, we come to
the shores of Africa; and over this vast space we meet with no well-defined
and distinct marine faunas. Although so few marine animals are common to
the above-named three approximate faunas of Eastern and Western America and
the eastern Pacific islands, yet many fishes range from the Pacific into
the Indian Ocean, and many shells are common to the eastern islands of the
Pacific and the eastern shores of Africa on almost exactly opposite
meridians of longitude.
A third great fact, partly included in the foregoing statement, is the
affinity of the productions of the same continent or of the same sea,
though the species themselves are distinct at different points and
stations. It is a law of the widest generality, and every continent offers
innumerable instances. Nevertheless, the naturalist, in travelling, for
instance, from north to south, never fails to be struck by the manner in
which successive groups of beings, specifically distinct, though nearly
related, replace each other. He hears from closely allied, yet distinct
kinds of birds, notes nearly similar, and sees their nests similarly
constructed, but not quite alike, with eggs coloured in nearly the same
manner. The plains near the Straits of Magellan are inhabited by one
species of Rhea (American ostrich), and northward the plains of La Plata by
another species of the same genus; and not by a true ostrich or emu, like
those inhabiting Africa and Australia under the same latitude. On these
same plains of La Plata we see the agouti and bizcacha, animals having
nearly the same habits as our hares and rabbits, and belonging to the same
order of Rodents, but they plainly display an American type of structure.
We ascend the lofty peaks of the Cordillera, and we find an alpine species
of bizcacha; we look to the waters, and we do not find the beaver or
muskrat, but the coypu and capybara, rodents of the South American type.
Innumerable other instances could be given. If we look to the islands off
the American shore, however much they may differ in geological structure,
the inhabitants are essentially American, though they may be all peculiar
species. We may look back to past ages, as shown in the last chapter, and
we find American types then prevailing on the American continent and in the
American seas. We see in these facts some deep organic bond, throughout
space and time, over the same areas of land and water, independently of
physical conditions. The naturalist must be dull who is not led to inquire
what this bond is.
The bond is simply inheritance, that cause which alone, as far as we
positively know, produces organisms quite like each other, or, as we see in
the case of varieties, nearly alike. The dissimilarity of the inhabitants
of different regions may be attributed to modification through variation
and natural selection, and probably in a subordinate degree to the definite
influence of different physical conditions. The degrees of dissimilarity
will depend on the migration of the more dominant forms of life from one
region into another having been more or less effectually prevented, at
periods more or less remote--on the nature and number of the former
immigrants--and on the action of the inhabitants on each other in leading
to the preservation of different modifications; the relation of organism to
organism in the struggle for life being, as I have already often remarked,
the most important of all relations. Thus the high importance of barriers
comes into play by checking migration; as does time for the slow process of
modification through natural selection. Widely-ranging species, abounding
in individuals, which have already triumphed over many competitors in their
own widely-extended homes, will have the best chance of seizing on new
places, when they spread out into new countries. In their new homes they
will be exposed to new conditions, and will frequently undergo further
modification and improvement; and thus they will become still further
victorious, and will produce groups of modified descendants. On this
principle of inheritance with modification we can understand how it is that
sections of genera, whole genera, and even families, are confined to the
same areas, as is so commonly and notoriously the case.
There is no evidence, as was remarked in the last chapter, of the existence
of any law of necessary development. As the variability of each species is
an independent property, and will be taken advantage of by natural
selection, only so far as it profits each individual in its complex
struggle for life, so the amount of modification in different species will
be no uniform quantity. If a number of species, after having long competed
with each other in their old home, were to migrate in a body into a new and
afterwards isolated country, they would be little liable to modification;
for neither migration nor isolation in themselves effect anything. These
principles come into play only by bringing organisms into new relations
with each other and in a lesser degree with the surrounding physical
conditions. As we have seen in the last chapter that some forms have
retained nearly the same character from an enormously remote geological
period, so certain species have migrated over vast spaces, and have not
become greatly or at all modified.
According to these views, it is obvious that the several species of the
same genus, though inhabiting the most distant quarters of the world, must
originally have proceeded from the same source, as they are descended from
the same progenitor. In the case of those species which have undergone,
during whole geological periods, little modification, there is not much
difficulty in believing that they have migrated from the same region; for
during the vast geographical and climatical changes which have supervened
since ancient times, almost any amount of migration is possible. But in
many other cases, in which we have reason to believe that the species of a
genus have been produced within comparatively recent times, there is great
difficulty on this head. It is also obvious that the individuals of the
same species, though now inhabiting distant and isolated regions, must have
proceeded from one spot, where their parents were first produced: for, as
has been explained, it is incredible that individuals identically the same
should have been produced from parents specifically distinct.
SINGLE CENTRES OF SUPPOSED CREATION.
We are thus brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at one or more
points of the earth's surface. Undoubtedly there are many cases of extreme
difficulty in understanding how the same species could possibly have
migrated from some one point to the several distant and isolated points,
where now found. Nevertheless the simplicity of the view that each species
was first produced within a single region captivates the mind. He who
rejects it, rejects the vera causa of ordinary generation with subsequent
migration, and calls in the agency of a miracle. It is universally
admitted, that in most cases the area inhabited by a species is continuous;
and that when a plant or animal inhabits two points so distant from each
other, or with an interval of such a nature, that the space could not have
been easily passed over by migration, the fact is given as something
remarkable and exceptional. The incapacity of migrating across a wide sea
is more clear in the case of terrestrial mammals than perhaps with any
other organic beings; and, accordingly, we find no inexplicable instances
of the same mammals inhabiting distant points of the world. No geologist
feels any difficulty in Great Britain possessing the same quadrupeds with
the rest of Europe, for they were no doubt once united. But if the same
species can be produced at two separate points, why do we not find a single
mammal common to Europe and Australia or South America? The conditions of
life are nearly the same, so that a multitude of European animals and
plants have become naturalised in America and Australia; and some of the
aboriginal plants are identically the same at these distant points of the
northern and southern hemispheres? The answer, as I believe, is, that
mammals have not been able to migrate, whereas some plants, from their
varied means of dispersal, have migrated across the wide and broken
interspaces. The great and striking influence of barriers of all kinds, is
intelligible only on the view that the great majority of species have been
produced on one side, and have not been able to migrate to the opposite
side. Some few families, many subfamilies, very many genera, a still
greater number of sections of genera, are confined to a single region; and
it has been observed by several naturalists that the most natural genera,
or those genera in which the species are most closely related to each
other, are generally confined to the same country, or if they have a wide
range that their range is continuous. What a strange anomaly it would be
if a directly opposite rule were to prevail when we go down one step lower
in the series, namely to the individuals of the same species, and these had
not been, at least at first, confined to some one region!
Hence, it seems to me, as it has to many other naturalists, that the view
of each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration and
subsistence under past and present conditions permitted, is the most
probable. Undoubtedly many cases occur in which we cannot explain how the
same species could have passed from one point to the other. But the
geographical and climatical changes which have certainly occurred within
recent geological times, must have rendered discontinuous the formerly
continuous range of many species. So that we are reduced to consider
whether the exceptions to continuity of range are so numerous, and of so
grave a nature, that we ought to give up the belief, rendered probable by
general considerations, that each species has been produced within one
area, and has migrated thence as far as it could. It would be hopelessly
tedious to discuss all the exceptional cases of the same species, now
living at distant and separated points; nor do I for a moment pretend that
any explanation could be offered of many instances. But, after some
preliminary remarks, I will discuss a few of the most striking classes of
facts, namely, the existence of the same species on the summits of distant
mountain ranges, and at distant points in the Arctic and Antarctic regions;
and secondly (in the following chapter), the wide distribution of fresh
water productions; and thirdly, the occurrence of the same terrestrial
species on islands and on the nearest mainland, though separated by
hundreds of miles of open sea. If the existence of the same species at
distant and isolated points of the earth's surface can in many instances be
explained on the view of each species having migrated from a single
birthplace; then, considering our ignorance with respect to former
climatical and geographical changes, and to the various occasional means of
transport, the belief that a single birthplace is the law seems to me
incomparably the safest.
In discussing this subject we shall be enabled at the same time to consider
a point equally important for us, namely, whether the several species of a
genus which must on our theory all be descended from a common progenitor,
can have migrated, undergoing modification during their migration from some
one area. If, when most of the species inhabiting one region are different
from those of another region, though closely allied to them, it can be
shown that migration from the one region to the other has probably occurred
at some former period, our general view will be much strengthened; for the
explanation is obvious on the principle of descent with modification. A
volcanic island, for instance, upheaved and formed at the distance of a few
hundreds of miles from a continent, would probably receive from it in the
course of time a few colonists, and their descendants, though modified,
would still be related by inheritance to the inhabitants of that continent.
Cases of this nature are common, and are, as we shall hereafter see,
inexplicable on the theory of independent creation. This view of the
relation of the species of one region to those of another, does not differ
much from that advanced by Mr. Wallace, who concludes that "every species
has come into existence coincident both in space and time with a
pre-existing closely allied species." And it is now well known that he
attributes this coincidence to descent with modification.
The question of single or multiple centres of creation differs from another
though allied question, namely, whether all the individuals of the same
species are descended from a single pair, or single hermaphrodite, or
whether, as some authors suppose, from many individuals simultaneously
created. With organic beings which never intercross, if such exist, each
species, must be descended from a succession of modified varieties, that
have supplanted each other, but have never blended with other individuals
or varieties of the same species, so that, at each successive stage of
modification, all the individuals of the same form will be descended from a
single parent. But in the great majority of cases, namely, with all
organisms which habitually unite for each birth, or which occasionally
intercross, the individuals of the same species inhabiting the same area
will be kept nearly uniform by intercrossing; so that many individuals will
go on simultaneously changing, and the whole amount of modification at each
stage will not be due to descent from a single parent. To illustrate what
I mean: our English race-horses differ from the horses of every other
breed; but they do not owe their difference and superiority to descent from
any single pair, but to continued care in the selecting and training of
many individuals during each generation.
Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of "single
centres of creation," I must say a few words on the means of dispersal.
MEANS OF DISPERSAL.
Sir C. Lyell and other authors have ably treated this subject. I can give
here only the briefest abstract of the more important facts. Change of
climate must have had a powerful influence on migration. A region now
impassable to certain organisms from the nature of its climate, might have
been a high road for migration, when the climate was different. I shall,
however, presently have to discuss this branch of the subject in some
detail. Changes of level in the land must also have been highly
influential: a narrow isthmus now separates two marine faunas; submerge
it, or let it formerly have been submerged, and the two faunas will now
blend together, or may formerly have blended. Where the sea now extends,
land may at a former period have connected islands or possibly even
continents together, and thus have allowed terrestrial productions to pass
from one to the other. No geologist disputes that great mutations of level
have occurred within the period of existing organisms. Edward Forbes
insisted that all the islands in the Atlantic must have been recently
connected with Europe or Africa, and Europe likewise with America. Other
authors have thus hypothetically bridged over every ocean, and united
almost every island with some mainland. If, indeed, the arguments used by
Forbes are to be trusted, it must be admitted that scarcely a single island
exists which has not recently been united to some continent. This view
cuts the Gordian knot of the dispersal of the same species to the most
distant points, and removes many a difficulty; but to the best of my
judgment we are not authorized in admitting such enormous geographical
changes within the period of existing species. It seems to me that we have
abundant evidence of great oscillations in the level of the land or sea;
but not of such vast changes in the position and extension of our
continents, as to have united them within the recent period to each other
and to the several intervening oceanic islands. I freely admit the former
existence of many islands, now buried beneath the sea, which may have
served as halting places for plants and for many animals during their
migration. In the coral-producing oceans such sunken islands are now
marked by rings of coral or atolls standing over them. Whenever it is
fully admitted, as it will some day be, that each species has proceeded
from a single birthplace, and when in the course of time we know something
definite about the means of distribution, we shall be enabled to speculate
with security on the former extension of the land. But I do not believe
that it will ever be proved that within the recent period most of our
continents which now stand quite separate, have been continuously, or
almost continuously united with each other, and with the many existing
oceanic islands. Several facts in distribution--such as the great
difference in the marine faunas on the opposite sides of almost every
continent--the close relation of the tertiary inhabitants of several lands
and even seas to their present inhabitants--the degree of affinity between
the mammals inhabiting islands with those of the nearest continent, being
in part determined (as we shall hereafter see) by the depth of the
intervening ocean--these and other such facts are opposed to the admission
of such prodigious geographical revolutions within the recent period, as
are necessary on the view advanced by Forbes and admitted by his followers.
The nature and relative proportions of the inhabitants of oceanic islands
are likewise opposed to the belief of their former continuity of
continents. Nor does the almost universally volcanic composition of such
islands favour the admission that they are the wrecks of sunken continents;
if they had originally existed as continental mountain ranges, some at
least of the islands would have been formed, like other mountain summits,
of granite, metamorphic schists, old fossiliferous and other rocks, instead
of consisting of mere piles of volcanic matter.
I must now say a few words on what are called accidental means, but which
more properly should be called occasional means of distribution. I shall
here confine myself to plants. In botanical works, this or that plant is
often stated to be ill adapted for wide dissemination; but the greater or
less facilities for transport across the sea may be said to be almost
wholly unknown. Until I tried, with Mr. Berkeley's aid, a few experiments,
it was not even known how far seeds could resist the injurious action of
sea-water. To my surprise I found that out of eighty-seven kinds, sixty-
four germinated after an immersion of twenty-eight days, and a few survived
an immersion of 137 days. It deserves notice that certain orders were far
more injured than others: nine Leguminosae were tried, and, with one
exception, they resisted the salt-water badly; seven species of the allied
orders, Hydrophyllaceae and Polemoniaceae, were all killed by a month's
immersion. For convenience sake I chiefly tried small seeds without the
capsules or fruit; and as all of these sank in a few days, they could not
have been floated across wide spaces of the sea, whether or not they were
injured by salt water. Afterwards I tried some larger fruits, capsules,
etc., and some of these floated for a long time. It is well known what a
difference there is in the buoyancy of green and seasoned timber; and it
occurred to me that floods would often wash into the sea dried plants or
branches with seed-capsules or fruit attached to them. Hence I was led to
dry the stems and branches of ninety-four plants with ripe fruit, and to
place them on sea-water. The majority sank quickly, but some which, whilst
green, floated for a very short time, when dried floated much longer; for
instance, ripe hazel-nuts sank immediately, but when dried they floated for
ninety days, and afterwards when planted germinated; an asparagus plant
with ripe berries floated for twenty-three days, when dried it floated for
eighty-five days, and the seeds afterwards germinated: the ripe seeds of
Helosciadium sank in two days, when dried they floated for above ninety
days, and afterwards germinated. Altogether, out of the ninety-four dried
plants, eighteen floated for above twenty-eight days; and some of the
eighteen floated for a very much longer period. So that as 64/87 kinds of
seeds germinated after an immersion of twenty-eight days; and as 18/94
distinct species with ripe fruit (but not all the same species as in the
foregoing experiment) floated, after being dried, for above twenty-eight
days, we may conclude, as far as anything can be inferred from these scanty
facts, that the seeds of 14/100 kinds of plants of any country might be
floated by sea-currents during twenty-eight days, and would retain their
power of germination. In Johnston's Physical Atlas, the average rate of
the several Atlantic currents is thirty-three miles per diem (some currents
running at the rate of sixty miles per diem); on this average, the seeds of
14/100 plants belonging to one country might be floated across 924 miles of
sea to another country; and when stranded, if blown by an inland gale to a
favourable spot, would germinate.
Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea, so
that they were alternately wet and exposed to the air like really floating
plants. He tried ninety-eight seeds, mostly different from mine, but he
chose many large fruits, and likewise seeds, from plants which live near
the sea; and this would have favoured both the average length of their
flotation and their resistance to the injurious action of the salt-water.
On the other hand, he did not previously dry the plants or branches with
the fruit; and this, as we have seen, would have caused some of them to
have floated much longer. The result was that 18/98 of his seeds of
different kinds floated for forty-two days, and were then capable of
germination. But I do not doubt that plants exposed to the waves would
float for a less time than those protected from violent movement as in our
experiments. Therefore, it would perhaps be safer to assume that the seeds
of about 10/100 plants of a flora, after having been dried, could be
floated across a space of sea 900 miles in width, and would then germinate.
The fact of the larger fruits often floating longer than the small, is
interesting; as plants with large seeds or fruit which, as Alph. de
Candolle has shown, generally have restricted ranges, could hardly be
transported by any other means.
Seeds may be occasionally transported in another manner. Drift timber is
thrown up on most islands, even on those in the midst of the widest oceans;
and the natives of the coral islands in the Pacific procure stones for
their tools, solely from the roots of drifted trees, these stones being a
valuable royal tax. I find that when irregularly shaped stones are
embedded in the roots of trees, small parcels of earth are very frequently
enclosed in their interstices and behind them, so perfectly that not a
particle could be washed away during the longest transport: out of one
small portion of earth thus COMPLETELY enclosed by the roots of an oak
about fifty years old, three dicotyledonous plants germinated: I am
certain of the accuracy of this observation. Again, I can show that the
carcasses of birds, when floating on the sea, sometimes escape being
immediately devoured; and many kinds of seeds in the crops of floating
birds long retain their vitality: peas and vetches, for instance, are
killed by even a few days' immersion in sea-water; but some taken out of
the crop of a pigeon, which had floated on artificial sea-water for thirty
days, to my surprise nearly all germinated.
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the ocean.
We may safely assume that under such circumstances their rate of flight
would often be thirty-five miles an hour; and some authors have given a far
higher estimate. I have never seen an instance of nutritious seeds passing
through the intestines of a bird; but hard seeds of fruit pass uninjured
through even the digestive organs of a turkey. In the course of two
months, I picked up in my garden twelve kinds of seeds, out of the
excrement of small birds, and these seemed perfect, and some of them, which
were tried, germinated. But the following fact is more important: the
crops of birds do not secrete gastric juice, and do not, as I know by
trial, injure in the least the germination of seeds; now, after a bird has
found and devoured a large supply of food, it is positively asserted that
all the grains do not pass into the gizzard for twelve or even eighteen
hours. A bird in this interval might easily be blown to the distance of
five hundred miles, and hawks are known to look out for tired birds, and
the contents of their torn crops might thus readily get scattered. Some
hawks and owls bolt their prey whole, and after an interval of from twelve
to twenty hours, disgorge pellets, which, as I know from experiments made
in the Zoological Gardens, include seeds capable of germination. Some
seeds of the oat, wheat, millet, canary, hemp, clover, and beet germinated
after having been from twelve to twenty-one hours in the stomachs of
different birds of prey; and two seeds of beet grew after having been thus
retained for two days and fourteen hours. Fresh-water fish, I find, eat
seeds of many land and water plants; fish are frequently devoured by birds,
and thus the seeds might be transported from place to place. I forced many
kinds of seeds into the stomachs of dead fish, and then gave their bodies
to fishing-eagles, storks, and pelicans; these birds, after an interval of
many hours, either rejected the seeds in pellets or passed them in their
excrement; and several of these seeds retained the power of germination.
Certain seeds, however, were always killed by this process.
Locusts are sometimes blown to great distances from the land. I myself
caught one 370 miles from the coast of Africa, and have heard of others
caught at greater distances. The Rev. R.T. Lowe informed Sir C. Lyell that
in November, 1844, swarms of locusts visited the island of Madeira. They
were in countless numbers, as thick as the flakes of snow in the heaviest
snowstorm, and extended upward as far as could be seen with a telescope.
During two or three days they slowly careered round and round in an immense
ellipse, at least five or six miles in diameter, and at night alighted on
the taller trees, which were completely coated with them. They then
disappeared over the sea, as suddenly as they had appeared, and have not
since visited the island. Now, in parts of Natal it is believed by some
farmers, though on insufficient evidence, that injurious seeds are
introduced into their grass-land in the dung left by the great flights of
locusts which often visit that country. In consequence of this belief Mr.
Weale sent me in a letter a small packet of the dried pellets, out of which
I extracted under the microscope several seeds, and raised from them seven
grass plants, belonging to two species, of two genera. Hence a swarm of
locusts, such as that which visited Madeira, might readily be the means of
introducing several kinds of plants into an island lying far from the
mainland.
Although the beaks and feet of birds are generally clean, earth sometimes
adheres to them: in one case I removed sixty-one grains, and in another
case twenty-two grains of dry argillaceous earth from the foot of a
partridge, and in the earth there was a pebble as large as the seed of a
vetch. Here is a better case: the leg of a woodcock was sent to me by a
friend, with a little cake of dry earth attached to the shank, weighing
only nine grains; and this contained a seed of the toad-rush (Juncus
bufonius) which germinated and flowered. Mr. Swaysland, of Brighton, who
during the last forty years has paid close attention to our migratory
birds, informs me that he has often shot wagtails (Motacillae), wheatears,
and whinchats (Saxicolae), on their first arrival on our shores, before
they had alighted; and he has several times noticed little cakes of earth
attached to their feet. Many facts could be given showing how generally
soil is charged with seeds. For instance, Professor Newton sent me the leg
of a red-legged partridge (Caccabis rufa) which had been wounded and could
not fly, with a ball of hard earth adhering to it, and weighing six and a
half ounces. The earth had been kept for three years, but when broken,
watered and placed under a bell glass, no less than eighty-two plants
sprung from it: these consisted of twelve monocotyledons, including the
common oat, and at least one kind of grass, and of seventy dicotyledons,
which consisted, judging from the young leaves, of at least three distinct
species. With such facts before us, can we doubt that the many birds which
are annually blown by gales across great spaces of ocean, and which
annually migrate--for instance, the millions of quails across the
Mediterranean--must occasionally transport a few seeds embedded in dirt
adhering to their feet or beaks? But I shall have to recur to this
subject.
As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, it can
hardly be doubted that they must occasionally, as suggested by Lyell, have
transported seeds from one part to another of the arctic and antarctic
regions; and during the Glacial period from one part of the now temperate
regions to another. In the Azores, from the large number of plants common
to Europe, in comparison with the species on the other islands of the
Atlantic, which stand nearer to the mainland, and (as remarked by Mr. H.C.
Watson) from their somewhat northern character, in comparison with the
latitude, I suspected that these islands had been partly stocked by
ice-borne seeds during the Glacial epoch. At my request Sir C. Lyell wrote
to M. Hartung to inquire whether he had observed erratic boulders on these
islands, and he answered that he had found large fragments of granite and
other rocks, which do not occur in the archipelago. Hence we may safely
infer that icebergs formerly landed their rocky burdens on the shores of
these mid-ocean islands, and it is at least possible that they may have
brought thither the seeds of northern plants.
Considering that these several means of transport, and that other means,
which without doubt remain to be discovered, have been in action year after
year for tens of thousands of years, it would, I think, be a marvellous
fact if many plants had not thus become widely transported. These means of
transport are sometimes called accidental, but this is not strictly
correct: the currents of the sea are not accidental, nor is the direction
of prevalent gales of wind. It should be observed that scarcely any means
of transport would carry seeds for very great distances; for seeds do not
retain their vitality when exposed for a great length of time to the action
of sea water; nor could they be long carried in the crops or intestines of
birds. These means, however, would suffice for occasional transport across
tracts of sea some hundred miles in breadth, or from island to island, or
from a continent to a neighbouring island, but not from one distant
continent to another. The floras of distant continents would not by such
means become mingled; but would remain as distinct as they now are. The
currents, from their course, would never bring seeds from North America to
Britain, though they might and do bring seeds from the West Indies to our
western shores, where, if not killed by their very long immersion in salt
water, they could not endure our climate. Almost every year, one or two
land-birds are blown across the whole Atlantic Ocean, from North America to
the western shores of Ireland and England; but seeds could be transported
by these rare wanderers only by one means, namely, by dirt adhering to
their feet or beaks, which is in itself a rare accident. Even in this
case, how small would be the chance of a seed falling on favourable soil,
and coming to maturity! But it would be a great error to argue that
because a well-stocked island, like Great Britain, has not, as far as is
known (and it would be very difficult to prove this), received within the
last few centuries, through occasional means of transport, immigrants from
Europe or any other continent, that a poorly-stocked island, though
standing more remote from the mainland, would not receive colonists by
similar means. Out of a hundred kinds of seeds or animals transported to
an island, even if far less well-stocked than Britain, perhaps not more
than one would be so well fitted to its new home, as to become naturalised.
But this is no valid argument against what would be effected by occasional
means of transport, during the long lapse of geological time, whilst the
island was being upheaved, and before it had become fully stocked with
inhabitants. On almost bare land, with few or no destructive insects or
birds living there, nearly every seed which chanced to arrive, if fitted
for the climate, would germinate and survive.
DISPERSAL DURING THE GLACIAL PERIOD.
The identity of many plants and animals, on mountain-summits, separated
from each other by hundreds of miles of lowlands, where Alpine species
could not possibly exist, is one of the most striking cases known of the
same species living at distant points, without the apparent possibility of
their having migrated from one point to the other. It is indeed a
remarkable fact to see so many plants of the same species living on the
snowy regions of the Alps or Pyrenees, and in the extreme northern parts of
Europe; but it is far more remarkable, that the plants on the White
Mountains, in the United States of America, are all the same with those of
Labrador, and nearly all the same, as we hear from Asa Gray, with those on
the loftiest mountains of Europe. Even as long ago as 1747, such facts led
Gmelin to conclude that the same species must have been independently
created at many distinct points; and we might have remained in this same
belief, had not Agassiz and others called vivid attention to the Glacial
period, which, as we shall immediately see, affords a simple explanation of
these facts. We have evidence of almost every conceivable kind, organic
and inorganic, that, within a very recent geological period, central Europe
and North America suffered under an Arctic climate. The ruins of a house
burnt by fire do not tell their tale more plainly than do the mountains of
Scotland and Wales, with their scored flanks, polished surfaces, and
perched boulders, of the icy streams with which their valleys were lately
filled. So greatly has the climate of Europe changed, that in Northern
Italy, gigantic moraines, left by old glaciers, are now clothed by the vine
and maize. Throughout a large part of the United States, erratic boulders
and scored rocks plainly reveal a former cold period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained by Edward Forbes, is substantially as
follows. But we shall follow the changes more readily, by supposing a new
glacial period slowly to come on, and then pass away, as formerly occurred.
As the cold came on, and as each more southern zone became fitted for the
inhabitants of the north, these would take the places of the former
inhabitants of the temperate regions. The latter, at the same time would
travel further and further southward, unless they were stopped by barriers,
in which case they would perish. The mountains would become covered with
snow and ice, and their former Alpine inhabitants would descend to the
plains. By the time that the cold had reached its maximum, we should have
an arctic fauna and flora, covering the central parts of Europe, as far
south as the Alps and Pyrenees, and even stretching into Spain. The now
temperate regions of the United States would likewise be covered by arctic
plants and animals and these would be nearly the same with those of Europe;
for the present circumpolar inhabitants, which we suppose to have
everywhere travelled southward, are remarkably uniform round the world.
As the warmth returned, the arctic forms would retreat northward, closely
followed up in their retreat by the productions of the more temperate
regions. And as the snow melted from the bases of the mountains, the
arctic forms would seize on the cleared and thawed ground, always
ascending, as the warmth increased and the snow still further disappeared,
higher and higher, whilst their brethren were pursuing their northern
journey. Hence, when the warmth had fully returned, the same species,
which had lately lived together on the European and North American
lowlands, would again be found in the arctic regions of the Old and New
Worlds, and on many isolated mountain-summits far distant from each other.
Thus we can understand the identity of many plants at points so immensely
remote as the mountains of the United States and those of Europe. We can
thus also understand the fact that the Alpine plants of each mountain-range
are more especially related to the arctic forms living due north or nearly
due north of them: for the first migration when the cold came on, and the
re-migration on the returning warmth, would generally have been due south
and north. The Alpine plants, for example, of Scotland, as remarked by Mr.
H.C. Watson, and those of the Pyrenees, as remarked by Ramond, are more
especially allied to the plants of northern Scandinavia; those of the
United States to Labrador; those of the mountains of Siberia to the arctic
regions of that country. These views, grounded as they are on the
perfectly well-ascertained occurrence of a former Glacial period, seem to
me to explain in so satisfactory a manner the present distribution of the
Alpine and Arctic productions of Europe and America, that when in other
regions we find the same species on distant mountain-summits, we may almost
conclude, without other evidence, that a colder climate formerly permitted
their migration across the intervening lowlands, now become too warm for
their existence.
As the arctic forms moved first southward and afterwards backward to the
north, in unison with the changing climate, they will not have been exposed
during their long migrations to any great diversity of temperature; and as
they all migrated in a body together, their mutual relations will not have
been much disturbed. Hence, in accordance with the principles inculcated
in this volume, these forms will not have been liable to much modification.
But with the Alpine productions, left isolated from the moment of the
returning warmth, first at the bases and ultimately on the summits of the
mountains, the case will have been somewhat different; for it is not likely
that all the same arctic species will have been left on mountain ranges far
distant from each other, and have survived there ever since; they will
also, in all probability, have become mingled with ancient Alpine species,
which must have existed on the mountains before the commencement of the
Glacial epoch, and which during the coldest period will have been
temporarily driven down to the plains; they will, also, have been
subsequently exposed to somewhat different climatical influences. Their
mutual relations will thus have been in some degree disturbed; consequently
they will have been liable to modification; and they have been modified;
for if we compare the present Alpine plants and animals of the several
great European mountain ranges, one with another, though many of the
species remain identically the same, some exist as varieties, some as
doubtful forms or sub-species and some as distinct yet closely allied
species representing each other on the several ranges.
In the foregoing illustration, I have assumed that at the commencement of
our imaginary Glacial period, the arctic productions were as uniform round
the polar regions as they are at the present day. But it is also necessary
to assume that many sub-arctic and some few temperate forms were the same
round the world, for some of the species which now exist on the lower
mountain slopes and on the plains of North America and Europe are the same;
and it may be asked how I account for this degree of uniformity of the
sub-arctic and temperate forms round the world, at the commencement of the
real Glacial period. At the present day, the sub-arctic and northern
temperate productions of the Old and New Worlds are separated from each
other by the whole Atlantic Ocean and by the northern part of the Pacific.
During the Glacial period, when the inhabitants of the Old and New Worlds
lived further southwards than they do at present, they must have been still
more completely separated from each other by wider spaces of ocean; so that
it may well be asked how the same species could then or previously have
entered the two continents. The explanation, I believe, lies in the nature
of the climate before the commencement of the Glacial period. At this, the
newer Pliocene period, the majority of the inhabitants of the world were
specifically the same as now, and we have good reason to believe that the
climate was warmer than at the present day. Hence, we may suppose that the
organisms which now live under latitude 60 degrees, lived during the
Pliocene period further north, under the Polar Circle, in latitude 66-67
degrees; and that the present arctic productions then lived on the broken
land still nearer to the pole. Now, if we look at a terrestrial globe, we
see under the Polar Circle that there is almost continuous land from
western Europe through Siberia, to eastern America. And this continuity of
the circumpolar land, with the consequent freedom under a more favourable
climate for intermigration, will account for the supposed uniformity of the
sub-arctic and temperate productions of the Old and New Worlds, at a period
anterior to the Glacial epoch.
Believing, from reasons before alluded to, that our continents have long
remained in nearly the same relative position, though subjected to great
oscillations of level, I am strongly inclined to extend the above view, and
to infer that during some earlier and still warmer period, such as the
older Pliocene period, a large number of the same plants and animals
inhabited the almost continuous circumpolar land; and that these plants and
animals, both in the Old and New Worlds, began slowly to migrate southwards
as the climate became less warm, long before the commencement of the
Glacial period. We now see, as I believe, their descendants, mostly in a
modified condition, in the central parts of Europe and the United States.
On this view we can understand the relationship with very little identity,
between the productions of North America and Europe--a relationship which
is highly remarkable, considering the distance of the two areas, and their
separation by the whole Atlantic Ocean. We can further understand the
singular fact remarked on by several observers that the productions of
Europe and America during the later tertiary stages were more closely
related to each other than they are at the present time; for during these
warmer periods the northern parts of the Old and New Worlds will have been
almost continuously united by land, serving as a bridge, since rendered
impassable by cold, for the intermigration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as soon as the
species in common, which inhabited the New and Old Worlds, migrated south
of the Polar Circle, they will have been completely cut off from each
other. This separation, as far as the more temperate productions are
concerned, must have taken place long ages ago. As the plants and animals
migrated southward, they will have become mingled in the one great region
with the native American productions, and would have had to compete with
them; and in the other great region, with those of the Old World.
Consequently we have here everything favourable for much modification--for
far more modification than with the Alpine productions, left isolated,
within a much more recent period, on the several mountain ranges and on the
arctic lands of Europe and North America. Hence, it has come, that when we
compare the now living productions of the temperate regions of the New and
Old Worlds, we find very few identical species (though Asa Gray has lately
shown that more plants are identical than was formerly supposed), but we
find in every great class many forms, which some naturalists rank as
geographical races, and others as distinct species; and a host of closely
allied or representative forms which are ranked by all naturalists as
specifically distinct.
As on the land, so in the waters of the sea, a slow southern migration of a
marine fauna, which, during the Pliocene or even a somewhat earlier period,
was nearly uniform along the continuous shores of the Polar Circle, will
account, on the theory of modification, for many closely allied forms now
living in marine areas completely sundered. Thus, I think, we can
understand the presence of some closely allied, still existing and extinct
tertiary forms, on the eastern and western shores of temperate North
America; and the still more striking fact of many closely allied
crustaceans (as described in Dana's admirable work), some fish and other
marine animals, inhabiting the Mediterranean and the seas of Japan--these
two areas being now completely separated by the breadth of a whole
continent and by wide spaces of ocean.
These cases of close relationship in species either now or formerly
inhabiting the seas on the eastern and western shores of North America, the
Mediterranean and Japan, and the temperate lands of North America and
Europe, are inexplicable on the theory of creation. We cannot maintain
that such species have been created alike, in correspondence with the
nearly similar physical conditions of the areas; for if we compare, for
instance, certain parts of South America with parts of South Africa or
Australia, we see countries closely similar in all their physical
conditions, with their inhabitants utterly dissimilar.
ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH.
But we must return to our more immediate subject. I am convinced that
Forbes's view may be largely extended. In Europe we meet with the plainest
evidence of the Glacial period, from the western shores of Britain to the
Ural range, and southward to the Pyrenees. We may infer from the frozen
mammals and nature of the mountain vegetation, that Siberia was similarly
affected. In the Lebanon, according to Dr. Hooker, perpetual snow formerly
covered the central axis, and fed glaciers which rolled 4,000 feet down the
valleys. The same observer has recently found great moraines at a low
level on the Atlas range in North Africa. Along the Himalaya, at points
900 miles apart, glaciers have left the marks of their former low descent;
and in Sikkim, Dr. Hooker saw maize growing on ancient and gigantic
moraines. Southward of the Asiatic continent, on the opposite side of the
equator, we know, from the excellent researches of Dr. J. Haast and Dr.
Hector, that in New Zealand immense glaciers formerly descended to a low
level; and the same plants, found by Dr. Hooker on widely separated
mountains in this island tell the same story of a former cold period. From
facts communicated to me by the Rev. W.B. Clarke, it appears also that
there are traces of former glacial action on the mountains of the south-
eastern corner of Australia.
Looking to America: in the northern half, ice-borne fragments of rock have
been observed on the eastern side of the continent, as far south as
latitude 36 and 37 degrees, and on the shores of the Pacific, where the
climate is now so different, as far south as latitude 46 degrees. Erratic
boulders have, also, been noticed on the Rocky Mountains. In the
Cordillera of South America, nearly under the equator, glaciers once
extended far below their present level. In central Chile I examined a vast
mound of detritus with great boulders, crossing the Portillo valley, which,
there can hardly be a doubt, once formed a huge moraine; and Mr. D. Forbes
informs me that he found in various parts of the Cordillera, from latitude
13 to 30 degrees south, at about the height of 12,000 feet, deeply-furrowed
rocks, resembling those with which he was familiar in Norway, and likewise
great masses of detritus, including grooved pebbles. Along this whole
space of the Cordillera true glaciers do not now exist even at much more
considerable heights. Further south, on both sides of the continent, from
latitude 41 degrees to the southernmost extremity, we have the clearest
evidence of former glacial action, in numerous immense boulders transported
far from their parent source.
>From these several facts, namely, from the glacial action having extended
all round the northern and southern hemispheres--from the period having
been in a geological sense recent in both hemispheres--from its having
lasted in both during a great length of time, as may be inferred from the
amount of work effected--and lastly, from glaciers having recently
descended to a low level along the whole line of the Cordillera, it at one
time appeared to me that we could not avoid the conclusion that the
temperature of the whole world had been simultaneously lowered during the
Glacial period. But now, Mr. Croll, in a series of admirable memoirs, has
attempted to show that a glacial condition of climate is the result of
various physical causes, brought into operation by an increase in the
eccentricity of the earth's orbit. All these causes tend towards the same
end; but the most powerful appears to be the indirect influence of the
eccentricity of the orbit upon oceanic currents. According to Mr. Croll,
cold periods regularly recur every ten or fifteen thousand years; and these
at long intervals are extremely severe, owing to certain contingencies, of
which the most important, as Sir C. Lyell has shown, is the relative
position of the land and water. Mr. Croll believes that the last great
glacial period occurred about 240,000 years ago, and endured, with slight
alterations of climate, for about 160,000 years. With respect to more
ancient glacial periods, several geologists are convinced, from direct
evidence, that such occurred during the miocene and eocene formations, not
to mention still more ancient formations. But the most important result
for us, arrived at by Mr. Croll, is that whenever the northern hemisphere
passes through a cold period the temperature of the southern hemisphere is
actually raised, with the winters rendered much milder, chiefly through
changes in the direction of the ocean currents. So conversely it will be
with the northern hemisphere, while the southern passes through a glacial
period. This conclusion throws so much light on geographical distribution
that I am strongly inclined to trust in it; but I will first give the facts
which demand an explanation.
In South America, Dr. Hooker has shown that besides many closely allied
species, between forty and fifty of the flowering plants of Tierra del
Fuego, forming no inconsiderable part of its scanty flora, are common to
North America and Europe, enormously remote as these areas in opposite
hemispheres are from each other. On the lofty mountains of equatorial
America a host of peculiar species belonging to European genera occur. On
the Organ Mountains of Brazil some few temperate European, some Antarctic
and some Andean genera were found by Gardner which do not exist in the low
intervening hot countries. On the Silla of Caraccas the illustrious
Humboldt long ago found species belonging to genera characteristic of the
Cordillera.
In Africa, several forms characteristic of Europe, and some few
representatives of the flora of the Cape of Good Hope, occur on the
mountains of Abyssinia. At the Cape of Good Hope a very few European
species, believed not to have been introduced by man, and on the mountains
several representative European forms are found which have not been
discovered in the intertropical parts of Africa. Dr. Hooker has also
lately shown that several of the plants living on the upper parts of the
lofty island of Fernando Po, and on the neighbouring Cameroon Mountains, in
the Gulf of Guinea, are closely related to those on the mountains of
Abyssinia, and likewise to those of temperate Europe. It now also appears,
as I hear from Dr. Hooker, that some of these same temperate plants have
been discovered by the Rev. R.T. Lowe on the mountains of the Cape Verde
Islands. This extension of the same temperate forms, almost under the
equator, across the whole continent of Africa and to the mountains of the
Cape Verde archipelago, is one of the most astonishing facts ever recorded
in the distribution of plants.
On the Himalaya, and on the isolated mountain ranges of the peninsula of
India, on the heights of Ceylon, and on the volcanic cones of Java, many
plants occur either identically the same or representing each other, and at
the same time representing plants of Europe not found in the intervening
hot lowlands. A list of the genera of plants collected on the loftier
peaks of Java, raises a picture of a collection made on a hillock in
Europe. Still more striking is the fact that peculiar Australian forms are
represented by certain plants growing on the summits of the mountains of
Borneo. Some of these Australian forms, as I hear from Dr. Hooker, extend
along the heights of the peninsula of Malacca, and are thinly scattered on
the one hand over India, and on the other hand as far north as Japan.
On the southern mountains of Australia, Dr. F. Muller has discovered
several European species; other species, not introduced by man, occur on
the lowlands; and a long list can be given, as I am informed by Dr. Hooker,
of European genera, found in Australia, but not in the intermediate torrid
regions. In the admirable "Introduction to the Flora of New Zealand," by
Dr. Hooker, analogous and striking facts are given in regard to the plants
of that large island. Hence, we see that certain plants growing on the
more lofty mountains of the tropics in all parts of the world, and on the
temperate plains of the north and south, are either the same species or
varieties of the same species. It should, however, be observed that these
plants are not strictly arctic forms; for, as Mr. H.C. Watson has remarked,
"in receding from polar toward equatorial latitudes, the Alpine or mountain
flora really become less and less Arctic." Besides these identical and
closely allied forms, many species inhabiting the same widely sundered
areas, belong to genera not now found in the intermediate tropical
lowlands.
These brief remarks apply to plants alone; but some few analogous facts
could be given in regard to terrestrial animals. In marine productions,
similar cases likewise occur; as an example, I may quote a statement by the
highest authority, Prof. Dana, that "it is certainly a wonderful fact that
New Zealand should have a closer resemblance in its crustacea to Great
Britain, its antipode, than to any other part of the world." Sir J.
Richardson, also, speaks of the reappearance on the shores of New Zealand,
Tasmania, etc., of northern forms of fish. Dr. Hooker informs me that
twenty-five species of Algae are common to New Zealand and to Europe, but
have not been found in the intermediate tropical seas.
>From the foregoing facts, namely, the presence of temperate forms on the
highlands across the whole of equatorial Africa, and along the Peninsula of
India, to Ceylon and the Malay Archipelago, and in a less well-marked
manner across the wide expanse of tropical South America, it appears almost
certain that at some former period, no doubt during the most severe part of
a Glacial period, the lowlands of these great continents were everywhere
tenanted under the equator by a considerable number of temperate forms. At
this period the equatorial climate at the level of the sea was probably
about the same with that now experienced at the height of from five to six
thousand feet under the same latitude, or perhaps even rather cooler.
During this, the coldest period, the lowlands under the equator must have
been clothed with a mingled tropical and temperate vegetation, like that
described by Hooker as growing luxuriantly at the height of from four to
five thousand feet on the lower slopes of the Himalaya, but with perhaps a
still greater preponderance of temperate forms. So again in the
mountainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann found
temperate European forms beginning to appear at the height of about five
thousand feet. On the mountains of Panama, at the height of only two
thousand feet, Dr. Seemann found the vegetation like that of Mexico, "with
forms of the torrid zone harmoniously blended with those of the temperate."
Now let us see whether Mr. Croll's conclusion that when the northern
hemisphere suffered from the extreme cold of the great Glacial period, the
southern hemisphere was actually warmer, throws any clear light on the
present apparently inexplicable distribution of various organisms in the
temperate parts of both hemispheres, and on the mountains of the tropics.
The Glacial period, as measured by years, must have been very long; and
when we remember over what vast spaces some naturalised plants and animals
have spread within a few centuries, this period will have been ample for
any amount of migration. As the cold became more and more intense, we know
that Arctic forms invaded the temperate regions; and from the facts just
given, there can hardly be a doubt that some of the more vigorous, dominant
and widest-spreading temperate forms invaded the equatorial lowlands. The
inhabitants of these hot lowlands would at the same time have migrated to
the tropical and subtropical regions of the south, for the southern
hemisphere was at this period warmer. On the decline of the Glacial
period, as both hemispheres gradually recovered their former temperature,
the northern temperate forms living on the lowlands under the equator,
would have been driven to their former homes or have been destroyed, being
replaced by the equatorial forms returning from the south. Some, however,
of the northern temperate forms would almost certainly have ascended any
adjoining high land, where, if sufficiently lofty, they would have long
survived like the Arctic forms on the mountains of Europe. They might have
survived, even if the climate was not perfectly fitted for them, for the
change of temperature must have been very slow, and plants undoubtedly
possess a certain capacity for acclimatisation, as shown by their
transmitting to their offspring different constitutional powers of
resisting heat and cold.
In the regular course of events the southern hemisphere would in its turn
be subjected to a severe Glacial period, with the northern hemisphere
rendered warmer; and then the southern temperate forms would invade the
equatorial lowlands. The northern forms which had before been left on the
mountains would now descend and mingle with the southern forms. These
latter, when the warmth returned, would return to their former homes,
leaving some few species on the mountains, and carrying southward with them
some of the northern temperate forms which had descended from their
mountain fastnesses. Thus, we should have some few species identically the
same in the northern and southern temperate zones and on the mountains of
the intermediate tropical regions. But the species left during a long time
on these mountains, or in opposite hemispheres, would have to compete with
many new forms and would be exposed to somewhat different physical
conditions; hence, they would be eminently liable to modification, and
would generally now exist as varieties or as representative species; and
this is the case. We must, also, bear in mind the occurrence in both
hemispheres of former Glacial periods; for these will account, in
accordance with the same principles, for the many quite distinct species
inhabiting the same widely separated areas, and belonging to genera not now
found in the intermediate torrid zones.
It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many more
identical or slightly modified species have migrated from the north to the
south, than in a reversed direction. We see, however, a few southern forms
on the mountains of Borneo and Abyssinia. I suspect that this preponderant
migration from the north to the south is due to the greater extent of land
in the north, and to the northern forms having existed in their own homes
in greater numbers, and having consequently been advanced through natural
selection and competition to a higher stage of perfection, or dominating
power, than the southern forms. And thus, when the two sets became
commingled in the equatorial regions, during the alternations of the
Glacial periods, the northern forms were the more powerful and were able to
hold their places on the mountains, and afterwards migrate southward with
the southern forms; but not so the southern in regard to the northern
forms. In the same manner, at the present day, we see that very many
European productions cover the ground in La Plata, New Zealand, and to a
lesser degree in Australia, and have beaten the natives; whereas extremely
few southern forms have become naturalised in any part of the northern
hemisphere, though hides, wool, and other objects likely to carry seeds
have been largely imported into Europe during the last two or three
centuries from La Plata and during the last forty or fifty years from
Australia. The Neilgherrie Mountains in India, however, offer a partial
exception; for here, as I hear from Dr. Hooker, Australian forms are
rapidly sowing themselves and becoming naturalised. Before the last great
Glacial period, no doubt the intertropical mountains were stocked with
endemic Alpine forms; but these have almost everywhere yielded to the more
dominant forms generated in the larger areas and more efficient workshops
of the north. In many islands the native productions are nearly equalled,
or even outnumbered, by those which have become naturalised; and this is
the first stage towards their extinction. Mountains are islands on the
land; and their inhabitants have yielded to those produced within the
larger areas of the north, just in the same way as the inhabitants of real
islands have everywhere yielded and are still yielding to continental forms
naturalised through man's agency.
The same principles apply to the distribution of terrestrial animals and of
marine productions, in the northern and southern temperate zones, and on
the intertropical mountains. When, during the height of the Glacial
period, the ocean-currents were widely different to what they now are, some
of the inhabitants of the temperate seas might have reached the equator; of
these a few would perhaps at once be able to migrate southwards, by keeping
to the cooler currents, while others might remain and survive in the colder
depths until the southern hemisphere was in its turn subjected to a glacial
climate and permitted their further progress; in nearly the same manner as,
according to Forbes, isolated spaces inhabited by Arctic productions exist
to the present day in the deeper parts of the northern temperate seas.
I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which now
live so widely separated in the north and south, and sometimes on the
intermediate mountain ranges, are removed on the views above given. The
exact lines of migration cannot be indicated. We cannot say why certain
species and not others have migrated; why certain species have been
modified and have given rise to new forms, while others have remained
unaltered. We cannot hope to explain such facts, until we can say why one
species and not another becomes naturalised by man's agency in a foreign
land; why one species ranges twice or thrice as far, and is twice or thrice
as common, as another species within their own homes.
Various special difficulties also remain to be solved; for instance, the
occurrence, as shown by Dr. Hooker, of the same plants at points so
enormously remote as Kerguelen Land, New Zealand, and Fuegia; but icebergs,
as suggested by Lyell, may have been concerned in their dispersal. The
existence at these and other distant points of the southern hemisphere, of
species, which, though distinct, belong to genera exclusively confined to
the south, is a more remarkable case. Some of these species are so
distinct, that we cannot suppose that there has been time since the
commencement of the last Glacial period for their migration and subsequent
modification to the necessary degree. The facts seem to indicate that
distinct species belonging to the same genera have migrated in radiating
lines from a common centre; and I am inclined to look in the southern, as
in the northern hemisphere, to a former and warmer period, before the
commencement of the last Glacial period, when the Antarctic lands, now
covered with ice, supported a highly peculiar and isolated flora. It may
be suspected that before this flora was exterminated during the last
Glacial epoch, a few forms had been already widely dispersed to various
points of the southern hemisphere by occasional means of transport, and by
the aid, as halting-places, of now sunken islands. Thus the southern
shores of America, Australia, and New Zealand may have become slightly
tinted by the same peculiar forms of life.
Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
throughout the world on geographical distribution. And we have now seen
that Mr. Croll's conclusion that successive Glacial periods in the one
hemisphere coincide with warmer periods in the opposite hemisphere,
together with the admission of the slow modification of species, explains a
multitude of facts in the distribution of the same and of the allied forms
of life in all parts of the globe. The living waters have flowed during
one period from the north and during another from the south, and in both
cases have reached the equator; but the stream of life has flowed with
greater force from the north than in the opposite direction, and has
consequently more freely inundated the south. As the tide leaves its drift
in horizontal lines, rising higher on the shores where the tide rises
highest, so have the living waters left their living drift on our mountain
summits, in a line gently rising from the Arctic lowlands to a great
latitude under the equator. The various beings thus left stranded may be
compared with savage races of man, driven up and surviving in the mountain
fastnesses of almost every land, which serves as a record, full of interest
to us, of the former inhabitants of the surrounding lowlands.
CHAPTER XIII.
GEOGRAPHICAL DISTRIBUTION--continued.
Distribution of fresh-water productions -- On the inhabitants of oceanic
islands -- Absence of Batrachians and of terrestrial Mammals -- On the
relation of the inhabitants of islands to those of the nearest mainland --
On colonisation from the nearest source with subsequent modification --
Summary of the last and present chapters.
FRESH-WATER PRODUCTIONS.
As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently a
still more formidable barrier, that they would never have extended to
distant countries. But the case is exactly the reverse. Not only have
many fresh-water species, belonging to different classes, an enormous
range, but allied species prevail in a remarkable manner throughout the
world. When first collecting in the fresh waters of Brazil, I well
remember feeling much surprise at the similarity of the fresh-water
insects, shells, etc., and at the dissimilarity of the surrounding
terrestrial beings, compared with those of Britain.
But the wide ranging power of fresh-water productions can, I think, in most
cases be explained by their having become fitted, in a manner highly useful
to them, for short and frequent migrations from pond to pond, or from
stream to stream, within their own countries; and liability to wide
dispersal would follow from this capacity as an almost necessary
consequence. We can here consider only a few cases; of these, some of the
most difficult to explain are presented by fish. It was formerly believed
that the same fresh-water species never existed on two continents distant
from each other. But Dr. Gunther has lately shown that the Galaxias
attenuatus inhabits Tasmania, New Zealand, the Falkland Islands and the
mainland of South America. This is a wonderful case, and probably
indicates dispersal from an Antarctic centre during a former warm period.
This case, however, is rendered in some degree less surprising by the
species of this genus having the power of crossing by some unknown means
considerable spaces of open ocean: thus there is one species common to New
Zealand and to the Auckland Islands, though separated by a distance of
about 230 miles. On the same continent fresh-water fish often range
widely, and as if capriciously; for in two adjoining river systems some of
the species may be the same and some wholly different.
It is probable that they are occasionally transported by what may be called
accidental means. Thus fishes still alive are not very rarely dropped at
distant points by whirlwinds; and it is known that the ova retain their
vitality for a considerable time after removal from the water. Their
dispersal may, however, be mainly attributed to changes in the level of the
land within the recent period, causing rivers to flow into each other.
Instances, also, could be given of this having occurred during floods,
without any change of level. The wide differences of the fish on the
opposite sides of most mountain-ranges, which are continuous and
consequently must, from an early period, have completely prevented the
inosculation of the river systems on the two sides, leads to the same
conclusion. Some fresh-water fish belong to very ancient forms, and in
such cases there will have been ample time for great geographical changes,
and consequently time and means for much migration. Moreover, Dr. Gunther
has recently been led by several considerations to infer that with fishes
the same forms have a long endurance. Salt-water fish can with care be
slowly accustomed to live in fresh water; and, according to Valenciennes,
there is hardly a single group of which all the members are confined to
fresh water, so that a marine species belonging to a fresh-water group
might travel far along the shores of the sea, and could, it is probable,
become adapted without much difficulty to the fresh waters of a distant
land.
Some species of fresh-water shells have very wide ranges, and allied
species which, on our theory, are descended from a common parent, and must
have proceeded from a single source, prevail throughout the world. Their
distribution at first perplexed me much, as their ova are not likely to be
transported by birds; and the ova, as well as the adults, are immediately
killed by sea-water. I could not even understand how some naturalised
species have spread rapidly throughout the same country. But two facts,
which I have observed--and many others no doubt will be discovered--throw
some light on this subject. When ducks suddenly emerge from a pond covered
with duck-weed, I have twice seen these little plants adhering to their
backs; and it has happened to me, in removing a little duck-weed from one
aquarium to another, that I have unintentionally stocked the one with
fresh-water shells from the other. But another agency is perhaps more
effectual: I suspended the feet of a duck in an aquarium, where many ova
of fresh-water shells were hatching; and I found that numbers of the
extremely minute and just-hatched shells crawled on the feet, and clung to
them so firmly that when taken out of the water they could not be jarred
off, though at a somewhat more advanced age they would voluntarily drop
off. These just-hatched molluscs, though aquatic in their nature, survived
on the duck's feet, in damp air, from twelve to twenty hours; and in this
length of time a duck or heron might fly at least six or seven hundred
miles, and if blown across the sea to an oceanic island, or to any other
distant point, would be sure to alight on a pool or rivulet. Sir Charles
Lyell informs me that a Dyticus has been caught with an Ancylus (a
fresh-water shell like a limpet) firmly adhering to it; and a water-beetle
of the same family, a Colymbetes, once flew on board the "Beagle," when
forty-five miles distant from the nearest land: how much farther it might
have been blown by a favouring gale no one can tell.
With respect to plants, it has long been known what enormous ranges many
fresh-water, and even marsh-species, have, both over continents and to the
most remote oceanic islands. This is strikingly illustrated, according to
Alph. de Candolle, in those large groups of terrestrial plants, which have
very few aquatic members; for the latter seem immediately to acquire, as if
in consequence, a wide range. I think favourable means of dispersal
explain this fact. I have before mentioned that earth occasionally adheres
in some quantity to the feet and beaks of birds. Wading birds, which
frequent the muddy edges of ponds, if suddenly flushed, would be the most
likely to have muddy feet. Birds of this order wander more than those of
any other; and are occasionally found on the most remote and barren islands
of the open ocean; they would not be likely to alight on the surface of the
sea, so that any dirt on their feet would not be washed off; and when
gaining the land, they would be sure to fly to their natural fresh-water
haunts. I do not believe that botanists are aware how charged the mud of
ponds is with seeds: I have tried several little experiments, but will
here give only the most striking case: I took in February three
tablespoonfuls of mud from three different points, beneath water, on the
edge of a little pond; this mud when dry weighed only 6 and 3/4 ounces; I
kept it covered up in my study for six months, pulling up and counting each
plant as it grew; the plants were of many kinds, and were altogether 537 in
number; and yet the viscid mud was all contained in a breakfast cup!
Considering these facts, I think it would be an inexplicable circumstance
if water-birds did not transport the seeds of fresh-water plants to
unstocked ponds and streams, situated at very distant points. The same
agency may have come into play with the eggs of some of the smaller
fresh-water animals.
Other and unknown agencies probably have also played a part. I have stated
that fresh-water fish eat some kinds of seeds, though they reject many
other kinds after having swallowed them; even small fish swallow seeds of
moderate size, as of the yellow water-lily and Potamogeton. Herons and
other birds, century after century, have gone on daily devouring fish; they
then take flight and go to other waters, or are blown across the sea; and
we have seen that seeds retain their power of germination, when rejected
many hours afterwards in pellets or in the excrement. When I saw the great
size of the seeds of that fine water-lily, the Nelumbium, and remembered
Alph. de Candolle's remarks on the distribution of this plant, I thought
that the means of its dispersal must remain inexplicable; but Audubon
states that he found the seeds of the great southern water-lily (probably
according to Dr. Hooker, the Nelumbium luteum) in a heron's stomach. Now
this bird must often have flown with its stomach thus well stocked to
distant ponds, and, then getting a hearty meal of fish, analogy makes me
believe that it would have rejected the seeds in the pellet in a fit state
for germination.
In considering these several means of distribution, it should be remembered
that when a pond or stream is first formed, for instance on a rising islet,
it will be unoccupied; and a single seed or egg will have a good chance of
succeeding. Although there will always be a struggle for life between the
inhabitants of the same pond, however few in kind, yet as the number even
in a well-stocked pond is small in comparison with the number of species
inhabiting an equal area of land, the competition between them will
probably be less severe than between terrestrial species; consequently an
intruder from the waters of a foreign country would have a better chance of
seizing on a new place, than in the case of terrestrial colonists. We
should also remember that many fresh-water productions are low in the scale
of nature, and we have reason to believe that such beings become modified
more slowly than the high; and this will give time for the migration of
aquatic species. We should not forget the probability of many fresh-water
forms having formerly ranged continuously over immense areas, and then
having become extinct at intermediate points. But the wide distribution of
fresh-water plants, and of the lower animals, whether retaining the same
identical form, or in some degree modified, apparently depends in main part
on the wide dispersal of their seeds and eggs by animals, more especially
by fresh-water birds, which have great powers of flight, and naturally
travel from one piece of water to another.
ON THE INHABITANTS OF OCEANIC ISLANDS.
We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty with respect to
distribution, on the view that not only all the individuals of the same
species have migrated from some one area, but that allied species, although
now inhabiting the most distant points, have proceeded from a single area,
the birthplace of their early progenitors. I have already given my reasons
for disbelieving in continental extensions within the period of existing
species on so enormous a scale that all the many islands of the several
oceans were thus stocked with their present terrestrial inhabitants. This
view removes many difficulties, but it does not accord with all the facts
in regard to the productions of islands. In the following remarks I shall
not confine myself to the mere question of dispersal, but shall consider
some other cases bearing on the truth of the two theories of independent
creation and of descent with modification.
The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. New Zealand, for instance,
with its lofty mountains and diversified stations, extending over 780 miles
of latitude, together with the outlying islands of Auckland, Campbell and
Chatham, contain altogether only 960 kinds of flowering plants; if we
compare this moderate number with the species which swarm over equal areas
in Southwestern Australia or at the Cape of Good Hope, we must admit that
some cause, independently of different physical conditions, has given rise
to so great a difference in number. Even the uniform county of Cambridge
has 847 plants, and the little island of Anglesea 764, but a few ferns and
a few introduced plants are included in these numbers, and the comparison
in some other respects is not quite fair. We have evidence that the barren
island of Ascension aboriginally possessed less than half-a-dozen flowering
plants; yet many species have now become naturalised on it, as they have in
New Zealand and on every other oceanic island which can be named. In St.
Helena there is reason to believe that the naturalised plants and animals
have nearly or quite exterminated many native productions. He who admits
the doctrine of the creation of each separate species, will have to admit
that a sufficient number of the best adapted plants and animals were not
created for oceanic islands; for man has unintentionally stocked them far
more fully and perfectly than did nature.
Although in oceanic islands the species are few in number, the proportion
of endemic kinds (i.e. those found nowhere else in the world) is often
extremely large. If we compare, for instance, the number of endemic
land-shells in Madeira, or of endemic birds in the Galapagos Archipelago,
with the number found on any continent, and then compare the area of the
island with that of the continent, we shall see that this is true. This
fact might have been theoretically expected, for, as already explained,
species occasionally arriving, after long intervals of time in the new and
isolated district, and having to compete with new associates, would be
eminently liable to modification, and would often produce groups of
modified descendants. But it by no means follows that, because in an
island nearly all the species of one class are peculiar, those of another
class, or of another section of the same class, are peculiar; and this
difference seems to depend partly on the species which are not modified
having immigrated in a body, so that their mutual relations have not been
much disturbed; and partly on the frequent arrival of unmodified immigrants
from the mother-country, with which the insular forms have intercrossed.
It should be borne in mind that the offspring of such crosses would
certainly gain in vigour; so that even an occasional cross would produce
more effect than might have been anticipated. I will give a few
illustrations of the foregoing remarks: in the Galapagos Islands there are
twenty-six land birds; of these twenty-one (or perhaps twenty-three) are
peculiar; whereas of the eleven marine birds only two are peculiar; and it
is obvious that marine birds could arrive at these islands much more easily
and frequently than land-birds. Bermuda, on the other hand, which lies at
about the same distance from North America as the Galapagos Islands do from
South America, and which has a very peculiar soil, does not possess a
single endemic land bird; and we know from Mr. J.M. Jones's admirable
account of Bermuda, that very many North American birds occasionally or
even frequently visit this island. Almost every year, as I am informed by
Mr. E.V. Harcourt, many European and African birds are blown to Madeira;
this island is inhabited by ninety-nine kinds, of which one alone is
peculiar, though very closely related to a European form; and three or four
other species are confined to this island and to the Canaries. So that the
islands of Bermuda and Madeira have been stocked from the neighbouring
continents with birds, which for long ages have there struggled together,
and have become mutually co-adapted. Hence, when settled in their new
homes, each kind will have been kept by the others to its proper place and
habits, and will consequently have been but little liable to modification.
Any tendency to modification will also have been checked by intercrossing
with the unmodified immigrants, often arriving from the mother-country.
Madeira again is inhabited by a wonderful number of peculiar land-shells,
whereas not one species of sea-shell is peculiar to its shores: now,
though we do not know how sea-shells are dispersed, yet we can see that
their eggs or larvae, perhaps attached to seaweed or floating timber, or to
the feet of wading birds, might be transported across three or four hundred
miles of open sea far more easily than land-shells. The different orders
of insects inhabiting Madeira present nearly parallel cases.
Oceanic islands are sometimes deficient in animals of certain whole
classes, and their places are occupied by other classes; thus in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless birds,
take, or recently took, the place of mammals. Although New Zealand is here
spoken of as an oceanic island, it is in some degree doubtful whether it
should be so ranked; it is of large size, and is not separated from
Australia by a profoundly deep sea; from its geological character and the
direction of its mountain ranges, the Rev. W.B. Clarke has lately
maintained that this island, as well as New Caledonia, should be considered
as appurtenances of Australia. Turning to plants, Dr. Hooker has shown
that in the Galapagos Islands the proportional numbers of the different
orders are very different from what they are elsewhere. All such
differences in number, and the absence of certain whole groups of animals
and plants, are generally accounted for by supposed differences in the
physical conditions of the islands; but this explanation is not a little
doubtful. Facility of immigration seems to have been fully as important as
the nature of the conditions.
Many remarkable little facts could be given with respect to the inhabitants
of oceanic islands. For instance, in certain islands not tenanted by a
single mammal, some of the endemic plants have beautifully hooked seeds;
yet few relations are more manifest than that hooks serve for the
transportal of seeds in the wool or fur of quadrupeds. But a hooked seed
might be carried to an island by other means; and the plant then becoming
modified would form an endemic species, still retaining its hooks, which
would form a useless appendage, like the shrivelled wings under the
soldered wing-covers of many insular beetles. Again, islands often possess
trees or bushes belonging to orders which elsewhere include only herbaceous
species; now trees, as Alph. de Candolle has shown, generally have,
whatever the cause may be, confined ranges. Hence trees would be little
likely to reach distant oceanic islands; and an herbaceous plant, which had
no chance of successfully competing with the many fully developed trees
growing on a continent, might, when established on an island, gain an
advantage over other herbaceous plants by growing taller and taller and
overtopping them. In this case, natural selection would tend to add to the
stature of the plant, to whatever order it belonged, and thus first convert
it into a bush and then into a tree.
ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON OCEANIC ISLANDS.
With respect to the absence of whole orders of animals on oceanic islands,
Bory St. Vincent long ago remarked that Batrachians (frogs, toads, newts)
are never found on any of the many islands with which the great oceans are
studded. I have taken pains to verify this assertion, and have found it
true, with the exception of New Zealand, New Caledonia, the Andaman
Islands, and perhaps the Solomon Islands and the Seychelles. But I have
already remarked that it is doubtful whether New Zealand and New Caledonia
ought to be classed as oceanic islands; and this is still more doubtful
with respect to the Andaman and Solomon groups and the Seychelles. This
general absence of frogs, toads and newts on so many true oceanic islands
cannot be accounted for by their physical conditions; indeed it seems that
islands are peculiarly fitted for these animals; for frogs have been
introduced into Madeira, the Azores, and Mauritius, and have multiplied so
as to become a nuisance. But as these animals and their spawn are
immediately killed (with the exception, as far as known, of one Indian
species) by sea-water, there would be great difficulty in their transportal
across the sea, and therefore we can see why they do not exist on strictly
oceanic islands. But why, on the theory of creation, they should not have
been created there, it would be very difficult to explain.
Mammals offer another and similar case. I have carefully searched the
oldest voyages, and have not found a single instance, free from doubt, of a
terrestrial mammal (excluding domesticated animals kept by the natives)
inhabiting an island situated above 300 miles from a continent or great
continental island; and many islands situated at a much less distance are
equally barren. The Falkland Islands, which are inhabited by a wolf-like
fox, come nearest to an exception; but this group cannot be considered as
oceanic, as it lies on a bank in connection with the mainland at a distance
of about 280 miles; moreover, icebergs formerly brought boulders to its
western shores, and they may have formerly transported foxes, as now
frequently happens in the arctic regions. Yet it cannot be said that small
islands will not support at least small mammals, for they occur in many
parts of the world on very small islands, when lying close to a continent;
and hardly an island can be named on which our smaller quadrupeds have not
become naturalised and greatly multiplied. It cannot be said, on the
ordinary view of creation, that there has not been time for the creation of
mammals; many volcanic islands are sufficiently ancient, as shown by the
stupendous degradation which they have suffered, and by their tertiary
strata: there has also been time for the production of endemic species
belonging to other classes; and on continents it is known that new species
of mammals appear and disappear at a quicker rate than other and lower
animals. Although terrestrial mammals do not occur on oceanic islands,
aerial mammals do occur on almost every island. New Zealand possesses two
bats found nowhere else in the world: Norfolk Island, the Viti
Archipelago, the Bonin Islands, the Caroline and Marianne Archipelagoes,
and Mauritius, all possess their peculiar bats. Why, it may be asked, has
the supposed creative force produced bats and no other mammals on remote
islands? On my view this question can easily be answered; for no
terrestrial mammal can be transported across a wide space of sea, but bats
can fly across. Bats have been seen wandering by day far over the Atlantic
Ocean; and two North American species, either regularly or occasionally,
visit Bermuda, at the distance of 600 miles from the mainland. I hear from
Mr. Tomes, who has specially studied this family, that many species have
enormous ranges, and are found on continents and on far distant islands.
Hence, we have only to suppose that such wandering species have been
modified in their new homes in relation to their new position, and we can
understand the presence of endemic bats on oceanic islands, with the
absence of all other terrestrial mammals.
Another interesting relation exists, namely, between the depth of the sea
separating islands from each other, or from the nearest continent, and the
degree of affinity of their mammalian inhabitants. Mr. Windsor Earl has
made some striking observations on this head, since greatly extended by Mr.
Wallace's admirable researches, in regard to the great Malay Archipelago,
which is traversed near Celebes by a space of deep ocean, and this
separates two widely distinct mammalian faunas. On either side, the
islands stand on a moderately shallow submarine bank, and these islands are
inhabited by the same or by closely allied quadrupeds. I have not as yet
had time to follow up this subject in all quarters of the world; but as far
as I have gone, the relation holds good. For instance, Britain is
separated by a shallow channel from Europe, and the mammals are the same on
both sides; and so it is with all the islands near the shores of Australia.
The West Indian Islands, on the other hand, stand on a deeply submerged
bank, nearly one thousand fathoms in depth, and here we find American
forms, but the species and even the genera are quite distinct. As the
amount of modification which animals of all kinds undergo partly depends on
the lapse of time, and as the islands which are separated from each other,
or from the mainland, by shallow channels, are more likely to have been
continuously united within a recent period than the islands separated by
deeper channels, we can understand how it is that a relation exists between
the depth of the sea separating two mammalian faunas, and the degree of
their affinity, a relation which is quite inexplicable on the theory of
independent acts of creation.
The foregoing statements in regard to the inhabitants of oceanic islands,
namely, the fewness of the species, with a large proportion consisting of
endemic forms--the members of certain groups, but not those of other groups
in the same class, having been modified--the absence of certain whole
orders, as of batrachians and of terrestrial mammals, notwithstanding the
presence of aerial bats, the singular proportions of certain orders of
plants, herbaceous forms having been developed into trees, etc., seem to me
to accord better with the belief in the efficiency of occasional means of
transport, carried on during a long course of time, than with the belief in
the former connection of all oceanic islands with the nearest continent;
for on this latter view it is probable that the various classes would have
immigrated more uniformly, and from the species having entered in a body,
their mutual relations would not have been much disturbed, and
consequently, they would either have not been modified, or all the species
in a more equable manner.
I do not deny that there are many and serious difficulties in understanding
how many of the inhabitants of the more remote islands, whether still
retaining the same specific form or subsequently modified, have reached
their present homes. But the probability of other islands having once
existed as halting-places, of which not a wreck now remains, must not be
overlooked. I will specify one difficult case. Almost all oceanic
islands, even the most isolated and smallest, are inhabited by land-shells,
generally by endemic species, but sometimes by species found elsewhere
striking instances of which have been given by Dr. A.A. Gould in relation
to the Pacific. Now it is notorious that land-shells are easily killed by
sea-water; their eggs, at least such as I have tried, sink in it and are
killed. Yet there must be some unknown, but occasionally efficient means
for their transportal. Would the just-hatched young sometimes adhere to
the feet of birds roosting on the ground and thus get transported? It
occurred to me that land-shells, when hybernating and having a membranous
diaphragm over the mouth of the shell, might be floated in chinks of
drifted timber across moderately wide arms of the sea. And I find that
several species in this state withstand uninjured an immersion in sea-water
during seven days. One shell, the Helix pomatia, after having been thus
treated, and again hybernating, was put into sea-water for twenty days and
perfectly recovered. During this length of time the shell might have been
carried by a marine country of average swiftness to a distance of 660
geographical miles. As this Helix has a thick calcareous operculum I
removed it, and when it had formed a new membranous one, I again immersed
it for fourteen days in sea-water, and again it recovered and crawled away.
Baron Aucapitaine has since tried similar experiments. He placed 100 land-
shells, belonging to ten species, in a box pierced with holes, and immersed
it for a fortnight in the sea. Out of the hundred shells twenty-seven
recovered. The presence of an operculum seems to have been of importance,
as out of twelve specimens of Cyclostoma elegans, which is thus furnished,
eleven revived. It is remarkable, seeing how well the Helix pomatia
resisted with me the salt-water, that not one of fifty-four specimens
belonging to four other species of Helix tried by Aucapitaine recovered.
It is, however, not at all probable that land-shells have often been thus
transported; the feet of birds offer a more probable method.
ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO THOSE OF THE NEAREST
MAINLAND.
The most striking and important fact for us is the affinity of the species
which inhabit islands to those of the nearest mainland, without being
actually the same. Numerous instances could be given. The Galapagos
Archipelago, situated under the equator, lies at a distance of between 500
and 600 miles from the shores of South America. Here almost every product
of the land and of the water bears the unmistakable stamp of the American
continent. There are twenty-six land birds. Of these twenty-one, or
perhaps twenty-three, are ranked as distinct species, and would commonly be
assumed to have been here created; yet the close affinity of most of these
birds to American species is manifest in every character in their habits,
gestures, and tones of voice. So it is with the other animals, and with a
large proportion of the plants, as shown by Dr. Hooker in his admirable
Flora of this archipelago. The naturalist, looking at the inhabitants of
these volcanic islands in the Pacific, distant several hundred miles from
the continent, feels that he is standing on American land. Why should this
be so? Why should the species which are supposed to have been created in
the Galapagos Archipelago, and nowhere else, bear so plainly the stamp of
affinity to those created in America? There is nothing in the conditions
of life, in the geological nature of the islands, in their height or
climate, or in the proportions in which the several classes are associated
together, which closely resembles the conditions of the South American
coast. In fact, there is a considerable dissimilarity in all these
respects. On the other hand, there is a considerable degree of resemblance
in the volcanic nature of the soil, in the climate, height, and size of the
islands, between the Galapagos and Cape Verde Archipelagos: but what an
entire and absolute difference in their inhabitants! The inhabitants of
the Cape Verde Islands are related to those of Africa, like those of the
Galapagos to America. Facts, such as these, admit of no sort of
explanation on the ordinary view of independent creation; whereas, on the
view here maintained, it is obvious that the Galapagos Islands would be
likely to receive colonists from America, whether by occasional means of
transport or (though I do not believe in this doctrine) by formerly
continuous land, and the Cape Verde Islands from Africa; such colonists
would be liable to modification--the principle of inheritance still
betraying their original birthplace.
Many analogous facts could be given: indeed it is an almost universal rule
that the endemic productions of islands are related to those of the nearest
continent, or of the nearest large island. The exceptions are few, and
most of them can be explained. Thus, although Kerguelen Land stands nearer
to Africa than to America, the plants are related, and that very closely,
as we know from Dr. Hooker's account, to those of America: but on the view
that this island has been mainly stocked by seeds brought with earth and
stones on icebergs, drifted by the prevailing currents, this anomaly
disappears. New Zealand in its endemic plants is much more closely related
to Australia, the nearest mainland, than to any other region: and this is
what might have been expected; but it is also plainly related to South
America, which, although the next nearest continent, is so enormously
remote, that the fact becomes an anomaly. But this difficulty partially
disappears on the view that New Zealand, South America, and the other
southern lands, have been stocked in part from a nearly intermediate though
distant point, namely, from the antarctic islands, when they were clothed
with vegetation, during a warmer tertiary period, before the commencement
of the last Glacial period. The affinity, which, though feeble, I am
assured by Dr. Hooker is real, between the flora of the south-western
corner of Australia and of the Cape of Good Hope, is a far more remarkable
case; but this affinity is confined to the plants, and will, no doubt, some
day be explained.
The same law which has determined the relationship between the inhabitants
of islands and the nearest mainland, is sometimes displayed on a small
scale, but in a most interesting manner, within the limits of the same
archipelago. Thus each separate island of the Galapagos Archipelago is
tenanted, and the fact is a marvellous one, by many distinct species; but
these species are related to each other in a very much closer manner than
to the inhabitants of the American continent, or of any other quarter of
the world. This is what might have been expected, for islands situated so
near to each other would almost necessarily receive immigrants from the
same original source, and from each other. But how is it that many of the
immigrants have been differently modified, though only in a small degree,
in islands situated within sight of each other, having the same geological
nature, the same height, climate, etc? This long appeared to me a great
difficulty: but it arises in chief part from the deeply-seated error of
considering the physical conditions of a country as the most important;
whereas it cannot be disputed that the nature of the other species with
which each has to compete, is at least as important, and generally a far
more important element of success. Now if we look to the species which
inhabit the Galapagos Archipelago, and are likewise found in other parts of
the world, we find that they differ considerably in the several islands.
This difference might indeed have been expected if the islands have been
stocked by occasional means of transport--a seed, for instance, of one
plant having been brought to one island, and that of another plant to
another island, though all proceeding from the same general source. Hence,
when in former times an immigrant first settled on one of the islands, or
when it subsequently spread from one to another, it would undoubtedly be
exposed to different conditions in the different islands, for it would have
to compete with a different set of organisms; a plant, for instance, would
find the ground best-fitted for it occupied by somewhat different species
in the different islands, and would be exposed to the attacks of somewhat
different enemies. If, then, it varied, natural selection would probably
favour different varieties in the different islands. Some species,
however, might spread and yet retain the same character throughout the
group, just as we see some species spreading widely throughout a continent
and remaining the same.
The really surprising fact in this case of the Galapagos Archipelago, and
in a lesser degree in some analogous cases, is that each new species after
being formed in any one island, did not spread quickly to the other
islands. But the islands, though in sight of each other, are separated by
deep arms of the sea, in most cases wider than the British Channel, and
there is no reason to suppose that they have at any former period been
continuously united. The currents of the sea are rapid and deep between
the islands, and gales of wind are extraordinarily rare; so that the
islands are far more effectually separated from each other than they appear
on a map. Nevertheless, some of the species, both of those found in other
parts of the world and of those confined to the archipelago, are common to
the several islands; and we may infer from the present manner of
distribution that they have spread from one island to the others. But we
often take, I think, an erroneous view of the probability of closely allied
species invading each other's territory, when put into free
intercommunication. Undoubtedly, if one species has any advantage over
another, it will in a very brief time wholly or in part supplant it; but if
both are equally well fitted for their own places, both will probably hold
their separate places for almost any length of time. Being familiar with
the fact that many species, naturalised through man's agency, have spread
with astonishing rapidity over wide areas, we are apt to infer that most
species would thus spread; but we should remember that the species which
become naturalised in new countries are not generally closely allied to the
aboriginal inhabitants, but are very distinct forms, belonging in a large
proportion of cases, as shown by Alph. de Candolle, to distinct genera. In
the Galapagos Archipelago, many even of the birds, though so well adapted
for flying from island to island, differ on the different islands; thus
there are three closely allied species of mocking-thrush, each confined to
its own island. Now let us suppose the mocking-thrush of Chatham Island to
be blown to Charles Island, which has its own mocking-thrush; why should it
succeed in establishing itself there? We may safely infer that Charles
Island is well stocked with its own species, for annually more eggs are
laid and young birds hatched than can possibly be reared; and we may infer
that the mocking-thrush peculiar to Charles Island is at least as well
fitted for its home as is the species peculiar to Chatham Island. Sir C.
Lyell and Mr. Wollaston have communicated to me a remarkable fact bearing
on this subject; namely, that Madeira and the adjoining islet of Porto
Santo possess many distinct but representative species of land-shells, some
of which live in crevices of stone; and although large quantities of stone
are annually transported from Porto Santo to Madeira, yet this latter
island has not become colonised by the Porto Santo species: nevertheless,
both islands have been colonised by some European land-shells, which no
doubt had some advantage over the indigenous species. From these
considerations I think we need not greatly marvel at the endemic species
which inhabit the several islands of the Galapagos Archipelago not having
all spread from island to island. On the same continent, also,
pre-occupation has probably played an important part in checking the
commingling of the species which inhabit different districts with nearly
the same physical conditions. Thus, the south-east and south-west corners
of Australia have nearly the same physical conditions, and are united by
continuous land, yet they are inhabited by a vast number of distinct
mammals, birds, and plants; so it is, according to Mr. Bates, with the
butterflies and other animals inhabiting the great, open, and continuous
valley of the Amazons.
The same principle which governs the general character of the inhabitants
of oceanic islands, namely, the relation to the source whence colonists
could have been most easily derived, together with their subsequent
modification, is of the widest application throughout nature. We see this
on every mountain-summit, in every lake and marsh. For Alpine species,
excepting in as far as the same species have become widely spread during
the Glacial epoch, are related to those of the surrounding lowlands; thus
we have in South America, Alpine humming-birds, Alpine rodents, Alpine
plants, etc., all strictly belonging to American forms; and it is obvious
that a mountain, as it became slowly upheaved, would be colonised from the
surrounding lowlands. So it is with the inhabitants of lakes and marshes,
excepting in so far as great facility of transport has allowed the same
forms to prevail throughout large portions of the world. We see the same
principle in the character of most of the blind animals inhabiting the
caves of America and of Europe. Other analogous facts could be given. It
will, I believe, be found universally true, that wherever in two regions,
let them be ever so distant, many closely allied or representative species
occur, there will likewise be found some identical species; and wherever
many closely-allied species occur, there will be found many forms which
some naturalists rank as distinct species, and others as mere varieties;
these doubtful forms showing us the steps in the process of modification.
The relation between the power and extent of migration in certain species,
either at the present or at some former period, and the existence at remote
points of the world of closely allied species, is shown in another and more
general way. Mr. Gould remarked to me long ago, that in those genera of
birds which range over the world, many of the species have very wide
ranges. I can hardly doubt that this rule is generally true, though
difficult of proof. Among mammals, we see it strikingly displayed in Bats,
and in a lesser degree in the Felidae and Canidae. We see the same rule in
the distribution of butterflies and beetles. So it is with most of the
inhabitants of fresh water, for many of the genera in the most distinct
classes range over the world, and many of the species have enormous ranges.
It is not meant that all, but that some of the species have very wide
ranges in the genera which range very widely. Nor is it meant that the
species in such genera have, on an average, a very wide range; for this
will largely depend on how far the process of modification has gone; for
instance, two varieties of the same species inhabit America and Europe, and
thus the species has an immense range; but, if variation were to be carried
a little further, the two varieties would be ranked as distinct species,
and their range would be greatly reduced. Still less is it meant, that
species which have the capacity of crossing barriers and ranging widely, as
in the case of certain powerfully-winged birds, will necessarily range
widely; for we should never forget that to range widely implies not only
the power of crossing barriers, but the more important power of being
victorious in distant lands in the struggle for life with foreign
associates. But according to the view that all the species of a genus,
though distributed to the most remote points of the world, are descended
from a single progenitor, we ought to find, and I believe as a general rule
we do find, that some at least of the species range very widely.
We should bear in mind that many genera in all classes are of ancient
origin, and the species in this case will have had ample time for dispersal
and subsequent modification. There is also reason to believe, from
geological evidence, that within each great class the lower organisms
change at a slower rate than the higher; consequently they will have had a
better chance of ranging widely and of still retaining the same specific
character. This fact, together with that of the seeds and eggs of most
lowly organised forms being very minute and better fitted for distant
transportal, probably accounts for a law which has long been observed, and
which has lately been discussed by Alph. de Candolle in regard to plants,
namely, that the lower any group of organisms stands the more widely it
ranges.
The relations just discussed--namely, lower organisms ranging more widely
than the higher--some of the species of widely-ranging genera themselves
ranging widely--such facts, as alpine, lacustrine, and marsh productions
being generally related to those which live on the surrounding low lands
and dry lands--the striking relationship between the inhabitants of islands
and those of the nearest mainland--the still closer relationship of the
distinct inhabitants of the islands of the same archipelago--are
inexplicable on the ordinary view of the independent creation of each
species, but are explicable if we admit colonisation from the nearest or
readiest source, together with the subsequent adaptation of the colonists
to their new homes.
SUMMARY OF THE LAST AND PRESENT CHAPTERS.
In these chapters I have endeavoured to show that if we make due allowance
for our ignorance of the full effects of changes of climate and of the
level of the land, which have certainly occurred within the recent period,
and of other changes which have probably occurred--if we remember how
ignorant we are with respect to the many curious means of occasional
transport--if we bear in mind, and this is a very important consideration,
how often a species may have ranged continuously over a wide area, and then
have become extinct in the intermediate tracts--the difficulty is not
insuperable in believing that all the individuals of the same species,
wherever found, are descended from common parents. And we are led to this
conclusion, which has been arrived at by many naturalists under the
designation of single centres of creation, by various general
considerations, more especially from the importance of barriers of all
kinds, and from the analogical distribution of subgenera, genera, and
families.
With respect to distinct species belonging to the same genus, which on our
theory have spread from one parent-source; if we make the same allowances
as before for our ignorance, and remember that some forms of life have
changed very slowly, enormous periods of time having been thus granted for
their migration, the difficulties are far from insuperable; though in this
case, as in that of the individuals of the same species, they are often
great.
As exemplifying the effects of climatical changes on distribution, I have
attempted to show how important a part the last Glacial period has played,
which affected even the equatorial regions, and which, during the
alternations of the cold in the north and the south, allowed the
productions of opposite hemispheres to mingle, and left some of them
stranded on the mountain-summits in all parts of the world. As showing how
diversified are the means of occasional transport, I have discussed at some
little length the means of dispersal of fresh-water productions.
If the difficulties be not insuperable in admitting that in the long course
of time all the individuals of the same species, and likewise of the
several species belonging to the same genus, have proceeded from some one
source; then all the grand leading facts of geographical distribution are
explicable on the theory of migration, together with subsequent
modification and the multiplication of new forms. We can thus understand
the high importance of barriers, whether of land or water, in not only
separating but in apparently forming the several zoological and botanical
provinces. We can thus understand the concentration of related species
within the same areas; and how it is that under different latitudes, for
instance, in South America, the inhabitants of the plains and mountains, of
the forests, marshes, and deserts, are linked together in so mysterious a
manner, and are likewise linked to the extinct beings which formerly
inhabited the same continent. Bearing in mind that the mutual relation of
organism to organism is of the highest importance, we can see why two
areas, having nearly the same physical conditions, should often be
inhabited by very different forms of life; for according to the length of
time which has elapsed since the colonists entered one of the regions, or
both; according to the nature of the communication which allowed certain
forms and not others to enter, either in greater or lesser numbers;
according or not as those which entered happened to come into more or less
direct competition with each other and with the aborigines; and according
as the immigrants were capable of varying more or less rapidly, there would
ensue in the to or more regions, independently of their physical
conditions, infinitely diversified conditions of life; there would be an
almost endless amount of organic action and reaction, and we should find
some groups of beings greatly, and some only slightly modified; some
developed in great force, some existing in scanty numbers--and this we do
find in the several great geographical provinces of the world.
On these same principles we can understand, as I have endeavoured to show,
why oceanic islands should have few inhabitants, but that of these, a large
proportion should be endemic or peculiar; and why, in relation to the means
of migration, one group of beings should have all its species peculiar, and
another group, even within the same class, should have all its species the
same with those in an adjoining quarter of the world. We can see why whole
groups of organisms, as batrachians and terrestrial mammals, should be
absent from oceanic islands, whilst the most isolated islands should
possess their own peculiar species of aerial mammals or bats. We can see
why, in islands, there should be some relation between the presence of
mammals, in a more or less modified condition, and the depth of the sea
between such islands and the mainland. We can clearly see why all the
inhabitants of an archipelago, though specifically distinct on the several
islets, should be closely related to each other, and should likewise be
related, but less closely, to those of the nearest continent, or other
source whence immigrants might have been derived. We can see why, if there
exist very closely allied or representative species in two areas, however
distant from each other, some identical species will almost always there be
found.
As the late Edward Forbes often insisted, there is a striking parallelism
in the laws of life throughout time and space; the laws governing the
succession of forms in past times being nearly the same with those
governing at the present time the differences in different areas. We see
this in many facts. The endurance of each species and group of species is
continuous in time; for the apparent exceptions to the rule are so few that
they may fairly be attributed to our not having as yet discovered in an
intermediate deposit certain forms which are absent in it, but which occur
above and below: so in space, it certainly is the general rule that the
area inhabited by a single species, or by a group of species, is
continuous, and the exceptions, which are not rare, may, as I have
attempted to show, be accounted for by former migrations under different
circumstances, or through occasional means of transport, or by the species
having become extinct in the intermediate tracts. Both in time and space
species and groups of species have their points of maximum development.
Groups of species, living during the same period of time, or living within
the same area, are often characterised by trifling features in common, as
of sculpture or colour. In looking to the long succession of past ages, as
in looking to distant provinces throughout the world, we find that species
in certain classes differ little from each other, whilst those in another
class, or only in a different section of the same order, differ greatly
from each other. In both time and space the lowly organised members of
each class generally change less than the highly organised; but there are
in both cases marked exceptions to the rule. According to our theory,
these several relations throughout time and space are intelligible; for
whether we look to the allied forms of life which have changed during
successive ages, or to those which have changed after having migrated into
distant quarters, in both cases they are connected by the same bond of
ordinary generation; in both cases the laws of variation have been the
same, and modifications have been accumulated by the same means of natural
selection.
CHAPTER XIV.
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY --
RUDIMENTARY ORGANS.
Classification, groups subordinate to groups -- Natural system -- Rules and
difficulties in classification, explained on the theory of descent with
modification -- Classification of varieties -- Descent always used in
classification -- Analogical or adaptive characters -- Affinities, general,
complex and radiating -- Extinction separates and defines groups --
Morphology, between members of the same class, between parts of the same
individual -- Embryology, laws of, explained by variations not supervening
at an early age, and being inherited at a corresponding age -- Rudimentary
organs; their origin explained -- Summary.
CLASSIFICATION.
>From the most remote period in the history of the world organic beings have
been found to resemble each other in descending degrees, so that they can
be classed in groups under groups. This classification is not arbitrary
like the grouping of the stars in constellations. The existence of groups
would have been of simple significance, if one group had been exclusively
fitted to inhabit the land, and another the water; one to feed on flesh,
another on vegetable matter, and so on; but the case is widely different,
for it is notorious how commonly members of even the same subgroup have
different habits. In the second and fourth chapters, on Variation and on
Natural Selection, I have attempted to show that within each country it is
the widely ranging, the much diffused and common, that is the dominant
species, belonging to the larger genera in each class, which vary most.
The varieties, or incipient species, thus produced, ultimately become
converted into new and distinct species; and these, on the principle of
inheritance, tend to produce other new and dominant species. Consequently
the groups which are now large, and which generally include many dominant
species, tend to go on increasing in size. I further attempted to show
that from the varying descendants of each species trying to occupy as many
and as different places as possible in the economy of nature, they
constantly tend to diverge in character. This latter conclusion is
supported by observing the great diversity of forms, which, in any small
area, come into the closest competition, and by certain facts in
naturalisation.
I attempted also to show that there is a steady tendency in the forms which
are increasing in number and diverging in character, to supplant and
exterminate the preceding, less divergent and less improved forms. I
request the reader to turn to the diagram illustrating the action, as
formerly explained, of these several principles; and he will see that the
inevitable result is, that the modified descendants proceeding from one
progenitor become broken up into groups subordinate to groups. In the
diagram each letter on the uppermost line may represent a genus including
several species; and the whole of the genera along this upper line form
together one class, for all are descended from one ancient parent, and,
consequently, have inherited something in common. But the three genera on
the left hand have, on this same principle, much in common, and form a
subfamily, distinct from that containing the next two genera on the right
hand, which diverged from a common parent at the fifth stage of descent.
These five genera have also much in common, though less than when grouped
in subfamilies; and they form a family distinct from that containing the
three genera still further to the right hand, which diverged at an earlier
period. And all these genera, descended from (A), form an order distinct
from the genera descended from (I). So that we here have many species
descended from a single progenitor grouped into genera; and the genera into
subfamilies, families and orders, all under one great class. The grand
fact of the natural subordination of organic beings in groups under groups,
which, from its familiarity, does not always sufficiently strike us, is in
my judgment thus explained. No doubt organic beings, like all other
objects, can be classed in many ways, either artificially by single
characters, or more naturally by a number of characters. We know, for
instance, that minerals and the elemental substances can be thus arranged.
In this case there is of course no relation to genealogical succession, and
no cause can at present be assigned for their falling into groups. But
with organic beings the case is different, and the view above given accords
with their natural arrangement in group under group; and no other
explanation has ever been attempted.
Naturalists, as we have seen, try to arrange the species, genera and
families in each class, on what is called the Natural System. But what is
meant by this system? Some authors look at it merely as a scheme for
arranging together those living objects which are most alike, and for
separating those which are most unlike; or as an artificial method of
enunciating, as briefly as possible, general propositions--that is, by one
sentence to give the characters common, for instance, to all mammals, by
another those common to all carnivora, by another those common to the
dog-genus, and then, by adding a single sentence, a full description is
given of each kind of dog. The ingenuity and utility of this system are
indisputable. But many naturalists think that something more is meant by
the Natural System; they believe that it reveals the plan of the Creator;
but unless it be specified whether order in time or space, or both, or what
else is meant by the plan of the Creator, it seems to me that nothing is
thus added to our knowledge. Expressions such as that famous one by
Linnaeus, which we often meet with in a more or less concealed form,
namely, that the characters do not make the genus, but that the genus gives
the characters, seem to imply that some deeper bond is included in our
classifications than mere resemblance. I believe that this is the case,
and that community of descent--the one known cause of close similarity in
organic beings--is the bond, which, though observed by various degrees of
modification, is partially revealed to us by our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification either
gives some unknown plan of creation, or is simply a scheme for enunciating
general propositions and of placing together the forms most like each
other. It might have been thought (and was in ancient times thought) that
those parts of the structure which determined the habits of life, and the
general place of each being in the economy of nature, would be of very high
importance in classification. Nothing can be more false. No one regards
the external similarity of a mouse to a shrew, of a dugong to a whale, of a
whale to a fish, as of any importance. These resemblances, though so
intimately connected with the whole life of the being, are ranked as merely
"adaptive or analogical characters;" but to the consideration of these
resemblances we shall recur. It may even be given as a general rule, that
the less any part of the organisation is concerned with special habits, the
more important it becomes for classification. As an instance: Owen, in
speaking of the dugong, says, "The generative organs, being those which are
most remotely related to the habits and food of an animal, I have always
regarded as affording very clear indications of its true affinities. We
are least likely in the modifications of these organs to mistake a merely
adaptive for an essential character." With plants how remarkable it is
that the organs of vegetation, on which their nutrition and life depend,
are of little signification; whereas the organs of reproduction, with their
product the seed and embryo, are of paramount importance! So again, in
formerly discussing certain morphological characters which are not
functionally important, we have seen that they are often of the highest
service in classification. This depends on their constancy throughout many
allied groups; and their constancy chiefly depends on any slight deviations
not having been preserved and accumulated by natural selection, which acts
only on serviceable characters.
That the mere physiological importance of an organ does not determine its
classificatory value, is almost proved by the fact, that in allied groups,
in which the same organ, as we have every reason to suppose, has nearly the
same physiological value, its classificatory value is widely different. No
naturalist can have worked at any group without being struck with this
fact; and it has been fully acknowledged in the writings of almost every
author. It will suffice to quote the highest authority, Robert Brown, who,
in speaking of certain organs in the Proteaceae, says their generic
importance, "like that of all their parts, not only in this, but, as I
apprehend in every natural family, is very unequal, and in some cases seems
to be entirely lost." Again, in another work he says, the genera of the
Connaraceae "differ in having one or more ovaria, in the existence or
absence of albumen, in the imbricate or valvular aestivation. Any one of
these characters singly is frequently of more than generic importance,
though here even, when all taken together, they appear insufficient to
separate Cnestis from Connarus." To give an example among insects: in one
great division of the Hymenoptera, the antennae, as Westwood has remarked,
are most constant in structure; in another division they differ much, and
the differences are of quite subordinate value in classification; yet no
one will say that the antennae in these two divisions of the same order are
of unequal physiological importance. Any number of instances could be
given of the varying importance for classification of the same important
organ within the same group of beings.
Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this
condition are often of much value in classification. No one will dispute
that the rudimentary teeth in the upper jaws of young ruminants, and
certain rudimentary bones of the leg, are highly serviceable in exhibiting
the close affinity between Ruminants and Pachyderms. Robert Brown has
strongly insisted on the fact that the position of the rudimentary florets
is of the highest importance in the classification of the Grasses.
Numerous instances could be given of characters derived from parts which
must be considered of very trifling physiological importance, but which are
universally admitted as highly serviceable in the definition of whole
groups. For instance, whether or not there is an open passage from the
nostrils to the mouth, the only character, according to Owen, which
absolutely distinguishes fishes and reptiles--the inflection of the angle
of the lower jaw in Marsupials--the manner in which the wings of insects
are folded--mere colour in certain Algae--mere pubescence on parts of the
flower in grasses--the nature of the dermal covering, as hair or feathers,
in the Vertebrata. If the Ornithorhynchus had been covered with feathers
instead of hair, this external and trifling character would have been
considered by naturalists as an important aid in determining the degree of
affinity of this strange creature to birds.
The importance, for classification, of trifling characters, mainly depends
on their being correlated with many other characters of more or less
importance. The value indeed of an aggregate of characters is very evident
in natural history. Hence, as has often been remarked, a species may
depart from its allies in several characters, both of high physiological
importance, and of almost universal prevalence, and yet leave us in no
doubt where it should be ranked. Hence, also, it has been found that a
classification founded on any single character, however important that may
be, has always failed; for no part of the organisation is invariably
constant. The importance of an aggregate of characters, even when none are
important, alone explains the aphorism enunciated by Linnaeus, namely, that
the characters do not give the genus, but the genus gives the character;
for this seems founded on the appreciation of many trifling points of
resemblance, too slight to be defined. Certain plants, belonging to the
Malpighiaceae, bear perfect and degraded flowers; in the latter, as A. de
Jussieu has remarked, "The greater number of the characters proper to the
species, to the genus, to the family, to the class, disappear, and thus
laugh at our classification." When Aspicarpa produced in France, during
several years, only these degraded flowers, departing so wonderfully in a
number of the most important points of structure from the proper type of
the order, yet M. Richard sagaciously saw, as Jussieu observes, that this
genus should still be retained among the Malpighiaceae. This case well
illustrates the spirit of our classifications.
Practically, when naturalists are at work, they do not trouble themselves
about the physiological value of the characters which they use in defining
a group or in allocating any particular species. If they find a character
nearly uniform, and common to a great number of forms, and not common to
others, they use it as one of high value; if common to some lesser number,
they use it as of subordinate value. This principle has been broadly
confessed by some naturalists to be the true one; and by none more clearly
than by that excellent botanist, Aug. St. Hilaire. If several trifling
characters are always found in combination, though no apparent bond of
connexion can be discovered between them, especial value is set on them.
As in most groups of animals, important organs, such as those for
propelling the blood, or for aerating it, or those for propagating the
race, are found nearly uniform, they are considered as highly serviceable
in classification; but in some groups all these, the most important vital
organs, are found to offer characters of quite subordinate value. Thus, as
Fritz Muller has lately remarked, in the same group of crustaceans,
Cypridina is furnished with a heart, while in two closely allied genera,
namely Cypris and Cytherea, there is no such organ; one species of
Cypridina has well-developed branchiae, while another species is destitute
of them.
We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for a natural classification
of course includes all ages. But it is by no means obvious, on the
ordinary view, why the structure of the embryo should be more important for
this purpose than that of the adult, which alone plays its full part in the
economy of nature. Yet it has been strongly urged by those great
naturalists, Milne Edwards and Agassiz, that embryological characters are
the most important of all; and this doctrine has very generally been
admitted as true. Nevertheless, their importance has sometimes been
exaggerated, owing to the adaptive characters of larvae not having been
excluded; in order to show this, Fritz Muller arranged, by the aid of such
characters alone, the great class of crustaceans, and the arrangement did
not prove a natural one. But there can be no doubt that embryonic,
excluding larval characters, are of the highest value for classification,
not only with animals but with plants. Thus the main divisions of
flowering plants are founded on differences in the embryo--on the number
and position of the cotyledons, and on the mode of development of the
plumule and radicle. We shall immediately see why these characters possess
so high a value in classification, namely, from the natural system being
genealogical in its arrangement.
Our classifications are often plainly influenced by chains of affinities.
Nothing can be easier than to define a number of characters common to all
birds; but with crustaceans, any such definition has hitherto been found
impossible. There are crustaceans at the opposite ends of the series,
which have hardly a character in common; yet the species at both ends, from
being plainly allied to others, and these to others, and so onwards, can be
recognised as unequivocally belonging to this, and to no other class of the
Articulata.
Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of
closely allied forms. Temminck insists on the utility or even necessity of
this practice in certain groups of birds; and it has been followed by
several entomologists and botanists.
Finally, with respect to the comparative value of the various groups of
species, such as orders, suborders, families, subfamilies, and genera, they
seem to be, at least at present, almost arbitrary. Several of the best
botanists, such as Mr. Bentham and others, have strongly insisted on their
arbitrary value. Instances could be given among plants and insects, of a
group first ranked by practised naturalists as only a genus, and then
raised to the rank of a subfamily or family; and this has been done, not
because further research has detected important structural differences, at
first overlooked, but because numerous allied species, with slightly
different grades of difference, have been subsequently discovered.
All the foregoing rules and aids and difficulties in classification may be
explained, if I do not greatly deceive myself, on the view that the natural
system is founded on descent with modification--that the characters which
naturalists consider as showing true affinity between any two or more
species, are those which have been inherited from a common parent, all true
classification being genealogical--that community of descent is the hidden
bond which naturalists have been unconsciously seeking, and not some
unknown plan of creation, or the enunciation of general propositions, and
the mere putting together and separating objects more or less alike.
But I must explain my meaning more fully. I believe that the ARRANGEMENT
of the groups within each class, in due subordination and relation to each
other, must be strictly genealogical in order to be natural; but that the
AMOUNT of difference in the several branches or groups, though allied in
the same degree in blood to their common progenitor, may differ greatly,
being due to the different degrees of modification which they have
undergone; and this is expressed by the forms being ranked under different
genera, families, sections or orders. The reader will best understand what
is meant, if he will take the trouble to refer to the diagram in the fourth
chapter. We will suppose the letters A to L to represent allied genera
existing during the Silurian epoch, and descended from some still earlier
form. In three of these genera (A, F, and I) a species has transmitted
modified descendants to the present day, represented by the fifteen genera
(a14 to z14) on the uppermost horizontal line. Now, all these modified
descendants from a single species are related in blood or descent in the
same degree. They may metaphorically be called cousins to the same
millionth degree, yet they differ widely and in different degrees from each
other. The forms descended from A, now broken up into two or three
families, constitute a distinct order from those descended from I, also
broken up into two families. Nor can the existing species descended from A
be ranked in the same genus with the parent A, or those from I with parent
I. But the existing genus F14 may be supposed to have been but slightly
modified, and it will then rank with the parent genus F; just as some few
still living organisms belong to Silurian genera. So that the comparative
value of the differences between these organic beings, which are all
related to each other in the same degree in blood, has come to be widely
different. Nevertheless, their genealogical ARRANGEMENT remains strictly
true, not only at the present time, but at each successive period of
descent. All the modified descendants from A will have inherited something
in common from their common parent, as will all the descendants from I; so
will it be with each subordinate branch of descendants at each successive
stage. If, however, we suppose any descendant of A or of I to have become
so much modified as to have lost all traces of its parentage in this case,
its place in the natural system will be lost, as seems to have occurred
with some few existing organisms. All the descendants of the genus F,
along its whole line of descent, are supposed to have been but little
modified, and they form a single genus. But this genus, though much
isolated, will still occupy its proper intermediate position. The
representation of the groups as here given in the diagram on a flat
surface, is much too simple. The branches ought to have diverged in all
directions. If the names of the groups had been simply written down in a
linear series the representation would have been still less natural; and it
is notoriously not possible to represent in a series, on a flat surface,
the affinities which we discover in nature among the beings of the same
group. Thus, the natural system is genealogical in its arrangement, like a
pedigree. But the amount of modification which the different groups have
undergone has to be expressed by ranking them under different so-called
genera, subfamilies, families, sections, orders, and classes.
It may be worth while to illustrate this view of classification, by taking
the case of languages. If we possessed a perfect pedigree of mankind, a
genealogical arrangement of the races of man would afford the best
classification of the various languages now spoken throughout the world;
and if all extinct languages, and all intermediate and slowly changing
dialects, were to be included, such an arrangement would be the only
possible one. Yet it might be that some ancient languages had altered very
little and had given rise to few new languages, whilst others had altered
much owing to the spreading, isolation and state of civilisation of the
several co-descended races, and had thus given rise to many new dialects
and languages. The various degrees of difference between the languages of
the same stock would have to be expressed by groups subordinate to groups;
but the proper or even the only possible arrangement would still be
genealogical; and this would be strictly natural, as it would connect
together all languages, extinct and recent, by the closest affinities, and
would give the filiation and origin of each tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are known or believed to be descended from a single
species. These are grouped under the species, with the subvarieties under
the varieties; and in some cases, as with the domestic pigeon, with several
other grades of difference. Nearly the same rules are followed as in
classifying species. Authors have insisted on the necessity of arranging
varieties on a natural instead of an artificial system; we are cautioned,
for instance, not to class two varieties of the pine-apple together, merely
because their fruit, though the most important part, happens to be nearly
identical; no one puts the Swedish and common turnip together, though the
esculent and thickened stems are so similar. Whatever part is found to be
most constant, is used in classing varieties: thus the great agriculturist
Marshall says the horns are very useful for this purpose with cattle,
because they are less variable than the shape or colour of the body, etc.;
whereas with sheep the horns are much less serviceable, because less
constant. In classing varieties, I apprehend that if we had a real
pedigree, a genealogical classification would be universally preferred; and
it has been attempted in some cases. For we might feel sure, whether there
had been more or less modification, that the principle of inheritance would
keep the forms together which were allied in the greatest number of points.
In tumbler pigeons, though some of the subvarieties differ in the important
character of the length of the beak, yet all are kept together from having
the common habit of tumbling; but the short-faced breed has nearly or quite
lost this habit; nevertheless, without any thought on the subject, these
tumblers are kept in the same group, because allied in blood and alike in
some other respects.
With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade, that
of species, the two sexes; and how enormously these sometimes differ in the
most important characters is known to every naturalist: scarcely a single
fact can be predicated in common of the adult males and hermaphrodites of
certain cirripedes, and yet no one dreams of separating them. As soon as
the three Orchidean forms, Monachanthus, Myanthus, and Catasetum, which had
previously been ranked as three distinct genera, were known to be sometimes
produced on the same plant, they were immediately considered as varieties;
and now I have been able to show that they are the male, female, and
hermaphrodite forms of the same species. The naturalist includes as one
species the various larval stages of the same individual, however much they
may differ from each other and from the adult; as well as the so-called
alternate generations of Steenstrup, which can only in a technical sense be
considered as the same individual. He includes monsters and varieties, not
from their partial resemblance to the parent-form, but because they are
descended from it.
As descent has universally been used in classing together the individuals
of the same species, though the males and females and larvae are sometimes
extremely different; and as it has been used in classing varieties which
have undergone a certain, and sometimes a considerable amount of
modification, may not this same element of descent have been unconsciously
used in grouping species under genera, and genera under higher groups, all
under the so-called natural system? I believe it has been unconsciously
used; and thus only can I understand the several rules and guides which
have been followed by our best systematists. As we have no written
pedigrees, we are forced to trace community of descent by resemblances of
any kind. Therefore, we choose those characters which are the least likely
to have been modified, in relation to the conditions of life to which each
species has been recently exposed. Rudimentary structures on this view are
as good as, or even sometimes better than other parts of the organisation.
We care not how trifling a character may be--let it be the mere inflection
of the angle of the jaw, the manner in which an insect's wing is folded,
whether the skin be covered by hair or feathers--if it prevail throughout
many and different species, especially those having very different habits
of life, it assumes high value; for we can account for its presence in so
many forms with such different habits, only by inheritance from a common
parent. We may err in this respect in regard to single points of
structure, but when several characters, let them be ever so trifling,
concur throughout a large group of beings having different habits, we may
feel almost sure, on the theory of descent, that these characters have been
inherited from a common ancestor; and we know that such aggregated
characters have especial value in classification.
We can understand why a species or a group of species may depart from its
allies, in several of its most important characteristics, and yet be safely
classed with them. This may be safely done, and is often done, as long as
a sufficient number of characters, let them be ever so unimportant, betrays
the hidden bond of community of descent. Let two forms have not a single
character in common, yet, if these extreme forms are connected together by
a chain of intermediate groups, we may at once infer their community of
descent, and we put them all into the same class. As we find organs of
high physiological importance--those which serve to preserve life under the
most diverse conditions of existence--are generally the most constant, we
attach especial value to them; but if these same organs, in another group
or section of a group, are found to differ much, we at once value them less
in our classification. We shall presently see why embryological characters
are of such high classificatory importance. Geographical distribution may
sometimes be brought usefully into play in classing large genera, because
all the species of the same genus, inhabiting any distinct and isolated
region, are in all probability descended from the same parents.
ANALOGICAL RESEMBLANCES.
We can understand, on the above views, the very important distinction
between real affinities and analogical or adaptive resemblances. Lamarck
first called attention to this subject, and he has been ably followed by
Macleay and others. The resemblance in the shape of the body and in the
fin-like anterior limbs between dugongs and whales, and between these two
orders of mammals and fishes, are analogical. So is the resemblance
between a mouse and a shrew-mouse (Sorex), which belong to different
orders; and the still closer resemblance, insisted on by Mr. Mivart,
between the mouse and a small marsupial animal (Antechinus) of Australia.
These latter resemblances may be accounted for, as it seems to me, by
adaptation for similarly active movements through thickets and herbage,
together with concealment from enemies.
Among insects there are innumerable instances; thus Linnaeus, misled by
external appearances, actually classed an homopterous insect as a moth. We
see something of the same kind even with our domestic varieties, as in the
strikingly similar shape of the body in the improved breeds of the Chinese
and common pig, which are descended from distinct species; and in the
similarly thickened stems of the common and specifically distinct Swedish
turnip. The resemblance between the greyhound and race-horse is hardly
more fanciful than the analogies which have been drawn by some authors
between widely different animals.
On the view of characters being of real importance for classification, only
in so far as they reveal descent, we can clearly understand why analogical
or adaptive characters, although of the utmost importance to the welfare of
the being, are almost valueless to the systematist. For animals, belonging
to two most distinct lines of descent, may have become adapted to similar
conditions, and thus have assumed a close external resemblance; but such
resemblances will not reveal--will rather tend to conceal their
blood-relationship. We can thus also understand the apparent paradox, that
the very same characters are analogical when one group is compared with
another, but give true affinities when the members of the same group are
compared together: thus the shape of the body and fin-like limbs are only
analogical when whales are compared with fishes, being adaptations in both
classes for swimming through the water; but between the the several
members of the whale family, the shape of the body and the fin-like limbs
offer characters exhibiting true affinity; for as these parts are so nearly
similar throughout the whole family, we cannot doubt that they have been
inherited from a common ancestor. So it is with fishes.
Numerous cases could be given of striking resemblances in quite distinct
beings between single parts or organs, which have been adapted for the same
functions. A good instance is afforded by the close resemblance of the
jaws of the dog and Tasmanian wolf or Thylacinus--animals which are widely
sundered in the natural system. But this resemblance is confined to
general appearance, as in the prominence of the canines, and in the cutting
shape of the molar teeth. For the teeth really differ much: thus the dog
has on each side of the upper jaw four pre-molars and only two molars;
while the Thylacinus has three pre-molars and four molars. The molars also
differ much in the two animals in relative size and structure. The adult
dentition is preceded by a widely different milk dentition. Any one may,
of course, deny that the teeth in either case have been adapted for tearing
flesh, through the natural selection of successive variations; but if this
be admitted in the one case, it is unintelligible to me that it should be
denied in the other. I am glad to find that so high an authority as
Professor Flower has come to this same conclusion.
The extraordinary cases given in a former chapter, of widely different
fishes possessing electric organs--of widely different insects possessing
luminous organs--and of orchids and asclepiads having pollen-masses with
viscid discs, come under this same head of analogical resemblances. But
these cases are so wonderful that they were introduced as difficulties or
objections to our theory. In all such cases some fundamental difference in
the growth or development of the parts, and generally in their matured
structure, can be detected. The end gained is the same, but the means,
though appearing superficially to be the same, are essentially different.
The principle formerly alluded to under the term of ANALOGICAL VARIATION
has probably in these cases often come into play; that is, the members of
the same class, although only distantly allied, have inherited so much in
common in their constitution, that they are apt to vary under similar
exciting causes in a similar manner; and this would obviously aid in the
acquirement through natural selection of parts or organs, strikingly like
each other, independently of their direct inheritance from a common
progenitor.
As species belonging to distinct classes have often been adapted by
successive slight modifications to live under nearly similar circumstances
--to inhabit, for instance, the three elements of land, air and water--we
can perhaps understand how it is that a numerical parallelism has sometimes
been observed between the subgroups of distinct classes. A naturalist,
struck with a parallelism of this nature, by arbitrarily raising or sinking
the value of the groups in several classes (and all our experience shows
that their valuation is as yet arbitrary), could easily extend the
parallelism over a wide range; and thus the septenary, quinary, quaternary
and ternary classifications have probably arisen.
There is another and curious class of cases in which close external
resemblance does not depend on adaptation to similar habits of life, but
has been gained for the sake of protection. I allude to the wonderful
manner in which certain butterflies imitate, as first described by Mr.
Bates, other and quite distinct species. This excellent observer has shown
that in some districts of South America, where, for instance, an Ithomia
abounds in gaudy swarms, another butterfly, namely, a Leptalis, is often
found mingled in the same flock; and the latter so closely resembles the
Ithomia in every shade and stripe of colour, and even in the shape of its
wings, that Mr. Bates, with his eyes sharpened by collecting during eleven
years, was, though always on his guard, continually deceived. When the
mockers and the mocked are caught and compared, they are found to be very
different in essential structure, and to belong not only to distinct
genera, but often to distinct families. Had this mimicry occurred in only
one or two instances, it might have been passed over as a strange
coincidence. But, if we proceed from a district where one Leptalis
imitates an Ithomia, another mocking and mocked species, belonging to the
same two genera, equally close in their resemblance, may be found.
Altogether no less than ten genera are enumerated, which include species
that imitate other butterflies. The mockers and mocked always inhabit the
same region; we never find an imitator living remote from the form which it
imitates. The mockers are almost invariably rare insects; the mocked in
almost every case abounds in swarms. In the same district in which a
species of Leptalis closely imitates an Ithomia, there are sometimes other
Lepidoptera mimicking the same Ithomia: so that in the same place, species
of three genera of butterflies and even a moth are found all closely
resembling a butterfly belonging to a fourth genus. It deserves especial
notice that many of the mimicking forms of the Leptalis, as well as of the
mimicked forms, can be shown by a graduated series to be merely varieties
of the same species; while others are undoubtedly distinct species. But
why, it may be asked, are certain forms treated as the mimicked and others
as the mimickers? Mr. Bates satisfactorily answers this question by
showing that the form which is imitated keeps the usual dress of the group
to which it belongs, while the counterfeiters have changed their dress and
do not resemble their nearest allies.
We are next led to enquire what reason can be assigned for certain
butterflies and moths so often assuming the dress of another and quite
distinct form; why, to the perplexity of naturalists, has nature
condescended to the tricks of the stage? Mr. Bates has, no doubt, hit on
the true explanation. The mocked forms, which always abound in numbers,
must habitually escape destruction to a large extent, otherwise they could
not exist in such swarms; and a large amount of evidence has now been
collected, showing that they are distasteful to birds and other insect-
devouring animals. The mocking forms, on the other hand, that inhabit the
same district, are comparatively rare, and belong to rare groups; hence,
they must suffer habitually from some danger, for otherwise, from the
number of eggs laid by all butterflies, they would in three or four
generations swarm over the whole country. Now if a member of one of these
persecuted and rare groups were to assume a dress so like that of a well-
protected species that it continually deceived the practised eyes of an
entomologist, it would often deceive predaceous birds and insects, and thus
often escape destruction. Mr. Bates may almost be said to have actually
witnessed the process by which the mimickers have come so closely to
resemble the mimicked; for he found that some of the forms of Leptalis
which mimic so many other butterflies, varied in an extreme degree. In one
district several varieties occurred, and of these one alone resembled, to a
certain extent, the common Ithomia of the same district. In another
district there were two or three varieties, one of which was much commoner
than the others, and this closely mocked another form of Ithomia. From
facts of this nature, Mr. Bates concludes that the Leptalis first varies;
and when a variety happens to resemble in some degree any common butterfly
inhabiting the same district, this variety, from its resemblance to a
flourishing and little persecuted kind, has a better chance of escaping
destruction from predaceous birds and insects, and is consequently oftener
preserved; "the less perfect degrees of resemblance being generation after
generation eliminated, and only the others left to propagate their kind."
So that here we have an excellent illustration of natural selection.
Messrs. Wallace and Trimen have likewise described several equally striking
cases of imitation in the Lepidoptera of the Malay Archipelago and Africa,
and with some other insects. Mr. Wallace has also detected one such case
with birds, but we have none with the larger quadrupeds. The much greater
frequency of imitation with insects than with other animals, is probably
the consequence of their small size; insects cannot defend themselves,
excepting indeed the kinds furnished with a sting, and I have never heard
of an instance of such kinds mocking other insects, though they are mocked;
insects cannot easily escape by flight from the larger animals which prey
on them; therefore, speaking metaphorically, they are reduced, like most
weak creatures, to trickery and dissimulation.
It should be observed that the process of imitation probably never
commenced between forms widely dissimilar in colour. But, starting with
species already somewhat like each other, the closest resemblance, if
beneficial, could readily be gained by the above means, and if the imitated
form was subsequently and gradually modified through any agency, the
imitating form would be led along the same track, and thus be altered to
almost any extent, so that it might ultimately assume an appearance or
colouring wholly unlike that of the other members of the family to which it
belonged. There is, however, some difficulty on this head, for it is
necessary to suppose in some cases that ancient members belonging to
several distinct groups, before they had diverged to their present extent,
accidentally resembled a member of another and protected group in a
sufficient degree to afford some slight protection, this having given the
basis for the subsequent acquisition of the most perfect resemblance.
ON THE NATURE OF THE AFFINITIES CONNECTING ORGANIC BEINGS.
As the modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages which made the groups to which they
belong large and their parents dominant, they are almost sure to spread
widely, and to seize on more and more places in the economy of nature. The
larger and more dominant groups within each class thus tend to go on
increasing in size, and they consequently supplant many smaller and feebler
groups. Thus, we can account for the fact that all organisms, recent and
extinct, are included under a few great orders and under still fewer
classes. As showing how few the higher groups are in number, and how
widely they are spread throughout the world, the fact is striking that the
discovery of Australia has not added an insect belonging to a new class,
and that in the vegetable kingdom, as I learn from Dr. Hooker, it has added
only two or three families of small size.
In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character during
the long-continued process of modification, how it is that the more ancient
forms of life often present characters in some degree intermediate between
existing groups. As some few of the old and intermediate forms having
transmitted to the present day descendants but little modified, these
constitute our so-called osculant or aberrant groups. The more aberrant
any form is, the greater must be the number of connecting forms which have
been exterminated and utterly lost. And we have evidence of aberrant
groups having suffered severely from extinction, for they are almost always
represented by extremely few species; and such species as do occur are
generally very distinct from each other, which again implies extinction.
The genera Ornithorhynchus and Lepidosiren, for example, would not have
been less aberrant had each been represented by a dozen species, instead of
as at present by a single one, or by two or three. We can, I think,
account for this fact only by looking at aberrant groups as forms which
have been conquered by more successful competitors, with a few members
still preserved under unusually favourable conditions.
Mr. Waterhouse has remarked that when a member belonging to one group of
animals exhibits an affinity to a quite distinct group, this affinity in
most cases is general and not special: thus, according to Mr. Waterhouse,
of all Rodents, the bizcacha is most nearly related to Marsupials; but in
the points in which it approaches this order, its relations are general,
that is, not to any one Marsupial species more than to another. As these
points of affinity are believed to be real and not merely adaptive, they
must be due in accordance with our view to inheritance from a common
progenitor. Therefore, we must suppose either that all Rodents, including
the bizcacha, branched off from some ancient Marsupial, which will
naturally have been more or less intermediate in character with respect to
all existing Marsupials; or that both Rodents and Marsupials branched off
from a common progenitor, and that both groups have since undergone much
modification in divergent directions. On either view we must suppose that
the bizcacha has retained, by inheritance, more of the character of its
ancient progenitor than have other Rodents; and therefore it will not be
specially related to any one existing Marsupial, but indirectly to all or
nearly all Marsupials, from having partially retained the character of
their common progenitor, or of some early member of the group. On the
other hand, of all Marsupials, as Mr. Waterhouse has remarked, the
Phascolomys resembles most nearly, not any one species, but the general
order of Rodents. In this case, however, it may be strongly suspected that
the resemblance is only analogical, owing to the Phascolomys having become
adapted to habits like those of a Rodent. The elder De Candolle has made
nearly similar observations on the general nature of the affinities of
distinct families of plants.
On the principle of the multiplication and gradual divergence in character
of the species descended from a common progenitor, together with their
retention by inheritance of some characters in common, we can understand
the excessively complex and radiating affinities by which all the members
of the same family or higher group are connected together. For the common
progenitor of a whole family, now broken up by extinction into distinct
groups and subgroups, will have transmitted some of its characters,
modified in various ways and degrees, to all the species; and they will
consequently be related to each other by circuitous lines of affinity of
various lengths (as may be seen in the diagram so often referred to),
mounting up through many predecessors. As it is difficult to show the
blood-relationship between the numerous kindred of any ancient and noble
family, even by the aid of a genealogical tree, and almost impossible to do
so without this aid, we can understand the extraordinary difficulty which
naturalists have experienced in describing, without the aid of a diagram,
the various affinities which they perceive between the many living and
extinct members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has played an important
part in defining and widening the intervals between the several groups in
each class. We may thus account for the distinctness of whole classes from
each other--for instance, of birds from all other vertebrate animals--by
the belief that many ancient forms of life have been utterly lost, through
which the early progenitors of birds were formerly connected with the early
progenitors of the other and at that time less differentiated vertebrate
classes. There has been much less extinction of the forms of life which
once connected fishes with Batrachians. There has been still less within
some whole classes, for instance the Crustacea, for here the most
wonderfully diverse forms are still linked together by a long and only
partially broken chain of affinities. Extinction has only defined the
groups: it has by no means made them; for if every form which has ever
lived on this earth were suddenly to reappear, though it would be quite
impossible to give definitions by which each group could be distinguished,
still a natural classification, or at least a natural arrangement, would be
possible. We shall see this by turning to the diagram: the letters, A to
L, may represent eleven Silurian genera, some of which have produced large
groups of modified descendants, with every link in each branch and
sub-branch still alive; and the links not greater than those between
existing varieties. In this case it would be quite impossible to give
definitions by which the several members of the several groups could be
distinguished from their more immediate parents and descendants. Yet the
arrangement in the diagram would still hold good and would be natural; for,
on the principle of inheritance, all the forms descended, for instance from
A, would have something in common. In a tree we can distinguish this or
that branch, though at the actual fork the two unite and blend together.
We could not, as I have said, define the several groups; but we could pick
out types, or forms, representing most of the characters of each group,
whether large or small, and thus give a general idea of the value of the
differences between them. This is what we should be driven to, if we were
ever to succeed in collecting all the forms in any one class which have
lived throughout all time and space. Assuredly we shall never succeed in
making so perfect a collection: nevertheless, in certain classes, we are
tending toward this end; and Milne Edwards has lately insisted, in an able
paper, on the high importance of looking to types, whether or not we can
separate and define the groups to which such types belong.
Finally, we have seen that natural selection, which follows from the
struggle for existence, and which almost inevitably leads to extinction and
divergence of character in the descendants from any one parent-species,
explains that great and universal feature in the affinities of all organic
beings, namely, their subordination in group under group. We use the
element of descent in classing the individuals of both sexes and of all
ages under one species, although they may have but few characters in
common; we use descent in classing acknowledged varieties, however
different they may be from their parents; and I believe that this element
of descent is the hidden bond of connexion which naturalists have sought
under the term of the Natural System. On this idea of the natural system
being, in so far as it has been perfected, genealogical in its arrangement,
with the grades of difference expressed by the terms genera, families,
orders, etc., we can understand the rules which we are compelled to follow
in our classification. We can understand why we value certain resemblances
far more than others; why we use rudimentary and useless organs, or others
of trifling physiological importance; why, in finding the relations between
one group and another, we summarily reject analogical or adaptive
characters, and yet use these same characters within the limits of the same
group. We can clearly see how it is that all living and extinct forms can
be grouped together within a few great classes; and how the several members
of each class are connected together by the most complex and radiating
lines of affinities. We shall never, probably, disentangle the
inextricable web of the affinities between the members of any one class;
but when we have a distinct object in view, and do not look to some unknown
plan of creation, we may hope to make sure but slow progress.
Professor Haeckel in his "Generelle Morphologie" and in another works, has
recently brought his great knowledge and abilities to bear on what he calls
phylogeny, or the lines of descent of all organic beings. In drawing up
the several series he trusts chiefly to embryological characters, but
receives aid from homologous and rudimentary organs, as well as from the
successive periods at which the various forms of life are believed to have
first appeared in our geological formations. He has thus boldly made a
great beginning, and shows us how classification will in the future be
treated.
MORPHOLOGY.
We have seen that the members of the same class, independently of their
habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term "unity of
type;" or by saying that the several parts and organs in the different
species of the class are homologous. The whole subject is included under
the general term of Morphology. This is one of the most interesting
departments of natural history, and may almost be said to be its very soul.
What can be more curious than that the hand of a man, formed for grasping,
that of a mole for digging, the leg of the horse, the paddle of the
porpoise, and the wing of the bat, should all be constructed on the same
pattern, and should include similar bones, in the same relative positions?
How curious it is, to give a subordinate though striking instance, that the
hind feet of the kangaroo, which are so well fitted for bounding over the
open plains--those of the climbing, leaf-eating koala, equally well fitted
for grasping the branches of trees--those of the ground-dwelling, insect or
root-eating, bandicoots--and those of some other Australian marsupials--
should all be constructed on the same extraordinary type, namely with the
bones of the second and third digits extremely slender and enveloped within
the same skin, so that they appear like a single toe furnished with two
claws. Notwithstanding this similarity of pattern, it is obvious that the
hind feet of these several animals are used for as widely different
purposes as it is possible to conceive. The case is rendered all the more
striking by the American opossums, which follow nearly the same habits of
life as some of their Australian relatives, having feet constructed on the
ordinary plan. Professor Flower, from whom these statements are taken,
remarks in conclusion: "We may call this conformity to type, without
getting much nearer to an explanation of the phenomenon;" and he then adds
"but is it not powerfully suggestive of true relationship, of inheritance
from a common ancestor?"
Geoffroy St. Hilaire has strongly insisted on the high importance of
relative position or connexion in homologous parts; they may differ to
almost any extent in form and size, and yet remain connected together in
the same invariable order. We never find, for instance, the bones of the
arm and forearm, or of the thigh and leg, transposed. Hence the same names
can be given to the homologous bones in widely different animals. We see
the same great law in the construction of the mouths of insects: what can
be more different than the immensely long spiral proboscis of a
sphinx-moth, the curious folded one of a bee or bug, and the great jaws of
a beetle? Yet all these organs, serving for such widely different
purposes, are formed by infinitely numerous modifications of an upper lip,
mandibles, and two pairs of maxillae. The same law governs the
construction of the mouths and limbs of crustaceans. So it is with the
flowers of plants.
Nothing can be more hopeless than to attempt to explain this similarity of
pattern in members of the same class, by utility or by the doctrine of
final causes. The hopelessness of the attempt has been expressly admitted
by Owen in his most interesting work on the "Nature of Limbs." On the
ordinary view of the independent creation of each being, we can only say
that so it is; that it has pleased the Creator to construct all the animals
and plants in each great class on a uniform plan; but this is not a
scientific explanation.
The explanation is to a large extent simple, on the theory of the selection
of successive slight modifications, each being profitable in some way to
the modified form, but often affecting by correlation other parts of the
organisation. In changes of this nature, there will be little or no
tendency to alter the original pattern, or to transpose the parts. The
bones of a limb might be shortened and flattened to any extent, becoming at
the same time enveloped in thick membrane, so as to serve as a fin; or a
webbed hand might have all its bones, or certain bones, lengthened to any
extent, with the membrane connecting them increased, so as to serve as a
wing; yet all these modifications would not tend to alter the framework of
the bones or the relative connexion of the parts. If we suppose that an
early progenitor--the archetype, as it may be called--of all mammals, birds
and reptiles, had its limbs constructed on the existing general pattern,
for whatever purpose they served, we can at once perceive the plain
signification of the homologous construction of the limbs throughout the
class. So with the mouths of insects, we have only to suppose that their
common progenitor had an upper lip, mandibles, and two pairs of maxillae,
these parts being perhaps very simple in form; and then natural selection
will account for the infinite diversity in structure and function of the
mouths of insects. Nevertheless, it is conceivable that the general
pattern of an organ might become so much obscured as to be finally lost, by
the reduction and ultimately by the complete abortion of certain parts, by
the fusion of other parts, and by the doubling or multiplication of others,
variations which we know to be within the limits of possibility. In the
paddles of the gigantic extinct sea-lizards, and in the mouths of certain
suctorial crustaceans, the general pattern seems thus to have become
partially obscured.
There is another and equally curious branch of our subject; namely, serial
homologies, or the comparison of the different parts or organs in the same
individual, and not of the same parts or organs in different members of the
same class. Most physiologists believe that the bones of the skull are
homologous--that is, correspond in number and in relative connexion--with
the elemental parts of a certain number of vertebrae. The anterior and
posterior limbs in all the higher vertebrate classes are plainly
homologous. So it is with the wonderfully complex jaws and legs of
crustaceans. It is familiar to almost every one, that in a flower the
relative position of the sepals, petals, stamens, and pistils, as well as
their intimate structure, are intelligible on the view that they consist of
metamorphosed leaves, arranged in a spire. In monstrous plants, we often
get direct evidence of the possibility of one organ being transformed into
another; and we can actually see, during the early or embryonic stages of
development in flowers, as well as in crustaceans and many other animals,
that organs, which when mature become extremely different are at first
exactly alike.
How inexplicable are the cases of serial homologies on the ordinary view of
creation! Why should the brain be enclosed in a box composed of such
numerous and such extraordinarily shaped pieces of bone apparently
representing vertebrae? As Owen has remarked, the benefit derived from the
yielding of the separate pieces in the act of parturition by mammals, will
by no means explain the same construction in the skulls of birds and
reptiles. Why should similar bones have been created to form the wing and
the leg of a bat, used as they are for such totally different purposes,
namely flying and walking? Why should one crustacean, which has an
extremely complex mouth formed of many parts, consequently always have
fewer legs; or conversely, those with many legs have simpler mouths? Why
should the sepals, petals, stamens, and pistils, in each flower, though
fitted for such distinct purposes, be all constructed on the same pattern?
On the theory of natural selection, we can, to a certain extent, answer
these questions. We need not here consider how the bodies of some animals
first became divided into a series of segments, or how they became divided
into right and left sides, with corresponding organs, for such questions
are almost beyond investigation. It is, however, probable that some serial
structures are the result of cells multiplying by division, entailing the
multiplication of the parts developed from such cells. It must suffice for
our purpose to bear in mind that an indefinite repetition of the same part
or organ is the common characteristic, as Owen has remarked, of all low or
little specialised forms; therefore the unknown progenitor of the
Vertebrata probably possessed many vertebrae; the unknown progenitor of the
Articulata, many segments; and the unknown progenitor of flowering plants,
many leaves arranged in one or more spires. We have also formerly seen
that parts many times repeated are eminently liable to vary, not only in
number, but in form. Consequently such parts, being already present in
considerable numbers, and being highly variable, would naturally afford the
materials for adaptation to the most different purposes; yet they would
generally retain, through the force of inheritance, plain traces of their
original or fundamental resemblance. They would retain this resemblance
all the more, as the variations, which afforded the basis for their
subsequent modification through natural selection, would tend from the
first to be similar; the parts being at an early stage of growth alike, and
being subjected to nearly the same conditions. Such parts, whether more or
less modified, unless their common origin became wholly obscured, would be
serially homologous.
In the great class of molluscs, though the parts in distinct species can be
shown to be homologous, only a few serial homologies; such as the valves of
Chitons, can be indicated; that is, we are seldom enabled to say that one
part is homologous with another part in the same individual. And we can
understand this fact; for in molluscs, even in the lowest members of the
class, we do not find nearly so much indefinite repetition of any one part
as we find in the other great classes of the animal and vegetable kingdoms.
But morphology is a much more complex subject than it at first appears, as
has lately been well shown in a remarkable paper by Mr. E. Ray Lankester,
who has drawn an important distinction between certain classes of cases
which have all been equally ranked by naturalists as homologous. He
proposes to call the structures which resemble each other in distinct
animals, owing to their descent from a common progenitor with subsequent
modification, "homogenous"; and the resemblances which cannot thus be
accounted for, he proposes to call "homoplastic". For instance, he
believes that the hearts of birds and mammals are as a whole homogenous--
that is, have been derived from a common progenitor; but that the four
cavities of the heart in the two classes are homoplastic--that is, have
been independently developed. Mr. Lankester also adduces the close
resemblance of the parts on the right and left sides of the body, and in
the successive segments of the same individual animal; and here we have
parts commonly called homologous which bear no relation to the descent of
distinct species from a common progenitor. Homoplastic structures are the
same with those which I have classed, though in a very imperfect manner, as
analogous modifications or resemblances. Their formation may be attributed
in part to distinct organisms, or to distinct parts of the same organism,
having varied in an analogous manner; and in part to similar modifications,
having been preserved for the same general purpose or function, of which
many instances have been given.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae; the jaws of crabs as metamorphosed legs; the stamens and pistils
in flowers as metamorphosed leaves; but it would in most cases be more
correct, as Professor Huxley has remarked, to speak of both skull and
vertebrae, jaws and legs, etc., as having been metamorphosed, not one from
the other, as they now exist, but from some common and simpler element.
Most naturalists, however, use such language only in a metaphorical sense:
they are far from meaning that during a long course of descent, primordial
organs of any kind--vertebrae in the one case and legs in the other--have
actually been converted into skulls or jaws. Yet so strong is the
appearance of this having occurred that naturalists can hardly avoid
employing language having this plain signification. According to the views
here maintained, such language may be used literally; and the wonderful
fact of the jaws, for instance, of a crab retaining numerous characters,
which they probably would have retained through inheritance, if they had
really been metamorphosed from true though extremely simple legs, is in
part explained.
DEVELOPMENT AND EMBRYOLOGY.
This is one of the most important subjects in the whole round of natural
history. The metamorphoses of insects, with which every one is familiar,
are generally effected abruptly by a few stages; but the transformations
are in reality numerous and gradual, though concealed. A certain
ephemerous insect (Chloeon) during its development, moults, as shown by Sir
J. Lubbock, above twenty times, and each time undergoes a certain amount of
change; and in this case we see the act of metamorphosis performed in a
primary and gradual manner. Many insects, and especially certain
crustaceans, show us what wonderful changes of structure can be effected
during development. Such changes, however, reach their acme in the so-
called alternate generations of some of the lower animals. It is, for
instance, an astonishing fact that a delicate branching coralline, studded
with polypi, and attached to a submarine rock, should produce, first by
budding and then by transverse division, a host of huge floating jelly-
fishes; and that these should produce eggs, from which are hatched swimming
animalcules, which attach themselves to rocks and become developed into
branching corallines; and so on in an endless cycle. The belief in the
essential identity of the process of alternate generation and of ordinary
metamorphosis has been greatly strengthened by Wagner's discovery of the
larva or maggot of a fly, namely the Cecidomyia, producing asexually other
larvae, and these others, which finally are developed into mature males and
females, propagating their kind in the ordinary manner by eggs.
It may be worth notice that when Wagner's remarkable discovery was first
announced, I was asked how was it possible to account for the larvae of
this fly having acquired the power of a sexual reproduction. As long as
the case remained unique no answer could be given. But already Grimm has
shown that another fly, a Chironomus, reproduces itself in nearly the same
manner, and he believes that this occurs frequently in the order. It is
the pupa, and not the larva, of the Chironomus which has this power; and
Grimm further shows that this case, to a certain extent, "unites that of
the Cecidomyia with the parthenogenesis of the Coccidae;" the term
parthenogenesis implying that the mature females of the Coccidae are
capable of producing fertile eggs without the concourse of the male.
Certain animals belonging to several classes are now known to have the
power of ordinary reproduction at an unusually early age; and we have only
to accelerate parthenogenetic reproduction by gradual steps to an earlier
and earlier age--Chironomus showing us an almost exactly intermediate
stage, viz., that of the pupa--and we can perhaps account for the
marvellous case of the Cecidomyia.
It has already been stated that various parts in the same individual, which
are exactly alike during an early embryonic period, become widely different
and serve for widely different purposes in the adult state. So again it
has been shown that generally the embryos of the most distinct species
belonging to the same class are closely similar, but become, when fully
developed, widely dissimilar. A better proof of this latter fact cannot be
given than the statement by Von Baer that "the embryos of mammalia, of
birds, lizards and snakes, probably also of chelonia, are in the earliest
states exceedingly like one another, both as a whole and in the mode of
development of their parts; so much so, in fact, that we can often
distinguish the embryos only by their size. In my possession are two
little embryos in spirit, whose names I have omitted to attach, and at
present I am quite unable to say to what class they belong. They may be
lizards or small birds, or very young mammalia, so complete is the
similarity in the mode of formation of the head and trunk in these animals.
The extremities, however, are still absent in these embryos. But even if
they had existed in the earliest stage of their development we should learn
nothing, for the feet of lizards and mammals, the wings and feet of birds,
no less than the hands and feet of man, all arise from the same fundamental
form." The larvae of most crustaceans, at corresponding stages of
development, closely resemble each other, however different the adults may
become; and so it is with very many other animals. A trace of the law of
embryonic resemblance occasionally lasts till a rather late age: thus
birds of the same genus, and of allied genera, often resemble each other in
their immature plumage; as we see in the spotted feathers in the young of
the thrush group. In the cat tribe, most of the species when adult are
striped or spotted in lines; and stripes or spots can be plainly
distinguished in the whelp of the lion and the puma. We occasionally,
though rarely, see something of the same kind in plants; thus the first
leaves of the ulex or furze, and the first leaves of the phyllodineous
acacias, are pinnate or divided like the ordinary leaves of the
leguminosae.
The points of structure, in which the embryos of widely different animals
within the same class resemble each other, often have no direct relation to
their conditions of existence. We cannot, for instance, suppose that in
the embryos of the vertebrata the peculiar loop-like courses of the
arteries near the branchial slits are related to similar conditions--in the
young mammal which is nourished in the womb of its mother, in the egg of
the bird which is hatched in a nest, and in the spawn of a frog under
water. We have no more reason to believe in such a relation than we have
to believe that the similar bones in the hand of a man, wing of a bat, and
fin of a porpoise, are related to similar conditions of life. No one
supposes that the stripes on the whelp of a lion, or the spots on the young
blackbird, are of any use to these animals.
The case, however, is different when an animal, during any part of its
embryonic career, is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on,
the adaptation of the larva to its conditions of life is just as perfect
and as beautiful as in the adult animal. In how important a manner this
has acted, has recently been well shown by Sir J. Lubbock in his remarks on
the close similarity of the larvae of some insects belonging to very
different orders, and on the dissimilarity of the larvae of other insects
within the same order, according to their habits of life. Owing to such
adaptations the similarity of the larvae of allied animals is sometimes
greatly obscured; especially when there is a division of labour during the
different stages of development, as when the same larva has during one
stage to search for food, and during another stage has to search for a
place of attachment. Cases can even be given of the larvae of allied
species, or groups of species, differing more from each other than do the
adults. In most cases, however, the larvae, though active, still obey,
more or less closely, the law of common embryonic resemblance. Cirripedes
afford a good instance of this: even the illustrious Cuvier did not
perceive that a barnacle was a crustacean: but a glance at the larva shows
this in an unmistakable manner. So again the two main divisions of
cirripedes, the pedunculated and sessile, though differing widely in
external appearance, have larvae in all their stages barely
distinguishable.
The embryo in the course of development generally rises in organisation. I
use this expression, though I am aware that it is hardly possible to define
clearly what is meant by organisation being higher or lower. But no one
probably will dispute that the butterfly is higher than the caterpillar.
In some cases, however, the mature animal must be considered as lower in
the scale than the larva, as with certain parasitic crustaceans. To refer
once again to cirripedes: the larvae in the first stage have three pairs
of locomotive organs, a simple single eye, and a probosciformed mouth, with
which they feed largely, for they increase much in size. In the second
stage, answering to the chrysalis stage of butterflies, they have six pairs
of beautifully constructed natatory legs, a pair of magnificent compound
eyes, and extremely complex antennae; but they have a closed and imperfect
mouth, and cannot feed: their function at this stage is, to search out by
their well-developed organs of sense, and to reach by their active powers
of swimming, a proper place on which to become attached and to undergo
their final metamorphosis. When this is completed they are fixed for life:
their legs are now converted into prehensile organs; they again obtain a
well-constructed mouth; but they have no antennae, and their two eyes are
now reconverted into a minute, single, simple eye-spot. In this last and
complete state, cirripedes may be considered as either more highly or more
lowly organised than they were in the larval condition. But in some genera
the larvae become developed into hermaphrodites having the ordinary
structure, or into what I have called complemental males; and in the latter
the development has assuredly been retrograde; for the male is a mere sack,
which lives for a short time and is destitute of mouth, stomach, and every
other organ of importance, excepting those for reproduction.
We are so much accustomed to see a difference in structure between the
embryo and the adult, that we are tempted to look at this difference as in
some necessary manner contingent on growth. But there is no reason why,
for instance, the wing of a bat, or the fin of a porpoise, should not have
been sketched out with all their parts in proper proportion, as soon as any
part became visible. In some whole groups of animals and in certain
members of other groups this is the case, and the embryo does not at any
period differ widely from the adult: thus Owen has remarked in regard to
cuttle-fish, "there is no metamorphosis; the cephalopodic character is
manifested long before the parts of the embryo are completed." Land-shells
and fresh-water crustaceans are born having their proper forms, while the
marine members of the same two great classes pass through considerable and
often great changes during their development. Spiders, again, barely
undergo any metamorphosis. The larvae of most insects pass through a
worm-like stage, whether they are active and adapted to diversified habits,
or are inactive from being placed in the midst of proper nutriment, or from
being fed by their parents; but in some few cases, as in that of Aphis, if
we look to the admirable drawings of the development of this insect, by
Professor Huxley, we see hardly any trace of the vermiform stage.
Sometimes it is only the earlier developmental stages which fail. Thus,
Fritz Muller has made the remarkable discovery that certain shrimp-like
crustaceans (allied to Penoeus) first appear under the simple nauplius-
form, and after passing through two or more zoea-stages, and then through
the mysis-stage, finally acquire their mature structure: now in the whole
great malacostracan order, to which these crustaceans belong, no other
member is as yet known to be first developed under the nauplius-form,
though many appear as zoeas; nevertheless Muller assigns reasons for his
belief, that if there had been no suppression of development, all these
crustaceans would have appeared as nauplii.
How, then, can we explain these several facts in embryology--namely, the
very general, though not universal, difference in structure between the
embryo and the adult; the various parts in the same individual embryo,
which ultimately become very unlike, and serve for diverse purposes, being
at an early period of growth alike; the common, but not invariable,
resemblance between the embryos or larvae of the most distinct species in
the same class; the embryo often retaining, while within the egg or womb,
structures which are of no service to it, either at that or at a later
period of life; on the other hand, larvae which have to provide for their
own wants, being perfectly adapted to the surrounding conditions; and
lastly, the fact of certain larvae standing higher in the scale of
organisation than the mature animal into which they are developed? I
believe that all these facts can be explained as follows.
It is commonly assumed, perhaps from monstrosities affecting the embryo at
a very early period, that slight variations or individual differences
necessarily appear at an equally early period. We have little evidence on
this head, but what we have certainly points the other way; for it is
notorious that breeders of cattle, horses and various fancy animals, cannot
positively tell, until some time after birth, what will be the merits and
demerits of their young animals. We see this plainly in our own children;
we cannot tell whether a child will be tall or short, or what its precise
features will be. The question is not, at what period of life any
variation may have been caused, but at what period the effects are
displayed. The cause may have acted, and I believe often has acted, on one
or both parents before the act of generation. It deserves notice that it
is of no importance to a very young animal, as long as it is nourished and
protected by its parent, whether most of its characters are acquired a
little earlier or later in life. It would not signify, for instance, to a
bird which obtained its food by having a much-curved beak whether or not
while young it possessed a beak of this shape, as long as it was fed by its
parents.
I have stated in the first chapter, that at whatever age any variation
first appears in the parent, it tends to reappear at a corresponding age in
the offspring. Certain variations can only appear at corresponding ages;
for instance, peculiarities in the caterpillar, cocoon, or imago states of
the silk-moth; or, again, in the full-grown horns of cattle. But
variations which, for all that we can see might have appeared either
earlier or later in life, likewise tend to reappear at a corresponding age
in the offspring and parent. I am far from meaning that this is invariably
the case, and I could give several exceptional cases of variations (taking
the word in the largest sense) which have supervened at an earlier age in
the child than in the parent.
These two principles, namely, that slight variations generally appear at a
not very early period of life, and are inherited at a corresponding not
early period, explain, as I believe, all the above specified leading facts
in embryology. But first let us look to a few analogous cases in our
domestic varieties. Some authors who have written on Dogs maintain that
the greyhound and bull-dog, though so different, are really closely allied
varieties, descended from the same wild stock, hence I was curious to see
how far their puppies differed from each other. I was told by breeders
that they differed just as much as their parents, and this, judging by the
eye, seemed almost to be the case; but on actually measuring the old dogs
and their six-days-old puppies, I found that the puppies had not acquired
nearly their full amount of proportional difference. So, again, I was told
that the foals of cart and race-horses--breeds which have been almost
wholly formed by selection under domestication--differed as much as the
full-grown animals; but having had careful measurements made of the dams
and of three-days-old colts of race and heavy cart-horses, I find that this
is by no means the case.
As we have conclusive evidence that the breeds of the Pigeon are descended
from a single wild species, I compared the young pigeons within twelve
hours after being hatched. I carefully measured the proportions (but will
not here give the details) of the beak, width of mouth, length of nostril
and of eyelid, size of feet and length of leg, in the wild parent species,
in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now,
some of these birds, when mature, differ in so extraordinary a manner in
the length and form of beak, and in other characters, that they would
certainly have been ranked as distinct genera if found in a state of
nature. But when the nestling birds of these several breeds were placed in
a row, though most of them could just be distinguished, the proportional
differences in the above specified points were incomparably less than in
the full-grown birds. Some characteristic points of difference--for
instance, that of the width of mouth--could hardly be detected in the
young. But there was one remarkable exception to this rule, for the young
of the short-faced tumbler differed from the young of the wild rock-pigeon,
and of the other breeds, in almost exactly the same proportions as in the
adult stage.
These facts are explained by the above two principles. Fanciers select
their dogs, horses, pigeons, etc., for breeding, when nearly grown up.
They are indifferent whether the desired qualities are acquired earlier or
later in life, if the full-grown animal possesses them. And the cases just
given, more especially that of the pigeons, show that the characteristic
differences which have been accumulated by man's selection, and which give
value to his breeds, do not generally appear at a very early period of
life, and are inherited at a corresponding not early period. But the case
of the short-faced tumbler, which when twelve hours old possessed its
proper characters, proves that this is not the universal rule; for here the
characteristic differences must either have appeared at an earlier period
than usual, or, if not so, the differences must have been inherited, not at
a corresponding, but at an earlier age.
Now, let us apply these two principles to species in a state of nature.
Let us take a group of birds, descended from some ancient form and modified
through natural selection for different habits. Then, from the many slight
successive variations having supervened in the several species at a not
early age, and having been inherited at a corresponding age, the young will
have been but little modified, and they will still resemble each other much
more closely than do the adults, just as we have seen with the breeds of
the pigeon. We may extend this view to widely distinct structures and to
whole classes. The fore-limbs, for instance, which once served as legs to
a remote progenitor, may have become, through a long course of
modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; but on the above two principles the
fore-limbs will not have been much modified in the embryos of these several
forms; although in each form the fore-limb will differ greatly in the adult
state. Whatever influence long continued use or disuse may have had in
modifying the limbs or other parts of any species, this will chiefly or
solely have affected it when nearly mature, when it was compelled to use
its full powers to gain its own living; and the effects thus produced will
have been transmitted to the offspring at a corresponding nearly mature
age. Thus the young will not be modified, or will be modified only in a
slight degree, through the effects of the increased use or disuse of parts.
With some animals the successive variations may have supervened at a very
early period of life, or the steps may have been inherited at an earlier
age than that at which they first occurred. In either of these cases the
young or embryo will closely resemble the mature parent-form, as we have
seen with the short-faced tumbler. And this is the rule of development in
certain whole groups, or in certain sub-groups alone, as with cuttle-fish,
land-shells, fresh-water crustaceans, spiders, and some members of the
great class of insects. With respect to the final cause of the young in
such groups not passing through any metamorphosis, we can see that this
would follow from the following contingencies: namely, from the young
having to provide at a very early age for their own wants, and from their
following the same habits of life with their parents; for in this case it
would be indispensable for their existence that they should be modified in
the same manner as their parents. Again, with respect to the singular fact
that many terrestrial and fresh-water animals do not undergo any
metamorphosis, while marine members of the same groups pass through various
transformations, Fritz Muller has suggested that the process of slowly
modifying and adapting an animal to live on the land or in fresh water,
instead of in the sea, would be greatly simplified by its not passing
through any larval stage; for it is not probable that places well adapted
for both the larval and mature stages, under such new and greatly changed
habits of life, would commonly be found unoccupied or ill-occupied by other
organisms. In this case the gradual acquirement at an earlier and earlier
age of the adult structure would be favoured by natural selection; and all
traces of former metamorphoses would finally be lost.
If, on the other hand, it profited the young of an animal to follow habits
of life slightly different from those of the parent-form, and consequently
to be constructed on a slightly different plan, or if it profited a larva
already different from its parent to change still further, then, on the
principle of inheritance at corresponding ages, the young or the larvae
might be rendered by natural selection more and more different from their
parents to any conceivable extent. Differences in the larva might, also,
become correlated with successive stages of its development; so that the
larva, in the first stage, might come to differ greatly from the larva in
the second stage, as is the case with many animals. The adult might also
become fitted for sites or habits, in which organs of locomotion or of the
senses, etc., would be useless; and in this case the metamorphosis would be
retrograde.
>From the remarks just made we can see how by changes of structure in the
young, in conformity with changed habits of life, together with inheritance
at corresponding ages, animals might come to pass through stages of
development, perfectly distinct from the primordial condition of their
adult progenitors. Most of our best authorities are now convinced that the
various larval and pupal stages of insects have thus been acquired through
adaptation, and not through inheritance from some ancient form. The
curious case of Sitaris--a beetle which passes through certain unusual
stages of development--will illustrate how this might occur. The first
larval form is described by M. Fabre, as an active, minute insect,
furnished with six legs, two long antennae, and four eyes. These larvae
are hatched in the nests of bees; and when the male bees emerge from their
burrows, in the spring, which they do before the females, the larvae spring
on them, and afterwards crawl on to the females while paired with the
males. As soon as the female bee deposits her eggs on the surface of the
honey stored in the cells, the larvae of the Sitaris leap on the eggs and
devour them. Afterwards they undergo a complete change; their eyes
disappear; their legs and antennae become rudimentary, and they feed on
honey; so that they now more closely resemble the ordinary larvae of
insects; ultimately they undergo a further transformation, and finally
emerge as the perfect beetle. Now, if an insect, undergoing
transformations like those of the Sitaris, were to become the progenitor of
a whole new class of insects, the course of development of the new class
would be widely different from that of our existing insects; and the first
larval stage certainly would not represent the former condition of any
adult and ancient form.
On the other hand it is highly probable that with many animals the
embryonic or larval stages show us, more or less completely, the condition
of the progenitor of the whole group in its adult state. In the great
class of the Crustacea, forms wonderfully distinct from each other, namely,
suctorial parasites, cirripedes, entomostraca, and even the malacostraca,
appear at first as larvae under the nauplius-form; and as these larvae live
and feed in the open sea, and are not adapted for any peculiar habits of
life, and from other reasons assigned by Fritz Muller, it is probable that
at some very remote period an independent adult animal, resembling the
Nauplius, existed, and subsequently produced, along several divergent lines
of descent, the above-named great Crustacean groups. So again, it is
probable, from what we know of the embryos of mammals, birds, fishes and
reptiles, that these animals are the modified descendants of some ancient
progenitor, which was furnished in its adult state with branchiae, a swim-
bladder, four fin-like limbs, and a long tail, all fitted for an aquatic
life.
As all the organic beings, extinct and recent, which have ever lived, can
be arranged within a few great classes; and as all within each class have,
according to our theory, been connected together by fine gradations, the
best, and, if our collections were nearly perfect, the only possible
arrangement, would be genealogical; descent being the hidden bond of
connexion which naturalists have been seeking under the term of the Natural
System. On this view we can understand how it is that, in the eyes of most
naturalists, the structure of the embryo is even more important for
classification than that of the adult. In two or more groups of animals,
however much they may differ from each other in structure and habits in
their adult condition, if they pass through closely similar embryonic
stages, we may feel assured that they are all descended from one parent-
form, and are therefore closely related. Thus, community in embryonic
structure reveals community of descent; but dissimilarity in embryonic
development does not prove discommunity of descent, for in one of two
groups the developmental stages may have been suppressed, or may have been
so greatly modified through adaptation to new habits of life as to be no
longer recognisable. Even in groups, in which the adults have been
modified to an extreme degree, community of origin is often revealed by the
structure of the larvae; we have seen, for instance, that cirripedes,
though externally so like shell-fish, are at once known by their larvae to
belong to the great class of crustaceans. As the embryo often shows us
more or less plainly the structure of the less modified and ancient
progenitor of the group, we can see why ancient and extinct forms so often
resemble in their adult state the embryos of existing species of the same
class. Agassiz believes this to be a universal law of nature; and we may
hope hereafter to see the law proved true. It can, however, be proved true
only in those cases in which the ancient state of the progenitor of the
group has not been wholly obliterated, either by successive variations
having supervened at a very early period of growth, or by such variations
having been inherited at an earlier age than that at which they first
appeared. It should also be borne in mind, that the law may be true, but
yet, owing to the geological record not extending far enough back in time,
may remain for a long period, or for ever, incapable of demonstration. The
law will not strictly hold good in those cases in which an ancient form
became adapted in its larval state to some special line of life, and
transmitted the same larval state to a whole group of descendants; for such
larval state will not resemble any still more ancient form in its adult
state.
Thus, as it seems to me, the leading facts in embryology, which are second
to none in importance, are explained on the principle of variations in the
many descendants from some one ancient progenitor, having appeared at a not
very early period of life, and having been inherited at a corresponding
period. Embryology rises greatly in interest, when we look at the embryo
as a picture, more or less obscured, of the progenitor, either in its adult
or larval state, of all the members of the same great class.
RUDIMENTARY, ATROPHIED, AND ABORTED ORGANS.
Organs or parts in this strange condition, bearing the plain stamp of
inutility, are extremely common, or even general, throughout nature. It
would be impossible to name one of the higher animals in which some part or
other is not in a rudimentary condition. In the mammalia, for instance,
the males possess rudimentary mammae; in snakes one lobe of the lungs is
rudimentary; in birds the "bastard-wing" may safely be considered as a
rudimentary digit, and in some species the whole wing is so far rudimentary
that it cannot be used for flight. What can be more curious than the
presence of teeth in foetal whales, which when grown up have not a tooth in
their heads; or the teeth, which never cut through the gums, in the upper
jaws of unborn calves?
Rudimentary organs plainly declare their origin and meaning in various
ways. There are beetles belonging to closely allied species, or even to
the same identical species, which have either full-sized and perfect wings,
or mere rudiments of membrane, which not rarely lie under wing-covers
firmly soldered together; and in these cases it is impossible to doubt,
that the rudiments represent wings. Rudimentary organs sometimes retain
their potentiality: this occasionally occurs with the mammae of male
mammals, which have been known to become well developed and to secrete
milk. So again in the udders of the genus Bos, there are normally four
developed and two rudimentary teats; but the latter in our domestic cows
sometimes become well developed and yield milk. In regard to plants, the
petals are sometimes rudimentary, and sometimes well developed in the
individuals of the same species. In certain plants having separated sexes
Kolreuter found that by crossing a species, in which the male flowers
included a rudiment of a pistil, with an hermaphrodite species, having of
course a well-developed pistil, the rudiment in the hybrid offspring was
much increased in size; and this clearly shows that the rudimentary and
perfect pistils are essentially alike in nature. An animal may possess
various parts in a perfect state, and yet they may in one sense be
rudimentary, for they are useless: thus the tadpole of the common
salamander or water-newt, as Mr. G.H. Lewes remarks, "has gills, and passes
its existence in the water; but the Salamandra atra, which lives high up
among the mountains, brings forth its young full-formed. This animal never
lives in the water. Yet if we open a gravid female, we find tadpoles
inside her with exquisitely feathered gills; and when placed in water they
swim about like the tadpoles of the water-newt. Obviously this aquatic
organisation has no reference to the future life of the animal, nor has it
any adaptation to its embryonic condition; it has solely reference to
ancestral adaptations, it repeats a phase in the development of its
progenitors."
An organ, serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose, and remain perfectly
efficient for the other. Thus, in plants, the office of the pistil is to
allow the pollen-tubes to reach the ovules within the ovarium. The pistil
consists of a stigma supported on the style; but in some Compositae, the
male florets, which of course cannot be fecundated, have a rudimentary
pistil, for it is not crowned with a stigma; but the style remains well
developed and is clothed in the usual manner with hairs, which serve to
brush the pollen out of the surrounding and conjoined anthers. Again, an
organ may become rudimentary for its proper purpose, and be used for a
distinct one: in certain fishes the swim-bladder seems to be rudimentary
for its proper function of giving buoyancy, but has become converted into a
nascent breathing organ or lung. Many similar instances could be given.
Useful organs, however little they may be developed, unless we have reason
to suppose that they were formerly more highly developed, ought not to be
considered as rudimentary. They may be in a nascent condition, and in
progress towards further development. Rudimentary organs, on the other
hand, are either quite useless, such as teeth which never cut through the
gums, or almost useless, such as the wings of an ostrich, which serve
merely as sails. As organs in this condition would formerly, when still
less developed, have been of even less use than at present, they cannot
formerly have been produced through variation and natural selection, which
acts solely by the preservation of useful modifications. They have been
partially retained by the power of inheritance, and relate to a former
state of things. It is, however, often difficult to distinguish between
rudimentary and nascent organs; for we can judge only by analogy whether a
part is capable of further development, in which case alone it deserves to
be called nascent. Organs in this condition will always be somewhat rare;
for beings thus provided will commonly have been supplanted by their
successors with the same organ in a more perfect state, and consequently
will have become long ago extinct. The wing of the penguin is of high
service, acting as a fin; it may, therefore, represent the nascent state of
the wing: not that I believe this to be the case; it is more probably a
reduced organ, modified for a new function: the wing of the Apteryx, on
the other hand, is quite useless, and is truly rudimentary. Owen considers
the simple filamentary limbs of the Lepidosiren as the "beginnings of
organs which attain full functional development in higher vertebrates;"
but, according to the view lately advocated by Dr. Gunther, they are
probably remnants, consisting of the persistent axis of a fin, with the
lateral rays or branches aborted. The mammary glands of the
Ornithorhynchus may be considered, in comparison with the udders of a cow,
as in a nascent condition. The ovigerous frena of certain cirripedes,
which have ceased to give attachment to the ova and are feebly developed,
are nascent branchiae.
Rudimentary organs in the individuals of the same species are very liable
to vary in the degree of their development and in other respects. In
closely allied species, also, the extent to which the same organ has been
reduced occasionally differs much. This latter fact is well exemplified in
the state of the wings of female moths belonging to the same family.
Rudimentary organs may be utterly aborted; and this implies, that in
certain animals or plants, parts are entirely absent which analogy would
lead us to expect to find in them, and which are occasionally found in
monstrous individuals. Thus in most of the Scrophulariaceae the fifth
stamen is utterly aborted; yet we may conclude that a fifth stamen once
existed, for a rudiment of it is found in many species of the family, and
this rudiment occasionally becomes perfectly developed, as may sometimes be
seen in the common snap-dragon. In tracing the homologies of any part in
different members of the same class, nothing is more common, or, in order
fully to understand the relations of the parts, more useful than the
discovery of rudiments. This is well shown in the drawings given by Owen
of the leg bones of the horse, ox, and rhinoceros.
It is an important fact that rudimentary organs, such as teeth in the upper
jaws of whales and ruminants, can often be detected in the embryo, but
afterwards wholly disappear. It is also, I believe, a universal rule, that
a rudimentary part is of greater size in the embryo relatively to the
adjoining parts, than in the adult; so that the organ at this early age is
less rudimentary, or even cannot be said to be in any degree rudimentary.
Hence rudimentary organs in the adult are often said to have retained their
embryonic condition.
I have now given the leading facts with respect to rudimentary organs. In
reflecting on them, every one must be struck with astonishment; for the
same reasoning power which tells us that most parts and organs are
exquisitely adapted for certain purposes, tells us with equal plainness
that these rudimentary or atrophied organs are imperfect and useless. In
works on natural history, rudimentary organs are generally said to have
been created "for the sake of symmetry," or in order "to complete the
scheme of nature." But this is not an explanation, merely a restatement of
the fact. Nor is it consistent with itself: thus the boa-constrictor has
rudiments of hind limbs and of a pelvis, and if it be said that these bones
have been retained "to complete the scheme of nature," why, as Professor
Weismann asks, have they not been retained by other snakes, which do not
possess even a vestige of these same bones? What would be thought of an
astronomer who maintained that the satellites revolve in elliptic courses
round their planets "for the sake of symmetry," because the planets thus
revolve round the sun? An eminent physiologist accounts for the presence
of rudimentary organs, by supposing that they serve to excrete matter in
excess, or matter injurious to the system; but can we suppose that the
minute papilla, which often represents the pistil in male flowers, and
which is formed of mere cellular tissue, can thus act? Can we suppose that
rudimentary teeth, which are subsequently absorbed, are beneficial to the
rapidly growing embryonic calf by removing matter so precious as phosphate
of lime? When a man's fingers have been amputated, imperfect nails have
been known to appear on the stumps, and I could as soon believe that these
vestiges of nails are developed in order to excrete horny matter, as that
the rudimentary nails on the fin of the manatee have been developed for
this same purpose.
On the view of descent with modification, the origin of rudimentary organs
is comparatively simple; and we can understand to a large extent the laws
governing their imperfect development. We have plenty of cases of
rudimentary organs in our domestic productions, as the stump of a tail in
tailless breeds, the vestige of an ear in earless breeds of sheep--the
reappearance of minute dangling horns in hornless breeds of cattle, more
especially, according to Youatt, in young animals--and the state of the
whole flower in the cauliflower. We often see rudiments of various parts
in monsters; but I doubt whether any of these cases throw light on the
origin of rudimentary organs in a state of nature, further than by showing
that rudiments can be produced; for the balance of evidence clearly
indicates that species under nature do not undergo great and abrupt
changes. But we learn from the study of our domestic productions that the
disuse of parts leads to their reduced size; and that the result is
inherited.
It appears probable that disuse has been the main agent in rendering organs
rudimentary. It would at first lead by slow steps to the more and more
complete reduction of a part, until at last it became rudimentary--as in
the case of the eyes of animals inhabiting dark caverns, and of the wings
of birds inhabiting oceanic islands, which have seldom been forced by
beasts of prey to take flight, and have ultimately lost the power of
flying. Again, an organ, useful under certain conditions, might become
injurious under others, as with the wings of beetles living on small and
exposed islands; and in this case natural selection will have aided in
reducing the organ, until it was rendered harmless and rudimentary.
Any change in structure and function, which can be effected by small
stages, is within the power of natural selection; so that an organ
rendered, through changed habits of life, useless or injurious for one
purpose, might be modified and used for another purpose. An organ might,
also, be retained for one alone of its former functions. Organs,
originally formed by the aid of natural selection, when rendered useless
may well be variable, for their variations can no longer be checked by
natural selection. All this agrees well with what we see under nature.
Moreover, at whatever period of life either disuse or selection reduces an
organ, and this will generally be when the being has come to maturity and
to exert its full powers of action, the principle of inheritance at
corresponding ages will tend to reproduce the organ in its reduced state at
the same mature age, but will seldom affect it in the embryo. Thus we can
understand the greater size of rudimentary organs in the embryo relatively
to the adjoining parts, and their lesser relative size in the adult. If,
for instance, the digit of an adult animal was used less and less during
many generations, owing to some change of habits, or if an organ or gland
was less and less functionally exercised, we may infer that it would become
reduced in size in the adult descendants of this animal, but would retain
nearly its original standard of development in the embryo.
There remains, however, this difficulty. After an organ has ceased being
used, and has become in consequence much reduced, how can it be still
further reduced in size until the merest vestige is left; and how can it be
finally quite obliterated? It is scarcely possible that disuse can go on
producing any further effect after the organ has once been rendered
functionless. Some additional explanation is here requisite which I cannot
give. If, for instance, it could be proved that every part of the
organisation tends to vary in a greater degree towards diminution than
toward augmentation of size, then we should be able to understand how an
organ which has become useless would be rendered, independently of the
effects of disuse, rudimentary and would at last be wholly suppressed; for
the variations towards diminished size would no longer be checked by
natural selection. The principle of the economy of growth, explained in a
former chapter, by which the materials forming any part, if not useful to
the possessor, are saved as far as is possible, will perhaps come into play
in rendering a useless part rudimentary. But this principle will almost
necessarily be confined to the earlier stages of the process of reduction;
for we cannot suppose that a minute papilla, for instance, representing in
a male flower the pistil of the female flower, and formed merely of
cellular tissue, could be further reduced or absorbed for the sake of
economising nutriment.
Finally, as rudimentary organs, by whatever steps they may have been
degraded into their present useless condition, are the record of a former
state of things, and have been retained solely through the power of
inheritance--we can understand, on the genealogical view of classification,
how it is that systematists, in placing organisms in their proper places in
the natural system, have often found rudimentary parts as useful as, or
even sometimes more useful than, parts of high physiological importance.
Rudimentary organs may be compared with the letters in a word, still
retained in the spelling, but become useless in the pronunciation, but
which serve as a clue for its derivation. On the view of descent with
modification, we may conclude that the existence of organs in a
rudimentary, imperfect, and useless condition, or quite aborted, far from
presenting a strange difficulty, as they assuredly do on the old doctrine
of creation, might even have been anticipated in accordance with the views
here explained.
SUMMARY.
In this chapter I have attempted to show that the arrangement of all
organic beings throughout all time in groups under groups--that the nature
of the relationships by which all living and extinct organisms are united
by complex, radiating, and circuitous lines of affinities into a few grand
classes--the rules followed and the difficulties encountered by naturalists
in their classifications--the value set upon characters, if constant and
prevalent, whether of high or of the most trifling importance, or, as with
rudimentary organs of no importance--the wide opposition in value between
analogical or adaptive characters, and characters of true affinity; and
other such rules--all naturally follow if we admit the common parentage of
allied forms, together with their modification through variation and
natural selection, with the contingencies of extinction and divergence of
character. In considering this view of classification, it should be borne
in mind that the element of descent has been universally used in ranking
together the sexes, ages, dimorphic forms, and acknowledged varieties of
the same species, however much they may differ from each other in
structure. If we extend the use of this element of descent--the one
certainly known cause of similarity in organic beings--we shall understand
what is meant by the Natural System: it is genealogical in its attempted
arrangement, with the grades of acquired difference marked by the terms,
varieties, species, genera, families, orders, and classes.
On this same view of descent with modification, most of the great facts in
Morphology become intelligible--whether we look to the same pattern
displayed by the different species of the same class in their homologous
organs, to whatever purpose applied, or to the serial and lateral
homologies in each individual animal and plant.
On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being inherited
at a corresponding period, we can understand the leading facts in
embryology; namely, the close resemblance in the individual embryo of the
parts which are homologous, and which when matured become widely different
in structure and function; and the resemblance of the homologous parts or
organs in allied though distinct species, though fitted in the adult state
for habits as different as is possible. Larvae are active embryos, which
have become specially modified in a greater or less degree in relation to
their habits of life, with their modifications inherited at a corresponding
early age. On these same principles, and bearing in mind that when organs
are reduced in size, either from disuse or through natural selection, it
will generally be at that period of life when the being has to provide for
its own wants, and bearing in mind how strong is the force of
inheritance--the occurrence of rudimentary organs might even have been
anticipated. The importance of embryological characters and of rudimentary
organs in classification is intelligible, on the view that a natural
arrangement must be genealogical.
Finally, the several classes of facts which have been considered in this
chapter, seem to me to proclaim so plainly, that the innumerable species,
genera and families, with which this world is peopled, are all descended,
each within its own class or group, from common parents, and have all been
modified in the course of descent, that I should without hesitation adopt
this view, even if it were unsupported by other facts or arguments.
CHAPTER XV.
RECAPITULATION AND CONCLUSION.
Recapitulation of the objections to the theory of Natural Selection --
Recapitulation of the general and special circumstances in its favour --
Causes of the general belief in the immutability of species -- How far the
theory of Natural Selection may be extended -- Effects of its adoption on
the study of Natural History -- Concluding remarks.
As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.
That many and serious objections may be advanced against the theory of
descent with modification through variation and natural selection, I do not
deny. I have endeavoured to give to them their full force. Nothing at
first can appear more difficult to believe than that the more complex
organs and instincts have been perfected, not by means superior to, though
analogous with, human reason, but by the accumulation of innumerable slight
variations, each good for the individual possessor. Nevertheless, this
difficulty, though appearing to our imagination insuperably great, cannot
be considered real if we admit the following propositions, namely, that all
parts of the organisation and instincts offer, at least individual
differences--that there is a struggle for existence leading to the
preservation of profitable deviations of structure or instinct--and,
lastly, that gradations in the state of perfection of each organ may have
existed, each good of its kind. The truth of these propositions cannot, I
think, be disputed.
It is, no doubt, extremely difficult even to conjecture by what gradations
many structures have been perfected, more especially among broken and
failing groups of organic beings, which have suffered much extinction; but
we see so many strange gradations in nature, that we ought to be extremely
cautious in saying that any organ or instinct, or any whole structure,
could not have arrived at its present state by many graduated steps. There
are, it must be admitted, cases of special difficulty opposed to the theory
of natural selection; and one of the most curious of these is the existence
in the same community of two or three defined castes of workers or sterile
female ants; but I have attempted to show how these difficulties can be
mastered.
With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the ninth chapter, which
seem to me conclusively to show that this sterility is no more a special
endowment than is the incapacity of two distinct kinds of trees to be
grafted together; but that it is incidental on differences confined to the
reproductive systems of the intercrossed species. We see the truth of this
conclusion in the vast difference in the results of crossing the same two
species reciprocally--that is, when one species is first used as the father
and then as the mother. Analogy from the consideration of dimorphic and
trimorphic plants clearly leads to the same conclusion, for when the forms
are illegitimately united, they yield few or no seed, and their offspring
are more or less sterile; and these forms belong to the same undoubted
species, and differ from each other in no respect except in their
reproductive organs and functions.
Although the fertility of varieties when intercrossed, and of their mongrel
offspring, has been asserted by so many authors to be universal, this
cannot be considered as quite correct after the facts given on the high
authority of Gartner and Kolreuter. Most of the varieties which have been
experimented on have been produced under domestication; and as
domestication (I do not mean mere confinement) almost certainly tends to
eliminate that sterility which, judging from analogy, would have affected
the parent-species if intercrossed, we ought not to expect that
domestication would likewise induce sterility in their modified descendants
when crossed. This elimination of sterility apparently follows from the
same cause which allows our domestic animals to breed freely under
diversified circumstances; and this again apparently follows from their
having been gradually accustomed to frequent changes in their conditions of
life.
A double and parallel series of facts seems to throw much light on the
sterility of species, when first crossed, and of their hybrid offspring.
On the one side, there is good reason to believe that slight changes in the
conditions of life give vigour and fertility to all organic beings. We
know also that a cross between the distinct individuals of the same
variety, and between distinct varieties, increases the number of their
offspring, and certainly gives to them increased size and vigour. This is
chiefly owing to the forms which are crossed having been exposed to
somewhat different conditions of life; for I have ascertained by a
labourious series of experiments that if all the individuals of the same
variety be subjected during several generations to the same conditions, the
good derived from crossing is often much diminished or wholly disappears.
This is one side of the case. On the other side, we know that species
which have long been exposed to nearly uniform conditions, when they are
subjected under confinement to new and greatly changed conditions, either
perish, or if they survive, are rendered sterile, though retaining perfect
health. This does not occur, or only in a very slight degree, with our
domesticated productions, which have long been exposed to fluctuating
conditions. Hence when we find that hybrids produced by a cross between
two distinct species are few in number, owing to their perishing soon after
conception or at a very early age, or if surviving that they are rendered
more or less sterile, it seems highly probable that this result is due to
their having been in fact subjected to a great change in their conditions
of life, from being compounded of two distinct organisations. He who will
explain in a definite manner why, for instance, an elephant or a fox will
not breed under confinement in its native country, whilst the domestic pig
or dog will breed freely under the most diversified conditions, will at the
same time be able to give a definite answer to the question why two
distinct species, when crossed, as well as their hybrid offspring, are
generally rendered more or less sterile, while two domesticated varieties
when crossed and their mongrel offspring are perfectly fertile.
Turning to geographical distribution, the difficulties encountered on the
theory of descent with modification are serious enough. All the
individuals of the same species, and all the species of the same genus, or
even higher group, are descended from common parents; and therefore, in
however distant and isolated parts of the world they may now be found, they
must in the course of successive generations have travelled from some one
point to all the others. We are often wholly unable even to conjecture how
this could have been effected. Yet, as we have reason to believe that some
species have retained the same specific form for very long periods of time,
immensely long as measured by years, too much stress ought not to be laid
on the occasional wide diffusion of the same species; for during very long
periods there will always have been a good chance for wide migration by
many means. A broken or interrupted range may often be accounted for by
the extinction of the species in the intermediate regions. It cannot be
denied that we are as yet very ignorant as to the full extent of the
various climatical and geographical changes which have affected the earth
during modern periods; and such changes will often have facilitated
migration. As an example, I have attempted to show how potent has been the
influence of the Glacial period on the distribution of the same and of
allied species throughout the world. We are as yet profoundly ignorant of
the many occasional means of transport. With respect to distinct species
of the same genus, inhabiting distant and isolated regions, as the process
of modification has necessarily been slow, all the means of migration will
have been possible during a very long period; and consequently the
difficulty of the wide diffusion of the species of the same genus is in
some degree lessened.
As according to the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species in
each group by gradations as fine as our existing varieties, it may be
asked, Why do we not see these linking forms all around us? Why are not
all organic beings blended together in an inextricable chaos? With respect
to existing forms, we should remember that we have no right to expect
(excepting in rare cases) to discover DIRECTLY connecting links between
them, but only between each and some extinct and supplanted form. Even on
a wide area, which has during a long period remained continuous, and of
which the climatic and other conditions of life change insensibly in
proceeding from a district occupied by one species into another district
occupied by a closely allied species, we have no just right to expect often
to find intermediate varieties in the intermediate zones. For we have
reason to believe that only a few species of a genus ever undergo change;
the other species becoming utterly extinct and leaving no modified progeny.
Of the species which do change, only a few within the same country change
at the same time; and all modifications are slowly effected. I have also
shown that the intermediate varieties which probably at first existed in
the intermediate zones, would be liable to be supplanted by the allied
forms on either hand; for the latter, from existing in greater numbers,
would generally be modified and improved at a quicker rate than the
intermediate varieties, which existed in lesser numbers; so that the
intermediate varieties would, in the long run, be supplanted and
exterminated.
On this doctrine of the extermination of an infinitude of connecting links,
between the living and extinct inhabitants of the world, and at each
successive period between the extinct and still older species, why is not
every geological formation charged with such links? Why does not every
collection of fossil remains afford plain evidence of the gradation and
mutation of the forms of life? Although geological research has
undoubtedly revealed the former existence of many links, bringing numerous
forms of life much closer together, it does not yield the infinitely many
fine gradations between past and present species required on the theory,
and this is the most obvious of the many objections which may be urged
against it. Why, again, do whole groups of allied species appear, though
this appearance is often false, to have come in suddenly on the successive
geological stages? Although we now know that organic beings appeared on
this globe, at a period incalculably remote, long before the lowest bed of
the Cambrian system was deposited, why do we not find beneath this system
great piles of strata stored with the remains of the progenitors of the
Cambrian fossils? For on the theory, such strata must somewhere have been
deposited at these ancient and utterly unknown epochs of the world's
history.
I can answer these questions and objections only on the supposition that
the geological record is far more imperfect than most geologists believe.
The number of specimens in all our museums is absolutely as nothing
compared with the countless generations of countless species which have
certainly existed. The parent form of any two or more species would not be
in all its characters directly intermediate between its modified offspring,
any more than the rock-pigeon is directly intermediate in crop and tail
between its descendants, the pouter and fantail pigeons. We should not be
able to recognise a species as the parent of another and modified species,
if we were to examine the two ever so closely, unless we possessed most of
the intermediate links; and owing to the imperfection of the geological
record, we have no just right to expect to find so many links. If two or
three, or even more linking forms were discovered, they would simply be
ranked by many naturalists as so many new species, more especially if found
in different geological substages, let their differences be ever so slight.
Numerous existing doubtful forms could be named which are probably
varieties; but who will pretend that in future ages so many fossil links
will be discovered, that naturalists will be able to decide whether or not
these doubtful forms ought to be called varieties? Only a small portion of
the world has been geologically explored. Only organic beings of certain
classes can be preserved in a fossil condition, at least in any great
number. Many species when once formed never undergo any further change but
become extinct without leaving modified descendants; and the periods during
which species have undergone modification, though long as measured by
years, have probably been short in comparison with the periods during which
they retained the same form. It is the dominant and widely ranging species
which vary most frequently and vary most, and varieties are often at first
local--both causes rendering the discovery of intermediate links in any one
formation less likely. Local varieties will not spread into other and
distant regions until they are considerably modified and improved; and when
they have spread, and are discovered in a geological formation, they appear
as if suddenly created there, and will be simply classed as new species.
Most formations have been intermittent in their accumulation; and their
duration has probably been shorter than the average duration of specific
forms. Successive formations are in most cases separated from each other
by blank intervals of time of great length, for fossiliferous formations
thick enough to resist future degradation can, as a general rule, be
accumulated only where much sediment is deposited on the subsiding bed of
the sea. During the alternate periods of elevation and of stationary level
the record will generally be blank. During these latter periods there will
probably be more variability in the forms of life; during periods of
subsidence, more extinction.
With respect to the absence of strata rich in fossils beneath the Cambrian
formation, I can recur only to the hypothesis given in the tenth chapter;
namely, that though our continents and oceans have endured for an enormous
period in nearly their present relative positions, we have no reason to
assume that this has always been the case; consequently formations much
older than any now known may lie buried beneath the great oceans. With
respect to the lapse of time not having been sufficient since our planet
was consolidated for the assumed amount of organic change, and this
objection, as urged by Sir William Thompson, is probably one of the gravest
as yet advanced, I can only say, firstly, that we do not know at what rate
species change, as measured by years, and secondly, that many philosophers
are not as yet willing to admit that we know enough of the constitution of
the universe and of the interior of our globe to speculate with safety on
its past duration.
That the geological record is imperfect all will admit; but that it is
imperfect to the degree required by our theory, few will be inclined to
admit. If we look to long enough intervals of time, geology plainly
declares that species have all changed; and they have changed in the manner
required by the theory, for they have changed slowly and in a graduated
manner. We clearly see this in the fossil remains from consecutive
formations invariably being much more closely related to each other than
are the fossils from widely separated formations.
Such is the sum of the several chief objections and difficulties which may
justly be urged against the theory; and I have now briefly recapitulated
the answers and explanations which, as far as I can see, may be given. I
have felt these difficulties far too heavily during many years to doubt
their weight. But it deserves especial notice that the more important
objections relate to questions on which we are confessedly ignorant; nor do
we know how ignorant we are. We do not know all the possible transitional
gradations between the simplest and the most perfect organs; it cannot be
pretended that we know all the varied means of Distribution during the long
lapse of years, or that we know how imperfect is the Geological Record.
Serious as these several objections are, in my judgment they are by no
means sufficient to overthrow the theory of descent with subsequent
modification.
Now let us turn to the other side of the argument. Under domestication we
see much variability, caused, or at least excited, by changed conditions of
life; but often in so obscure a manner, that we are tempted to consider the
variations as spontaneous. Variability is governed by many complex laws,
by correlated growth, compensation, the increased use and disuse of parts,
and the definite action of the surrounding conditions. There is much
difficulty in ascertaining how largely our domestic productions have been
modified; but we may safely infer that the amount has been large, and that
modifications can be inherited for long periods. As long as the conditions
of life remain the same, we have reason to believe that a modification,
which has already been inherited for many generations, may continue to be
inherited for an almost infinite number of generations. On the other hand
we have evidence that variability, when it has once come into play, does
not cease under domestication for a very long period; nor do we know that
it ever ceases, for new varieties are still occasionally produced by our
oldest domesticated productions.
Variability is not actually caused by man; he only unintentionally exposes
organic beings to new conditions of life and then nature acts on the
organisation and causes it to vary. But man can and does select the
variations given to him by nature, and thus accumulates them in any desired
manner. He thus adapts animals and plants for his own benefit or pleasure.
He may do this methodically, or he may do it unconsciously by preserving
the individuals most useful or pleasing to him without any intention of
altering the breed. It is certain that he can largely influence the
character of a breed by selecting, in each successive generation,
individual differences so slight as to be inappreciable except by an
educated eye. This unconscious process of selection has been the great
agency in the formation of the most distinct and useful domestic breeds.
That many breeds produced by man have to a large extent the character of
natural species, is shown by the inextricable doubts whether many of them
are varieties or aboriginally distinct species.
There is no reason why the principles which have acted so efficiently under
domestication should not have acted under nature. In the survival of
favoured individuals and races, during the constantly recurrent Struggle
for Existence, we see a powerful and ever-acting form of Selection. The
struggle for existence inevitably follows from the high geometrical ratio
of increase which is common to all organic beings. This high rate of
increase is proved by calculation--by the rapid increase of many animals
and plants during a succession of peculiar seasons, and when naturalised in
new countries. More individuals are born than can possibly survive. A
grain in the balance may determine which individuals shall live and which
shall die--which variety or species shall increase in number, and which
shall decrease, or finally become extinct. As the individuals of the same
species come in all respects into the closest competition with each other,
the struggle will generally be most severe between them; it will be almost
equally severe between the varieties of the same species, and next in
severity between the species of the same genus. On the other hand the
struggle will often be severe between beings remote in the scale of nature.
The slightest advantage in certain individuals, at any age or during any
season, over those with which they come into competition, or better
adaptation in however slight a degree to the surrounding physical
conditions, will, in the long run, turn the balance.
With animals having separated sexes, there will be in most cases a struggle
between the males for the possession of the females. The most vigorous
males, or those which have most successfully struggled with their
conditions of life, will generally leave most progeny. But success will
often depend on the males having special weapons or means of defence or
charms; and a slight advantage will lead to victory.
As geology plainly proclaims that each land has undergone great physical
changes, we might have expected to find that organic beings have varied
under nature, in the same way as they have varied under domestication. And
if there has been any variability under nature, it would be an
unaccountable fact if natural selection had not come into play. It has
often been asserted, but the assertion is incapable of proof, that the
amount of variation under nature is a strictly limited quantity. Man,
though acting on external characters alone and often capriciously, can
produce within a short period a great result by adding up mere individual
differences in his domestic productions; and every one admits that species
present individual differences. But, besides such differences, all
naturalists admit that natural varieties exist, which are considered
sufficiently distinct to be worthy of record in systematic works. No one
has drawn any clear distinction between individual differences and slight
varieties; or between more plainly marked varieties and subspecies and
species. On separate continents, and on different parts of the same
continent, when divided by barriers of any kind, and on outlying islands,
what a multitude of forms exist, which some experienced naturalists rank as
varieties, others as geographical races or sub species, and others as
distinct, though closely allied species!
If, then, animals and plants do vary, let it be ever so slightly or slowly,
why should not variations or individual differences, which are in any way
beneficial, be preserved and accumulated through natural selection, or the
survival of the fittest? If man can by patience select variations useful
to him, why, under changing and complex conditions of life, should not
variations useful to nature's living products often arise, and be preserved
or selected? What limit can be put to this power, acting during long ages
and rigidly scrutinising the whole constitution, structure, and habits of
each creature, favouring the good and rejecting the bad? I can see no
limit to this power, in slowly and beautifully adapting each form to the
most complex relations of life. The theory of natural selection, even if
we look no further than this, seems to be in the highest degree probable.
I have already recapitulated, as fairly as I could, the opposed
difficulties and objections: now let us turn to the special facts and
arguments in favour of the theory.
On the view that species are only strongly marked and permanent varieties,
and that each species first existed as a variety, we can see why it is that
no line of demarcation can be drawn between species, commonly supposed to
have been produced by special acts of creation, and varieties which are
acknowledged to have been produced by secondary laws. On this same view we
can understand how it is that in a region where many species of a genus
have been produced, and where they now flourish, these same species should
present many varieties; for where the manufactory of species has been
active, we might expect, as a general rule, to find it still in action; and
this is the case if varieties be incipient species. Moreover, the species
of the larger genera, which afford the greater number of varieties or
incipient species, retain to a certain degree the character of varieties;
for they differ from each other by a less amount of difference than do the
species of smaller genera. The closely allied species also of a larger
genera apparently have restricted ranges, and in their affinities they are
clustered in little groups round other species--in both respects resembling
varieties. These are strange relations on the view that each species was
independently created, but are intelligible if each existed first as a
variety.
As each species tends by its geometrical rate of reproduction to increase
inordinately in number; and as the modified descendants of each species
will be enabled to increase by as much as they become more diversified in
habits and structure, so as to be able to seize on many and widely
different places in the economy of nature, there will be a constant
tendency in natural selection to preserve the most divergent offspring of
any one species. Hence during a long-continued course of modification, the
slight differences characteristic of varieties of the same species, tend to
be augmented into the greater differences characteristic of the species of
the same genus. New and improved varieties will inevitably supplant and
exterminate the older, less improved and intermediate varieties; and thus
species are rendered to a large extent defined and distinct objects.
Dominant species belonging to the larger groups within each class tend to
give birth to new and dominant forms; so that each large group tends to
become still larger, and at the same time more divergent in character. But
as all groups cannot thus go on increasing in size, for the world would not
hold them, the more dominant groups beat the less dominant. This tendency
in the large groups to go on increasing in size and diverging in character,
together with the inevitable contingency of much extinction, explains the
arrangement of all the forms of life in groups subordinate to groups, all
within a few great classes, which has prevailed throughout all time. This
grand fact of the grouping of all organic beings under what is called the
Natural System, is utterly inexplicable on the theory of creation.
As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modifications; it
can act only by short and slow steps. Hence, the canon of "Natura non
facit saltum," which every fresh addition to our knowledge tends to
confirm, is on this theory intelligible. We can see why throughout nature
the same general end is gained by an almost infinite diversity of means,
for every peculiarity when once acquired is long inherited, and structures
already modified in many different ways have to be adapted for the same
general purpose. We can, in short, see why nature is prodigal in variety,
though niggard in innovation. But why this should be a law of nature if
each species has been independently created no man can explain.
Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of a woodpecker, should prey on
insects on the ground; that upland geese, which rarely or never swim, would
possess webbed feet; that a thrush-like bird should dive and feed on
sub-aquatic insects; and that a petrel should have the habits and structure
fitting it for the life of an auk! and so in endless other cases. But on
the view of each species constantly trying to increase in number, with
natural selection always ready to adapt the slowly varying descendants of
each to any unoccupied or ill-occupied place in nature, these facts cease
to be strange, or might even have been anticipated.
We can to a certain extent understand how it is that there is so much
beauty throughout nature; for this may be largely attributed to the agency
of selection. That beauty, according to our sense of it, is not universal,
must be admitted by every one who will look at some venomous snakes, at
some fishes, and at certain hideous bats with a distorted resemblance to
the human face. Sexual selection has given the most brilliant colours,
elegant patterns, and other ornaments to the males, and sometimes to both
sexes of many birds, butterflies and other animals. With birds it has
often rendered the voice of the male musical to the female, as well as to
our ears. Flowers and fruit have been rendered conspicuous by brilliant
colours in contrast with the green foliage, in order that the flowers may
be easily seen, visited and fertilised by insects, and the seeds
disseminated by birds. How it comes that certain colours, sounds and forms
should give pleasure to man and the lower animals, that is, how the sense
of beauty in its simplest form was first acquired, we do not know any more
than how certain odours and flavours were first rendered agreeable.
As natural selection acts by competition, it adapts and improves the
inhabitants of each country only in relation to their co-inhabitants; so
that we need feel no surprise at the species of any one country, although
on the ordinary view supposed to have been created and specially adapted
for that country, being beaten and supplanted by the naturalised
productions from another land. Nor ought we to marvel if all the
contrivances in nature be not, as far as we can judge, absolutely perfect;
as in the case even of the human eye; or if some of them be abhorrent to
our ideas of fitness. We need not marvel at the sting of the bee, when
used against the enemy, causing the bee's own death; at drones being
produced in such great numbers for one single act, and being then
slaughtered by their sterile sisters; at the astonishing waste of pollen by
our fir-trees; at the instinctive hatred of the queen-bee for her own
fertile daughters; at ichneumonidae feeding within the living bodies of
caterpillars; and at other such cases. The wonder, indeed, is, on the
theory of natural selection, that more cases of the want of absolute
perfection have not been detected.
The complex and little known laws governing the production of varieties are
the same, as far as we can judge, with the laws which have governed the
production of distinct species. In both cases physical conditions seem to
have produced some direct and definite effect, but how much we cannot say.
Thus, when varieties enter any new station, they occasionally assume some
of the characters proper to the species of that station. With both
varieties and species, use and disuse seem to have produced a considerable
effect; for it is impossible to resist this conclusion when we look, for
instance, at the logger-headed duck, which has wings incapable of flight,
in nearly the same condition as in the domestic duck; or when we look at
the burrowing tucu-tucu, which is occasionally blind, and then at certain
moles, which are habitually blind and have their eyes covered with skin; or
when we look at the blind animals inhabiting the dark caves of America and
Europe. With varieties and species, correlated variation seems to have
played an important part, so that when one part has been modified other
parts have been necessarily modified. With both varieties and species,
reversions to long-lost characters occasionally occur. How inexplicable on
the theory of creation is the occasional appearance of stripes on the
shoulders and legs of the several species of the horse-genus and of their
hybrids! How simply is this fact explained if we believe that these
species are all descended from a striped progenitor, in the same manner as
the several domestic breeds of the pigeon are descended from the blue and
barred rock-pigeon!
On the ordinary view of each species having been independently created, why
should specific characters, or those by which the species of the same genus
differ from each other, be more variable than the generic characters in
which they all agree? Why, for instance, should the colour of a flower be
more likely to vary in any one species of a genus, if the other species
possess differently coloured flowers, than if all possessed the same
coloured flowers? If species are only well-marked varieties, of which the
characters have become in a high degree permanent, we can understand this
fact; for they have already varied since they branched off from a common
progenitor in certain characters, by which they have come to be
specifically distinct from each other; therefore these same characters
would be more likely again to vary than the generic characters which have
been inherited without change for an immense period. It is inexplicable on
the theory of creation why a part developed in a very unusual manner in one
species alone of a genus, and therefore, as we may naturally infer, of
great importance to that species, should be eminently liable to variation;
but, on our view, this part has undergone, since the several species
branched off from a common progenitor, an unusual amount of variability and
modification, and therefore we might expect the part generally to be still
variable. But a part may be developed in the most unusual manner, like the
wing of a bat, and yet not be more variable than any other structure, if
the part be common to many subordinate forms, that is, if it has been
inherited for a very long period; for in this case it will have been
rendered constant by long-continued natural selection.
Glancing at instincts, marvellous as some are, they offer no greater
difficulty than do corporeal structures on the theory of the natural
selection of successive, slight, but profitable modifications. We can thus
understand why nature moves by graduated steps in endowing different
animals of the same class with their several instincts. I have attempted
to show how much light the principle of gradation throws on the admirable
architectural powers of the hive-bee. Habit no doubt often comes into play
in modifying instincts; but it certainly is not indispensable, as we see in
the case of neuter insects, which leave no progeny to inherit the effects
of long-continued habit. On the view of all the species of the same genus
having descended from a common parent, and having inherited much in common,
we can understand how it is that allied species, when placed under widely
different conditions of life, yet follow nearly the same instincts; why the
thrushes of tropical and temperate South America, for instance, line their
nests with mud like our British species. On the view of instincts having
been slowly acquired through natural selection, we need not marvel at some
instincts being not perfect and liable to mistakes, and at many instincts
causing other animals to suffer.
If species be only well-marked and permanent varieties, we can at once see
why their crossed offspring should follow the same complex laws in their
degrees and kinds of resemblance to their parents--in being absorbed into
each other by successive crosses, and in other such points--as do the
crossed offspring of acknowledged varieties. This similarity would be a
strange fact, if species had been independently created and varieties had
been produced through secondary laws.
If we admit that the geological record is imperfect to an extreme degree,
then the facts, which the record does give, strongly support the theory of
descent with modification. New species have come on the stage slowly and
at successive intervals; and the amount of change after equal intervals of
time, is widely different in different groups. The extinction of species
and of whole groups of species, which has played so conspicuous a part in
the history of the organic world, almost inevitably follows from the
principle of natural selection; for old forms are supplanted by new and
improved forms. Neither single species nor groups of species reappear when
the chain of ordinary generation is once broken. The gradual diffusion of
dominant forms, with the slow modification of their descendants, causes the
forms of life, after long intervals of time, to appear as if they had
changed simultaneously throughout the world. The fact of the fossil
remains of each formation being in some degree intermediate in character
between the fossils in the formations above and below, is simply explained
by their intermediate position in the chain of descent. The grand fact
that all extinct beings can be classed with all recent beings, naturally
follows from the living and the extinct being the offspring of common
parents. As species have generally diverged in character during their long
course of descent and modification, we can understand why it is that the
more ancient forms, or early progenitors of each group, so often occupy a
position in some degree intermediate between existing groups. Recent forms
are generally looked upon as being, on the whole, higher in the scale of
organisation than ancient forms; and they must be higher, in so far as the
later and more improved forms have conquered the older and less improved
forms in the struggle for life; they have also generally had their organs
more specialised for different functions. This fact is perfectly
compatible with numerous beings still retaining simple and but little
improved structures, fitted for simple conditions of life; it is likewise
compatible with some forms having retrograded in organisation, by having
become at each stage of descent better fitted for new and degraded habits
of life. Lastly, the wonderful law of the long endurance of allied forms
on the same continent--of marsupials in Australia, of edentata in America,
and other such cases--is intelligible, for within the same country the
existing and the extinct will be closely allied by descent.
Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world to
another, owing to former climatical and geographical changes and to the
many occasional and unknown means of dispersal, then we can understand, on
the theory of descent with modification, most of the great leading facts in
Distribution. We can see why there should be so striking a parallelism in
the distribution of organic beings throughout space, and in their
geological succession throughout time; for in both cases the beings have
been connected by the bond of ordinary generation, and the means of
modification have been the same. We see the full meaning of the wonderful
fact, which has struck every traveller, namely, that on the same continent,
under the most diverse conditions, under heat and cold, on mountain and
lowland, on deserts and marshes, most of the inhabitants within each great
class are plainly related; for they are the descendants of the same
progenitors and early colonists. On this same principle of former
migration, combined in most cases with modification, we can understand, by
the aid of the Glacial period, the identity of some few plants, and the
close alliance of many others, on the most distant mountains, and in the
northern and southern temperate zones; and likewise the close alliance of
some of the inhabitants of the sea in the northern and southern temperate
latitudes, though separated by the whole intertropical ocean. Although two
countries may present physical conditions as closely similar as the same
species ever require, we need feel no surprise at their inhabitants being
widely different, if they have been for a long period completely sundered
from each other; for as the relation of organism to organism is the most
important of all relations, and as the two countries will have received
colonists at various periods and in different proportions, from some other
country or from each other, the course of modification in the two areas
will inevitably have been different.
On this view of migration, with subsequent modification, we see why oceanic
islands are inhabited by only few species, but of these, why many are
peculiar or endemic forms. We clearly see why species belonging to those
groups of animals which cannot cross wide spaces of the ocean, as frogs and
terrestrial mammals, do not inhabit oceanic islands; and why, on the other
hand, new and peculiar species of bats, animals which can traverse the
ocean, are often found on islands far distant from any continent. Such
cases as the presence of peculiar species of bats on oceanic islands and
the absence of all other terrestrial mammals, are facts utterly
inexplicable on the theory of independent acts of creation.
The existence of closely allied representative species in any two areas,
implies, on the theory of descent with modification, that the same parent-
forms formerly inhabited both areas; and we almost invariably find that
wherever many closely allied species inhabit two areas, some identical
species are still common to both. Wherever many closely allied yet
distinct species occur, doubtful forms and varieties belonging to the same
groups likewise occur. It is a rule of high generality that the
inhabitants of each area are related to the inhabitants of the nearest
source whence immigrants might have been derived. We see this in the
striking relation of nearly all the plants and animals of the Galapagos
Archipelago, of Juan Fernandez, and of the other American islands, to the
plants and animals of the neighbouring American mainland; and of those of
the Cape de Verde Archipelago, and of the other African islands to the
African mainland. It must be admitted that these facts receive no
explanation on the theory of creation.
The fact, as we have seen, that all past and present organic beings can be
arranged within a few great classes, in groups subordinate to groups, and
with the extinct groups often falling in between the recent groups, is
intelligible on the theory of natural selection with its contingencies of
extinction and divergence of character. On these same principles we see
how it is that the mutual affinities of the forms within each class are so
complex and circuitous. We see why certain characters are far more
serviceable than others for classification; why adaptive characters, though
of paramount importance to the beings, are of hardly any importance in
classification; why characters derived from rudimentary parts, though of no
service to the beings, are often of high classificatory value; and why
embryological characters are often the most valuable of all. The real
affinities of all organic beings, in contradistinction to their adaptive
resemblances, are due to inheritance or community of descent. The Natural
System is a genealogical arrangement, with the acquired grades of
difference, marked by the terms, varieties, species, genera, families,
etc.; and we have to discover the lines of descent by the most permanent
characters, whatever they may be, and of however slight vital importance.
The similar framework of bones in the hand of a man, wing of a bat, fin of
the porpoise, and leg of the horse--the same number of vertebrae forming
the neck of the giraffe and of the elephant--and innumerable other such
facts, at once explain themselves on the theory of descent with slow and
slight successive modifications. The similarity of pattern in the wing and
in the leg of a bat, though used for such different purpose--in the jaws
and legs of a crab--in the petals, stamens, and pistils of a flower, is
likewise, to a large extent, intelligible on the view of the gradual
modification of parts or organs, which were aboriginally alike in an early
progenitor in each of these classes. On the principle of successive
variations not always supervening at an early age, and being inherited at a
corresponding not early period of life, we clearly see why the embryos of
mammals, birds, reptiles, and fishes should be so closely similar, and so
unlike the adult forms. We may cease marvelling at the embryo of an
air-breathing mammal or bird having branchial slits and arteries running in
loops, like those of a fish which has to breathe the air dissolved in water
by the aid of well-developed branchiae.
Disuse, aided sometimes by natural selection, will often have reduced
organs when rendered useless under changed habits or conditions of life;
and we can understand on this view the meaning of rudimentary organs. But
disuse and selection will generally act on each creature, when it has come
to maturity and has to play its full part in the struggle for existence,
and will thus have little power on an organ during early life; hence the
organ will not be reduced or rendered rudimentary at this early age. The
calf, for instance, has inherited teeth, which never cut through the gums
of the upper jaw, from an early progenitor having well-developed teeth; and
we may believe, that the teeth in the mature animal were formerly reduced
by disuse owing to the tongue and palate, or lips, having become
excellently fitted through natural selection to browse without their aid;
whereas in the calf, the teeth have been left unaffected, and on the
principle of inheritance at corresponding ages have been inherited from a
remote period to the present day. On the view of each organism with all
its separate parts having been specially created, how utterly inexplicable
is it that organs bearing the plain stamp of inutility, such as the teeth
in the embryonic calf or the shrivelled wings under the soldered
wing-covers of many beetles, should so frequently occur. Nature may be
said to have taken pains to reveal her scheme of modification, by means of
rudimentary organs, of embryological and homologous structures, but we are
too blind to understand her meaning.
I have now recapitulated the facts and considerations which have thoroughly
convinced me that species have been modified, during a long course of
descent. This has been effected chiefly through the natural selection of
numerous successive, slight, favourable variations; aided in an important
manner by the inherited effects of the use and disuse of parts; and in an
unimportant manner, that is, in relation to adaptive structures, whether
past or present, by the direct action of external conditions, and by
variations which seem to us in our ignorance to arise spontaneously. It
appears that I formerly underrated the frequency and value of these latter
forms of variation, as leading to permanent modifications of structure
independently of natural selection. But as my conclusions have lately been
much misrepresented, and it has been stated that I attribute the
modification of species exclusively to natural selection, I may be
permitted to remark that in the first edition of this work, and
subsequently, I placed in a most conspicuous position--namely, at the close
of the Introduction--the following words: "I am convinced that natural
selection has been the main but not the exclusive means of modification."
This has been of no avail. Great is the power of steady misrepresentation;
but the history of science shows that fortunately this power does not long
endure.
It can hardly be supposed that a false theory would explain, in so
satisfactory a manner as does the theory of natural selection, the several
large classes of facts above specified. It has recently been objected that
this is an unsafe method of arguing; but it is a method used in judging of
the common events of life, and has often been used by the greatest natural
philosophers. The undulatory theory of light has thus been arrived at; and
the belief in the revolution of the earth on its own axis was until lately
supported by hardly any direct evidence. It is no valid objection that
science as yet throws no light on the far higher problem of the essence or
origin of life. Who can explain what is the essence of the attraction of
gravity? No one now objects to following out the results consequent on
this unknown element of attraction; notwithstanding that Leibnitz formerly
accused Newton of introducing "occult qualities and miracles into
philosophy."
I see no good reasons why the views given in this volume should shock the
religious feelings of any one. It is satisfactory, as showing how
transient such impressions are, to remember that the greatest discovery
ever made by man, namely, the law of the attraction of gravity, was also
attacked by Leibnitz, "as subversive of natural, and inferentially of
revealed, religion." A celebrated author and divine has written to me that
"he has gradually learned to see that it is just as noble a conception of
the Deity to believe that He created a few original forms capable of self-
development into other and needful forms, as to believe that He required a
fresh act of creation to supply the voids caused by the action of His
laws."
Why, it may be asked, until recently did nearly all the most eminent living
naturalists and geologists disbelieve in the mutability of species? It
cannot be asserted that organic beings in a state of nature are subject to
no variation; it cannot be proved that the amount of variation in the
course of long ages is a limited quantity; no clear distinction has been,
or can be, drawn between species and well-marked varieties. It cannot be
maintained that species when intercrossed are invariably sterile and
varieties invariably fertile; or that sterility is a special endowment and
sign of creation. The belief that species were immutable productions was
almost unavoidable as long as the history of the world was thought to be of
short duration; and now that we have acquired some idea of the lapse of
time, we are too apt to assume, without proof, that the geological record
is so perfect that it would have afforded us plain evidence of the mutation
of species, if they had undergone mutation.
But the chief cause of our natural unwillingness to admit that one species
has given birth to other and distinct species, is that we are always slow
in admitting any great changes of which we do not see the steps. The
difficulty is the same as that felt by so many geologists, when Lyell first
insisted that long lines of inland cliffs had been formed, and great
valleys excavated, by the agencies which we still see at work. The mind
cannot possibly grasp the full meaning of the term of even a million years;
it cannot add up and perceive the full effects of many slight variations,
accumulated during an almost infinite number of generations.
Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of facts
all viewed, during a long course of years, from a point of view directly
opposite to mine. It is so easy to hide our ignorance under such
expressions as the "plan of creation," "unity of design," etc., and to
think that we give an explanation when we only restate a fact. Any one
whose disposition leads him to attach more weight to unexplained
difficulties than to the explanation of a certain number of facts will
certainly reject the theory. A few naturalists, endowed with much
flexibility of mind, and who have already begun to doubt the immutability
of species, may be influenced by this volume; but I look with confidence to
the future, to young and rising naturalists, who will be able to view both
sides of the question with impartiality. Whoever is led to believe that
species are mutable will do good service by conscientiously expressing his
conviction; for thus only can the load of prejudice by which this subject
is overwhelmed be removed.
Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but that
other species are real, that is, have been independently created. This
seems to me a strange conclusion to arrive at. They admit that a multitude
of forms, which till lately they themselves thought were special creations,
and which are still thus looked at by the majority of naturalists, and
which consequently have all the external characteristic features of true
species--they admit that these have been produced by variation, but they
refuse to extend the same view to other and slightly different forms.
Nevertheless, they do not pretend that they can define, or even conjecture,
which are the created forms of life, and which are those produced by
secondary laws. They admit variation as a vera causa in one case, they
arbitrarily reject it in another, without assigning any distinction in the
two cases. The day will come when this will be given as a curious
illustration of the blindness of preconceived opinion. These authors seem
no more startled at a miraculous act of creation than at an ordinary birth.
But do they really believe that at innumerable periods in the earth's
history certain elemental atoms have been commanded suddenly to flash into
living tissues? Do they believe that at each supposed act of creation one
individual or many were produced? Were all the infinitely numerous kinds
of animals and plants created as eggs or seed, or as full grown? and in the
case of mammals, were they created bearing the false marks of nourishment
from the mother's womb? Undoubtedly some of these same questions cannot be
answered by those who believe in the appearance or creation of only a few
forms of life or of some one form alone. It has been maintained by several
authors that it is as easy to believe in the creation of a million beings
as of one; but Maupertuis' philosophical axiom "of least action" leads the
mind more willingly to admit the smaller number; and certainly we ought not
to believe that innumerable beings within each great class have been
created with plain, but deceptive, marks of descent from a single parent.
As a record of a former state of things, I have retained in the foregoing
paragraphs, and elsewhere, several sentences which imply that naturalists
believe in the separate creation of each species; and I have been much
censured for having thus expressed myself. But undoubtedly this was the
general belief when the first edition of the present work appeared. I
formerly spoke to very many naturalists on the subject of evolution, and
never once met with any sympathetic agreement. It is probable that some
did then believe in evolution, but they were either silent or expressed
themselves so ambiguously that it was not easy to understand their meaning.
Now, things are wholly changed, and almost every naturalist admits the
great principle of evolution. There are, however, some who still think
that species have suddenly given birth, through quite unexplained means, to
new and totally different forms. But, as I have attempted to show, weighty
evidence can be opposed to the admission of great and abrupt modifications.
Under a scientific point of view, and as leading to further investigation,
but little advantage is gained by believing that new forms are suddenly
developed in an inexplicable manner from old and widely different forms,
over the old belief in the creation of species from the dust of the earth.
It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct
the forms are which we consider, by so much the arguments in favour of
community of descent become fewer in number and less in force. But some
arguments of the greatest weight extend very far. All the members of whole
classes are connected together by a chain of affinities, and all can be
classed on the same principle, in groups subordinate to groups. Fossil
remains sometimes tend to fill up very wide intervals between existing
orders.
Organs in a rudimentary condition plainly show that an early progenitor had
the organ in a fully developed condition, and this in some cases implies an
enormous amount of modification in the descendants. Throughout whole
classes various structures are formed on the same pattern, and at a very
early age the embryos closely resemble each other. Therefore I cannot
doubt that the theory of descent with modification embraces all the members
of the same great class or kingdom. I believe that animals are descended
from at most only four or five progenitors, and plants from an equal or
lesser number.
Analogy would lead me one step further, namely, to the belief that all
animals and plants are descended from some one prototype. But analogy may
be a deceitful guide. Nevertheless all living things have much in common,
in their chemical composition, their cellular structure, their laws of
growth, and their liability to injurious influences. We see this even in
so trifling a fact as that the same poison often similarly affects plants
and animals; or that the poison secreted by the gall-fly produces monstrous
growths on the wild rose or oak-tree. With all organic beings, excepting
perhaps some of the very lowest, sexual reproduction seems to be
essentially similar. With all, as far as is at present known, the germinal
vesicle is the same; so that all organisms start from a common origin. If
we look even to the two main divisions--namely, to the animal and vegetable
kingdoms--certain low forms are so far intermediate in character that
naturalists have disputed to which kingdom they should be referred. As
Professor Asa Gray has remarked, "the spores and other reproductive bodies
of many of the lower algae may claim to have first a characteristically
animal, and then an unequivocally vegetable existence." Therefore, on the
principle of natural selection with divergence of character, it does not
seem incredible that, from some such low and intermediate form, both
animals and plants may have been developed; and, if we admit this, we must
likewise admit that all the organic beings which have ever lived on this
earth may be descended from some one primordial form. But this inference
is chiefly grounded on analogy, and it is immaterial whether or not it be
accepted. No doubt it is possible, as Mr. G.H. Lewes has urged, that at
the first commencement of life many different forms were evolved; but if
so, we may conclude that only a very few have left modified descendants.
For, as I have recently remarked in regard to the members of each great
kingdom, such as the Vertebrata, Articulata, etc., we have distinct
evidence in their embryological, homologous, and rudimentary structures,
that within each kingdom all the members are descended from a single
progenitor.
When the views advanced by me in this volume, and by Mr. Wallace or when
analogous views on the origin of species are generally admitted, we can
dimly foresee that there will be a considerable revolution in natural
history. Systematists will be able to pursue their labours as at present;
but they will not be incessantly haunted by the shadowy doubt whether this
or that form be a true species. This, I feel sure and I speak after
experience, will be no slight relief. The endless disputes whether or not
some fifty species of British brambles are good species will cease.
Systematists will have only to decide (not that this will be easy) whether
any form be sufficiently constant and distinct from other forms, to be
capable of definition; and if definable, whether the differences be
sufficiently important to deserve a specific name. This latter point will
become a far more essential consideration than it is at present; for
differences, however slight, between any two forms, if not blended by
intermediate gradations, are looked at by most naturalists as sufficient to
raise both forms to the rank of species.
Hereafter we shall be compelled to acknowledge that the only distinction
between species and well-marked varieties is, that the latter are known, or
believed to be connected at the present day by intermediate gradations,
whereas species were formerly thus connected. Hence, without rejecting the
consideration of the present existence of intermediate gradations between
any two forms, we shall be led to weigh more carefully and to value higher
the actual amount of difference between them. It is quite possible that
forms now generally acknowledged to be merely varieties may hereafter be
thought worthy of specific names; and in this case scientific and common
language will come into accordance. In short, we shall have to treat
species in the same manner as those naturalists treat genera, who admit
that genera are merely artificial combinations made for convenience. This
may not be a cheering prospect; but we shall at least be freed from the
vain search for the undiscovered and undiscoverable essence of the term
species.
The other and more general departments of natural history will rise greatly
in interest. The terms used by naturalists, of affinity, relationship,
community of type, paternity, morphology, adaptive characters, rudimentary
and aborted organs, etc., will cease to be metaphorical and will have a
plain signification. When we no longer look at an organic being as a
savage looks at a ship, as something wholly beyond his comprehension; when
we regard every production of nature as one which has had a long history;
when we contemplate every complex structure and instinct as the summing up
of many contrivances, each useful to the possessor, in the same way as any
great mechanical invention is the summing up of the labour, the experience,
the reason, and even the blunders of numerous workmen; when we thus view
each organic being, how far more interesting--I speak from experience--does
the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the causes
and laws of variation, on correlation, on the effects of use and disuse, on
the direct action of external conditions, and so forth. The study of
domestic productions will rise immensely in value. A new variety raised by
man will be a far more important and interesting subject for study than one
more species added to the infinitude of already recorded species. Our
classifications will come to be, as far as they can be so made,
genealogies; and will then truly give what may be called the plan of
creation. The rules for classifying will no doubt become simpler when we
have a definite object in view. We possess no pedigree or armorial
bearings; and we have to discover and trace the many diverging lines of
descent in our natural genealogies, by characters of any kind which have
long been inherited. Rudimentary organs will speak infallibly with respect
to the nature of long-lost structures. Species and groups of species which
are called aberrant, and which may fancifully be called living fossils,
will aid us in forming a picture of the ancient forms of life. Embryology
will often reveal to us the structure, in some degree obscured, of the
prototypes of each great class.
When we can feel assured that all the individuals of the same species, and
all the closely allied species of most genera, have, within a not very
remote period descended from one parent, and have migrated from some one
birth-place; and when we better know the many means of migration, then, by
the light which geology now throws, and will continue to throw, on former
changes of climate and of the level of the land, we shall surely be enabled
to trace in an admirable manner the former migrations of the inhabitants of
the whole world. Even at present, by comparing the differences between the
inhabitants of the sea on the opposite sides of a continent, and the nature
of the various inhabitants of that continent in relation to their apparent
means of immigration, some light can be thrown on ancient geography.
The noble science of geology loses glory from the extreme imperfection of
the record. The crust of the earth, with its embedded remains, must not be
looked at as a well-filled museum, but as a poor collection made at hazard
and at rare intervals. The accumulation of each great fossiliferous
formation will be recognised as having depended on an unusual occurrence of
favourable circumstances, and the blank intervals between the successive
stages as having been of vast duration. But we shall be able to gauge with
some security the duration of these intervals by a comparison of the
preceding and succeeding organic forms. We must be cautious in attempting
to correlate as strictly contemporaneous two formations, which do not
include many identical species, by the general succession of the forms of
life. As species are produced and exterminated by slowly acting and still
existing causes, and not by miraculous acts of creation; and as the most
important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism--the improvement of one
organism entailing the improvement or the extermination of others; it
follows, that the amount of organic change in the fossils of consecutive
formations probably serves as a fair measure of the relative, though not
actual lapse of time. A number of species, however, keeping in a body
might remain for a long period unchanged, whilst within the same period,
several of these species, by migrating into new countries and coming into
competition with foreign associates, might become modified; so that we must
not overrate the accuracy of organic change as a measure of time.
In the future I see open fields for far more important researches.
Psychology will be securely based on the foundation already well laid by
Mr. Herbert Spencer, that of the necessary acquirement of each mental power
and capacity by gradation. Much light will be thrown on the origin of man
and his history.
Authors of the highest eminence seem to be fully satisfied with the view
that each species has been independently created. To my mind it accords
better with what we know of the laws impressed on matter by the Creator,
that the production and extinction of the past and present inhabitants of
the world should have been due to secondary causes, like those determining
the birth and death of the individual. When I view all beings not as
special creations, but as the lineal descendants of some few beings which
lived long before the first bed of the Cambrian system was deposited, they
seem to me to become ennobled. Judging from the past, we may safely infer
that not one living species will transmit its unaltered likeness to a
distinct futurity. And of the species now living very few will transmit
progeny of any kind to a far distant futurity; for the manner in which all
organic beings are grouped, shows that the greater number of species in
each genus, and all the species in many genera, have left no descendants,
but have become utterly extinct. We can so far take a prophetic glance
into futurity as to foretell that it will be the common and widely spread
species, belonging to the larger and dominant groups within each class,
which will ultimately prevail and procreate new and dominant species. As
all the living forms of life are the lineal descendants of those which
lived long before the Cambrian epoch, we may feel certain that the ordinary
succession by generation has never once been broken, and that no cataclysm
has desolated the whole world. Hence, we may look with some confidence to
a secure future of great length. And as natural selection works solely by
and for the good of each being, all corporeal and mental endowments will
tend to progress towards perfection.
It is interesting to contemplate a tangled bank, clothed with many plants
of many kinds, with birds singing on the bushes, with various insects
flitting about, and with worms crawling through the damp earth, and to
reflect that these elaborately constructed forms, so different from each
other, and dependent upon each other in so complex a manner, have all been
produced by laws acting around us. These laws, taken in the largest sense,
being Growth with reproduction; Inheritance which is almost implied by
reproduction; Variability from the indirect and direct action of the
conditions of life, and from use and disuse; a Ratio of Increase so high as
to lead to a Struggle for Life, and as a consequence to Natural Selection,
entailing Divergence of Character and the Extinction of less improved
forms. Thus, from the war of nature, from famine and death, the most
exalted object which we are capable of conceiving, namely, the production
of the higher animals, directly follows. There is grandeur in this view of
life, with its several powers, having been originally breathed by the
Creator into a few forms or into one; and that, whilst this planet has gone
circling on according to the fixed law of gravity, from so simple a
beginning endless forms most beautiful and most wonderful have been, and
are being evolved.
GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME.
(I am indebted to the kindness of Mr. W.S. Dallas for this Glossary, which
has been given because several readers have complained to me that some of
the terms used were unintelligible to them. Mr. Dallas has endeavoured to
give the explanations of the terms in as popular a form as possible.)
ABERRANT.--Forms or groups of animals or plants which deviate in important
characters from their nearest allies, so as not to be easily included in
the same group with them, are said to be aberrant.
ABERRATION (in Optics).--In the refraction of light by a convex lens the
rays passing through different parts of the lens are brought to a focus at
slightly different distances--this is called SPHERICAL ABERRATION; at the
same time the coloured rays are separated by the prismatic action of the
lens and likewise brought to a focus at different distances--this is
CHROMATIC ABERRATION.
ABNORMAL.--Contrary to the general rule.
ABORTED.--An organ is said to be aborted, when its development has been
arrested at a very early stage.
ALBINISM.--Albinos are animals in which the usual colouring matters
characteristic of the species have not been produced in the skin and its
appendages. Albinism is the state of being an albino.
ALGAE.--A class of plants including the ordinary sea-weeds and the
filamentous fresh-water weeds.
ALTERNATION OF GENERATIONS.--This term is applied to a peculiar mode of
reproduction which prevails among many of the lower animals, in which the
egg produces a living form quite different from its parent, but from which
the parent-form is reproduced by a process of budding, or by the division
of the substance of the first product of the egg.
AMMONITES.--A group of fossil, spiral, chambered shells, allied to the
existing pearly Nautilus, but having the partitions between the chambers
waved in complicated patterns at their junction with the outer wall of the
shell.
ANALOGY.--That resemblance of structures which depends upon similarity of
function, as in the wings of insects and birds. Such structures are said
to be ANALOGOUS, and to be ANALOGUES of each other.
ANIMALCULE.--A minute animal: generally applied to those visible only by
the microscope.
ANNELIDS.--A class of worms in which the surface of the body exhibits a
more or less distinct division into rings or segments, generally provided
with appendages for locomotion and with gills. It includes the ordinary
marine worms, the earth-worms, and the leeches.
ANTENNAE.--Jointed organs appended to the head in Insects, Crustacea and
Centipedes, and not belonging to the mouth.
ANTHERS.--The summits of the stamens of flowers, in which the pollen or
fertilising dust is produced.
APLACENTALIA, APLACENTATA or APLACENTAL MAMMALS.--See MAMMALIA.
ARCHETYPAL.--Of or belonging to the Archetype, or ideal primitive form upon
which all the beings of a group seem to be organised.
ARTICULATA.--A great division of the Animal Kingdom characterised generally
by having the surface of the body divided into rings called segments, a
greater or less number of which are furnished with jointed legs (such as
Insects, Crustaceans and Centipedes).
ASYMMETRICAL.--Having the two sides unlike.
ATROPHIED.--Arrested in development at a very early stage.
BALANUS.--The genus including the common Acorn-shells which live in
abundance on the rocks of the sea-coast.
BATRACHIANS.--A class of animals allied to the Reptiles, but undergoing a
peculiar metamorphosis, in which the young animal is generally aquatic and
breathes by gills. (Examples, Frogs, Toads, and Newts.)
BOULDERS.--Large transported blocks of stone generally embedded in clays or
gravels.
BRACHIOPODA.--A class of marine Mollusca, or soft-bodied animals, furnished
with a bivalve shell, attached to submarine objects by a stalk which passes
through an aperture in one of the valves, and furnished with fringed arms,
by the action of which food is carried to the mouth.
BRANCHIAE.--Gills or organs for respiration in water.
BRANCHIAL.--Pertaining to gills or branchiae.
CAMBRIAN SYSTEM.--A series of very ancient Palaeozoic rocks, between the
Laurentian and the Silurian. Until recently these were regarded as the
oldest fossiliferous rocks.
CANIDAE.--The Dog-family, including the Dog, Wolf, Fox, Jackal, etc.
CARAPACE.--The shell enveloping the anterior part of the body in
Crustaceans generally; applied also to the hard shelly pieces of the
Cirripedes.
CARBONIFEROUS.--This term is applied to the great formation which includes,
among other rocks, the coal-measures. It belongs to the oldest, or
Palaeozoic, system of formations.
CAUDAL.--Of or belonging to the tail.
CEPHALOPODS.--The highest class of the Mollusca, or soft-bodied animals,
characterised by having the mouth surrounded by a greater or less number of
fleshy arms or tentacles, which, in most living species, are furnished with
sucking-cups. (Examples, Cuttle-fish, Nautilus.)
CETACEA.--An order of Mammalia, including the Whales, Dolphins, etc.,
having the form of the body fish-like, the skin naked, and only the fore
limbs developed.
CHELONIA.--An order of Reptiles including the Turtles, Tortoises, etc.
CIRRIPEDES.--An order of Crustaceans including the Barnacles and Acorn-
shells. Their young resemble those of many other Crustaceans in form; but
when mature they are always attached to other objects, either directly or
by means of a stalk, and their bodies are enclosed by a calcareous shell
composed of several pieces, two of which can open to give issue to a bunch
of curled, jointed tentacles, which represent the limbs.
COCCUS.--The genus of Insects including the Cochineal. In these the male
is a minute, winged fly, and the female generally a motionless, berry-like
mass.
COCOON.--A case usually of silky material, in which insects are frequently
enveloped during the second or resting-stage (pupa) of their existence.
The term "cocoon-stage" is here used as equivalent to "pupa-stage."
COELOSPERMOUS.--A term applied to those fruits of the Umbelliferae which
have the seed hollowed on the inner face.
COLEOPTERA.--Beetles, an order of Insects, having a biting mouth and the
first pair of wings more or less horny, forming sheaths for the second
pair, and usually meeting in a straight line down the middle of the back.
COLUMN.--A peculiar organ in the flowers of Orchids, in which the stamens,
style and stigma (or the reproductive parts) are united.
COMPOSITAE or COMPOSITOUS PLANTS.--Plants in which the inflorescence
consists of numerous small flowers (florets) brought together into a dense
head, the base of which is enclosed by a common envelope. (Examples, the
Daisy, Dandelion, etc.)
CONFERVAE.--The filamentous weeds of fresh water.
CONGLOMERATE.--A rock made up of fragments of rock or pebbles, cemented
together by some other material.
COROLLA.--The second envelope of a flower usually composed of coloured,
leaf-like organs (petals), which may be united by their edges either in the
basal part or throughout.
CORRELATION.--The normal coincidence of one phenomenon, character, etc.,
with another.
CORYMB.--A bunch of flowers in which those springing from the lower part of
the flower stalks are supported on long stalks so as to be nearly on a
level with the upper ones.
COTYLEDONS.--The first or seed-leaves of plants.
CRUSTACEANS.--A class of articulated animals, having the skin of the body
generally more or less hardened by the deposition of calcareous matter,
breathing by means of gills. (Examples, Crab, Lobster, Shrimp, etc.)
CURCULIO.--The old generic term for the Beetles known as Weevils,
characterised by their four-jointed feet, and by the head being produced
into a sort of beak, upon the sides of which the antennae are inserted.
CUTANEOUS.--Of or belonging to the skin.
DEGRADATION.--The wearing down of land by the action of the sea or of
meteoric agencies.
DENUDATION.--The wearing away of the surface of the land by water.
DEVONIAN SYSTEM or FORMATION.--A series of Palaeozoic rocks, including the
Old Red Sandstone.
DICOTYLEDONS, or DICOTYLEDONOUS PLANTS.--A class of plants characterised by
having two seed-leaves, by the formation of new wood between the bark and
the old wood (exogenous growth) and by the reticulation of the veins of the
leaves. The parts of the flowers are generally in multiples of five.
DIFFERENTATION.--The separation or discrimination of parts or organs which
in simpler forms of life are more or less united.
DIMORPHIC.--Having two distinct forms.--DIMORPHISM is the condition of the
appearance of the same species under two dissimilar forms.
DIOECIOUS.--Having the organs of the sexes upon distinct individuals.
DIORITE.--A peculiar form of Greenstone.
DORSAL.--Of or belonging to the back.
EDENTATA.--A peculiar order of Quadrupeds, characterised by the absence of
at least the middle incisor (front) teeth in both jaws. (Examples, the
Sloths and Armadillos.)
ELYTRA.--The hardened fore-wings of Beetles, serving as sheaths for the
membranous hind-wings, which constitute the true organs of flight.
EMBRYO.--The young animal undergoing development within the egg or womb.
EMBRYOLOGY.--The study of the development of the embryo.
ENDEMIC.--Peculiar to a given locality.
ENTOMOSTRACA.--A division of the class Crustacea, having all the segments
of the body usually distinct, gills attached to the feet or organs of the
mouth, and the feet fringed with fine hairs. They are generally of small
size.
EOCENE.--The earliest of the three divisions of the Tertiary epoch of
geologists. Rocks of this age contain a small proportion of shells
identical with species now living.
EPHEMEROUS INSECTS.--Insects allied to the May-fly.
FAUNA.--The totality of the animals naturally inhabiting a certain country
or region, or which have lived during a given geological period.
FELIDAE.--The Cat-family.
FERAL.--Having become wild from a state of cultivation or domestication.
FLORA.--The totality of the plants growing naturally in a country, or
during a given geological period.
FLORETS.--Flowers imperfectly developed in some respects, and collected
into a dense spike or head, as in the Grasses, the Dandelion, etc.
FOETAL.--Of or belonging to the foetus, or embryo in course of development.
FORAMINIFERA.--A class of animals of very low organisation and generally of
small size, having a jelly-like body, from the surface of which delicate
filaments can be given off and retracted for the prehension of external
objects, and having a calcareous or sandy shell, usually divided into
chambers and perforated with small apertures.
FOSSILIFEROUS.--Containing fossils.
FOSSORIAL.--Having a faculty of digging. The Fossorial Hymenoptera are a
group of Wasp-like Insects, which burrow in sandy soil to make nests for
their young.
FRENUM (pl. FRENA).--A small band or fold of skin.
FUNGI (sing. FUNGUS).--A class of cellular plants, of which Mushrooms,
Toadstools, and Moulds, are familiar examples.
FURCULA.--The forked bone formed by the union of the collar-bones in many
birds, such as the common Fowl.
GALLINACEOUS BIRDS.--An order of birds of which the common Fowl, Turkey,
and Pheasant, are well-known examples.
GALLUS.--The genus of birds which includes the common Fowl.
GANGLION.--A swelling or knot from which nerves are given off as from a
centre.
GANOID FISHES.--Fishes covered with peculiar enamelled bony scales. Most
of them are extinct.
GERMINAL VESICLE.--A minute vesicle in the eggs of animals, from which the
development of the embryo proceeds.
GLACIAL PERIOD.--A period of great cold and of enormous extension of ice
upon the surface of the earth. It is believed that glacial periods have
occurred repeatedly during the geological history of the earth, but the
term is generally applied to the close of the Tertiary epoch, when nearly
the whole of Europe was subjected to an arctic climate.
GLAND.--An organ which secretes or separates some peculiar product from the
blood or sap of animals or plants.
GLOTTIS.--The opening of the windpipe into the oesophagus or gullet.
GNEISS.--A rock approaching granite in composition, but more or less
laminated, and really produced by the alteration of a sedimentary deposit
after its consolidation.
GRALLATORES.--The so-called wading-birds (storks, cranes, snipes, etc.),
which are generally furnished with long legs, bare of feathers above the
heel, and have no membranes between the toes.
GRANITE.--A rock consisting essentially of crystals of felspar and mica in
a mass of quartz.
HABITAT.--The locality in which a plant or animal naturally lives.
HEMIPTERA.--An order or sub-order of insects, characterised by the
possession of a jointed beak or rostrum, and by having the fore-wings horny
in the basal portion and membranous at the extremity, where they cross each
other. This group includes the various species of bugs.
HERMAPHRODITE.--Possessing the organs of both sexes.
HOMOLOGY.--That relation between parts which results from their development
from corresponding embryonic parts, either in different animals, as in the
case of the arm of man, the fore-leg of a quadruped, and the wing of a
bird; or in the same individual, as in the case of the fore and hind legs
in quadrupeds, and the segments or rings and their appendages of which the
body of a worm, a centipede, etc., is composed. The latter is called
serial homology. The parts which stand in such a relation to each other
are said to be homologous, and one such part or organ is called the
homologue of the other. In different plants the parts of the flower are
homologous, and in general these parts are regarded as homologous with
leaves.
HOMOPTERA.--An order or sub-order of insects having (like the Hemiptera) a
jointed beak, but in which the fore-wings are either wholly membranous or
wholly leathery, The Cicadae, frog-hoppers, and Aphides, are well-known
examples.
HYBRID.--The offspring of the union of two distinct species.
HYMENOPTERA.--An order of insects possessing biting jaws and usually four
membranous wings in which there are a few veins. Bees and wasps are
familiar examples of this group.
HYPERTROPHIED.--Excessively developed.
ICHNEUMONIDAE.--A family of hymenopterous insects, the members of which lay
their eggs in the bodies or eggs of other insects.
IMAGO.--The perfect (generally winged) reproductive state of an insect.
INDIGENES.--The aboriginal animal or vegetable inhabitants of a country or
region.
INFLORESCENCE.--The mode of arrangement of the flowers of plants.
INFUSORIA.--A class of microscopic animalcules, so called from their having
originally been observed in infusions of vegetable matters. They consist
of a gelatinous material enclosed in a delicate membrane, the whole or part
of which is furnished with short vibrating hairs (called cilia), by means
of which the animalcules swim through the water or convey the minute
particles of their food to the orifice of the mouth.
INSECTIVOROUS.--Feeding on insects.
INVERTEBRATA, or INVERTEBRATE ANIMALS.--Those animals which do not possess
a backbone or spinal column.
LACUNAE.--Spaces left among the tissues in some of the lower animals and
serving in place of vessels for the circulation of the fluids of the body.
LAMELLATED.--Furnished with lamellae or little plates.
LARVA (pl. LARVAE).--The first condition of an insect at its issuing from
the egg, when it is usually in the form of a grub, caterpillar, or maggot.
LARYNX.--The upper part of the windpipe opening into the gullet.
LAURENTIAN.--A group of greatly altered and very ancient rocks, which is
greatly developed along the course of the St. Laurence, whence the name.
It is in these that the earliest known traces of organic bodies have been
found.
LEGUMINOSAE.--An order of plants represented by the common peas and beans,
having an irregular flower in which one petal stands up like a wing, and
the stamens and pistil are enclosed in a sheath formed by two other petals.
The fruit is a pod (or legume).
LEMURIDAE.--A group of four-handed animals, distinct from the monkeys and
approaching the insectivorous quadrupeds in some of their characters and
habits. Its members have the nostrils curved or twisted, and a claw
instead of a nail upon the first finger of the hind hands.
LEPIDOPTERA.--An order of insects, characterised by the possession of a
spiral proboscis, and of four large more or less scaly wings. It includes
the well-known butterflies and moths.
LITTORAL.--Inhabiting the seashore.
LOESS.--A marly deposit of recent (Post-Tertiary) date, which occupies a
great part of the valley of the Rhine.
MALACOSTRACA.--The higher division of the Crustacea, including the ordinary
crabs, lobsters, shrimps, etc., together with the woodlice and
sand-hoppers.
MAMMALIA.--The highest class of animals, including the ordinary hairy
quadrupeds, the whales and man, and characterised by the production of
living young which are nourished after birth by milk from the teats
(MAMMAE, MAMMARY GLANDS) of the mother. A striking difference in embryonic
development has led to the division of this class into two great groups; in
one of these, when the embryo has attained a certain stage, a vascular
connection, called the PLACENTA, is formed between the embryo and the
mother; in the other this is wanting, and the young are produced in a very
incomplete state. The former, including the greater part of the class, are
called PLACENTAL MAMMALS; the latter, or APLACENTAL MAMMALS, include the
Marsupials and Monotremes (ORNITHORHYNCHUS).
MAMMIFEROUS.--Having mammae or teats (see MAMMALIA).
MANDIBLES.--in insects, the first or uppermost pair of jaws, which are
generally solid, horny, biting organs. In birds the term is applied to
both jaws with their horny coverings. In quadrupeds the mandible is
properly the lower jaw.
MARSUPIALS.--An order of Mammalia in which the young are born in a very
incomplete state of development, and carried by the mother, while sucking,
in a ventral pouch (marsupium), such as the kangaroos, opossums, etc. (see
MAMMALIA).
MAXILLAE.--in insects, the second or lower pair of jaws, which are composed
of several joints and furnished with peculiar jointed appendages called
palpi, or feelers.
MELANISM.--The opposite of albinism; an undue development of colouring
material in the skin and its appendages.
METAMORPHIC ROCKS.--Sedimentary rocks which have undergone alteration,
generally by the action of heat, subsequently to their deposition and
consolidation.
MOLLUSCA.--One of the great divisions of the animal kingdom, including
those animals which have a soft body, usually furnished with a shell, and
in which the nervous ganglia, or centres, present no definite general
arrangement. They are generally known under the denomination of
"shellfish"; the cuttle-fish, and the common snails, whelks, oysters,
mussels, and cockles, may serve as examples of them.
MONOCOTYLEDONS, or MONOCOTYLEDONOUS PLANTS.--Plants in which the seed sends
up only a single seed-leaf (or cotyledon); characterised by the absence of
consecutive layers of wood in the stem (endogenous growth), by the veins of
the leaves being generally straight, and by the parts of the flowers being
generally in multiples of three. (Examples, grasses, lilies, orchids,
palms, etc.)
MORAINES.--The accumulations of fragments of rock brought down by glaciers.
MORPHOLOGY.--The law of form or structure independent of function.
MYSIS-STAGE.--A stage in the development of certain crustaceans (prawns),
in which they closely resemble the adults of a genus (Mysis) belonging to a
slightly lower group.
NASCENT.--Commencing development.
NATATORY.--Adapted for the purpose of swimming.
NAUPLIUS-FORM.--The earliest stage in the development of many Crustacea,
especially belonging to the lower groups. In this stage the animal has a
short body, with indistinct indications of a division into segments, and
three pairs of fringed limbs. This form of the common fresh-water CYCLOPS
was described as a distinct genus under the name of NAUPLIUS.
NEURATION.--The arrangement of the veins or nervures in the wings of
insects.
NEUTERS.--Imperfectly developed females of certain social insects (such as
ants and bees), which perform all the labours of the community. Hence,
they are also called WORKERS.
NICTITATING MEMBRANE.--A semi-transparent membrane, which can be drawn
across the eye in birds and reptiles, either to moderate the effects of a
strong light or to sweep particles of dust, etc., from the surface of the
eye.
OCELLI.--The simple eyes or stemmata of insects, usually situated on the
crown of the head between the great compound eyes.
OESOPHAGUS.--The gullet.
OOLITIC.--A great series of secondary rocks, so called from the texture of
some of its members, which appear to be made up of a mass of small EGG-LIKE
calcareous bodies.
OPERCULUM.--A calcareous plate employed by many Molluscae to close the
aperture of their shell. The OPERCULAR VALVES of Cirripedes are those
which close the aperture of the shell.
ORBIT.--The bony cavity for the reception of the eye.
ORGANISM.--An organised being, whether plant or animal.
ORTHOSPERMOUS.--A term applied to those fruits of the Umbelliferae which
have the seed straight.
OSCULANT.--Forms or groups apparently intermediate between and connecting
other groups are said to be osculant.
OVA.--Eggs.
OVARIUM or OVARY (in plants).--The lower part of the pistil or female organ
of the flower, containing the ovules or incipient seeds; by growth after
the other organs of the flower have fallen, it usually becomes converted
into the fruit.
OVIGEROUS.--Egg-bearing.
OVULES (of plants).--The seeds in the earliest condition.
PACHYDERMS.--A group of Mammalia, so called from their thick skins, and
including the elephant, rhinoceros, hippopotamus, etc.
PALAEOZOIC.--The oldest system of fossiliferous rocks.
PALPI.--Jointed appendages to some of the organs of the mouth in insects
and Crustacea.
PAPILIONACEAE.--An order of plants (see LEGUMINOSAE), The flowers of these
plants are called PAPILIONACEOUS, or butterfly-like, from the fancied
resemblance of the expanded superior petals to the wings of a butterfly.
PARASITE.--An animal or plant living upon or in, and at the expense of,
another organism.
PARTHENOGENESIS.--The production of living organisms from unimpregnated
eggs or seeds.
PEDUNCULATED.--Supported upon a stem or stalk. The pedunculated oak has
its acorns borne upon a footstool.
PELORIA or PELORISM.--The appearance of regularity of structure in the
flowers of plants which normally bear irregular flowers.
PELVIS.--The bony arch to which the hind limbs of vertebrate animals are
articulated.
PETALS.--The leaves of the corolla, or second circle of organs in a flower.
They are usually of delicate texture and brightly coloured.
PHYLLODINEOUS.--Having flattened, leaf-like twigs or leafstalks instead of
true leaves.
PIGMENT.--The colouring material produced generally in the superficial
parts of animals. The cells secreting it are called PIGMENT-CELLS.
PINNATE.--Bearing leaflets on each side of a central stalk.
PISTILS.--The female organs of a flower, which occupy a position in the
centre of the other floral organs. The pistil is generally divisible into
the ovary or germen, the style and the stigma.
PLACENTALIA, PLACENTATA.--or PLACENTAL MAMMALS, See MAMMALIA.
PLANTIGRADES.--Quadrupeds which walk upon the whole sole of the foot, like
the bears.
PLASTIC.--Readily capable of change.
PLEISTOCENE PERIOD.--The latest portion of the Tertiary epoch.
PLUMULE (in plants).--The minute bud between the seed-leaves of
newly-germinated plants.
PLUTONIC ROCKS.--Rocks supposed to have been produced by igneous action in
the depths of the earth.
POLLEN.--The male element in flowering plants; usually a fine dust produced
by the anthers, which, by contact with the stigma effects the fecundation
of the seeds. This impregnation is brought about by means of tubes
(POLLEN-TUBES) which issue from the pollen-grains adhering to the stigma,
and penetrate through the tissues until they reach the ovary.
POLYANDROUS (flowers).--Flowers having many stamens.
POLYGAMOUS PLANTS.--Plants in which some flowers are unisexual and others
hermaphrodite. The unisexual (male and female) flowers, may be on the same
or on different plants.
POLYMORPHIC.--Presenting many forms.
POLYZOARY.--The common structure formed by the cells of the Polyzoa, such
as the well-known seamats.
PREHENSILE.--Capable of grasping.
PREPOTENT.--Having a superiority of power.
PRIMARIES.--The feathers forming the tip of the wing of a bird, and
inserted upon that part which represents the hand of man.
PROCESSES.--Projecting portions of bones, usually for the attachment of
muscles, ligaments, etc.
PROPOLIS.--A resinous material collected by the hivebees from the opening
buds of various trees.
PROTEAN.--Exceedingly variable.
PROTOZOA.--The lowest great division of the animal kingdom. These animals
are composed of a gelatinous material, and show scarcely any trace of
distinct organs. The Infusoria, Foraminifera, and sponges, with some other
forms, belong to this division.
PUPA (pl. PUPAE).--The second stage in the development of an insect, from
which it emerges in the perfect (winged) reproductive form. In most
insects the PUPAL STAGE is passed in perfect repose. The CHRYSALIS is the
pupal state of butterflies.
RADICLE.--The minute root of an embryo plant.
RAMUS.--One half of the lower jaw in the Mammalia. The portion which rises
to articulate with the skull is called the ASCENDING RAMUS.
RANGE.--The extent of country over which a plant or animal is naturally
spread. RANGE IN TIME expresses the distribution of a species or group
through the fossiliferous beds of the earth's crust.
RETINA.--The delicate inner coat of the eye, formed by nervous filaments
spreading from the optic nerve, and serving for the perception of the
impressions produced by light.
RETROGRESSION.--Backward development. When an animal, as it approaches
maturity, becomes less perfectly organised than might be expected from its
early stages and known relationships, it is said to undergo a RETROGRADE
DEVELOPMENT or METAMORPHOSIS.
RHIZOPODS.--A class of lowly organised animals (Protozoa), having a
gelatinous body, the surface of which can be protruded in the form of
root-like processes or filaments, which serve for locomotion and the
prehension of food. The most important order is that of the Foraminifera.
RODENTS.--The gnawing Mammalia, such as the rats, rabbits, and squirrels.
They are especially characterised by the possession of a single pair of
chisel-like cutting teeth in each jaw, between which and the grinding teeth
there is a great gap.
RUBUS.--The bramble genus.
RUDIMENTARY.--Very imperfectly developed.
RUMINANTS.--The group of quadrupeds which ruminate or chew the cud, such as
oxen, sheep, and deer. They have divided hoofs, and are destitute of front
teeth in the upper jaw.
SACRAL.--Belonging to the sacrum, or the bone composed usually of two or
more united vertebrae to which the sides of the pelvis in vertebrate
animals are attached.
SARCODE.--The gelatinous material of which the bodies of the lowest animals
(Protozoa) are composed.
SCUTELLAE.--The horny plates with which the feet of birds are generally
more or less covered, especially in front.
SEDIMENTARY FORMATIONS.--Rocks deposited as sediments from water.
SEGMENTS.--The transverse rings of which the body of an articulate animal
or annelid is composed.
SEPALS.--The leaves or segments of the calyx, or outermost envelope of an
ordinary flower. They are usually green, but sometimes brightly coloured.
SERRATURES.--Teeth like those of a saw.
SESSILE.--Not supported on a stem or footstalk.
SILURIAN SYSTEM.--A very ancient system of fossiliferous rocks belonging to
the earlier part of the Palaeozoic series.
SPECIALISATION.--The setting apart of a particular organ for the
performance of a particular function.
SPINAL CORD.--The central portion of the nervous system in the Vertebrata,
which descends from the brain through the arches of the vertebrae, and
gives off nearly all the nerves to the various organs of the body.
STAMENS.--The male organs of flowering plants, standing in a circle within
the petals. They usually consist of a filament and an anther, the anther
being the essential part in which the pollen, or fecundating dust, is
formed.
STERNUM.--The breast-bone.
STIGMA.--The apical portion of the pistil in flowering plants.
STIPULES.--Small leafy organs placed at the base of the footstalks of the
leaves in many plants.
STYLE.--The middle portion of the perfect pistil, which rises like a column
from the ovary and supports the stigma at its summit.
SUBCUTANEOUS.--Situated beneath the skin.
SUCTORIAL.--Adapted for sucking.
SUTURES (in the skull).--The lines of junction of the bones of which the
skull is composed.
TARSUS (pl. TARSI).--The jointed feet of articulate animals, such as
insects.
TELEOSTEAN FISHES.--Fishes of the kind familiar to us in the present day,
having the skeleton usually completely ossified and the scales horny.
TENTACULA or TENTACLES.--Delicate fleshy organs of prehension or touch
possessed by many of the lower animals.
TERTIARY.--The latest geological epoch, immediately preceding the
establishment of the present order of things.
TRACHEA.--The windpipe or passage for the admission of air to the lungs.
TRIDACTYLE.--Three-fingered, or composed of three movable parts attached to
a common base.
TRILOBITES.--A peculiar group of extinct crustaceans, somewhat resembling
the woodlice in external form, and, like some of them, capable of rolling
themselves up into a ball. Their remains are found only in the Palaeozoic
rocks, and most abundantly in those of Silurian age.
TRIMORPHIC.--Presenting three distinct forms.
UMBELLIFERAE.--An order of plants in which the flowers, which contain five
stamens and a pistil with two styles, are supported upon footstalks which
spring from the top of the flower stem and spread out like the wires of an
umbrella, so as to bring all the flowers in the same head (UMBEL) nearly to
the same level. (Examples, parsley and carrot.)
UNGULATA.--Hoofed quadrupeds.
UNICELLULAR.--Consisting of a single cell.
VASCULAR.--Containing blood-vessels.
VERMIFORM.--Like a worm.
VERTEBRATA or VERTEBRATE ANIMALS.--The highest division of the animal
kingdom, so called from the presence in most cases of a backbone composed
of numerous joints or VERTEBRAE, which constitutes the centre of the
skeleton and at the same time supports and protects the central parts of
the nervous system.
WHORLS.--The circles or spiral lines in which the parts of plants are
arranged upon the axis of growth.
WORKERS.--See neuters.
ZOEA-STAGE.--The earliest stage in the development of many of the higher
Crustacea, so called from the name of ZOEA applied to these young animals
when they were supposed to constitute a peculiar genus.
ZOOIDS.--In many of the lower animals (such as the Corals, Medusae, etc.)
reproduction takes place in two ways, namely, by means of eggs and by a
process of budding with or without separation from the parent of the
product of the latter, which is often very different from that of the egg.
The individuality of the species is represented by the whole of the form
produced between two sexual reproductions; and these forms, which are
apparently individual animals, have been called ZOOIDE.
INDEX.
Aberrant groups
Abyssinia, plants of
Acclimatisation
Adoxa
Affinities of extinct species
--of organic beings
Agassiz on Amblyopsis
--on groups of species suddenly appearing
--on prophetic forms
--on embryological succession
--on the Glacial period
--on embryological characters
--on the latest tertiary forms
--on parallelism of embryological development and geological succession
--Alex., on pedicellariae
Algae of New Zealand
Alligators, males, fighting
Alternate generations
Amblyopsis, blind fish
America, North, productions allied to those of Europe
--boulders and glaciers of
--South, no modern formations on west coast
Ammonites, sudden extinction of
Anagallis, sterility of
Analogy of variations
Andaman Islands inhabited by a toad
Ancylus
Animals, not domesticated from being variable
--domestic; descended from several stocks
--acclimatisation of
Animals of Australia
--with thicker fur in cold climates
--blind, in caves
--extinct, of Australia
Anomma
Antarctic islands, ancient flora of
Antechinus
Ants attending aphides
--slave-making instinct
--neuters, structure of
Apes, not having acquired intellectual powers
Aphides attended by ants
Aphis, development of
Apteryx
Arab horses
Aralo-Caspian Sea
Archeopteryx
Archiac, M. de, on the succession of species
Artichoke, Jerusalem
Ascension, plants of
Asclepias, pollen of
Asparagus
Aspicarpa
Asses, striped
--improved by selection
Ateuchus
Aucapitaine, on land-shells
Audubon, on habits of frigate-bird
--on variation in birds' nests
--on heron eating seeds
Australia, animals of
--dogs of
--extinct animals of
--European plants in
--glaciers of
Azara, on flies destroying cattle
Azores, flora of
Babington, Mr., on British plants
Baer, Von, standard of Highness
--comparison of bee and fish
--embryonic similarity of the Vertebrata
Baker, Sir S., on the giraffe
Balancement of growth
Baleen
Barberry, flowers of
Barrande, M., on Silurian colonies
--on the succession of species
--on parallelism of palaeozoic formations
--on affinities of ancient species
Barriers, importance of
Bates, Mr., on mimetic butterflies
Batrachians on islands
Bats, how structure acquired
--distribution of
Bear, catching water-insects
Beauty, how acquired
Bee, sting of
--queen, killing rivals
--Australian, extermination of
Bees, fertilizing flowers
--hive, not sucking the red clover
--hive, cell-making instinct
--Ligurian
--variation in habits
Bees, parasitic
--humble, cells of
Beetles, wingless, in Madeira
--with deficient tarsi
Bentham, Mr., on British plants
--on classification
Berkeley, Mr., on seeds in salt-water
Bermuda, birds of
Birds acquiring fear
--beauty of
--annually cross the Atlantic
--colour of, on continents
--footsteps, and remains of, in secondary rocks
--fossil, in caves of Brazil
--of Madeira, Bermuda, and Galapagos
--song of males
--transporting seeds
--waders
--wingless
Bizcacha, affinities of
Bladder for swimming, in fish
Blindness of cave animals
Blyth, Mr., on distinctness of Indian cattle
--on striped Hemionus
--on crossed geese
Borrow, Mr., on the Spanish pointer
Bory St. Vincent, on Batrachians
Bosquet, M., on fossil Chthamalus
Boulders, erratic, on the Azores
Branchiae
--of crustaceans
Braun, Prof., on the seeds of Fumariaceae
Brent, Mr., on house-tumblers
Britain, mammals of
Broca, Prof., on Natural Selection
Bronn, Prof., on duration of specific forms
--various objections by
Brown, Robert, on classification
Brown-Sequard, on inherited mutilations
Busk, Mr., on the Polyzoa
Butterflies, mimetic
Buzareingues, on sterility of varieties
Cabbage, varieties of, crossed
Calceolaria
Canary-birds, sterility of hybrids
Cape de Verde Islands, productions of
--plants of, on mountains
Cape of Good Hope, plants of
Carpenter, Dr., on foraminifera
Carthemus
Catasetum
Cats, with blue eyes, deaf
--variation in habits of
--curling tail when going to spring
Cattle destroying fir-trees
--destroyed by flies in Paraguay
--breeds of, locally extinct
--fertility of Indian and European breeds
--Indian
Cave, inhabitants of, blind
Cecidomyia
Celts, proving antiquity of man
Centres of creation
Cephalopodae, structures of eyes
--development of
Cercopithecus, tail of
Ceroxylus laceratus
Cervulus
Cetacea, teeth and hair
--development of the whalebone
Cetaceans
Ceylon, plants of
Chalk formation
Characters, divergence of
--sexual, variable
--adaptive or analogical
Charlock
Checks to increase
--mutual
Chelae of Crustaceans
Chickens, instinctive tameness of
Chironomus, its asexual reproduction
Chthamalinae
Chthamalus, cretacean species of
Circumstances favourable to selection of domestic products
--to natural selection
Cirripedes capable of crossing
--carapace aborted
--their ovigerous frena
--fossil
--larvae of
Claparede, Prof., on the hair-claspers of the Acaridae
Clarke, Rev. W.B., on old glaciers in Australia
Classification
Clift, Mr., on the succession of types
Climate, effects of, in checking increase of beings
--adaptation of, to organisms
Climbing plants
--development of
Clover visited by bees
Cobites, intestine of
Cockroach
Collections, palaeontological, poor
Colour, influenced by climate
--in relation to attacks by flies
Columba livia, parent of domestic pigeons
Colymbetes
Compensation of growth
Compositae, flowers and seeds of
--outer and inner florets of
--male flowers of
Conclusion, general
Conditions, slight changes in, favourable to fertility
Convergence of genera
Coot
Cope, Prof., on the acceleration or retardation of the period of
reproduction
Coral-islands, seeds drifted to
--reefs, indicating movements of earth
Corn-crake
Correlated variation in domestic productions
Coryanthes
Creation, single centres of
Crinum
Croll, Mr., on subaerial denudation
--on the age of our oldest formations
--on alternate Glacial periods in the North and South
Crosses, reciprocal
Crossing of domestic animals, importance in altering breeds
--advantages of
--unfavourable to selection
Cruger, Dr., on Coryanthes
Crustacea of New Zealand
Crustacean, blind
air-breathers
Crustaceans, their chelae
Cryptocerus
Ctenomys, blind
Cuckoo, instinct of
Cunningham, Mr., on the flight of the logger-headed duck
Currants, grafts of
Currents of sea, rate of
Cuvier on conditions of existence
--on fossil monkeys
Cuvier, Fred., on instinct
Cyclostoma, resisting salt water
Dana, Prof., on blind cave-animals
--on relations of crustaceans of Japan
--on crustaceans of New Zealand
Dawson, Dr., on eozoon
De Candolle, Aug. Pyr., on struggle for existence
--on umbelliferae
--on general affinities
De Candolle, Alph., on the variability of oaks
--on low plants, widely dispersed
--on widely-ranging plants being variable
--on naturalisation
--on winged seeds
--on Alpine species suddenly becoming rare
--on distribution of plants with large seeds
--on vegetation of Australia
--on fresh-water plants
--on insular plants
Degradation of rocks
Denudation, rate of
--of oldest rocks
--of granite areas
Development of ancient forms
Devonian system
Dianthus, fertility of crosses
Dimorphism in plants
Dirt on feet of birds
Dispersal, means of
--during Glacial period
Distribution, geographical
--means of
Disuse, effect of, under nature
Diversification of means for same general purpose
Division, physiological, of labour
Divergence of character
Dog, resemblance of jaw to that of the Thylacinus
Dogs, hairless, with imperfect teeth
--descended from several wild stocks
--domestic instincts of
--inherited civilisation of
--fertility of breeds together
--of crosses
--proportions of body in different breeds, when young
Domestication, variation under
Double flowers
Downing, Mr., on fruit-trees in America
Dragon-flies, intestines of
Drift-timber
Driver-ant
Drones killed by other bees
Duck, domestic, wings of, reduced
--beak of
--logger-headed
Duckweed
Dugong, affinities of
Dung-beetles with deficient tarsi
Dyticus
Earl, Mr., W., on the Malay Archipelago
Ears, drooping, in domestic animals
--rudimentary
Earth, seeds in roots of trees
--charged with seeds
Echinodermata, their pedicellariae
Eciton
Economy of organisation
Edentata, teeth and hair
--fossil species of
Edwards, Milne, on physiological division of labour
--on gradations of structure
Edwards, on embryological characters
Eggs, young birds escaping from
Egypt, productions of, not modified
Electric organs
Elephant, rate of increase
--of Glacial period
Embryology
Eozoon Canadense
Epilipsy inherited
Existence, struggle for
--condition of
Extinction, as bearing on natural selection
--of domestic varieties
Eye, structure of
--correction for aberration
Eyes, reduced, in moles
Fabre, M., on hymenoptera fighting
--on parasitic sphex
--on Sitaris
Falconer, Dr., on naturalisation of plants in India
--on elephants and mastodons
--and Cautley on mammals of sub-Himalayan beds
Falkland Islands, wolf of
Faults
Faunas, marine
Fear, instinctive, in birds
Feet of birds, young molluscs adhering to
Fertilisation variously effected
Fertility of hybrids
--from slight changes in conditions
--of crossed varieties
Fir-trees destroyed by cattle
--pollen of
Fish, flying
--teleostean, sudden appearance of
--eating seeds
--fresh-water, distribution of
Fishes, ganoid, now confined to fresh water
--ganoid, living in fresh water
--electric organs of
--of southern hemisphere
Flight, powers of, how acquired
Flint-tools, proving antiquity of man
Flower, Prof., on the larynx
--on Halitherium
--on the resemblance between the jaws of the dog and Thylacinus
--on the homology of the feet of certain marsupials
Flowers, structure of
--in relation to crossing
--of composite and umbelliferae
--beauty of
--double
Flysch formation, destitute of organic remains
Forbes, Mr. D., on glacial action in the Andes
Forbes, E., on colours of shells
--on abrupt range of shells in depth
--on poorness of palaeontological collections
--on continuous succession of genera
--on continental extensions
--on distribution during Glacial period
--on parallelism in time and space
Forests, changes in, in America
Formation, Devonian
--Cambrian
--intermittent
--thickness of, in Britain
Formica
--rufescens
--sanguinea
--flava, neuter of
Forms, lowly organised, long enduring
Frena, ovigerous, of cirripedes
Fresh-water productions, dispersal of
Fries on species in large genera being closely allied to other species
Frigate-bird
Frogs on islands
Fruit-trees, gradual improvement of
--in United States
--varieties of, acclimatised in United States
Fuci, crossed
Fur, thicker in cold climates
Furze
Galapagos Archipelago, birds of
--productions of
Galaxias, its wide range
Galeopithecus
Game, increase of, checked by vermin
Gartner on sterility of hybrids
--on reciprocal crosses
--on crossed maize and verbascum
--on comparison of hybrids and mongrels
Gaudry, Prof., on intermediate genera of fossil mammals in Attica
Geese, fertility when crossed
--upland
Geikie, Mr., on subaerial denudation
Genealogy, important in classification
Generations, alternate
Geoffroy St. Hilaire, on balancement
--on homologous organs
Geoffroy St. Hilaire, Isidore, on variability of repeated parts
--on correlation, in monstrosities
--on correlation
--on variable parts being often monstrous
Geographical distribution
Geography, ancient
Geology, future progress of
--imperfection of the record
Gervais, Prof., on Typotherium
Giraffe, tail of
--structure of
Glacial period
--affecting the North and South
Glands, mammary
Gmelin, on distribution
Godwin-Austin, Mr., on the Malay Archipelago
Goethe, on compensation of growth
Gomphia
Gooseberry, grafts of
Gould, Dr. Aug. A., on land-shells
Gould, Mr., on colours of birds
--on instincts of cuckoo
--on distribution of genera of birds
Gourds, crossed
Graba, on the Uria lacrymans
Grafting, capacity of
Granite, areas of denuded
Grasses, varieties of
Gray, Dr. Asa, on the variability of oaks
--on man not causing variability
--on sexes of the holly
--on trees of the United States
--on naturalised plants in the United States
--on aestivation
--on rarity of intermediate varieties
--on Alpine plants
Gray, Dr. J.E., on striped mule
Grebe
Grimm, on asexual reproduction
Groups, aberrant
Grouse, colours of
--red, a doubtful species
Growth, compensation of
Gunther, Dr., on flat-fish
--on prehensile tails
--on the fishes of Panama
--on the range of fresh-water fishes
--on the limbs of Lepidosiren
Haast, Dr., on glaciers of New Zealand
Habit, effect of, under domestication
--effect of, under nature
--diversified, of same species
Hackel, Prof., on classification and the lines of descent
Hair and teeth, correlated
Halitherium
Harcourt, Mr. E.V., on the birds of Madeira
Hartung, M., on boulders in the Azores
Hazel-nuts
Hearne, on habits of bears
Heath, changes in vegetation
Hector, Dr., on glaciers of New Zealand
Heer, Oswald, on ancient cultivated plants
--on plants of Madeira
Helianthemum
Helix pomatia, resisting salt water
Helmholtz, M., on the imperfection of the human eye
Helosciadium
Hemionus, striped
Hensen, Dr., on the eyes of Cephalopods
Herbert, W., on struggle for existence
--on sterility of hybrids
Hermaphrodites crossing
Heron eating seed
Heron, Sir R., on peacocks
Heusinger, on white animals poisoned by certain plants
Hewitt, Mr., on sterility of first crosses
Hildebrand, Prof., on the self-sterility of Corydalis
Hilgendorf, on intermediate varieties
Himalaya, glaciers of
--plants of
Hippeastrum
Hippocampus
Hofmeister, Prof., on the movements of plants
Holly-trees, sexes of
Hooker, Dr., on trees of New Zealand
--on acclimatisation of Himalayan trees
--on flowers of umbelliferae
--on the position of ovules
--on glaciers of Himalaya
--on algae of New Zealand
--on vegetation at the base of the Himalaya
--on plants of Tierra del Fuego
--on Australian plants
--on relations of flora of America
--on flora of the Antarctic lands
--on the plants of the Galapagos
--on glaciers of the Lebanon
--on man not causing variability
--on plants of mountains of Fernando Po
Hooks on palms
--on seeds, on islands
Hopkins, Mr., on denudation
Hornbill, remarkable instinct of
Horns, rudimentary
Horse, fossil in La Plata
--proportions of, when young
Horses destroyed by flies in Paraguay
--striped
Horticulturists, selection applied by
Huber on cells of bees
Huber, P., on reason blended with instinct
--on habitual nature of instincts
--on slave-making ants
--on Melipona domestica
Hudson, Mr., on the Ground-woodpecker of La Plata
--on the Molothrus
Humble-bees, cells of
Hunter, J., on secondary sexual characters
Hutton, Captain, on crossed geese
Huxley, Prof., on structure of hermaphrodites
--on the affinities of the Sirenia
--on forms connecting birds and reptiles
--on homologous organs
--on the development of aphis
Hybrids and mongrels compared
Hybridism
Hydra, structure of
Hymenoptera, fighting
Hymenopterous insect, diving
Hyoseris
Ibla
Icebergs transporting seeds
Increase, rate of
Individuals, numbers favourable to selection
--many, whether simultaneously created
Inheritance, laws of
--at corresponding ages
Insects, colour of, fitted for their stations
--sea-side, colours of
--blind, in caves
--luminous
--their resemblance to certain objects
--neuter
Instinct, not varying simultaneously with structure
Instincts, domestic
Intercrossing, advantages of
Islands, oceanic
Isolation favourable to selection
Japan, productions of
Java, plants of
Jones, Mr. J.M., on the birds of Bermuda
Jordain, M., on the eye-spots of star fishes
Jukes, Prof., on subaerial denudation
Jussieu on classification
Kentucky, caves of
Kerguelen-land, flora of
Kidney-bean, acclimatisation of
Kidneys of birds
Kirby, on tarsi deficient in beetles
Knight, Andrew, on cause of variation
Kolreuter, on intercrossing
--on the barberry
--on sterility of hybrids
--on reciprocal crosses
--on crossed varieties of nicotiana
--on crossing male and hermaphrodite flowers
Lamarck, on adaptive characters
Lancelet, eyes of
Landois, on the development of the wings of insects
Land-shells, distribution of
--of Madeira, naturalised
--resisting salt water
Languages, classification of
Lankester, Mr. E. Ray, on longevity
--on homologies
Lapse, great, of time
Larvae
Laurel, nectar secreted by the leaves
Laurentian formation
Laws of variation
Leech, varieties of
Leguminosae, nectar secreted by glands
Leibnitz, attack on Newton
Lepidosiren, limbs in a nascent condition
Lewes, Mr. G.H., on species not having changed in Egypt
--on the Salamandra atra
--on many forms of life having been at first evolved
Life, struggle for
Lingula, Silurian
Linnaeus, aphorism of
Lion, mane of
--young of, striped
Lobelia fulgens
Lobelia, sterility of crosses
Lockwood, Mr., on the ova of the Hippocampus
Locusts transporting seeds
Logan, Sir W., on Laurentian formation
Lowe, Rev. R.T., on locusts visiting Madeira
Lowness, of structure connected with variability
--related to wide distribution
Lubbock, Sir J., on the nerves of coccus
--on secondary sexual characters
--on a diving hymenopterous insect
--on affinities
--on metamorphoses
Lucas, Dr. P., on inheritance
--on resemblance of child to parent
Lund and Clausen, on fossils of Brazil
Lyell, Sir C., on the struggle for existence
--on modern changes of the earth
--on terrestrial animals not having been developed on islands
--on a carboniferous land-shell
--on strata beneath Silurian system
--on the imperfection of the geological record
--on the appearance of species
--on Barrande's colonies
--on tertiary formations of Europe and North America
--on parallelism of tertiary formations
--on transport of seeds by icebergs
--on great alternations of climate
--on the distribution of fresh-water shells
--on land-shells of Madeira
Lyell and Dawson, on fossilized trees in Nova Scotia
Lythrum salicaria, trimorphic
Macleay, on analogical characters
Macrauchenia
McDonnell, Dr., on electric organs
Madeira, plants of
--beetles of, wingless
--fossil land-shells of
--birds of
Magpie tame in Norway
Males, fighting
Maize, crossed
Malay Archipelago, compared with Europe
--mammals of
Malm, on flat-fish
Malpighiaceae, small imperfect flowers of
Mammae, their development
--rudimentary
Mammals, fossil, in secondary formation
--insular
Man, origin of
Manatee, rudimentary nails of
Marsupials, fossil species of
Marsupials of Australia, structure of their feet
Martens, M., experiment on seeds
Martin, Mr. W.C., on striped mules
Masters, Dr., on Saponaria
Matteucci, on the electric organs of rays
Matthiola, reciprocal crosses of
Maurandia
Means of dispersal
Melipona domestica
Merrill, Dr., on the American cuckoo
Metamorphism of oldest rocks
Mice destroying bees
--acclimatisation of
--tails of
Miller, Prof., on the cells of bees
Mirabilis, crosses of
Missel-thrush
Mistletoe, complex relations of
Mivart, Mr., on the relation of hair and teeth
--on the eyes of cephalopods
--various objections to Natural Selection
--on abrupt modifications
--on the resemblance of the mouse and antechinus
Mocking-thrush of the Galapagos
Modification of species, not abrupt
Moles, blind
Molothrus, habits of
Mongrels, fertility and sterility of
--and hybrids compared
Monkeys, fossil
Monachanthus
Mons, Van, on the origin of fruit-trees
Monstrosities
Moquin-Tandon, on sea-side plants
Morphology
Morren, on the leaves of Oxalis
Moths, hybrid
Mozart, musical powers of
Mud, seeds in
Mules, striped
Muller, Adolph, on the instincts of the cuckoo
Muller, Dr. Ferdinand, on Alpine Australian plants
Muller, Fritz, on dimorphic crustaceans
--on the lancelet
--on air-breathing crustaceans
--on the self-sterility of orchids
--on embryology in relation to classification
--on the metamorphoses of crustaceans
--on terrestrial and fresh-water organisms not undergoing any metamorphosis
--on climbing plants
Multiplication of species not indefinite
Murchison, Sir, R., on the formations of Russia
--on azoic formations
--on extinction
Murie, Dr., on the modification of the skull in old age
Murray, Mr. A., on cave-insects
Mustela vison
Myanthus
Myrmecocystus
Myrmica, eyes of
Nageli, on morphological characters
Nails, rudimentary
Nathusius, Von, on pigs
Natural history, future progress of
--selection
--system
Naturalisation of forms distinct from the indigenous species
--in New Zealand
Naudin, on analagous variations in gourds
--on hybrid gourds
--on reversion
Nautilus, Silurian
Nectar of plants
Nectaries, how formed
Nelumbium luteum
Nests, variation in
Neuter insects
New Zealand, productions of, not perfect
--naturalised products of
--fossil birds of
--glaciers of
--crustaceans of
--algae of
--number of plants of
--flora of
Newman, Col., on humble-bees
Newton, Prof., on earth attached to a partridge's foot
Newton, Sir I., attacked for irreligion
Nicotiana, crossed varieties of
--certain species very sterile
Nitsche, Dr., on the Polyzoa
Noble, Mr., on fertility of Rhododendron
Nodules, phosphatic, in azoic rocks
Oak, varieties of
Onites apelles
Orchids, fertilisation of
--the development of their flowers
--forms of
Orchis, pollen of
Organisation, tendency to advance
Organs of extreme perfection
--electric, of fishes
--of little importance
--homologous
--rudiments of, and nascent
Ornithorhynchus, mammae of
Ostrich not capable of flight
--habit of laying eggs together
--American, two species of
Otter, habits of, how acquired
Ouzel, water
Owen, Prof., on birds not flying
--on vegetative repetition
--on variability of unusually developed parts
--on the eyes of fishes
--on the swim-bladder of fishes
--on fossil horse of La Plata
--on generalised form
--on relation of ruminants and pachyderms
--on fossil birds of New Zealand
--on succession of types
--on affinities of the dugong
--on homologous organs
--on the metamorphosis of cephalopods
Pacific Ocean, faunas of
Pacini, on electric organs
Paley, on no organ formed to give pain
Pallas, on the fertility of the domesticated descendants of wild stocks
Palm with hooks
Papaver bracteatum
Paraguay, cattle destroyed by flies
Parasites
Partridge, with ball of dirt attached to foot
Parts greatly developed, variable
Parus major
Passiflora
Peaches in United States
Pear, grafts of
Pedicellariae
Pelargonium, flowers of
--sterility of
Peloria
Pelvis of women
Period, glacial
Petrels, habits of
Phasianus, fertility of hybrids
Pheasant, young, wild
Pictet, Prof., on groups of species suddenly appearing
--on rate of organic change
--on continuous succession of genera
--on close alliance of fossils in consecutive formations
--on change in latest tertiary forms
--on early transitional links
Pierce, Mr., on varieties of wolves
Pigeons with feathered feet and skin between toes
--breeds described, and origin of
--breeds of, how produced
--tumbler, not being able to get out of egg
--reverting to blue colour
--instinct of tumbling
--young of
Pigs, black, not affected by the paint-root
--modified by want of exercise
Pistil, rudimentary
Plants, poisonous, not affecting certain coloured animals
--selection, applied to
--gradual improvement of
--not improved in barbarous countries
--dimorphic
--destroyed by insects
--in midst of range, have to struggle with other plants
--nectar of
--fleshy, on sea-shores
--climbing
--fresh-water, distribution of
--low in scale, widely distributed
Pleuronectidae, their structure
Plumage, laws of change in sexes of birds
Plums in the United States
Pointer dog, origin of
--habits of
Poison not affecting certain coloured animals
Poison, similar effect of, on animals and plants
Pollen of fir-trees
--transported by various means
Pollinia, their development
Polyzoa, their avicularia
Poole, Col., on striped hemionus
Potemogeton
Pouchet, on the colours of flat-fish
Prestwich, Mr., on English and French eocene formations
Proctotrupes
Proteolepas
Proteus
Psychology, future progress of
Pyrgoma, found in the chalk
Quagga, striped
Quatrefages, M., on hybrid moths
Quercus, variability of
Quince, grafts of
Rabbits, disposition of young
Races, domestic, characters of
Race-horses, Arab
--English
Radcliffe, Dr., the electrical organs of the torpedo
Ramond, on plants of Pyrenees
Ramsay, Prof., on subaerial denudation
--on thickness of the British formations
--on faults
Ramsay, Mr., on instincts of cuckoo
Ratio of increase
Rats, supplanting each other
--acclimatisation of
--blind, in cave
Rattle-snake
Reason and instinct
Recapitulation, general
Reciprocity of crosses
Record, geological, imperfect
Rengger, on flies destroying cattle
Reproduction, rate of
Resemblance, protective, of insects
--to parents in mongrels and hybrids
Reversion, law of inheritance
--in pigeons, to blue colour
Rhododendron, sterility of
Richard, Prof., on Aspicarpa
Richardson, Sir J., on structure of squirrels
--on fishes of the southern hemisphere
Robinia, grafts of
Rodents, blind
Rogers, Prof., Map of N. America
Rudimentary organs
Rudiments important for classification
Rutimeyer, on Indian cattle
Sageret, on grafts
Salamandra atra
Saliva used in nests
Salmons, males fighting, and hooked jaws of
Salt-water, how far injurious to seeds
--not destructive to land-shells
Salter, Mr., on early death of hybrid embryos
Salvin, Mr., on the beaks of ducks
Saurophagus sulphuratus
Schacht, Prof., on Phyllotaxy
Schiodte, on blind insects
--on flat-fish
Schlegel, on snakes
Schobl, Dr., on the ears of mice
Scott, Mr. J., on the self-sterility of orchids
--on the crossing of varieties of verbascum
Sea-water, how far injurious to seeds
--not destructive to land-shells
Sebright, Sir J., on crossed animals
Sedgwick, Prof., on groups of species suddenly appearing
Seedlings destroyed by insects
Seeds, nutriment in
--winged
--means of dissemination
--power of resisting salt-water
--in crops and intestines of birds
--eaten by fish
--in mud
--hooked, on islands
Selection of domestic products
--principle not of recent origin
--unconscious
--natural
--sexual
--objections to term
--natural, has not induced sterility
Sexes, relations of
Sexual characters variable
--selection
Sheep, Merino, their selection
--two sub-breeds, unintentionally produced
--mountain, varieties of
Shells, colours of, littoral
--hinges of
--seldom embedded
Shells, fresh-water, long retain the same forms
--fresh-water, dispersal of
--of Madeira
--land, distribution of
--land, resisting salt water
Shrew-mouse
Silene, infertility of crosses
Silliman, Prof., on blind rat
Sirenia, their affinities
Sitaris, metamorphosis of
Skulls of young mammals
Slave-making instinct
Smith, Col. Hamilton, on striped horses
Smith, Dr., on the Polyzoa
Smith, Mr. Fred., on slave-making ants
--on neuter ants
Snake with tooth for cutting through egg-shell
Somerville, Lord, on selection of sheep
Sorbus, grafts of
Sorex
Spaniel, King Charles' breed
Specialisation of organs
Species, polymorphic
--dominant
--common, variable
--in large genera variable
--groups of, suddenly appearing
--beneath Silurian formations
--successively appearing
--changing simultaneously throughout the world
Spencer, Lord, on increase in size of cattle
Spencer, Mr. Herbert, on the first steps in differentiation
--on the tendency to an equilibrium in all forces
Sphex, parasitic
Spiders, development of
Sports in plants
Sprengel, C.C., on crossing
--on ray-florets
Squalodon
Squirrels, gradations in structure
Staffordshire, heath, changes in
Stag-beetles, fighting
Star fishes, eyes of
--their pedicellariae
Sterility from changed conditions of life
--of hybrids
--laws of
--causes of
--from unfavourable conditions
--not induced through natural selection
St. Helena, productions of
St. Hilaire, Aug., on variability of certain plants
--on classification
St. John, Mr., on habits of cats
Sting of bee
Stocks, aboriginal, of domestic animals
Strata, thickness of, in Britain
Stripes on horses
Structure, degrees of utility of
Struggle for existence
Succession, geological
--of types in same areas
Swallow, one species supplanting another
Swaysland, Mr., on earth adhering to the feet of migratory birds
Swifts, nests of
Swim-bladder
Switzerland, lake inhabitants of
System, natural
Tail of giraffe
--of aquatic animals
--prehensile
--rudimentary
Tanais, dimorphic
Tarsi deficient
Tausch, Dr., on umbelliferae
Teeth and hair correlated
--rudimentary, in embryonic calf
Tegetmeier, Mr., on cells of bees
Temminck, on distribution aiding classification
Tendrils, their development
Thompson, Sir W., on the age of the habitable world
--on the consolidation of the crust of the earth
Thouin, on grafts
Thrush, aquatic species of
--mocking, of the Galapagos
--young of, spotted
--nest of
Thuret, M., on crossed fuci
Thwaites, Mr., on acclimatisation
Thylacinus
Tierra del Fuego, dogs of
--plants of
Timber-drift
Time, lapse of
--by itself not causing modification
Titmouse
Toads on islands
Tobacco, crossed varieties of
Tomes, Mr., on the distribution of bats
Transitions in varieties rare
Traquair, Dr., on flat-fish
Trautschold, on intermediate varieties
Trees on islands belong to peculiar orders
--with separated sexes
Trifolium pratense
--incarnatum
Trigonia
Trilobites
--sudden extinction of
Trimen, Mr., on imitating-insects
Trimorphism in plants
Troglodytes
Tuco-tuco, blind
Tumbler pigeons, habits of, hereditary
--young of
Turkey-cock, tuft of hair on breast
Turkey, naked skin on head
--young of, instinctively wild
Turnip and cabbage, analogous variations of
Type, unity of
Types, succession of, in same areas
Typotherium
Udders enlarged by use
--rudimentary
Ulex, young leaves of
Umbelliferae, flowers and seeds of
--outer and inner florets of
Unity of type
Uria lacrymans
Use, effects of
--under domestication
--in a state of nature
Utility, how far important in the construction of each part
Valenciennes, on fresh-water fish
Variability of mongrels and hybrids
Variation, under domestication
--caused by reproductive system being affected by conditions of life
--under nature
--laws of
--correlated
Variations appear at corresponding ages
--analogous in distinct species
Varieties, natural
--struggle between
--domestic, extinction of
--transitional, rarity of
Varieties, when crossed
--fertile
--sterile
--classification of
Verbascum, sterility of
--varieties of, crossed
Verlot, M., on double stocks
Verneuil, M. de, on the succession of species
Vibracula of the Polyzoa
Viola, small imperfect flowers of
--tricolor
Virchow, on the structure of the crystalline lens
Virginia, pigs of
Volcanic islands, denudation of
Vulture, naked skin on head
Wading-birds
Wagner, Dr., on Cecidomyia
Wagner, Moritz, on the importance of isolation
Wallace, Mr., on origin of species
--on the limit of variation under domestication
--on dimorphic lepidoptera
--on races in the Malay Archipelago
--on the improvement of the eye
--on the walking-stick insect
--on laws of geographical distribution
--on the Malay Archipelago
--on mimetic animals
Walsh, Mr. B.D., on phytophagic forms
--on equal variability
Water, fresh, productions of
Water-hen
Waterhouse, Mr., on Australian marsupials
--on greatly developed parts being variable
--on the cells of bees
--on general affinities
Water-ouzel
Watson, Mr. H.C., on range of varieties of British plants
--on acclimatisation
--on flora of Azores
--on rarity of intermediate varieties
--on Alpine plants
--on convergence
--on the indefinite multiplication of species
Weale, Mr., on locusts transporting seeds
Web of feet in water-birds
Weismann, Prof., on the causes of variability
--on rudimentary organs
West Indian islands, mammals of
Westwood, on species in large genera being closely allied to others
--on the tarsi of Engidae
--on the antennae of hymenopterous insects
Whales
Wheat, varieties of
White Mountains, flora of
Whittaker, Mr., on lines of escarpment
Wichura, Max, on hybrids
Wings, reduction of size
--of insects homologous with branchiae
--rudimentary, in insects
Wolf crossed with dog
--of Falkland Isles
Wollaston, Mr., on varieties of insects
--on fossil varieties of shells in Madeira
Wollaston, Mr., on colours of insects on sea-shore
--on wingless beetles
--on rarity of intermediate varieties
--on insular insects
--on land-shells of Madeira naturalised
Wolves, varieties of
Woodcock with earth attached to leg
Woodpecker, habits of
--green colour of
Woodward, Mr., on the duration of specific forms
--on Pyrgoma
--on the continuous succession of genera
--on the succession of types
World, species changing simultaneously throughout
Wright, Mr. Chauncey, on the giraffe
--on abrupt modifications
Wrens, nest of
Wyman, Prof., on correlation of colour and effects of poison
--on the cells of the bee
Youatt, Mr., on selection
--on sub-breeds of sheep
--on rudimentary horns in young cattle
Zanthoxylon
Zebra, stripes on
Zeuglodons
End of Project Gutenberg Etext Origin of Species, 6th Edition, by Darwin
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