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The Origin of Species by Means of Natural Selection

by Charles Darwin · English · Gutenberg #2009 · 1,276,524 chars · public domain (US)

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Title: The Origin of Species by Means of Natural Selection

Author: Charles Darwin


        
Release date: December 1, 1999 [eBook #2009]
                Most recently updated: October 7, 2026

Language: English

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On 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.




“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,[1] 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.

 [1] Aristotle, in his “Physicæ 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.


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 Vertébres”. 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.[2]

 [2] I have taken the date of the first publication of Lamarck from
 Isidore Geoffroy Saint-Hilaire’s (“Hist. Nat. Générale”, 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 problème à réserver entièrement à l’avenir, supposé même 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 “Amaryllidaceæ” (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 palæontologists 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,” &c.(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 Résumé appeared in the “Revue et Mag. de Zoolog.”, Jan.,
1851), briefly gives his reason for believing that specific characters
“sont fixés, pour chaque espèce, tant qu’elle se perpétue au milieu des
mêmes circonstances: ils se modifient, si les circonstances ambiantes
viennent à changer. En résumé, _l’observation_ des animaux sauvages
démontre deja la variabilité _limitée_ des espèces. Les _expériences_
sur les animaux sauvages devenus domestiques, et sur les animaux
domestiques redevenus sauvages, la démontrent plus clairment encore.
Ces mêmes expériences prouvent, de plus, que les différences produites
peuvent être de _valeur générique_.” In his “Hist. Nat. Générale” (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 Muséum”, tom. i,
p. 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 mystérieuse, indéterminée;
fatalité pour les uns; pour les autres volonté providentielle, dont
l’action incessante sur les êtres vivantes détermine, à toutes les
époques de l’existence du monde, la forme, le volume, et la durée de
chacun d’eux, en raison de sa destinée dans l’ordre de choses dont il
fait partie. C’est cette puissance qui harmonise chaque membre à
l’ensemble, en l’appropriant à la fonction qu’il doit remplir dans
l’organisme général de la nature, fonction qui est pour lui sa raison
d’être.”[3]

 [3] From references in Bronn’s “Untersuchungen über die
 Entwickelungs-Gesetze”, it appears that the celebrated botanist and
 palæontologist 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’Espèce”, 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”,
&c.), 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
Géograph.” Bot. tom. i, page 250), “On voit que nos recherches sur la
fixité ou la variation de l’espéce, nous conduisent directement aux
idées émises par deux hommes justement célèbres, 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


AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES
INTRODUCTION.

CHAPTER I. VARIATION UNDER DOMESTICATION
CHAPTER II. VARIATION UNDER NATURE
CHAPTER III. STRUGGLE FOR EXISTENCE
CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST
CHAPTER V. LAWS OF VARIATION
CHAPTER VI. DIFFICULTIES OF THE THEORY
CHAPTER VII. MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION
CHAPTER VIII. INSTINCT
CHAPTER IX. HYBRIDISM
CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD
CHAPTER XI. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS
CHAPTER XII. GEOGRAPHICAL DISTRIBUTION
CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION—continued
CHAPTER XIV. MUTUAL AFFINITIES OF ORGANIC BEINGS
CHAPTER XV. RECAPITULATION AND CONCLUSION

GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT VOLUME.
INDEX.




DETAILED 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
palæontological 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, &c.,
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, &c. 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 Kölreuter 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,
&c., 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, &c.;
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, &c., 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 Canidæ 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 Rütimeyer. 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, &c., but that each of these kingdoms possesses several
peculiar breeds of cattle, sheep, &c., 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, &c.—so unlike all wild
Canidæ—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, &c., 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 œsophagus. 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 vertebræ
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 œsophagus; 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 scutellæ 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 Columbidæ
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 Columbidæ, 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 Ægyptian
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 encyclopædia. 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, &c., 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 œsophagus—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, &c., 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, &c., 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 larvæ 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 Müller 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 antennæ 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, &c., 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 palæontology, 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 towards 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. Linnæus 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, &c., 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

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