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THE ORIGIN OF SPECIES BY MEANS OF NATURAL SELECTION;
 
or, the PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE.
 
By Charles Darwin, M.A., F.R.S.,
 
Author of "The Descent of Man," etc., etc.
 
 
Sixth London Edition, with all Additions and Corrections.
 
The 6th Edition is often considered the definitive edition.
 
Also see Project Gutenberg Etext #1228 for the First Edition.
 
 
 
"But with regard to the material world, we can at least go so far as
this--we can perceive that events are brought about not by insulated
interpositions of Divine power, exerted in each particular case, but by
the establishment of general laws."--Whewell: "Bridgewater Treatise".
 
"The only distinct meaning of the word 'natural' is STATED, FIXED or
SETTLED; since what is natural as much requires and presupposes an
intelligent agent to render it so, i.e., to effect it continually or at
stated times, as what is supernatural or miraculous does to effect it
for once."--Butler: "Analogy of Revealed Religion".
 
"To conclude, therefore, let no man out of a weak conceit of sobriety,
or an ill-applied moderation, think or maintain, that a man can search
too far or be too well studied in the book of God's word, or in the book
of God's works; divinity or philosophy; but rather let men endeavour
an endless progress or proficience in both."--Bacon: "Advancement of
Learning".
 
 
 
 
AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION ON THE ORIGIN OF
SPECIES,
 
PREVIOUSLY TO THE PUBLICATION OF THE FIRST EDITION OF THIS WORK.
 
I will here give a brief sketch of the progress of opinion on the Origin
of Species. Until recently the great majority of naturalists believed
that species were immutable productions, and had been separately
created. This view has been ably maintained by many authors. Some few
naturalists, on the other hand, have believed that species undergo
modification, and that the existing forms of life are the descendants
by true generation of pre existing forms. Passing over allusions to
the subject in the classical writers (Aristotle, in his "Physicae
Auscultationes" (lib.2, cap.8, s.2), after remarking that rain does not
fall in order to make the corn grow, any more than it falls to spoil the
farmer's corn when threshed out of doors, applies the same argument
to organisation; and adds (as translated by Mr. Clair Grece, who first
pointed out the passage to me), "So what hinders the different parts (of
the body) from having this merely accidental relation in nature? as the
teeth, for example, grow by necessity, the front ones sharp, adapted
for dividing, and the grinders flat, and serviceable for masticating
the food; since they were not made for the sake of this, but it was the
result of accident. And in like manner as to other parts in which there
appears to exist an adaptation to an end. Wheresoever, therefore, all
things together (that is all the parts of one whole) happened like as if
they were made for the sake of something, these were preserved,
having been appropriately constituted by an internal spontaneity; and
whatsoever things were not thus constituted, perished and still perish."
We here see the principle of natural selection shadowed forth, but
how little Aristotle fully comprehended the principle, is shown by his
remarks on the formation of the teeth.), the first author who in modern
times has treated it in a scientific spirit was Buffon. But as his
opinions fluctuated greatly at different periods, and as he does not
enter on the causes or means of the transformation of species, I need
not here enter on details.
 
Lamarck was the first man whose conclusions on the subject excited much
attention. This justly celebrated naturalist first published his views
in 1801; he much enlarged them in 1809 in his "Philosophie Zoologique",
and subsequently, 1815, in the Introduction to his "Hist. Nat. des
Animaux sans Vertebres". In these works he up holds the doctrine that
all species, including man, are descended from other species. He first
did the eminent service of arousing attention to the probability of
all change in the organic, as well as in the inorganic world, being the
result of law, and not of miraculous interposition. Lamarck seems
to have been chiefly led to his conclusion on the gradual change of
species, by the difficulty of distinguishing species and varieties,
by the almost perfect gradation of forms in certain groups, and by
the analogy of domestic productions. With respect to the means of
modification, he attributed something to the direct action of the
physical conditions of life, something to the crossing of already
existing forms, and much to use and disuse, that is, to the effects of
habit. To this latter agency he seems to attribute all the beautiful
adaptations in nature; such as the long neck of the giraffe for
browsing on the branches of trees. But he likewise believed in a law
of progressive development, and as all the forms of life thus tend to
progress, in order to account for the existence at the present day of
simple productions, he maintains that such forms are now spontaneously
generated. (I have taken the date of the first publication of Lamarck
from Isidore Geoffroy Saint-Hilaire's ("Hist. Nat. Generale", tom. ii.
page 405, 1859) excellent history of opinion on this subject. In
this work a full account is given of Buffon's conclusions on the same
subject. It is curious how largely my grandfather, Dr. Erasmus Darwin,
anticipated the views and erroneous grounds of opinion of Lamarck in
his "Zoonomia" (vol. i. pages 500-510), published in 1794. According to
Isid. Geoffroy there is no doubt that Goethe was an extreme partisan of
similar views, as shown in the introduction to a work written in 1794
and 1795, but not published till long afterward; he has pointedly
remarked ("Goethe als Naturforscher", von Dr. Karl Meding, s. 34) that
the future question for naturalists will be how, for instance, cattle
got their horns and not for what they are used. It is rather a singular
instance of the manner in which similar views arise at about the same
time, that Goethe in Germany, Dr. Darwin in England, and Geoffroy
Saint-Hilaire (as we shall immediately see) in France, came to the same
conclusion on the origin of species, in the years 1794-5.)
 
Geoffroy Saint-Hilaire, as is stated in his "Life", written by his
son, suspected, as early as 1795, that what we call species are various
degenerations of the same type. It was not until 1828 that he published
his conviction that the same forms have not been perpetuated since
the origin of all things. Geoffroy seems to have relied chiefly on the
conditions of life, or the "monde ambiant" as the cause of change. He
was cautious in drawing conclusions, and did not believe that existing
species are now undergoing modification; and, as his son adds, "C'est
donc un probleme a reserver entierement a l'avenir, suppose meme que
l'avenir doive avoir prise sur lui."
 
In 1813 Dr. W.C. Wells read before the Royal Society "An Account of a
White Female, part of whose skin resembles that of a Negro"; but his
paper was not published until his famous "Two Essays upon Dew and Single
Vision" appeared in 1818. In this paper he distinctly recognises the
principle of natural selection, and this is the first recognition which
has been indicated; but he applies it only to the races of man, and to
certain characters alone. After remarking that negroes and mulattoes
enjoy an immunity from certain tropical diseases, he observes, firstly,
that all animals tend to vary in some degree, and, secondly, that
agriculturists improve their domesticated animals by selection; and
then, he adds, but what is done in this latter case "by art, seems to
be done with equal efficacy, though more slowly, by nature, in the
formation of varieties of mankind, fitted for the country which they
inhabit. Of the accidental varieties of man, which would occur among
the first few and scattered inhabitants of the middle regions of Africa,
some one would be better fitted than others to bear the diseases of the
country. This race would consequently multiply, while the others would
decrease; not only from their in ability to sustain the attacks of
disease, but from their incapacity of contending with their more
vigorous neighbours. The colour of this vigorous race I take for
granted, from what has been already said, would be dark. But the same
disposition to form varieties still existing, a darker and a darker race
would in the course of time occur: and as the darkest would be the best
fitted for the climate, this would at length become the most prevalent,
if not the only race, in the particular country in which it had
originated." He then extends these same views to the white inhabitants
of colder climates. I am indebted to Mr. Rowley, of the United States,
for having called my attention, through Mr. Brace, to the above passage
of Dr. Wells' work.
 
The Hon. and Rev. W. Herbert, afterward Dean of Manchester, in the
fourth volume of the "Horticultural Transactions", 1822, and in his
work on the "Amaryllidaceae" (1837, pages 19, 339), declares that
"horticultural experiments have established, beyond the possibility of
refutation, that botanical species are only a higher and more permanent
class of varieties." He extends the same view to animals. The dean
believes that single species of each genus were created in an originally
highly plastic condition, and that these have produced, chiefly by
inter-crossing, but likewise by variation, all our existing species.
 
In 1826 Professor Grant, in the concluding paragraph in his well-known
paper ("Edinburgh Philosophical Journal", vol. XIV, page 283) on the
Spongilla, clearly declares his belief that species are descended
from other species, and that they become improved in the course of
modification. This same view was given in his Fifty-fifth Lecture,
published in the "Lancet" in 1834.
 
In 1831 Mr. Patrick Matthew published his work on "Naval Timber and
Arboriculture", in which he gives precisely the same view on the origin
of species as that (presently to be alluded to) propounded by Mr.
Wallace and myself in the "Linnean Journal", and as that enlarged in
the present volume. Unfortunately the view was given by Mr. Matthew very
briefly in scattered passages in an appendix to a work on a different
subject, so that it remained unnoticed until Mr. Matthew himself drew
attention to it in the "Gardeners' Chronicle", on April 7, 1860. The
differences of Mr. Matthew's views from mine are not of much importance:
he seems to consider that the world was nearly depopulated at successive
periods, and then restocked; and he gives as an alternative, that new
forms may be generated "without the presence of any mold or germ of
former aggregates." I am not sure that I understand some passages; but
it seems that he attributes much influence to the direct action of
the conditions of life. He clearly saw, however, the full force of the
principle of natural selection.
 
The celebrated geologist and naturalist, Von Buch, in his excellent
"Description Physique des Isles Canaries" (1836, page 147), clearly
expresses his belief that varieties slowly become changed into permanent
species, which are no longer capable of intercrossing.
 
Rafinesque, in his "New Flora of North America", published in 1836,
wrote (page 6) as follows: "All species might have been varieties once,
and many varieties are gradually becoming species by assuming constant
and peculiar characters;" but further on (page 18) he adds, "except the
original types or ancestors of the genus."
 
In 1843-44 Professor Haldeman ("Boston Journal of Nat. Hist. U. States",
vol. iv, page 468) has ably given the arguments for and against the
hypothesis of the development and modification of species: he seems to
lean toward the side of change.
 
The "Vestiges of Creation" appeared in 1844. In the tenth and much
improved edition (1853) the anonymous author says (page 155): "The
proposition determined on after much consideration is, that the several
series of animated beings, from the simplest and oldest up to the
highest and most recent, are, under the providence of God, the results,
FIRST, of an impulse which has been imparted to the forms of life,
advancing them, in definite times, by generation, through grades of
organisation terminating in the highest dicotyledons and vertebrata,
these grades being few in number, and generally marked by intervals
of organic character, which we find to be a practical difficulty in
ascertaining affinities; SECOND, of another impulse connected with the
vital forces, tending, in the course of generations, to modify organic
structures in accordance with external circumstances, as food, the
nature of the habitat, and the meteoric agencies, these being the
'adaptations' of the natural theologian." The author apparently believes
that organisation progresses by sudden leaps, but that the effects
produced by the conditions of life are gradual. He argues with much
force on general grounds that species are not immutable productions.
But I cannot see how the two supposed "impulses" account in a scientific
sense for the numerous and beautiful coadaptations which we see
throughout nature; I cannot see that we thus gain any insight how, for
instance, a woodpecker has become adapted to its peculiar habits of
life. The work, from its powerful and brilliant style, though displaying
in the early editions little accurate knowledge and a great want of
scientific caution, immediately had a very wide circulation. In my
opinion it has done excellent service in this country in calling
attention to the subject, in removing prejudice, and in thus preparing
the ground for the reception of analogous views.
 
In 1846 the veteran geologist M.J. d'Omalius d'Halloy published in an
excellent though short paper ("Bulletins de l'Acad. Roy. Bruxelles",
tom. xiii, page 581) his opinion that it is more probable that new
species have been produced by descent with modification than that they
have been separately created: the author first promulgated this opinion
in 1831.
 
Professor Owen, in 1849 ("Nature of Limbs", page 86), wrote as follows:
"The archetypal idea was manifested in the flesh under diverse such
modifications, upon this planet, long prior to the existence of those
animal species that actually exemplify it. To what natural laws or
secondary causes the orderly succession and progression of such organic
phenomena may have been committed, we, as yet, are ignorant." In his
address to the British Association, in 1858, he speaks (page li) of "the
axiom of the continuous operation of creative power, or of the ordained
becoming of living things." Further on (page xc), after referring
to geographical distribution, he adds, "These phenomena shake our
confidence in the conclusion that the Apteryx of New Zealand and the
Red Grouse of England were distinct creations in and for those islands
respectively. Always, also, it may be well to bear in mind that by the
word 'creation' the zoologist means 'a process he knows not what.'" He
amplifies this idea by adding that when such cases as that of the Red
Grouse are "enumerated by the zoologist as evidence of distinct creation
of the bird in and for such islands, he chiefly expresses that he
knows not how the Red Grouse came to be there, and there exclusively;
signifying also, by this mode of expressing such ignorance, his belief
that both the bird and the islands owed their origin to a great first
Creative Cause." If we interpret these sentences given in the same
address, one by the other, it appears that this eminent philosopher felt
in 1858 his confidence shaken that the Apteryx and the Red Grouse first
appeared in their respective homes "he knew not how," or by some process
"he knew not what."
 
This address was delivered after the papers by Mr. Wallace and myself on
the Origin of Species, presently to be referred to, had been read before
the Linnean Society. When the first edition of this work was published,
I was so completely deceived, as were many others, by such expressions
as "the continuous operation of creative power," that I included
Professor Owen with other palaeontologists as being firmly convinced
of the immutability of species; but it appears ("Anat. of Vertebrates",
vol. iii, page 796) that this was on my part a preposterous error. In
the last edition of this work I inferred, and the inference still seems
to me perfectly just, from a passage beginning with the words "no doubt
the type-form," etc.(Ibid., vol. i, page xxxv), that Professor Owen
admitted that natural selection may have done something in the formation
of a new species; but this it appears (Ibid., vol. iii. page 798)
is inaccurate and without evidence. I also gave some extracts from a
correspondence between Professor Owen and the editor of the "London
Review", from which it appeared manifest to the editor as well as to
myself, that Professor Owen claimed to have promulgated the theory of
natural selection before I had done so; and I expressed my surprise
and satisfaction at this announcement; but as far as it is possible to
understand certain recently published passages (Ibid., vol. iii. page
798) I have either partially or wholly again fallen into error. It
is consolatory to me that others find Professor Owen's controversial
writings as difficult to understand and to reconcile with each other,
as I do. As far as the mere enunciation of the principle of natural
selection is concerned, it is quite immaterial whether or not Professor
Owen preceded me, for both of us, as shown in this historical sketch,
were long ago preceded by Dr. Wells and Mr. Matthews.
 
M. Isidore Geoffroy Saint-Hilaire, in his lectures delivered in 1850 (of
which a Resume appeared in the "Revue et Mag. de Zoolog.", Jan., 1851),
briefly gives his reason for believing that specific characters "sont
fixes, pour chaque espece, tant qu'elle se perpetue au milieu des memes
circonstances: ils se modifient, si les circonstances ambiantes viennent
a changer. En resume, L'OBSERVATION des animaux sauvages demontre deja
la variabilite LIMITEE des especes. Les EXPERIENCES sur les animaux
sauvages devenus domestiques, et sur les animaux domestiques redevenus
sauvages, la demontrent plus clairment encore. Ces memes experiences
prouvent, de plus, que les differences produites peuvent etre de VALEUR
GENERIQUE." In his "Hist. Nat. Generale" (tom. ii, page 430, 1859) he
amplifies analogous conclusions.
 
From a circular lately issued it appears that Dr. Freke, in 1851
("Dublin Medical Press", page 322), propounded the doctrine that all
organic beings have descended from one primordial form. His grounds of
belief and treatment of the subject are wholly different from mine;
but as Dr. Freke has now (1861) published his Essay on the "Origin of
Species by means of Organic Affinity", the difficult attempt to give any
idea of his views would be superfluous on my part.
 
Mr. Herbert Spencer, in an Essay (originally published in the "Leader",
March, 1852, and republished in his "Essays", in 1858), has contrasted
the theories of the Creation and the Development of organic beings
with remarkable skill and force. He argues from the analogy of domestic
productions, from the changes which the embryos of many species undergo,
from the difficulty of distinguishing species and varieties, and from
the principle of general gradation, that species have been modified;
and he attributes the modification to the change of circumstances.
The author (1855) has also treated Psychology on the principle of the
necessary acquirement of each mental power and capacity by gradation.
 
In 1852 M. Naudin, a distinguished botanist, expressly stated, in an
admirable paper on the Origin of Species ("Revue Horticole", page 102;
since partly republished in the "Nouvelles Archives du Museum", tom. i,
page 171), his belief that species are formed in an analogous manner as
varieties are under cultivation; and the latter process he attributes to
man's power of selection. But he does not show how selection acts under
nature. He believes, like Dean Herbert, that species, when nascent,
were more plastic than at present. He lays weight on what he calls the
principle of finality, "puissance mysterieuse, indeterminee; fatalite
pour les uns; pour les autres volonte providentielle, dont l'action
incessante sur les etres vivantes determine, a toutes les epoques de
l'existence du monde, la forme, le volume, et la duree de chacun d'eux,
en raison de sa destinee dans l'ordre de choses dont il fait partie.
C'est cette puissance qui harmonise chaque membre a l'ensemble, en
l'appropriant a la fonction qu'il doit remplir dans l'organisme general
de la nature, fonction qui est pour lui sa raison d'etre." (From
references in Bronn's "Untersuchungen uber die Entwickelungs-Gesetze",
it appears that the celebrated botanist and palaeontologist Unger
published, in 1852, his belief that species undergo development and
modification. Dalton, likewise, in Pander and Dalton's work on Fossil
Sloths, expressed, in 1821, a similar belief. Similar views have, as is
well known, been maintained by Oken in his mystical "Natur-Philosophie".
From other references in Godron's work "Sur l'Espece", it seems that
Bory St. Vincent, Burdach, Poiret and Fries, have all admitted that
new species are continually being produced. I may add, that of the
thirty-four authors named in this Historical Sketch, who believe in
the modification of species, or at least disbelieve in separate acts
of creation, twenty-seven have written on special branches of natural
history or geology.)
 
In 1853 a celebrated geologist, Count Keyserling ("Bulletin de la Soc.
Geolog.", 2nd Ser., tom. x, page 357), suggested that as new diseases,
supposed to have been caused by some miasma have arisen and spread over
the world, so at certain periods the germs of existing species may have
been chemically affected by circumambient molecules of a particular
nature, and thus have given rise to new forms.
 
In this same year, 1853, Dr. Schaaffhausen published an excellent
pamphlet ("Verhand. des Naturhist. Vereins der Preuss. Rheinlands",
etc.), in which he maintains the development of organic forms on the
earth. He infers that many species have kept true for long periods,
whereas a few have become modified. The distinction of species he
explains by the destruction of intermediate graduated forms. "Thus
living plants and animals are not separated from the extinct by new
creations, but are to be regarded as their descendants through continued
reproduction."
 
A well-known French botanist, M. Lecoq, writes in 1854 ("Etudes sur
Geograph." Bot. tom. i, page 250), "On voit que nos recherches sur la
fixite ou la variation de l'espece, nous conduisent directement aux
idees emises par deux hommes justement celebres, Geoffroy Saint-Hilaire
et Goethe." Some other passages scattered through M. Lecoq's large
work make it a little doubtful how far he extends his views on the
modification of species.
 
The "Philosophy of Creation" has been treated in a masterly manner by
the Rev. Baden Powell, in his "Essays on the Unity of Worlds", 1855.
Nothing can be more striking than the manner in which he shows that the
introduction of new species is "a regular, not a casual phenomenon," or,
as Sir John Herschel expresses it, "a natural in contradistinction to a
miraculous process."
 
The third volume of the "Journal of the Linnean Society" contains
papers, read July 1, 1858, by Mr. Wallace and myself, in which, as
stated in the introductory remarks to this volume, the theory of
Natural Selection is promulgated by Mr. Wallace with admirable force and
clearness.
 
Von Baer, toward whom all zoologists feel so profound a respect,
expressed about the year 1859 (see Prof. Rudolph Wagner,
"Zoologisch-Anthropologische Untersuchungen", 1861, s. 51) his
conviction, chiefly grounded on the laws of geographical distribution,
that forms now perfectly distinct have descended from a single
parent-form.
 
In June, 1859, Professor Huxley gave a lecture before the Royal
Institution on the "Persistent Types of Animal Life". Referring to such
cases, he remarks, "It is difficult to comprehend the meaning of such
facts as these, if we suppose that each species of animal and plant, or
each great type of organisation, was formed and placed upon the surface
of the globe at long intervals by a distinct act of creative power; and
it is well to recollect that such an assumption is as unsupported by
tradition or revelation as it is opposed to the general analogy of
nature. If, on the other hand, we view "Persistent Types" in relation to
that hypothesis which supposes the species living at any time to be the
result of the gradual modification of pre-existing species, a hypothesis
which, though unproven, and sadly damaged by some of its supporters,
is yet the only one to which physiology lends any countenance; their
existence would seem to show that the amount of modification which
living beings have undergone during geological time is but very small in
relation to the whole series of changes which they have suffered."
 
In December, 1859, Dr. Hooker published his "Introduction to the
Australian Flora". In the first part of this great work he admits the
truth of the descent and modification of species, and supports this
doctrine by many original observations.
 
The first edition of this work was published on November 24, 1859, and
the second edition on January 7, 1860.
 
 
 
CONTENTS.
 
 
  INTRODUCTION
 
 
  CHAPTER I.
 
  VARIATION UNDER DOMESTICATION.
 
  Causes of Variability--Effects of Habit and the use or disuse of
  Parts--Correlated Variation--Inheritance--Character of Domestic
  Varieties--Difficulty of distinguishing between Varieties and
  Species--Origin of Domestic Varieties from one or more Species--Domestic
  Pigeons, their Differences and Origin--Principles of Selection,
  anciently followed, their Effects--Methodical and Unconscious
  Selection--Unknown Origin of our Domestic Productions--Circumstances
  favourable to Man's power of Selection.
 
 
  CHAPTER II.
 
  VARIATION UNDER NATURE.
 
  Variability--Individual Differences--Doubtful species--Wide ranging,
  much diffused, and common species, vary most--Species of the larger
  genera in each country vary more frequently than the species of the
  smaller genera--Many of the species of the larger genera resemble
  varieties in being very closely, but unequally, related to each other,
  and in having restricted ranges.
 
 
  CHAPTER III.
 
  STRUGGLE FOR EXISTENCE.
 
  Its bearing on natural selection--The term used in a wide
  sense--Geometrical ratio of increase--Rapid increase of naturalised
  animals and plants--Nature of the checks to increase--Competition
  universal--Effects of climate--Protection from the number of
  individuals--Complex relations of all animals and plants throughout
  nature--Struggle for life most severe between individuals and varieties
  of the same species; often severe between species of the same genus--The
  relation of organism to organism the most important of all relations.
 
 
  CHAPTER IV.
 
  NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
 
  Natural Selection--its power compared with man's selection--its power
  on characters of trifling importance--its power at all ages and on
  both sexes--Sexual Selection--On the generality of intercrosses
  between individuals of the same species--Circumstances favourable and
  unfavourable to the results of Natural Selection, namely, intercrossing,
  isolation, number of individuals--Slow action--Extinction caused by
  Natural Selection--Divergence of Character, related to the diversity of
  inhabitants of any small area and to naturalisation--Action of Natural
  Selection, through Divergence of Character and Extinction, on the
  descendants from a common parent--Explains the Grouping of all organic
  beings--Advance in organisation--Low forms preserved--Convergence of
  character--Indefinite multiplication of species--Summary.
 
 
  CHAPTER V.
 
  LAWS OF VARIATION.
 
  Effects of changed conditions--Use and disuse, combined with natural
  selection; organs of flight and of vision--Acclimatisation--Correlated
  variation--Compensation and economy of growth--False
  correlations--Multiple, rudimentary, and lowly organised structures
  variable--Parts developed in an unusual manner are highly variable;
  specific characters more variable than generic; secondary sexual
  characters variable--Species of the same genus vary in an analogous
  manner--Reversions to long-lost characters--Summary.
 
 
  CHAPTER VI.
 
  DIFFICULTIES OF THE THEORY.
 
  Difficulties of the theory of descent with modification--Absence
  or rarity of transitional varieties--Transitions in habits of
  life--Diversified habits in the same species--Species with habits
  widely different from those of their allies--Organs of extreme
  perfection--Modes of transition--Cases of difficulty--Natura non facit
  saltum--Organs of small importance--Organs not in all cases absolutely
  perfect--The law of Unity of Type and of the Conditions of Existence
  embraced by the theory of Natural Selection.
 
 
  CHAPTER VII.
 
  MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.
 
  Longevity--Modifications not necessarily simultaneous--Modifications
  apparently of no direct service--Progressive development--Characters of
  small functional importance, the most constant--Supposed incompetence
  of natural selection to account for the incipient stages of useful
  structures--Causes which interfere with the acquisition through natural
  selection of useful structures--Gradations of structure with changed
  functions--Widely different organs in members of the same class,
  developed from one and the same source--Reasons for disbelieving in
  great and abrupt modifications.
 
 
  CHAPTER VIII.
 
  INSTINCT.
 
  Instincts comparable with habits, but different in their
  origin--Instincts graduated--Aphides and ants--Instincts
  variable--Domestic instincts, their origin--Natural instincts of
  the cuckoo, molothrus, ostrich, and parasitic bees--Slave-making
  ants--Hive-bee, its cell-making instinct--Changes of instinct and
  structure not necessarily simultaneous--Difficulties on the theory of
  the Natural Selection of instincts--Neuter or sterile insects--Summary.
 
 
  CHAPTER IX.
 
  HYBRIDISM.
 
  Distinction between the sterility of first crosses and of
  hybrids--Sterility various in degree, not universal, affected by close
  interbreeding, removed by domestication--Laws governing the sterility
  of hybrids--Sterility not a special endowment, but incidental on
  other differences, not accumulated by natural selection--Causes of
  the sterility of first crosses and of hybrids--Parallelism between the
  effects of changed conditions of life and of crossing--Dimorphism and
  Trimorphism--Fertility of varieties when crossed and of their mongrel
  offspring not universal--Hybrids and mongrels compared independently of
  their fertility--Summary.
 
 
  CHAPTER X.
 
  ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
 
  On the absence of intermediate varieties at the present day--On the
  nature of extinct intermediate varieties; on their number--On the lapse
  of time, as inferred from the rate of denudation and of deposition--On
  the lapse of time as estimated in years--On the poorness of our
  palaeontological collections--On the intermittence of geological
  formations--On the denudation of granitic areas--On the absence of
  intermediate varieties in any one formation--On the sudden appearance
  of groups of species--On their sudden appearance in the lowest known
  fossiliferous strata--Antiquity of the habitable earth.
 
 
  CHAPTER XI.
 
  ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
 
  On the slow and successive appearance of new species--On their different
  rates of change--Species once lost do not reappear--Groups of species
  follow the same general rules in their appearance and disappearance as
  do single species--On extinction--On simultaneous changes in the forms
  of life throughout the world--On the affinities of extinct species to
  each other and to living species--On the state of development of
  ancient forms--On the succession of the same types within the same
  areas--Summary of preceding and present chapter.
 
 
  CHAPTER XII.
 
  GEOGRAPHICAL DISTRIBUTION.
 
  Present distribution cannot be accounted for by differences in physical
  conditions--Importance of barriers--Affinity of the productions of the
  same continent--Centres of creation--Means of dispersal by changes of
  climate and of the level of the land, and by occasional means--Dispersal
  during the Glacial period--Alternate Glacial periods in the north and
  south.
 
 
  CHAPTER XIII.
 
  GEOGRAPHICAL DISTRIBUTION--CONTINUED.
 
  Distribution of fresh-water productions--On the inhabitants of oceanic
  islands--Absence of Batrachians and of terrestrial Mammals--On
  the relation of the inhabitants of islands to those of the nearest
  mainland--On colonisation from the nearest source with subsequent
  modification--Summary of the last and present chapter.
 
 
  CHAPTER XIV.
 
  MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY--EMBRYOLOGY--RUDIMENTARY
  ORGANS.
 
  Classification, groups subordinate to groups--Natural system--Rules and
  difficulties in classification, explained on the theory of descent
  with modification--Classification of varieties--Descent always used in
  classification--Analogical or adaptive characters--Affinities,
  general, complex and radiating--Extinction separates and defines
  groups--Morphology, between members of the same class, between parts of
  the same individual--Embryology, laws of, explained by variations not
  supervening at an early age, and being inherited at a corresponding
  age--Rudimentary Organs; their origin explained--Summary.
 
 
  CHAPTER XV.
 
  RECAPITULATION AND CONCLUSION.
 
  Recapitulation of the objections to the theory of Natural
  Selection--Recapitulation of the general and special circumstances
  in its favour--Causes of the general belief in the immutability
  of species--How far the theory of Natural Selection may be
  extended--Effects of its adoption on the study of Natural
  history--Concluding remarks.
 
 
  GLOSSARY OF SCIENTIFIC TERMS.
 
 
  INDEX.
 
 
 
 
ORIGIN OF SPECIES.
 
 
 
 
INTRODUCTION.
 
When on board H.M.S. Beagle, as naturalist, I was much struck with
certain facts in the distribution of the organic beings inhabiting South
America, and in the geological relations of the present to the past
inhabitants of that continent. These facts, as will be seen in the
latter chapters of this volume, seemed to throw some light on the origin
of species--that mystery of mysteries, as it has been called by one
of our greatest philosophers. On my return home, it occurred to me,
in 1837, that something might perhaps be made out on this question by
patiently accumulating and reflecting on all sorts of facts which could
possibly have any bearing on it. After five years' work I allowed myself
to speculate on the subject, and drew up some short notes; these I
enlarged in 1844 into a sketch of the conclusions, which then seemed to
me probable: from that period to the present day I have steadily pursued
the same object. I hope that I may be excused for entering on these
personal details, as I give them to show that I have not been hasty in
coming to a decision.
 
My work is now (1859) nearly finished; but as it will take me many more
years to complete it, and as my health is far from strong, I have been
urged to publish this abstract. I have more especially been induced to
do this, as Mr. Wallace, who is now studying the natural history of
the Malay Archipelago, has arrived at almost exactly the same general
conclusions that I have on the origin of species. In 1858 he sent me a
memoir on this subject, with a request that I would forward it to Sir
Charles Lyell, who sent it to the Linnean Society, and it is published
in the third volume of the Journal of that Society. Sir C. Lyell and Dr.
Hooker, who both knew of my work--the latter having read my sketch
of 1844--honoured me by thinking it advisable to publish, with Mr.
Wallace's excellent memoir, some brief extracts from my manuscripts.
 
This abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements;
and I must trust to the reader reposing some confidence in my accuracy.
No doubt errors may have crept in, though I hope I have always been
cautious in trusting to good authorities alone. I can here give only
the general conclusions at which I have arrived, with a few facts in
illustration, but which, I hope, in most cases will suffice. No one can
feel more sensible than I do of the necessity of hereafter publishing in
detail all the facts, with references, on which my conclusions have been
grounded; and I hope in a future work to do this. For I am well aware
that scarcely a single point is discussed in this volume on which facts
cannot be adduced, often apparently leading to conclusions directly
opposite to those at which I have arrived. A fair result can be obtained
only by fully stating and balancing the facts and arguments on both
sides of each question; and this is here impossible.
 
I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very
many naturalists, some of them personally unknown to me. I cannot,
however, let this opportunity pass without expressing my deep
obligations to Dr. Hooker, who, for the last fifteen years, has aided me
in every possible way by his large stores of knowledge and his excellent
judgment.
 
In considering the origin of species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings,
on their embryological relations, their geographical distribution,
geological succession, and other such facts, might come to the
conclusion that species had not been independently created, but had
descended, like varieties, from other species. Nevertheless, such a
conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species, inhabiting this world
have been modified, so as to acquire that perfection of structure
and coadaptation which justly excites our admiration. Naturalists
continually refer to external conditions, such as climate, food, etc.,
as the only possible cause of variation. In one limited sense, as
we shall hereafter see, this may be true; but it is preposterous to
attribute to mere external conditions, the structure, for instance,
of the woodpecker, with its feet, tail, beak, and tongue, so admirably
adapted to catch insects under the bark of trees. In the case of the
mistletoe, which draws its nourishment from certain trees, which has
seeds that must be transported by certain birds, and which has flowers
with separate sexes absolutely requiring the agency of certain insects
to bring pollen from one flower to the other, it is equally preposterous
to account for the structure of this parasite, with its relations to
several distinct organic beings, by the effects of external conditions,
or of habit, or of the volition of the plant itself.
 
It is, therefore, of the highest importance to gain a clear insight into
the means of modification and coadaptation. At the commencement of
my observations it seemed to me probable that a careful study of
domesticated animals and of cultivated plants would offer the best
chance of making out this obscure problem. Nor have I been disappointed;
in this and in all other perplexing cases I have invariably found that
our knowledge, imperfect though it be, of variation under domestication,
afforded the best and safest clue. I may venture to express my
conviction of the high value of such studies, although they have been
very commonly neglected by naturalists.
 
From these considerations, I shall devote the first chapter of this
abstract to variation under domestication. We shall thus see that a
large amount of hereditary modification is at least possible; and, what
is equally or more important, we shall see how great is the power of man
in accumulating by his selection successive slight variations. I will
then pass on to the variability of species in a state of nature; but
I shall, unfortunately, be compelled to treat this subject far too
briefly, as it can be treated properly only by giving long catalogues of
facts. We shall, however, be enabled to discuss what circumstances
are most favourable to variation. In the next chapter the struggle
for existence among all organic beings throughout the world, which
inevitably follows from the high geometrical ratio of their increase,
will be considered. This is the doctrine of Malthus, applied to the
whole animal and vegetable kingdoms. As many more individuals of each
species are born than can possibly survive; and as, consequently, there
is a frequently recurring struggle for existence, it follows that any
being, if it vary however slightly in any manner profitable to itself,
under the complex and sometimes varying conditions of life, will have
a better chance of surviving, and thus be NATURALLY SELECTED. From
the strong principle of inheritance, any selected variety will tend to
propagate its new and modified form.
 
This fundamental subject of natural selection will be treated at
some length in the fourth chapter; and we shall then see how natural
selection almost inevitably causes much extinction of the less improved
forms of life, and leads to what I have called divergence of character.
In the next chapter I shall discuss the complex and little known laws
of variation. In the five succeeding chapters, the most apparent and
gravest difficulties in accepting the theory will be given: namely,
first, the difficulties of transitions, or how a simple being or a
simple organ can be changed and perfected into a highly developed
being or into an elaborately constructed organ; secondly the subject of
instinct, or the mental powers of animals; thirdly, hybridism, or the
infertility of species and the fertility of varieties when intercrossed;
and fourthly, the imperfection of the geological record. In the next
chapter I shall consider the geological succession of organic beings
throughout time; in the twelfth and thirteenth, their geographical
distribution throughout space; in the fourteenth, their classification
or mutual affinities, both when mature and in an embryonic condition. In
the last chapter I shall give a brief recapitulation of the whole work,
and a few concluding remarks.
 
No one ought to feel surprise at much remaining as yet unexplained in
regard to the origin of species and varieties, if he make due allowance
for our profound ignorance in regard to the mutual relations of the
many beings which live around us. Who can explain why one species ranges
widely and is very numerous, and why another allied species has a narrow
range and is rare? Yet these relations are of the highest importance,
for they determine the present welfare and, as I believe, the future
success and modification of every inhabitant of this world. Still less
do we know of the mutual relations of the innumerable inhabitants of the
world during the many past geological epochs in its history. Although
much remains obscure, and will long remain obscure, I can entertain no
doubt, after the most deliberate study and dispassionate judgment of
which I am capable, that the view which most naturalists until recently
entertained, and which I formerly entertained--namely, that each species
has been independently created--is erroneous. I am fully convinced that
species are not immutable; but that those belonging to what are called
the same genera are lineal descendants of some other and generally
extinct species, in the same manner as the acknowledged varieties of
any one species are the descendants of that species. Furthermore, I am
convinced that natural selection has been the most important, but not
the exclusive, means of modification.
 
 
 
 
CHAPTER I. VARIATION UNDER DOMESTICATION.
 
 Causes of Variability--Effects of Habit and the use and disuse of
 Parts--Correlated Variation--Inheritance--Character of Domestic
 Varieties--Difficulty of distinguishing between Varieties and
 Species--Origin of Domestic Varieties from one or more Species--Domestic
 Pigeons, their Differences and Origin--Principles of Selection,
 anciently followed, their Effects--Methodical and Unconscious
 Selection--Unknown Origin of our Domestic Productions--Circumstances
 favourable to Man's power of Selection.
 
 
CAUSES OF VARIABILITY.
 
When we compare the individuals of the same variety or sub-variety of
our older cultivated plants and animals, one of the first points which
strikes us is, that they generally differ more from each other than do
the individuals of any one species or variety in a state of nature. And
if we reflect on the vast diversity of the plants and animals which have
been cultivated, and which have varied during all ages under the most
different climates and treatment, we are driven to conclude that this
great variability is due to our domestic productions having been raised
under conditions of life not so uniform as, and somewhat different from,
those to which the parent species had been exposed under nature. There
is, also, some probability in the view propounded by Andrew Knight, that
this variability may be partly connected with excess of food. It seems
clear that organic beings must be exposed during several generations to
new conditions to cause any great amount of variation; and that, when
the organisation has once begun to vary, it generally continues varying
for many generations. No case is on record of a variable organism
ceasing to vary under cultivation. Our oldest cultivated plants, such
as wheat, still yield new varieties: our oldest domesticated animals are
still capable of rapid improvement or modification.
 
As far as I am able to judge, after long attending to the subject, the
conditions of life appear to act in two ways--directly on the whole
organisation or on certain parts alone and in directly by affecting the
reproductive system. With respect to the direct action, we must bear in
mind that in every case, as Professor Weismann has lately insisted,
and as I have incidently shown in my work on "Variation under
Domestication," there are two factors: namely, the nature of the
organism and the nature of the conditions. The former seems to be much
the more important; for nearly similar variations sometimes arise under,
as far as we can judge, dissimilar conditions; and, on the other hand,
dissimilar variations arise under conditions which appear to be
nearly uniform. The effects on the offspring are either definite or in
definite. They may be considered as definite when all or nearly all the
offspring of individuals exposed to certain conditions during several
generations are modified in the same manner. It is extremely difficult
to come to any conclusion in regard to the extent of the changes which
have been thus definitely induced. There can, however, be little doubt
about many slight changes, such as size from the amount of food,
colour from the nature of the food, thickness of the skin and hair from
climate, etc. Each of the endless variations which we see in the plumage
of our fowls must have had some efficient cause; and if the same cause
were to act uniformly during a long series of generations on many
individuals, all probably would be modified in the same manner. Such
facts as the complex and extraordinary out growths which variably
follow from the insertion of a minute drop of poison by a gall-producing
insect, shows us what singular modifications might result in the case of
plants from a chemical change in the nature of the sap.
 
In definite variability is a much more common result of changed
conditions than definite variability, and has probably played a more
important part in the formation of our domestic races. We see in
definite variability in the endless slight peculiarities which
distinguish the individuals of the same species, and which cannot be
accounted for by inheritance from either parent or from some more remote
ancestor. Even strongly-marked differences occasionally appear in the
young of the same litter, and in seedlings from the same seed-capsule.
At long intervals of time, out of millions of individuals reared in the
same country and fed on nearly the same food, deviations of structure so
strongly pronounced as to deserve to be called monstrosities arise; but
monstrosities cannot be separated by any distinct line from slighter
variations. All such changes of structure, whether extremely slight or
strongly marked, which appear among many individuals living together,
may be considered as the in definite effects of the conditions of life
on each individual organism, in nearly the same manner as the chill
effects different men in an in definite manner, according to their
state of body or constitution, causing coughs or colds, rheumatism, or
inflammation of various organs.
 
With respect to what I have called the in direct action of changed
conditions, namely, through the reproductive system of being affected,
we may infer that variability is thus induced, partly from the fact of
this system being extremely sensitive to any change in the conditions,
and partly from the similarity, as Kolreuter and others have remarked,
between the variability which follows from the crossing of distinct
species, and that which may be observed with plants and animals when
reared under new or unnatural conditions. Many facts clearly show how
eminently susceptible the reproductive system is to very slight changes
in the surrounding conditions. Nothing is more easy than to tame an
animal, and few things more difficult than to get it to breed freely
under confinement, even when the male and female unite. How many animals
there are which will not breed, though kept in an almost free state in
their native country! This is generally, but erroneously attributed to
vitiated instincts. Many cultivated plants display the utmost vigour,
and yet rarely or never seed! In some few cases it has been discovered
that a very trifling change, such as a little more or less water at some
particular period of growth, will determine whether or not a plant will
produce seeds. I cannot here give the details which I have collected and
elsewhere published on this curious subject; but to show how singular
the laws are which determine the reproduction of animals under
confinement, I may mention that carnivorous animals, even from the
tropics, breed in this country pretty freely under confinement, with
the exception of the plantigrades or bear family, which seldom produce
young; whereas, carnivorous birds, with the rarest exception, hardly
ever lay fertile eggs. Many exotic plants have pollen utterly worthless,
in the same condition as in the most sterile hybrids. When, on the one
hand, we see domesticated animals and plants, though often weak and
sickly, breeding freely under confinement; and when, on the other hand,
we see individuals, though taken young from a state of nature perfectly
tamed, long-lived, and healthy (of which I could give numerous
instances), yet having their reproductive system so seriously affected
by unperceived causes as to fail to act, we need not be surprised at
this system, when it does act under confinement, acting irregularly,
and producing offspring somewhat unlike their parents. I may add that
as some organisms breed freely under the most unnatural conditions--for
instance, rabbits and ferrets kept in hutches--showing that their
reproductive organs are not easily affected; so will some animals
and plants withstand domestication or cultivation, and vary very
slightly--perhaps hardly more than in a state of nature.
 
Some naturalists have maintained that all variations are connected with
the act of sexual reproduction; but this is certainly an error; for I
have given in another work a long list of "sporting plants;" as they are
called by gardeners; that is, of plants which have suddenly produced a
single bud with a new and sometimes widely different character from that
of the other buds on the same plant. These bud variations, as they may
be named, can be propagated by grafts, offsets, etc., and sometimes
by seed. They occur rarely under nature, but are far from rare under
culture. As a single bud out of many thousands produced year after year
on the same tree under uniform conditions, has been known suddenly to
assume a new character; and as buds on distinct trees, growing
under different conditions, have sometimes yielded nearly the same
variety--for instance, buds on peach-trees producing nectarines, and
buds on common roses producing moss-roses--we clearly see that the
nature of the conditions is of subordinate importance in comparison
with the nature of the organism in determining each particular form of
variation; perhaps of not more importance than the nature of the spark,
by which a mass of combustible matter is ignited, has in determining the
nature of the flames.
 
EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS; CORRELATED
VARIATION; INHERITANCE.
 
Changed habits produce an inherited effect as in the period of the
flowering of plants when transported from one climate to another. With
animals the increased use or disuse of parts has had a more marked
influence; thus I find in the domestic duck that the bones of the wing
weigh less and the bones of the leg more, in proportion to the whole
skeleton, than do the same bones in the wild duck; and this change may
be safely attributed to the domestic duck flying much less, and walking
more, than its wild parents. The great and inherited development of the
udders in cows and goats in countries where they are habitually milked,
in comparison with these organs in other countries, is probably another
instance of the effects of use. Not one of our domestic animals can be
named which has not in some country drooping ears; and the view which
has been suggested that the drooping is due to disuse of the muscles of
the ear, from the animals being seldom much alarmed, seems probable.
 
Many laws regulate variation, some few of which can be dimly seen, and
will hereafter be briefly discussed. I will here only allude to what may
be called correlated variation. Important changes in the embryo or larva
will probably entail changes in the mature animal. In monstrosities,
the correlations between quite distinct parts are very curious; and many
instances are given in Isidore Geoffroy St. Hilaire's great work on this
subject. Breeders believe that long limbs are almost always accompanied
by an elongated head. Some instances of correlation are quite whimsical;
thus cats which are entirely white and have blue eyes are generally
deaf; but it has been lately stated by Mr. Tait that this is confined to
the males. Colour and constitutional peculiarities go together, of which
many remarkable cases could be given among animals and plants. From
facts collected by Heusinger, it appears that white sheep and pigs
are injured by certain plants, while dark-coloured individuals escape:
Professor Wyman has recently communicated to me a good illustration of
this fact; on asking some farmers in Virginia how it was that all their
pigs were black, they informed him that the pigs ate the paint-root
(Lachnanthes), which coloured their bones pink, and which caused
the hoofs of all but the black varieties to drop off; and one of the
"crackers" (i.e. Virginia squatters) added, "we select the black members
of a litter for raising, as they alone have a good chance of living."
Hairless dogs have imperfect teeth; long-haired and coarse-haired
animals are apt to have, as is asserted, long or many horns; pigeons
with feathered feet have skin between their outer toes; pigeons with
short beaks have small feet, and those with long beaks large feet. Hence
if man goes on selecting, and thus augmenting, any peculiarity, he will
almost certainly modify unintentionally other parts of the structure,
owing to the mysterious laws of correlation.
 
The results of the various, unknown, or but dimly understood laws of
variation are infinitely complex and diversified. It is well worth while
carefully to study the several treatises on some of our old cultivated
plants, as on the hyacinth, potato, even the dahlia, etc.; and it
is really surprising to note the endless points of structure and
constitution in which the varieties and sub-varieties differ slightly
from each other. The whole organisation seems to have become plastic,
and departs in a slight degree from that of the parental type.
 
Any variation which is not inherited is unimportant for us. But the
number and diversity of inheritable deviations of structure, both
those of slight and those of considerable physiological importance,
are endless. Dr. Prosper Lucas' treatise, in two large volumes, is the
fullest and the best on this subject. No breeder doubts how strong is
the tendency to inheritance; that like produces like is his fundamental
belief: doubts have been thrown on this principle only by theoretical
writers. When any deviation of structure often appears, and we see it
in the father and child, we cannot tell whether it may not be due to the
same cause having acted on both; but when among individuals, apparently
exposed to the same conditions, any very rare deviation, due to some
extraordinary combination of circumstances, appears in the parent--say,
once among several million individuals--and it reappears in the
child, the mere doctrine of chances almost compels us to attribute
its reappearance to inheritance. Every one must have heard of cases of
albinism, prickly skin, hairy bodies, etc., appearing in several members
of the same family. If strange and rare deviations of structure are
truly inherited, less strange and commoner deviations may be freely
admitted to be inheritable. Perhaps the correct way of viewing the whole
subject would be, to look at the inheritance of every character whatever
as the rule, and non-inheritance as the anomaly.
 
The laws governing inheritance are for the most part unknown; no one
can say why the same peculiarity in different individuals of the same
species, or in different species, is sometimes inherited and sometimes
not so; why the child often reverts in certain characteristics to its
grandfather or grandmother or more remote ancestor; why a peculiarity is
often transmitted from one sex to both sexes, or to one sex alone,
more commonly but not exclusively to the like sex. It is a fact of
some importance to us, that peculiarities appearing in the males of our
domestic breeds are often transmitted, either exclusively or in a much
greater degree, to the males alone. A much more important rule, which I
think may be trusted, is that, at whatever period of life a peculiarity
first appears, it tends to reappear in the offspring at a corresponding
age, though sometimes earlier. In many cases this could not be
otherwise; thus the inherited peculiarities in the horns of cattle could
appear only in the offspring when nearly mature; peculiarities in the
silk-worm are known to appear at the corresponding caterpillar or cocoon
stage. But hereditary diseases and some other facts make me believe
that the rule has a wider extension, and that, when there is no apparent
reason why a peculiarity should appear at any particular age, yet that
it does tend to appear in the offspring at the same period at which it
first appeared in the parent. I believe this rule to be of the highest
importance in explaining the laws of embryology. These remarks are of
course confined to the first APPEARANCE of the peculiarity, and not
to the primary cause which may have acted on the ovules or on the male
element; in nearly the same manner as the increased length of the horns
in the offspring from a short-horned cow by a long-horned bull, though
appearing late in life, is clearly due to the male element.
 
Having alluded to the subject of reversion, I may here refer to
a statement often made by naturalists--namely, that our domestic
varieties, when run wild, gradually but invariably revert in character
to their aboriginal stocks. Hence it has been argued that no deductions
can be drawn from domestic races to species in a state of nature. I
have in vain endeavoured to discover on what decisive facts the above
statement has so often and so boldly been made. There would be great
difficulty in proving its truth: we may safely conclude that very many
of the most strongly marked domestic varieties could not possibly live
in a wild state. In many cases we do not know what the aboriginal stock
was, and so could not tell whether or not nearly perfect reversion
had ensued. It would be necessary, in order to prevent the effects of
intercrossing, that only a single variety should be turned loose in
its new home. Nevertheless, as our varieties certainly do occasionally
revert in some of their characters to ancestral forms, it seems to me
not improbable that if we could succeed in naturalising, or were to
cultivate, during many generations, the several races, for instance,
of the cabbage, in very poor soil--in which case, however, some
effect would have to be attributed to the DEFINITE action of the poor
soil--that they would, to a large extent, or even wholly, revert to the
wild aboriginal stock. Whether or not the experiment would succeed is
not of great importance for our line of argument; for by the experiment
itself the conditions of life are changed. If it could be shown that our
domestic varieties manifested a strong tendency to reversion--that is,
to lose their acquired characters, while kept under the same conditions
and while kept in a considerable body, so that free intercrossing might
check, by blending together, any slight deviations in their structure,
in such case, I grant that we could deduce nothing from domestic
varieties in regard to species. But there is not a shadow of evidence
in favour of this view: to assert that we could not breed our cart
and race-horses, long and short-horned cattle, and poultry of various
breeds, and esculent vegetables, for an unlimited number of generations,
would be opposed to all experience.
 
CHARACTER OF DOMESTIC VARIETIES; DIFFICULTY OF DISTINGUISHING BETWEEN
VARIETIES AND SPECIES; ORIGIN OF DOMESTIC VARIETIES FROM ONE OR MORE
SPECIES.
 
When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with closely allied species, we
generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races often
have a somewhat monstrous character; by which I mean, that, although
differing from each other and from other species of the same genus, in
several trifling respects, they often differ in an extreme degree in
some one part, both when compared one with another, and more especially
when compared with the species under nature to which they are nearest
allied. With these exceptions (and with that of the perfect fertility of
varieties when crossed--a subject hereafter to be discussed), domestic
races of the same species differ from each other in the same manner as
do the closely allied species of the same genus in a state of nature,
but the differences in most cases are less in degree. This must be
admitted as true, for the domestic races of many animals and plants have
been ranked by some competent judges as the descendants of aboriginally
distinct species, and by other competent judges as mere varieties.
If any well marked distinction existed between a domestic race and a
species, this source of doubt would not so perpetually recur. It has
often been stated that domestic races do not differ from each other in
characters of generic value. It can be shown that this statement is not
correct; but naturalists differ much in determining what characters are
of generic value; all such valuations being at present empirical. When
it is explained how genera originate under nature, it will be seen that
we have no right to expect often to find a generic amount of difference
in our domesticated races.
 
In attempting to estimate the amount of structural difference between
allied domestic races, we are soon involved in doubt, from not knowing
whether they are descended from one or several parent species. This
point, if it could be cleared up, would be interesting; if, for
instance, it could be shown that the greyhound, bloodhound, terrier,
spaniel and bull-dog, which we all know propagate their kind truly, were
the offspring of any single species, then such facts would have great
weight in making us doubt about the immutability of the many closely
allied natural species--for instance, of the many foxes--inhabiting the
different quarters of the world. I do not believe, as we shall presently
see, that the whole amount of difference between the several breeds of
the dog has been produced under domestication; I believe that a small
part of the difference is due to their being descended from distinct
species. In the case of strongly marked races of some other domesticated
species, there is presumptive or even strong evidence that all are
descended from a single wild stock.
 
It has often been assumed that man has chosen for domestication animals
and plants having an extraordinary inherent tendency to vary, and
likewise to withstand diverse climates. I do not dispute that these
capacities have added largely to the value of most of our domesticated
productions; but how could a savage possibly know, when he first tamed
an animal, whether it would vary in succeeding generations, and whether
it would endure other climates? Has the little variability of the ass
and goose, or the small power of endurance of warmth by the reindeer,
or of cold by the common camel, prevented their domestication? I
cannot doubt that if other animals and plants, equal in number to our
domesticated productions, and belonging to equally diverse classes and
countries, were taken from a state of nature, and could be made to breed
for an equal number of generations under domestication, they would on
an average vary as largely as the parent species of our existing
domesticated productions have varied.
 
In the case of most of our anciently domesticated animals and plants,
it is not possible to come to any definite conclusion, whether they are
descended from one or several wild species. The argument mainly relied
on by those who believe in the multiple origin of our domestic animals
is, that we find in the most ancient times, on the monuments of Egypt,
and in the lake-habitations of Switzerland, much diversity in the
breeds; and that some of these ancient breeds closely resemble, or are
even identical with, those still existing. But this only throws far
backward the history of civilisation, and shows that animals were
domesticated at a much earlier period than has hitherto been supposed.
The lake-inhabitants of Switzerland cultivated several kinds of wheat
and barley, the pea, the poppy for oil and flax; and they possessed
several domesticated animals. They also carried on commerce with other
nations. All this clearly shows, as Heer has remarked, that they had at
this early age progressed considerably in civilisation; and this again
implies a long continued previous period of less advanced civilisation,
during which the domesticated animals, kept by different tribes in
different districts, might have varied and given rise to distinct races.
Since the discovery of flint tools in the superficial formations of many
parts of the world, all geologists believe that barbarian men existed at
an enormously remote period; and we know that at the present day there
is hardly a tribe so barbarous as not to have domesticated at least the
dog.
 
The origin of most of our domestic animals will probably forever remain
vague. But I may here state that, looking to the domestic dogs of the
whole world, I have, after a laborious collection of all known facts,
come to the conclusion that several wild species of Canidae have been
tamed, and that their blood, in some cases mingled together, flows in
the veins of our domestic breeds. In regard to sheep and goats I can
form no decided opinion. From facts communicated to me by Mr. Blyth,
on the habits, voice, constitution and structure of the humped Indian
cattle, it is almost certain that they are descended from a different
aboriginal stock from our European cattle; and some competent judges
believe that these latter have had two or three wild progenitors,
whether or not these deserve to be called species. This conclusion, as
well as that of the specific distinction between the humped and common
cattle, may, indeed, be looked upon as established by the admirable
researches of Professor Rutimeyer. With respect to horses, from reasons
which I cannot here give, I am doubtfully inclined to believe, in
opposition to several authors, that all the races belong to the same
species. Having kept nearly all the English breeds of the fowl alive,
having bred and crossed them, and examined their skeletons, it appears
to me almost certain that all are the descendants of the wild Indian
fowl, Gallus bankiva; and this is the conclusion of Mr. Blyth, and
of others who have studied this bird in India. In regard to ducks and
rabbits, some breeds of which differ much from each other, the evidence
is clear that they are all descended from the common duck and wild
rabbit.
 
The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some
authors. They believe that every race which breeds true, let the
distinctive characters be ever so slight, has had its wild prototype.
At this rate there must have existed at least a score of species of wild
cattle, as many sheep, and several goats, in Europe alone, and several
even within Great Britain. One author believes that there formerly
existed eleven wild species of sheep peculiar to Great Britain! When we
bear in mind that Britain has now not one peculiar mammal, and France
but few distinct from those of Germany, and so with Hungary, Spain,
etc., but that each of these kingdoms possesses several peculiar breeds
of cattle, sheep, etc., we must admit that many domestic breeds must
have originated in Europe; for whence otherwise could they have been
derived? So it is in India. Even in the case of the breeds of the
domestic dog throughout the world, which I admit are descended from
several wild species, it cannot be doubted that there has been an
immense amount of inherited variation; for who will believe that animals
closely resembling the Italian greyhound, the bloodhound, the bull-dog,
pug-dog, or Blenheim spaniel, etc.--so unlike all wild Canidae--ever
existed in a state of nature? It has often been loosely said that all
our races of dogs have been produced by the crossing of a few
aboriginal species; but by crossing we can only get forms in some degree
intermediate between their parents; and if we account for our several
domestic races by this process, we must admit the former existence of
the most extreme forms, as the Italian greyhound, bloodhound, bull-dog,
etc., in the wild state. Moreover, the possibility of making distinct
races by crossing has been greatly exaggerated. Many cases are on record
showing that a race may be modified by occasional crosses if aided
by the careful selection of the individuals which present the desired
character; but to obtain a race intermediate between two quite distinct
races would be very difficult. Sir J. Sebright expressly experimented
with this object and failed. The offspring from the first cross between
two pure breeds is tolerably and sometimes (as I have found with
pigeons) quite uniform in character, and every thing seems simple
enough; but when these mongrels are crossed one with another for several
generations, hardly two of them are alike, and then the difficulty of
the task becomes manifest.
 
BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES AND ORIGIN.
 
Believing that it is always best to study some special group, I have,
after deliberation, taken up domestic pigeons. I have kept every breed
which I could purchase or obtain, and have been most kindly favoured
with skins from several quarters of the world, more especially by the
Hon. W. Elliot from India, and by the Hon. C. Murray from Persia. Many
treatises in different languages have been published on pigeons, and
some of them are very important, as being of considerable antiquity. I
have associated with several eminent fanciers, and have been permitted
to join two of the London Pigeon Clubs. The diversity of the breeds is
something astonishing. Compare the English carrier and the short-faced
tumbler, and see the wonderful difference in their beaks, entailing
corresponding differences in their skulls. The carrier, more especially
the male bird, is also remarkable from the wonderful development of the
carunculated skin about the head, and this is accompanied by greatly
elongated eyelids, very large external orifices to the nostrils, and a
wide gape of mouth. The short-faced tumbler has a beak in outline almost
like that of a finch; and the common tumbler has the singular inherited
habit of flying at a great height in a compact flock, and tumbling in
the air head over heels. The runt is a bird of great size, with long,
massive beak and large feet; some of the sub-breeds of runts have very
long necks, others very long wings and tails, others singularly short
tails. The barb is allied to the carrier, but, instead of a long beak,
has a very short and broad one. The pouter has a much elongated body,
wings, and legs; and its enormously developed crop, which it glories in
inflating, may well excite astonishment and even laughter. The turbit
has a short and conical beak, with a line of reversed feathers down the
breast; and it has the habit of continually expanding, slightly, the
upper part of the oesophagus. The Jacobin has the feathers so much
reversed along the back of the neck that they form a hood, and it
has, proportionally to its size, elongated wing and tail feathers. The
trumpeter and laugher, as their names express, utter a very different
coo from the other breeds. The fantail has thirty or even forty
tail-feathers, instead of twelve or fourteen, the normal number in all
the members of the great pigeon family: these feathers are kept expanded
and are carried so erect that in good birds the head and tail touch: the
oil-gland is quite aborted. Several other less distinct breeds might be
specified.
 
In the skeletons of the several breeds, the development of the bones of
the face, in length and breadth and curvature, differs enormously. The
shape, as well as the breadth and length of the ramus of the lower jaw,
varies in a highly remarkable manner. The caudal and sacral vertebrae
vary in number; as does the number of the ribs, together with their
relative breadth and the presence of processes. The size and shape of
the apertures in the sternum are highly variable; so is the degree
of divergence and relative size of the two arms of the furcula. The
proportional width of the gape of mouth, the proportional length of the
eyelids, of the orifice of the nostrils, of the tongue (not always in
strict correlation with the length of beak), the size of the crop and
of the upper part of the oesophagus; the development and abortion of
the oil-gland; the number of the primary wing and caudal feathers; the
relative length of the wing and tail to each other and to the body;
the relative length of the leg and foot; the number of scutellae on
the toes, the development of skin between the toes, are all points of
structure which are variable. The period at which the perfect plumage is
acquired varies, as does the state of the down with which the nestling
birds are clothed when hatched. The shape and size of the eggs vary. The
manner of flight, and in some breeds the voice and disposition, differ
remarkably. Lastly, in certain breeds, the males and females have come
to differ in a slight degree from each other.
 
Altogether at least a score of pigeons might be chosen, which, if shown
to an ornithologist, and he were told that they were wild birds, would
certainly be ranked by him as well-defined species. Moreover, I do not
believe that any ornithologist would in this case place the English
carrier, the short-faced tumbler, the runt, the barb, pouter, and
fantail in the same genus; more especially as in each of these breeds
several truly-inherited sub-breeds, or species, as he would call them,
could be shown him.
 
Great as are the differences between the breeds of the pigeon, I am
fully convinced that the common opinion of naturalists is correct,
namely, that all are descended from the rock-pigeon (Columba livia),
including under this term several geographical races or sub-species,
which differ from each other in the most trifling respects. As several
of the reasons which have led me to this belief are in some degree
applicable in other cases, I will here briefly give them. If the several
breeds are not varieties, and have not proceeded from the rock-pigeon,
they must have descended from at least seven or eight aboriginal stocks;
for it is impossible to make the present domestic breeds by the crossing
of any lesser number: how, for instance, could a pouter be produced
by crossing two breeds unless one of the parent-stocks possessed the
characteristic enormous crop? The supposed aboriginal stocks must all
have been rock-pigeons, that is, they did not breed or willingly perch
on trees. But besides C. livia, with its geographical sub-species, only
two or three other species of rock-pigeons are known; and these have
not any of the characters of the domestic breeds. Hence the supposed
aboriginal stocks must either still exist in the countries where they
were originally domesticated, and yet be unknown to ornithologists; and
this, considering their size, habits and remarkable characters, seems
improbable; or they must have become extinct in the wild state. But
birds breeding on precipices, and good flyers, are unlikely to be
exterminated; and the common rock-pigeon, which has the same habits with
the domestic breeds, has not been exterminated even on several of the
smaller British islets, or on the shores of the Mediterranean. Hence the
supposed extermination of so many species having similar habits with
the rock-pigeon seems a very rash assumption. Moreover, the several
above-named domesticated breeds have been transported to all parts of
the world, and, therefore, some of them must have been carried back
again into their native country; but not one has become wild or feral,
though the dovecot-pigeon, which is the rock-pigeon in a very slightly
altered state, has become feral in several places. Again, all recent
experience shows that it is difficult to get wild animals to breed
freely under domestication; yet on the hypothesis of the multiple origin
of our pigeons, it must be assumed that at least seven or eight species
were so thoroughly domesticated in ancient times by half-civilized man,
as to be quite prolific under confinement.
 
An argument of great weight, and applicable in several other cases, is,
that the above-specified breeds, though agreeing generally with the wild
rock-pigeon in constitution, habits, voice, colouring, and in most parts
of their structure, yet are certainly highly abnormal in other parts; we
may look in vain through the whole great family of Columbidae for a beak
like that of the English carrier, or that of the short-faced tumbler, or
barb; for reversed feathers like those of the Jacobin; for a crop like
that of the pouter; for tail-feathers like those of the fantail. Hence
it must be assumed, not only that half-civilized man succeeded in
thoroughly domesticating several species, but that he intentionally or
by chance picked out extraordinarily abnormal species; and further, that
these very species have since all become extinct or unknown. So many
strange contingencies are improbable in the highest degree.
 
Some facts in regard to the colouring of pigeons well deserve
consideration. The rock-pigeon is of a slaty-blue, with white loins;
but the Indian sub-species, C. intermedia of Strickland, has this
part bluish. The tail has a terminal dark bar, with the outer feathers
externally edged at the base with white. The wings have two black bars.
Some semi-domestic breeds, and some truly wild breeds, have, besides the
two black bars, the wings chequered with black. These several marks do
not occur together in any other species of the whole family. Now, in
every one of the domestic breeds, taking thoroughly well-bred birds, all
the above marks, even to the white edging of the outer tail-feathers,
sometimes concur perfectly developed. Moreover, when birds belonging to
two or more distinct breeds are crossed, none of which are blue or have
any of the above-specified marks, the mongrel offspring are very apt
suddenly to acquire these characters. To give one instance out of
several which I have observed: I crossed some white fantails, which
breed very true, with some black barbs--and it so happens that blue
varieties of barbs are so rare that I never heard of an instance in
England; and the mongrels were black, brown and mottled. I also crossed
a barb with a spot, which is a white bird with a red tail and red spot
on the forehead, and which notoriously breeds very true; the mongrels
were dusky and mottled. I then crossed one of the mongrel barb-fantails
with a mongrel barb-spot, and they produced a bird of as beautiful a
blue colour, with the white loins, double black wing-bar, and barred and
white-edged tail-feathers, as any wild rock-pigeon! We can understand
these facts, on the well-known principle of reversion to ancestral
characters, if all the domestic breeds are descended from the
rock-pigeon. But if we deny this, we must make one of the two following
highly improbable suppositions. Either, first, that all the several
imagined aboriginal stocks were coloured and marked like the
rock-pigeon, although no other existing species is thus coloured and
marked, so that in each separate breed there might be a tendency to
revert to the very same colours and markings. Or, secondly, that each
breed, even the purest, has within a dozen, or at most within a score,
of generations, been crossed by the rock-pigeon: I say within a dozen
or twenty generations, for no instance is known of crossed descendants
reverting to an ancestor of foreign blood, removed by a greater number
of generations. In a breed which has been crossed only once the tendency
to revert to any character derived from such a cross will naturally
become less and less, as in each succeeding generation there will be
less of the foreign blood; but when there has been no cross, and there
is a tendency in the breed to revert to a character which was lost
during some former generation, this tendency, for all that we can see to
the contrary, may be transmitted undiminished for an indefinite
number of generations. These two distinct cases of reversion are often
confounded together by those who have written on inheritance.
 
Lastly, the hybrids or mongrels from between all the breeds of the
pigeon are perfectly fertile, as I can state from my own observations,
purposely made, on the most distinct breeds. Now, hardly any cases
have been ascertained with certainty of hybrids from two quite distinct
species of animals being perfectly fertile. Some authors believe
that long-continued domestication eliminates this strong tendency to
sterility in species. From the history of the dog, and of some other
domestic animals, this conclusion is probably quite correct, if applied
to species closely related to each other. But to extend it so far as to
suppose that species, aboriginally as distinct as carriers, tumblers,
pouters, and fantails now are, should yield offspring perfectly fertile,
inter se, seems to me rash in the extreme.
 
From these several reasons, namely, the improbability of man having
formerly made seven or eight supposed species of pigeons to breed freely
under domestication--these supposed species being quite unknown in a
wild state, and their not having become anywhere feral--these species
presenting certain very abnormal characters, as compared with all other
Columbidae, though so like the rock-pigeon in most other respects--the
occasional reappearance of the blue colour and various black marks in
all the breeds, both when kept pure and when crossed--and lastly, the
mongrel offspring being perfectly fertile--from these several reasons,
taken together, we may safely conclude that all our domestic breeds are
descended from the rock-pigeon or Columba livia with its geographical
sub-species.
 
In favour of this view, I may add, firstly, that the wild C. livia has
been found capable of domestication in Europe and in India; and that it
agrees in habits and in a great number of points of structure with all
the domestic breeds. Secondly, that although an English carrier or a
short-faced tumbler differs immensely in certain characters from the
rock-pigeon, yet that by comparing the several sub-breeds of these two
races, more especially those brought from distant countries, we can
make, between them and the rock-pigeon, an almost perfect series; so
we can in some other cases, but not with all the breeds. Thirdly,
those characters which are mainly distinctive of each breed are in each
eminently variable, for instance, the wattle and length of beak of
the carrier, the shortness of that of the tumbler, and the number of
tail-feathers in the fantail; and the explanation of this fact will be
obvious when we treat of selection. Fourthly, pigeons have been watched
and tended with the utmost care, and loved by many people. They have
been domesticated for thousands of years in several quarters of the
world; the earliest known record of pigeons is in the fifth Aegyptian
dynasty, about 3000 B.C., as was pointed out to me by Professor Lepsius;
but Mr. Birch informs me that pigeons are given in a bill of fare in
the previous dynasty. In the time of the Romans, as we hear from Pliny,
immense prices were given for pigeons; "nay, they are come to this pass,
that they can reckon up their pedigree and race." Pigeons were much
valued by Akber Khan in India, about the year 1600; never less than
20,000 pigeons were taken with the court. "The monarchs of Iran and
Turan sent him some very rare birds;" and, continues the courtly
historian, "His Majesty, by crossing the breeds, which method was never
practised before, has improved them astonishingly." About this same
period the Dutch were as eager about pigeons as were the old Romans. The
paramount importance of these considerations in explaining the immense
amount of variation which pigeons have undergone, will likewise be
obvious when we treat of selection. We shall then, also, see how it is
that the several breeds so often have a somewhat monstrous character.
It is also a most favourable circumstance for the production of distinct
breeds, that male and female pigeons can be easily mated for life; and
thus different breeds can be kept together in the same aviary.
 
I have discussed the probable origin of domestic pigeons at some,
yet quite insufficient, length; because when I first kept pigeons and
watched the several kinds, well knowing how truly they breed, I
felt fully as much difficulty in believing that since they had been
domesticated they had all proceeded from a common parent, as any
naturalist could in coming to a similar conclusion in regard to the
many species of finches, or other groups of birds, in nature. One
circumstance has struck me much; namely, that nearly all the breeders of
the various domestic animals and the cultivators of plants, with whom
I have conversed, or whose treatises I have read, are firmly convinced
that the several breeds to which each has attended, are descended
from so many aboriginally distinct species. Ask, as I have asked, a
celebrated raiser of Hereford cattle, whether his cattle might not have
descended from Long-horns, or both from a common parent-stock, and he
will laugh you to scorn. I have never met a pigeon, or poultry, or duck,
or rabbit fancier, who was not fully convinced that each main breed was
descended from a distinct species. Van Mons, in his treatise on pears
and apples, shows how utterly he disbelieves that the several sorts,
for instance a Ribston-pippin or Codlin-apple, could ever have proceeded
from the seeds of the same tree. Innumerable other examples could be
given. The explanation, I think, is simple: from long-continued study
they are strongly impressed with the differences between the several
races; and though they well know that each race varies slightly, for
they win their prizes by selecting such slight differences, yet they
ignore all general arguments, and refuse to sum up in their minds slight
differences accumulated during many successive generations. May not
those naturalists who, knowing far less of the laws of inheritance than
does the breeder, and knowing no more than he does of the intermediate
links in the long lines of descent, yet admit that many of our domestic
races are descended from the same parents--may they not learn a lesson
of caution, when they deride the idea of species in a state of nature
being lineal descendants of other species?
 
PRINCIPLES OF SELECTION ANCIENTLY FOLLOWED, AND THEIR EFFECTS.
 
Let us now briefly consider the steps by which domestic races have been
produced, either from one or from several allied species. Some effect
may be attributed to the direct and definite action of the external
conditions of life, and some to habit; but he would be a bold man who
would account by such agencies for the differences between a dray and
race-horse, a greyhound and bloodhound, a carrier and tumbler pigeon.
One of the most remarkable features in our domesticated races is that we
see in them adaptation, not indeed to the animal's or plant's own good,
but to man's use or fancy. Some variations useful to him have probably
arisen suddenly, or by one step; many botanists, for instance, believe
that the fuller's teasel, with its hooks, which can not be rivalled by
any mechanical contrivance, is only a variety of the wild Dipsacus; and
this amount of change may have suddenly arisen in a seedling. So it has
probably been with the turnspit dog; and this is known to have been
the case with the ancon sheep. But when we compare the dray-horse and
race-horse, the dromedary and camel, the various breeds of sheep fitted
either for cultivated land or mountain pasture, with the wool of one
breed good for one purpose, and that of another breed for another
purpose; when we compare the many breeds of dogs, each good for man
in different ways; when we compare the game-cock, so pertinacious in
battle, with other breeds so little quarrelsome, with "everlasting
layers" which never desire to sit, and with the bantam so small and
elegant; when we compare the host of agricultural, culinary, orchard,
and flower-garden races of plants, most useful to man at different
seasons and for different purposes, or so beautiful in his eyes, we
must, I think, look further than to mere variability. We can not suppose
that all the breeds were suddenly produced as perfect and as useful as
we now see them; indeed, in many cases, we know that this has not been
their history. The key is man's power of accumulative selection: nature
gives successive variations; man adds them up in certain directions
useful to him. In this sense he may be said to have made for himself
useful breeds.
 
The great power of this principle of selection is not hypothetical.
It is certain that several of our eminent breeders have, even within a
single lifetime, modified to a large extent their breeds of cattle
and sheep. In order fully to realise what they have done it is almost
necessary to read several of the many treatises devoted to this subject,
and to inspect the animals. Breeders habitually speak of an animal's
organisation as something plastic, which they can model almost as they
please. If I had space I could quote numerous passages to this effect
from highly competent authorities. Youatt, who was probably better
acquainted with the works of agriculturalists than almost any other
individual, and who was himself a very good judge of animals, speaks of
the principle of selection as "that which enables the agriculturist, not
only to modify the character of his flock, but to change it altogether.
It is the magician's wand, by means of which he may summon into life
whatever form and mould he pleases." Lord Somerville, speaking of
what breeders have done for sheep, says: "It would seem as if they had
chalked out upon a wall a form perfect in itself, and then had given it
existence." In Saxony the importance of the principle of selection in
regard to merino sheep is so fully recognised, that men follow it as a
trade: the sheep are placed on a table and are studied, like a picture
by a connoisseur; this is done three times at intervals of months, and
the sheep are each time marked and classed, so that the very best may
ultimately be selected for breeding.
 
What English breeders have actually effected is proved by the enormous
prices given for animals with a good pedigree; and these have been
exported to almost every quarter of the world. The improvement is by no
means generally due to crossing different breeds; all the best breeders
are strongly opposed to this practice, except sometimes among closely
allied sub-breeds. And when a cross has been made, the closest selection
is far more indispensable even than in ordinary cases. If selection
consisted merely in separating some very distinct variety and breeding
from it, the principle would be so obvious as hardly to be worth
notice; but its importance consists in the great effect produced by
the accumulation in one direction, during successive generations, of
differences absolutely inappreciable by an uneducated eye--differences
which I for one have vainly attempted to appreciate. Not one man in
a thousand has accuracy of eye and judgment sufficient to become an
eminent breeder. If gifted with these qualities, and he studies his
subject for years, and devotes his lifetime to it with indomitable
perseverance, he will succeed, and may make great improvements; if he
wants any of these qualities, he will assuredly fail. Few would readily
believe in the natural capacity and years of practice requisite to
become even a skilful pigeon-fancier.
 
The same principles are followed by horticulturists; but the variations
are here often more abrupt. No one supposes that our choicest
productions have been produced by a single variation from the aboriginal
stock. We have proofs that this is not so in several cases in which
exact records have been kept; thus, to give a very trifling instance,
the steadily increasing size of the common gooseberry may be quoted.
We see an astonishing improvement in many florists' flowers, when the
flowers of the present day are compared with drawings made only
twenty or thirty years ago. When a race of plants is once pretty well
established, the seed-raisers do not pick out the best plants, but
merely go over their seed-beds, and pull up the "rogues," as they call
the plants that deviate from the proper standard. With animals this kind
of selection is, in fact, likewise followed; for hardly any one is so
careless as to breed from his worst animals.
 
In regard to plants, there is another means of observing the accumulated
effects of selection--namely, by comparing the diversity of flowers in
the different varieties of the same species in the flower-garden; the
diversity of leaves, pods, or tubers, or whatever part is valued, in the
kitchen-garden, in comparison with the flowers of the same varieties;
and the diversity of fruit of the same species in the orchard, in
comparison with the leaves and flowers of the same set of varieties. See
how different the leaves of the cabbage are, and how extremely alike the
flowers; how unlike the flowers of the heartsease are, and how alike the
leaves; how much the fruit of the different kinds of gooseberries differ
in size, colour, shape, and hairiness, and yet the flowers present very
slight differences. It is not that the varieties which differ largely
in some one point do not differ at all in other points; this is hardly
ever--I speak after careful observation--perhaps never, the case. The
law of correlated variation, the importance of which should never be
overlooked, will ensure some differences; but, as a general rule, it
cannot be doubted that the continued selection of slight variations,
either in the leaves, the flowers, or the fruit, will produce races
differing from each other chiefly in these characters.
 
It may be objected that the principle of selection has been reduced to
methodical practice for scarcely more than three-quarters of a century;
it has certainly been more attended to of late years, and many treatises
have been published on the subject; and the result has been, in a
corresponding degree, rapid and important. But it is very far from
true that the principle is a modern discovery. I could give several
references to works of high antiquity, in which the full importance of
the principle is acknowledged. In rude and barbarous periods of English
history choice animals were often imported, and laws were passed to
prevent their exportation: the destruction of horses under a certain
size was ordered, and this may be compared to the "roguing" of plants
by nurserymen. The principle of selection I find distinctly given in an
ancient Chinese encyclopaedia. Explicit rules are laid down by some of
the Roman classical writers. From passages in Genesis, it is clear that
the colour of domestic animals was at that early period attended to.
Savages now sometimes cross their dogs with wild canine animals, to
improve the breed, and they formerly did so, as is attested by passages
in Pliny. The savages in South Africa match their draught cattle by
colour, as do some of the Esquimaux their teams of dogs. Livingstone
states that good domestic breeds are highly valued by the negroes in the
interior of Africa who have not associated with Europeans. Some of these
facts do not show actual selection, but they show that the breeding of
domestic animals was carefully attended to in ancient times, and is now
attended to by the lowest savages. It would, indeed, have been a strange
fact, had attention not been paid to breeding, for the inheritance of
good and bad qualities is so obvious.
 
UNCONSCIOUS SELECTION.
 
At the present time, eminent breeders try by methodical selection, with
a distinct object in view, to make a new strain or sub-breed, superior
to anything of the kind in the country. But, for our purpose, a form of
selection, which may be called unconscious, and which results from every
one trying to possess and breed from the best individual animals, is
more important. Thus, a man who intends keeping pointers naturally tries
to get as good dogs as he can, and afterwards breeds from his own best
dogs, but he has no wish or expectation of permanently altering the
breed. Nevertheless we may infer that this process, continued during
centuries, would improve and modify any breed, in the same way as
Bakewell, Collins, etc., by this very same process, only carried on more
methodically, did greatly modify, even during their lifetimes, the forms
and qualities of their cattle. Slow and insensible changes of this kind
could never be recognised unless actual measurements or careful drawings
of the breeds in question have been made long ago, which may serve for
comparison. In some cases, however, unchanged, or but little changed,
individuals of the same breed exist in less civilised districts, where
the breed has been less improved. There is reason to believe that King
Charles' spaniel has been unconsciously modified to a large extent
since the time of that monarch. Some highly competent authorities are
convinced that the setter is directly derived from the spaniel, and
has probably been slowly altered from it. It is known that the English
pointer has been greatly changed within the last century, and in this
case the change has, it is believed, been chiefly effected by crosses
with the foxhound; but what concerns us is, that the change has been
effected unconsciously and gradually, and yet so effectually that,
though the old Spanish pointer certainly came from Spain, Mr. Borrow
has not seen, as I am informed by him, any native dog in Spain like our
pointer.
 
By a similar process of selection, and by careful training, English
race-horses have come to surpass in fleetness and size the parent Arabs,
so that the latter, by the regulations for the Goodwood Races, are
favoured in the weights which they carry. Lord Spencer and others have
shown how the cattle of England have increased in weight and in early
maturity, compared with the stock formerly kept in this country. By
comparing the accounts given in various old treatises of the former
and present state of carrier and tumbler pigeons in Britain, India,
and Persia, we can trace the stages through which they have insensibly
passed, and come to differ so greatly from the rock-pigeon.
 
Youatt gives an excellent illustration of the effects of a course of
selection which may be considered as unconscious, in so far that the
breeders could never have expected, or even wished, to produce the
result which ensued--namely, the production of the distinct strains. The
two flocks of Leicester sheep kept by Mr. Buckley and Mr. Burgess, as
Mr. Youatt remarks, "Have been purely bred from the original stock
of Mr. Bakewell for upwards of fifty years. There is not a suspicion
existing in the mind of any one at all acquainted with the subject that
the owner of either of them has deviated in any one instance from the
pure blood of Mr. Bakewell's flock, and yet the difference between the
sheep possessed by these two gentlemen is so great that they have the
appearance of being quite different varieties."
 
If there exist savages so barbarous as never to think of the inherited
character of the offspring of their domestic animals, yet any one animal
particularly useful to them, for any special purpose, would be carefully
preserved during famines and other accidents, to which savages are
so liable, and such choice animals would thus generally leave more
offspring than the inferior ones; so that in this case there would be a
kind of unconscious selection going on. We see the value set on animals
even by the barbarians of Tierra del Fuego, by their killing and
devouring their old women, in times of dearth, as of less value than
their dogs.
 
In plants the same gradual process of improvement through the occasional
preservation of the best individuals, whether or not sufficiently
distinct to be ranked at their first appearance as distinct varieties,
and whether or not two or more species or races have become blended
together by crossing, may plainly be recognised in the increased size
and beauty which we now see in the varieties of the heartsease, rose,
pelargonium, dahlia, and other plants, when compared with the older
varieties or with their parent-stocks. No one would ever expect to get
a first-rate heartsease or dahlia from the seed of a wild plant. No one
would expect to raise a first-rate melting pear from the seed of a wild
pear, though he might succeed from a poor seedling growing wild, if it
had come from a garden-stock. The pear, though cultivated in classical
times, appears, from Pliny's description, to have been a fruit of very
inferior quality. I have seen great surprise expressed in horticultural
works at the wonderful skill of gardeners in having produced such
splendid results from such poor materials; but the art has been simple,
and, as far as the final result is concerned, has been followed almost
unconsciously. It has consisted in always cultivating the best known
variety, sowing its seeds, and, when a slightly better variety chanced
to appear, selecting it, and so onwards. But the gardeners of the
classical period, who cultivated the best pears which they could
procure, never thought what splendid fruit we should eat; though we
owe our excellent fruit in some small degree to their having naturally
chosen and preserved the best varieties they could anywhere find.
 
A large amount of change, thus slowly and unconsciously accumulated,
explains, as I believe, the well-known fact, that in a number of cases
we cannot recognise, and therefore do not know, the wild parent-stocks
of the plants which have been longest cultivated in our flower and
kitchen gardens. If it has taken centuries or thousands of years to
improve or modify most of our plants up to their present standard of
usefulness to man, we can understand how it is that neither Australia,
the Cape of Good Hope, nor any other region inhabited by quite
uncivilised man, has afforded us a single plant worth culture. It is
not that these countries, so rich in species, do not by a strange chance
possess the aboriginal stocks of any useful plants, but that the native
plants have not been improved by continued selection up to a standard
of perfection comparable with that acquired by the plants in countries
anciently civilised.
 
In regard to the domestic animals kept by uncivilised man, it should
not be overlooked that they almost always have to struggle for their
own food, at least during certain seasons. And in two countries very
differently circumstanced, individuals of the same species, having
slightly different constitutions or structure, would often succeed
better in the one country than in the other, and thus by a process of
"natural selection," as will hereafter be more fully explained, two
sub-breeds might be formed. This, perhaps, partly explains why the
varieties kept by savages, as has been remarked by some authors,
have more of the character of true species than the varieties kept in
civilised countries.
 
On the view here given of the important part which selection by man has
played, it becomes at once obvious, how it is that our domestic races
show adaptation in their structure or in their habits to man's wants
or fancies. We can, I think, further understand the frequently abnormal
character of our domestic races, and likewise their differences being so
great in external characters, and relatively so slight in internal parts
or organs. Man can hardly select, or only with much difficulty, any
deviation of structure excepting such as is externally visible; and
indeed he rarely cares for what is internal. He can never act by
selection, excepting on variations which are first given to him in some
slight degree by nature. No man would ever try to make a fantail till he
saw a pigeon with a tail developed in some slight degree in an unusual
manner, or a pouter till he saw a pigeon with a crop of somewhat unusual
size; and the more abnormal or unusual any character was when it first
appeared, the more likely it would be to catch his attention. But to use
such an expression as trying to make a fantail is, I have no doubt, in
most cases, utterly incorrect. The man who first selected a pigeon
with a slightly larger tail, never dreamed what the descendants of
that pigeon would become through long-continued, partly unconscious and
partly methodical, selection. Perhaps the parent bird of all fantails
had only fourteen tail-feathers somewhat expanded, like the present Java
fantail, or like individuals of other and distinct breeds, in which as
many as seventeen tail-feathers have been counted. Perhaps the first
pouter-pigeon did not inflate its crop much more than the turbit now
does the upper part of its oesophagus--a habit which is disregarded by
all fanciers, as it is not one of the points of the breed.
 
Nor let it be thought that some great deviation of structure would
be necessary to catch the fancier's eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however
slight, in one's own possession. Nor must the value which would formerly
have been set on any slight differences in the individuals of the same
species, be judged of by the value which is now set on them, after
several breeds have fairly been established. It is known that with
pigeons many slight variations now occasionally appear, but these are
rejected as faults or deviations from the standard of perfection in
each breed. The common goose has not given rise to any marked varieties;
hence the Toulouse and the common breed, which differ only in colour,
that most fleeting of characters, have lately been exhibited as distinct
at our poultry-shows.
 
These views appear to explain what has sometimes been noticed, namely,
that we know hardly anything about the origin or history of any of our
domestic breeds. But, in fact, a breed, like a dialect of a language,
can hardly be said to have a distinct origin. A man preserves and breeds
from an individual with some slight deviation of structure, or takes
more care than usual in matching his best animals, and thus improves
them, and the improved animals slowly spread in the immediate
neighbourhood. But they will as yet hardly have a distinct name,
and from being only slightly valued, their history will have been
disregarded. When further improved by the same slow and gradual process,
they will spread more widely, and will be recognised as something
distinct and valuable, and will then probably first receive a provincial
name. In semi-civilised countries, with little free communication, the
spreading of a new sub-breed will be a slow process. As soon as the
points of value are once acknowledged, the principle, as I have called
it, of unconscious selection will always tend--perhaps more at one
period than at another, as the breed rises or falls in fashion--perhaps
more in one district than in another, according to the state of
civilisation of the inhabitants--slowly to add to the characteristic
features of the breed, whatever they may be. But the chance will be
infinitely small of any record having been preserved of such slow,
varying, and insensible changes.
 
CIRCUMSTANCES FAVOURABLE TO MAN'S POWER OF SELECTION.
 
I will now say a few words on the circumstances, favourable or the
reverse, to man's power of selection. A high degree of variability is
obviously favourable, as freely giving the materials for selection to
work on; not that mere individual differences are not amply sufficient,
with extreme care, to allow of the accumulation of a large amount
of modification in almost any desired direction. But as variations
manifestly useful or pleasing to man appear only occasionally, the
chance of their appearance will be much increased by a large number of
individuals being kept. Hence number is of the highest importance for
success. On this principle Marshall formerly remarked, with respect
to the sheep of part of Yorkshire, "As they generally belong to poor
people, and are mostly IN SMALL LOTS, they never can be improved." On
the other hand, nurserymen, from keeping large stocks of the same plant,
are generally far more successful than amateurs in raising new and
valuable varieties. A large number of individuals of an animal or
plant can be reared only where the conditions for its propagation are
favourable. When the individuals are scanty all will be allowed to
breed, whatever their quality may be, and this will effectually prevent
selection. But probably the most important element is that the animal or
plant should be so highly valued by man, that the closest attention is
paid to even the slightest deviations in its qualities or structure.
Unless such attention be paid nothing can be effected. I have seen it
gravely remarked, that it was most fortunate that the strawberry began
to vary just when gardeners began to attend to this plant. No doubt the
strawberry had always varied since it was cultivated, but the slight
varieties had been neglected. As soon, however, as gardeners picked out
individual plants with slightly larger, earlier, or better fruit, and
raised seedlings from them, and again picked out the best seedlings and
bred from them, then (with some aid by crossing distinct species)
those many admirable varieties of the strawberry were raised which have
appeared during the last half-century.
 
With animals, facility in preventing crosses is an important element
in the formation of new races--at least, in a country which is already
stocked with other races. In this respect enclosure of the land plays a
part. Wandering savages or the inhabitants of open plains rarely possess
more than one breed of the same species. Pigeons can be mated for life,
and this is a great convenience to the fancier, for thus many races may
be improved and kept true, though mingled in the same aviary; and this
circumstance must have largely favoured the formation of new breeds.
Pigeons, I may add, can be propagated in great numbers and at a very
quick rate, and inferior birds may be freely rejected, as when killed
they serve for food. On the other hand, cats, from their nocturnal
rambling habits, can not be easily matched, and, although so much valued
by women and children, we rarely see a distinct breed long kept up; such
breeds as we do sometimes see are almost always imported from some other
country. Although I do not doubt that some domestic animals vary less
than others, yet the rarity or absence of distinct breeds of the cat,
the donkey, peacock, goose, etc., may be attributed in main part
to selection not having been brought into play: in cats, from the
difficulty in pairing them; in donkeys, from only a few being kept by
poor people, and little attention paid to their breeding; for recently
in certain parts of Spain and of the United States this animal has been
surprisingly modified and improved by careful selection; in peacocks,
from not being very easily reared and a large stock not kept; in geese,
from being valuable only for two purposes, food and feathers, and more
especially from no pleasure having been felt in the display of distinct
breeds; but the goose, under the conditions to which it is exposed when
domesticated, seems to have a singularly inflexible organisation, though
it has varied to a slight extent, as I have elsewhere described.
 
Some authors have maintained that the amount of variation in our
domestic productions is soon reached, and can never afterward be
exceeded. It would be somewhat rash to assert that the limit has been
attained in any one case; for almost all our animals and plants have
been greatly improved in many ways within a recent period; and this
implies variation. It would be equally rash to assert that characters
now increased to their utmost limit, could not, after remaining fixed
for many centuries, again vary under new conditions of life. No doubt,
as Mr. Wallace has remarked with much truth, a limit will be at last
reached. For instance, there must be a limit to the fleetness of any
terrestrial animal, as this will be determined by the friction to
be overcome, the weight of the body to be carried, and the power of
contraction in the muscular fibres. But what concerns us is that the
domestic varieties of the same species differ from each other in almost
every character, which man has attended to and selected, more than do
the distinct species of the same genera. Isidore Geoffroy St. Hilaire
has proved this in regard to size, and so it is with colour, and
probably with the length of hair. With respect to fleetness, which
depends on many bodily characters, Eclipse was far fleeter, and
a dray-horse is comparably stronger, than any two natural species
belonging to the same genus. So with plants, the seeds of the different
varieties of the bean or maize probably differ more in size than do the
seeds of the distinct species in any one genus in the same two families.
The same remark holds good in regard to the fruit of the several
varieties of the plum, and still more strongly with the melon, as well
as in many other analogous cases.
 
To sum up on the origin of our domestic races of animals and plants.
Changed conditions of life are of the highest importance in causing
variability, both by acting directly on the organisation, and
indirectly by affecting the reproductive system. It is not probable
that variability is an inherent and necessary contingent, under all
circumstances. The greater or less force of inheritance and reversion
determine whether variations shall endure. Variability is governed
by many unknown laws, of which correlated growth is probably the most
important. Something, but how much we do not know, may be attributed to
the definite action of the conditions of life. Some, perhaps a great,
effect may be attributed to the increased use or disuse of parts. The
final result is thus rendered infinitely complex. In some cases the
intercrossing of aboriginally distinct species appears to have played
an important part in the origin of our breeds. When several breeds have
once been formed in any country, their occasional intercrossing, with
the aid of selection, has, no doubt, largely aided in the formation
of new sub-breeds; but the importance of crossing has been much
exaggerated, both in regard to animals and to those plants which are
propagated by seed. With plants which are temporarily propagated by
cuttings, buds, etc., the importance of crossing is immense; for the
cultivator may here disregard the extreme variability both of hybrids
and of mongrels, and the sterility of hybrids; but plants not propagated
by seed are of little importance to us, for their endurance is only
temporary. Over all these causes of change, the accumulative action of
selection, whether applied methodically and quickly, or unconsciously
and slowly, but more efficiently, seems to have been the predominant
power.
 
 
 
 
CHAPTER II. VARIATION UNDER NATURE.
 
 Variability--Individual differences--Doubtful species--Wide ranging,
 much diffused, and common species, vary most--Species of the larger
 genera in each country vary more frequently than the species of the
 smaller genera--Many of the species of the larger genera resemble
 varieties in being very closely, but unequally, related to each other,
 and in having restricted ranges.
 
 
Before applying the principles arrived at in the last chapter to organic
beings in a state of nature, we must briefly discuss whether these
latter are subject to any variation. To treat this subject properly, a
long catalogue of dry facts ought to be given; but these I shall reserve
for a future work. Nor shall I here discuss the various definitions
which have been given of the term species. No one definition has
satisfied all naturalists; yet every naturalist knows vaguely what
he means when he speaks of a species. Generally the term includes the
unknown element of a distinct act of creation. The term "variety" is
almost equally difficult to define; but here community of descent is
almost universally implied, though it can rarely be proved. We have also
what are called monstrosities; but they graduate into varieties. By a
monstrosity I presume is meant some considerable deviation of structure,
generally injurious, or not useful to the species. Some authors use
the term "variation" in a technical sense, as implying a modification
directly due to the physical conditions of life; and "variations" in
this sense are supposed not to be inherited; but who can say that the
dwarfed condition of shells in the brackish waters of the Baltic, or
dwarfed plants on Alpine summits, or the thicker fur of an animal from
far northwards, would not in some cases be inherited for at least a few
generations? And in this case I presume that the form would be called a
variety.
 
It may be doubted whether sudden and considerable deviations of
structure, such as we occasionally see in our domestic productions, more
especially with plants, are ever permanently propagated in a state
of nature. Almost every part of every organic being is so beautifully
related to its complex conditions of life that it seems as improbable
that any part should have been suddenly produced perfect, as that a
complex machine should have been invented by man in a perfect state.
Under domestication monstrosities sometimes occur which resemble normal
structures in widely different animals. Thus pigs have occasionally been
born with a sort of proboscis, and if any wild species of the same genus
had naturally possessed a proboscis, it might have been argued that this
had appeared as a monstrosity; but I have as yet failed to find, after
diligent search, cases of monstrosities resembling normal structures in
nearly allied forms, and these alone bear on the question. If monstrous
forms of this kind ever do appear in a state of nature and are capable
of reproduction (which is not always the case), as they occur rarely
and singly, their preservation would depend on unusually favourable
circumstances. They would, also, during the first and succeeding
generations cross with the ordinary form, and thus their abnormal
character would almost inevitably be lost. But I shall have to return
in a future chapter to the preservation and perpetuation of single or
occasional variations.
 
INDIVIDUAL DIFFERENCES.
 
The many slight differences which appear in the offspring from the
same parents, or which it may be presumed have thus arisen, from being
observed in the individuals of the same species inhabiting the same
confined locality, may be called individual differences. No one supposes
that all the individuals of the same species are cast in the same actual
mould. These individual differences are of the highest importance for
us, for they are often inherited, as must be familiar to every one;
and they thus afford materials for natural selection to act on and
accumulate, in the same manner as man accumulates in any given direction
individual differences in his domesticated productions. These individual
differences generally affect what naturalists consider unimportant
parts; but I could show, by a long catalogue of facts, that parts
which must be called important, whether viewed under a physiological or
classificatory point of view, sometimes vary in the individuals of the
same species. I am convinced that the most experienced naturalist
would be surprised at the number of the cases of variability, even in
important parts of structure, which he could collect on good authority,
as I have collected, during a course of years. It should be remembered
that systematists are far from being pleased at finding variability
in important characters, and that there are not many men who will
laboriously examine internal and important organs, and compare them in
many specimens of the same species. It would never have been expected
that the branching of the main nerves close to the great central
ganglion of an insect would have been variable in the same species;
it might have been thought that changes of this nature could have been
effected only by slow degrees; yet Sir J. Lubbock has shown a degree of
variability in these main nerves in Coccus, which may almost be compared
to the irregular branching of the stem of a tree. This philosophical
naturalist, I may add, has also shown that the muscles in the larvae
of certain insects are far from uniform. Authors sometimes argue in a
circle when they state that important organs never vary; for these
same authors practically rank those parts as important (as some few
naturalists have honestly confessed) which do not vary; and, under
this point of view, no instance will ever be found of an important part
varying; but under any other point of view many instances assuredly can
be given.
 
There is one point connected with individual differences which is
extremely perplexing: I refer to those genera which have been called
"protean" or "polymorphic," in which species present an inordinate
amount of variation. With respect to many of these forms, hardly two
naturalists agree whether to rank them as species or as varieties. We
may instance Rubus, Rosa, and Hieracium among plants, several genera of
insects, and of Brachiopod shells. In most polymorphic genera some
of the species have fixed and definite characters. Genera which
are polymorphic in one country seem to be, with a few exceptions,
polymorphic in other countries, and likewise, judging from Brachiopod
shells, at former periods of time. These facts are very perplexing, for
they seem to show that this kind of variability is independent of the
conditions of life. I am inclined to suspect that we see, at least in
some of these polymorphic genera, variations which are of no service or
disservice to the species, and which consequently have not been seized
on and rendered definite by natural selection, as hereafter to be
explained.
 
Individuals of the same species often present, as is known to every one,
great differences of structure, independently of variation, as in the
two sexes of various animals, in the two or three castes of sterile
females or workers among insects, and in the immature and larval states
of many of the lower animals. There are, also, cases of dimorphism and
trimorphism, both with animals and plants. Thus, Mr. Wallace, who has
lately called attention to the subject, has shown that the females of
certain species of butterflies, in the Malayan Archipelago, regularly
appear under two or even three conspicuously distinct forms, not
connected by intermediate varieties. Fritz Muller has described
analogous but more extraordinary cases with the males of certain
Brazilian Crustaceans: thus, the male of a Tanais regularly occurs
under two distinct forms; one of these has strong and differently shaped
pincers, and the other has antennae much more abundantly furnished with
smelling-hairs. Although in most of these cases, the two or three forms,
both with animals and plants, are not now connected by intermediate
gradations, it is possible that they were once thus connected. Mr.
Wallace, for instance, describes a certain butterfly which presents in
the same island a great range of varieties connected by intermediate
links, and the extreme links of the chain closely resemble the two forms
of an allied dimorphic species inhabiting another part of the Malay
Archipelago. Thus also with ants, the several worker-castes are
generally quite distinct; but in some cases, as we shall hereafter see,
the castes are connected together by finely graduated varieties. So it
is, as I have myself observed, with some dimorphic plants. It certainly
at first appears a highly remarkable fact that the same female butterfly
should have the power of producing at the same time three distinct
female forms and a male; and that an hermaphrodite plant should produce
from the same seed-capsule three distinct hermaphrodite forms, bearing
three different kinds of females and three or even six different kinds
of males. Nevertheless these cases are only exaggerations of the common
fact that the female produces offspring of two sexes which sometimes
differ from each other in a wonderful manner.
 
DOUBTFUL SPECIES.
 
The forms which possess in some considerable degree the character of
species, but which are so closely similar to other forms, or are so
closely linked to them by intermediate gradations, that naturalists do
not like to rank them as distinct species, are in several respects the
most important for us. We have every reason to believe that many of
these doubtful and closely allied forms have permanently retained their
characters for a long time; for as long, as far as we know, as have good
and true species. Practically, when a naturalist can unite by means of
intermediate links any two forms, he treats the one as a variety of the
other, ranking the most common, but sometimes the one first described
as the species, and the other as the variety. But cases of great
difficulty, which I will not here enumerate, sometimes arise in deciding
whether or not to rank one form as a variety of another, even when
they are closely connected by intermediate links; nor will the commonly
assumed hybrid nature of the intermediate forms always remove the
difficulty. In very many cases, however, one form is ranked as a variety
of another, not because the intermediate links have actually been found,
but because analogy leads the observer to suppose either that they do
now somewhere exist, or may formerly have existed; and here a wide door
for the entry of doubt and conjecture is opened.
 
Hence, in determining whether a form should be ranked as a species or
a variety, the opinion of naturalists having sound judgment and wide
experience seems the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-marked and
well-known varieties can be named which have not been ranked as species
by at least some competent judges.
 
That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France, or
of the United States, drawn up by different botanists, and see what
a surprising number of forms have been ranked by one botanist as good
species, and by another as mere varieties. Mr. H.C. Watson, to whom I
lie under deep obligation for assistance of all kinds, has marked for
me 182 British plants, which are generally considered as varieties, but
which have all been ranked by botanists as species; and in making this
list he has omitted many trifling varieties, but which nevertheless have
been ranked by some botanists as species, and he has entirely omitted
several highly polymorphic genera. Under genera, including the most
polymorphic forms, Mr. Babington gives 251 species, whereas Mr. Bentham
gives only 112--a difference of 139 doubtful forms! Among animals which
unite for each birth, and which are highly locomotive, doubtful forms,
ranked by one zoologist as a species and by another as a variety, can
rarely be found within the same country, but are common in separated
areas. How many of the birds and insects in North America and Europe,
which differ very slightly from each other, have been ranked by one
eminent naturalist as undoubted species, and by another as varieties,
or, as they are often called, geographical races! Mr. Wallace, in
several valuable papers on the various animals, especially on the
Lepidoptera, inhabiting the islands of the great Malayan Archipelago,
shows that they may be classed under four heads, namely, as variable
forms, as local forms, as geographical races or sub-species, and as true
representative species. The first or variable forms vary much within the
limits of the same island. The local forms are moderately constant and
distinct in each separate island; but when all from the several islands
are compared together, the differences are seen to be so slight and
graduated that it is impossible to define or describe them, though
at the same time the extreme forms are sufficiently distinct. The
geographical races or sub-species are local forms completely fixed and
isolated; but as they do not differ from each other by strongly marked
and important characters, "There is no possible test but individual
opinion to determine which of them shall be considered as species and
which as varieties." Lastly, representative species fill the same
place in the natural economy of each island as do the local forms and
sub-species; but as they are distinguished from each other by a greater
amount of difference than that between the local forms and sub-species,
they are almost universally ranked by naturalists as true species.
Nevertheless, no certain criterion can possibly be given by which
variable forms, local forms, sub species and representative species can
be recognised.
 
Many years ago, when comparing, and seeing others compare, the birds
from the closely neighbouring islands of the Galapagos Archipelago,
one with another, and with those from the American mainland, I was
much struck how entirely vague and arbitrary is the distinction between
species and varieties. On the islets of the little Madeira group there
are many insects which are characterized as varieties in Mr. Wollaston's
admirable work, but which would certainly be ranked as distinct species
by many entomologists. Even Ireland has a few animals, now generally
regarded as varieties, but which have been ranked as species by some
zoologists. Several experienced ornithologists consider our British red
grouse as only a strongly marked race of a Norwegian species, whereas
the greater number rank it as an undoubted species peculiar to Great
Britain. A wide distance between the homes of two doubtful forms leads
many naturalists to rank them as distinct species; but what distance, it
has been well asked, will suffice if that between America and Europe
is ample, will that between Europe and the Azores, or Madeira, or the
Canaries, or between the several islets of these small archipelagos, be
sufficient?
 
Mr. B.D. Walsh, a distinguished entomologist of the United States, has
described what he calls Phytophagic varieties and Phytophagic species.
Most vegetable-feeding insects live on one kind of plant or on one
group of plants; some feed indiscriminately on many kinds, but do not
in consequence vary. In several cases, however, insects found living on
different plants, have been observed by Mr. Walsh to present in their
larval or mature state, or in both states, slight, though constant
differences in colour, size, or in the nature of their secretions.
In some instances the males alone, in other instances, both males and
females, have been observed thus to differ in a slight degree. When the
differences are rather more strongly marked, and when both sexes and
all ages are affected, the forms are ranked by all entomologists as good
species. But no observer can determine for another, even if he can do so
for himself, which of these Phytophagic forms ought to be called species
and which varieties. Mr. Walsh ranks the forms which it may be supposed
would freely intercross, as varieties; and those which appear to have
lost this power, as species. As the differences depend on the insects
having long fed on distinct plants, it cannot be expected that
intermediate links connecting the several forms should now be found.
The naturalist thus loses his best guide in determining whether to rank
doubtful forms as varieties or species. This likewise necessarily occurs
with closely allied organisms, which inhabit distinct continents or
islands. When, on the other hand, an animal or plant ranges over the
same continent, or inhabits many islands in the same archipelago, and
presents different forms in the different areas, there is always a
good chance that intermediate forms will be discovered which will link
together the extreme states; and these are then degraded to the rank of
varieties.
 
Some few naturalists maintain that animals never present varieties; but
then these same naturalists rank the slightest difference as of specific
value; and when the same identical form is met with in two distant
countries, or in two geological formations, they believe that two
distinct species are hidden under the same dress. The term species thus
comes to be a mere useless abstraction, implying and assuming a separate
act of creation. It is certain that many forms, considered by highly
competent judges to be varieties, resemble species so completely in
character that they have been thus ranked by other highly competent
judges. But to discuss whether they ought to be called species or
varieties, before any definition of these terms has been generally
accepted, is vainly to beat the air.
 
Many of the cases of strongly marked varieties or doubtful species well
deserve consideration; for several interesting lines of argument, from
geographical distribution, analogical variation, hybridism, etc., have
been brought to bear in the attempt to determine their rank; but space
does not here permit me to discuss them. Close investigation, in many
cases, will no doubt bring naturalists to agree how to rank doubtful
forms. Yet it must be confessed that it is in the best known countries
that we find the greatest number of them. I have been struck with the
fact that if any animal or plant in a state of nature be highly useful
to man, or from any cause closely attracts his attention, varieties of
it will almost universally be found recorded. These varieties, moreover,
will often be ranked by some authors as species. Look at the common oak,
how closely it has been studied; yet a German author makes more than a
dozen species out of forms, which are almost universally considered
by other botanists to be varieties; and in this country the highest
botanical authorities and practical men can be quoted to show that the
sessile and pedunculated oaks are either good and distinct species or
mere varieties.
 
I may here allude to a remarkable memoir lately published by A. de
Candolle, on the oaks of the whole world. No one ever had more ample
materials for the discrimination of the species, or could have worked on
them with more zeal and sagacity. He first gives in detail all the many
points of structure which vary in the several species, and estimates
numerically the relative frequency of the variations. He specifies above
a dozen characters which may be found varying even on the same branch,
sometimes according to age or development, sometimes without any
assignable reason. Such characters are not of course of specific value,
but they are, as Asa Gray has remarked in commenting on this memoir,
such as generally enter into specific definitions. De Candolle then goes
on to say that he gives the rank of species to the forms that differ by
characters never varying on the same tree, and never found connected
by intermediate states. After this discussion, the result of so much
labour, he emphatically remarks: "They are mistaken, who repeat that the
greater part of our species are clearly limited, and that the doubtful
species are in a feeble minority. This seemed to be true, so long as
a genus was imperfectly known, and its species were founded upon a few
specimens, that is to say, were provisional. Just as we come to know
them better, intermediate forms flow in, and doubts as to specific
limits augment." He also adds that it is the best known species which
present the greatest number of spontaneous varieties and sub-varieties.
Thus Quercus robur has twenty-eight varieties, all of which, excepting
six, are clustered round three sub-species, namely Q. pedunculata,
sessiliflora and pubescens. The forms which connect these three
sub-species are comparatively rare; and, as Asa Gray again remarks, if
these connecting forms which are now rare were to become totally extinct
the three sub-species would hold exactly the same relation to each other
as do the four or five provisionally admitted species which closely
surround the typical Quercus robur. Finally, De Candolle admits that
out of the 300 species, which will be enumerated in his Prodromus
as belonging to the oak family, at least two-thirds are provisional
species, that is, are not known strictly to fulfil the definition above
given of a true species. It should be added that De Candolle no longer
believes that species are immutable creations, but concludes that the
derivative theory is the most natural one, "and the most accordant with
the known facts in palaeontology, geographical botany and zoology, of
anatomical structure and classification."
 
When a young naturalist commences the study of a group of organisms
quite unknown to him he is at first much perplexed in determining what
differences to consider as specific and what as varietal; for he knows
nothing of the amount and kind of variation to which the group is
subject; and this shows, at least, how very generally there is some
variation. But if he confine his attention to one class within one
country he will soon make up his mind how to rank most of the doubtful
forms. His general tendency will be to make many species, for he will
become impressed, just like the pigeon or poultry fancier before alluded
to, with the amount of difference in the forms which he is continually
studying; and he has little general knowledge of analogical variation
in other groups and in other countries by which to correct his first
impressions. As he extends the range of his observations he will meet
with more cases of difficulty; for he will encounter a greater number of
closely-allied forms. But if his observations be widely extended he
will in the end generally be able to make up his own mind; but he will
succeed in this at the expense of admitting much variation, and the
truth of this admission will often be disputed by other naturalists.
When he comes to study allied forms brought from countries not now
continuous, in which case he cannot hope to find intermediate links,
he will be compelled to trust almost entirely to analogy, and his
difficulties will rise to a climax.
 
Certainly no clear line of demarcation has as yet been drawn between
species and sub-species--that is, the forms which in the opinion of some
naturalists come very near to, but do not quite arrive at, the rank of
species; or, again, between sub-species and well-marked varieties, or
between lesser varieties and individual differences. These differences
blend into each other by an insensible series; and a series impresses
the mind with the idea of an actual passage.
 
Hence I look at individual differences, though of small interest to the
systematist, as of the highest importance for us, as being the first
step towards such slight varieties as are barely thought worth recording
in works on natural history. And I look at varieties which are in any
degree more distinct and permanent, as steps toward more strongly marked
and permanent varieties; and at the latter, as leading to sub-species,
and then to species. The passage from one stage of difference to another
may, in many cases, be the simple result of the nature of the organism
and of the different physical conditions to which it has long been
exposed; but with respect to the more important and adaptive characters,
the passage from one stage of difference to another may be safely
attributed to the cumulative action of natural selection, hereafter
to be explained, and to the effects of the increased use or disuse
of parts. A well-marked variety may therefore be called an incipient
species; but whether this belief is justifiable must be judged by the
weight of the various facts and considerations to be given throughout
this work.
 
It need not be supposed that all varieties or incipient species attain
the rank of species. They may become extinct, or they may endure as
varieties for very long periods, as has been shown to be the case by Mr.
Wollaston with the varieties of certain fossil land-shells in Madeira,
and with plants by Gaston de Saporta. If a variety were to flourish so
as to exceed in numbers the parent species, it would then rank as the
species, and the species as the variety; or it might come to supplant
and exterminate the parent species; or both might co-exist, and both
rank as independent species. But we shall hereafter return to this
subject.
 
From these remarks it will be seen that I look at the term species
as one arbitrarily given, for the sake of convenience, to a set
of individuals closely resembling each other, and that it does not
essentially differ from the term variety, which is given to less
distinct and more fluctuating forms. The term variety, again,
in comparison with mere individual differences, is also applied
arbitrarily, for convenience sake.
 
WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST.
 
Guided by theoretical considerations, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr. H.C.
Watson, to whom I am much indebted for valuable advice and assistance
on this subject, soon convinced me that there were many difficulties, as
did subsequently Dr. Hooker, even in stronger terms. I shall reserve for
a future work the discussion of these difficulties, and the tables of
the proportional numbers of the varying species. Dr. Hooker permits me
to add that after having carefully read my manuscript, and examined
the tables, he thinks that the following statements are fairly well
established. The whole subject, however, treated as it necessarily here
is with much brevity, is rather perplexing, and allusions cannot be
avoided to the "struggle for existence," "divergence of character," and
other questions, hereafter to be discussed.
 
Alphonse de Candolle and others have shown that plants which have
very wide ranges generally present varieties; and this might have been
expected, as they are exposed to diverse physical conditions, and as
they come into competition (which, as we shall hereafter see, is a far
more important circumstance) with different sets of organic beings. But
my tables further show that, in any limited country, the species which
are the most common, that is abound most in individuals, and the species
which are most widely diffused within their own country (and this is a
different consideration from wide range, and to a certain extent from
commonness), oftenest give rise to varieties sufficiently well-marked to
have been recorded in botanical works. Hence it is the most flourishing,
or, as they may be called, the dominant species--those which range
widely, are the most diffused in their own country, and are the most
numerous in individuals--which oftenest produce well-marked varieties,
or, as I consider them, incipient species. And this, perhaps, might have
been anticipated; for, as varieties, in order to become in any degree
permanent, necessarily have to struggle with the other inhabitants of
the country, the species which are already dominant will be the most
likely to yield offspring, which, though in some slight degree modified,
still inherit those advantages that enabled their parents to become
dominant over their compatriots. In these remarks on predominence, it
should be understood that reference is made only to the forms which come
into competition with each other, and more especially to the members
of the same genus or class having nearly similar habits of life. With
respect to the number of individuals or commonness of species, the
comparison of course relates only to the members of the same group. One
of the higher plants may be said to be dominant if it be more numerous
in individuals and more widely diffused than the other plants of the
same country, which live under nearly the same conditions. A plant of
this kind is not the less dominant because some conferva inhabiting
the water or some parasitic fungus is infinitely more numerous in
individuals, and more widely diffused. But if the conferva or parasitic
fungus exceeds its allies in the above respects, it will then be
dominant within its own class.
 
SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY MORE FREQUENTLY THAN
THE SPECIES OF THE SMALLER GENERA.
 
If the plants inhabiting a country as described in any Flora, be divided
into two equal masses, all those in the larger genera (i.e., those
including many species) being placed on one side, and all those in the
smaller genera on the other side, the former will be found to include a
somewhat larger number of the very common and much diffused or dominant
species. This might have been anticipated, for the mere fact of many
species of the same genus inhabiting any country, shows that there
is something in the organic or inorganic conditions of that country
favourable to the genus; and, consequently, we might have expected to
have found in the larger genera, or those including many species, a
larger proportional number of dominant species. But so many causes tend
to obscure this result, that I am surprised that my tables show even a
small majority on the side of the larger genera. I will here allude
to only two causes of obscurity. Fresh water and salt-loving plants
generally have very wide ranges and are much diffused, but this seems to
be connected with the nature of the stations inhabited by them, and has
little or no relation to the size of the genera to which the species
belong. Again, plants low in the scale of organisation are generally
much more widely diffused than plants higher in the scale; and here
again there is no close relation to the size of the genera. The cause of
lowly-organised plants ranging widely will be discussed in our chapter
on Geographical Distribution.
 
From looking at species as only strongly marked and well-defined
varieties, I was led to anticipate that the species of the larger genera
in each country would oftener present varieties, than the species of the
smaller genera; for wherever many closely related species (i.e., species
of the same genus) have been formed, many varieties or incipient species
ought, as a general rule, to be now forming. Where many large trees
grow, we expect to find saplings. Where many species of a genus have
been formed through variation, circumstances have been favourable
for variation; and hence we might expect that the circumstances would
generally still be favourable to variation. On the other hand, if we
look at each species as a special act of creation, there is no apparent
reason why more varieties should occur in a group having many species,
than in one having few.
 
To test the truth of this anticipation I have arranged the plants of
twelve countries, and the coleopterous insects of two districts, into
two nearly equal masses, the species of the larger genera on one side,
and those of the smaller genera on the other side, and it has invariably
proved to be the case that a larger proportion of the species on the
side of the larger genera presented varieties, than on the side of the
smaller genera. Moreover, the species of the large genera which present
any varieties, invariably present a larger average number of varieties
than do the species of the small genera. Both these results follow when
another division is made, and when all the least genera, with from only
one to four species, are altogether excluded from the tables. These
facts are of plain signification on the view that species are only
strongly marked and permanent varieties; for wherever many species of
the same genus have been formed, or where, if we may use the expression,
the manufactory of species has been active, we ought generally to find
the manufactory still in action, more especially as we have every reason
to believe the process of manufacturing new species to be a slow one.
And this certainly holds true if varieties be looked at as incipient
species; for my tables clearly show, as a general rule, that, wherever
many species of a genus have been formed, the species of that genus
present a number of varieties, that is, of incipient species, beyond the
average. It is not that all large genera are now varying much, and are
thus increasing in the number of their species, or that no small genera
are now varying and increasing; for if this had been so, it would have
been fatal to my theory; inasmuch as geology plainly tells us that small
genera have in the lapse of time often increased greatly in size;
and that large genera have often come to their maxima, declined, and
disappeared. All that we want to show is, that where many species of a
genus have been formed, on an average many are still forming; and this
certainly holds good.
 
MANY OF THE SPECIES INCLUDED WITHIN THE LARGER GENERA RESEMBLE VARIETIES
IN BEING VERY CLOSELY, BUT UNEQUALLY, RELATED TO EACH OTHER, AND IN
HAVING RESTRICTED RANGES.
 
There are other relations between the species of large genera and their
recorded varieties which deserve notice. We have seen that there is no
infallible criterion by which to distinguish species and well-marked
varieties; and when intermediate links have not been found between
doubtful forms, naturalists are compelled to come to a determination by
the amount of difference between them, judging by analogy whether or not
the amount suffices to raise one or both to the rank of species. Hence
the amount of difference is one very important criterion in settling
whether two forms should be ranked as species or varieties. Now Fries
has remarked in regard to plants, and Westwood in regard to insects,
that in large genera the amount of difference between the species is
often exceedingly small. I have endeavoured to test this numerically by
averages, and, as far as my imperfect results go, they confirm the view.
I have also consulted some sagacious and experienced observers,
and, after deliberation, they concur in this view. In this respect,
therefore, the species of the larger genera resemble varieties, more
than do the species of the smaller genera. Or the case may be put in
another way, and it may be said, that in the larger genera, in which a
number of varieties or incipient species greater than the average are
now manufacturing, many of the species already manufactured still to a
certain extent resemble varieties, for they differ from each other by a
less than the usual amount of difference.
 
Moreover, the species of the larger genera are related to each other, in
the same manner as the varieties of any one species are related to
each other. No naturalist pretends that all the species of a genus are
equally distinct from each other; they may generally be divided into
sub-genera, or sections, or lesser groups. As Fries has well remarked,
little groups of species are generally clustered like satellites around
other species. And what are varieties but groups of forms, unequally
related to each other, and clustered round certain forms--that is, round
their parent-species. Undoubtedly there is one most important point of
difference between varieties and species, namely, that the amount of
difference between varieties, when compared with each other or with
their parent-species, is much less than that between the species of the
same genus. But when we come to discuss the principle, as I call it, of
divergence of character, we shall see how this may be explained, and
how the lesser differences between varieties tend to increase into the
greater differences between species.
 
There is one other point which is worth notice. Varieties generally have
much restricted ranges. This statement is indeed scarcely more than a
truism, for if a variety were found to have a wider range than that of
its supposed parent-species, their denominations would be reversed.
But there is reason to believe that the species which are very closely
allied to other species, and in so far resemble varieties, often have
much restricted ranges. For instance, Mr. H.C. Watson has marked for me
in the well-sifted London catalogue of Plants (4th edition) sixty-three
plants which are therein ranked as species, but which he considers as
so closely allied to other species as to be of doubtful value: these
sixty-three reputed species range on an average over 6.9 of the
provinces into which Mr. Watson has divided Great Britain. Now, in this
same catalogue, fifty-three acknowledged varieties are recorded, and
these range over 7.7 provinces; whereas, the species to which these
varieties belong range over 14.3 provinces. So that the acknowledged
varieties have very nearly the same restricted average range, as have
the closely allied forms, marked for me by Mr. Watson as doubtful
species, but which are almost universally ranked by British botanists as
good and true species.
 
SUMMARY.
 
Finally, varieties cannot be distinguished from species--except, first,
by the discovery of intermediate linking forms; and, secondly, by a
certain indefinite amount of difference between them; for two forms,
if differing very little, are generally ranked as varieties,
notwithstanding that they cannot be closely connected; but the amount
of difference considered necessary to give to any two forms the rank of
species cannot be defined. In genera having more than the average number
of species in any country, the species of these genera have more than
the average number of varieties. In large genera the species are apt to
be closely but unequally allied together, forming little clusters round
other species. Species very closely allied to other species apparently
have restricted ranges. In all these respects the species of large
genera present a strong analogy with varieties. And we can clearly
understand these analogies, if species once existed as varieties, and
thus originated; whereas, these analogies are utterly inexplicable if
species are independent creations.
 
We have also seen that it is the most flourishing or dominant species
of the larger genera within each class which on an average yield the
greatest number of varieties, and varieties, as we shall hereafter see,
tend to become converted into new and distinct species. Thus the larger
genera tend to become larger; and throughout nature the forms of life
which are now dominant tend to become still more dominant by leaving
many modified and dominant descendants. But, by steps hereafter to be
explained, the larger genera also tend to break up into smaller genera.
And thus, the forms of life throughout the universe become divided into
groups subordinate to groups.
 
 
 
 
CHAPTER III. STRUGGLE FOR EXISTENCE.
 
 Its bearing on natural selection--The term used in a wide
 sense--Geometrical ratio of increase--Rapid increase of naturalised
 animals and plants--Nature of the checks to increase--Competition
 universal--Effects of climate--Protection from the number of
 individuals--Complex relations of all animals and plants throughout
 nature--Struggle for life most severe between individuals and varieties
 of the same species: often severe between species of the same genus--The
 relation of organism to organism the most important of all relations.
 
Before entering on the subject of this chapter I must make a few
preliminary remarks to show how the struggle for existence bears on
natural selection. It has been seen in the last chapter that
among organic beings in a state of nature there is some individual
variability: indeed I am not aware that this has ever been disputed.
It is immaterial for us whether a multitude of doubtful forms be called
species or sub-species or varieties; what rank, for instance, the two or
three hundred doubtful forms of British plants are entitled to hold,
if the existence of any well-marked varieties be admitted. But the
mere existence of individual variability and of some few well-marked
varieties, though necessary as the foundation for the work, helps us but
little in understanding how species arise in nature. How have all those
exquisite adaptations of one part of the organisation to another part,
and to the conditions of life and of one organic being to another being,
been perfected? We see these beautiful co-adaptations most plainly in
the woodpecker and the mistletoe; and only a little less plainly in the
humblest parasite which clings to the hairs of a quadruped or feathers
of a bird; in the structure of the beetle which dives through the water;
in the plumed seed which is wafted by the gentlest breeze; in short, we
see beautiful adaptations everywhere and in every part of the organic
world.
 
Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far
more than do the varieties of the same species? How do those groups
of species, which constitute what are called distinct genera and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next
chapter, follow from the struggle for life. Owing to this struggle,
variations, however slight and from whatever cause proceeding, if they
be in any degree profitable to the individuals of a species, in their
infinitely complex relations to other organic beings and to their
physical conditions of life, will tend to the preservation of such
individuals, and will generally be inherited by the offspring. The
offspring, also, will thus have a better chance of surviving, for, of
the many individuals of any species which are periodically born, but a
small number can survive. I have called this principle, by which
each slight variation, if useful, is preserved, by the term natural
selection, in order to mark its relation to man's power of selection.
But the expression often used by Mr. Herbert Spencer, of the Survival of
the Fittest, is more accurate, and is sometimes equally convenient. We
have seen that man by selection can certainly produce great results, and
can adapt organic beings to his own uses, through the accumulation of
slight but useful variations, given to him by the hand of Nature. But
Natural Selection, we shall hereafter see, is a power incessantly ready
for action, and is as immeasurably superior to man's feeble efforts, as
the works of Nature are to those of Art.
 
We will now discuss in a little more detail the struggle for existence.
In my future work this subject will be treated, as it well deserves,
at greater length. The elder De Candolle and Lyell have largely and
philosophically shown that all organic beings are exposed to severe
competition. In regard to plants, no one has treated this subject with
more spirit and ability than W. Herbert, Dean of Manchester, evidently
the result of his great horticultural knowledge. Nothing is easier than
to admit in words the truth of the universal struggle for life, or
more difficult--at least I found it so--than constantly to bear this
conclusion in mind. Yet unless it be thoroughly engrained in the mind,
the whole economy of nature, with every fact on distribution, rarity,
abundance, extinction, and variation, will be dimly seen or quite
misunderstood. We behold the face of nature bright with gladness, we
often see superabundance of food; we do not see or we forget that the
birds which are idly singing round us mostly live on insects or seeds,
and are thus constantly destroying life; or we forget how largely these
songsters, or their eggs, or their nestlings, are destroyed by birds and
beasts of prey; we do not always bear in mind, that, though food may be
now superabundant, it is not so at all seasons of each recurring year.
 
THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE SENSE.
 
I should premise that I use this term in a large and metaphorical sense,
including dependence of one being on another, and including (which is
more important) not only the life of the individual, but success in
leaving progeny. Two canine animals, in a time of dearth, may be truly
said to struggle with each other which shall get food and live. But a
plant on the edge of a desert is said to struggle for life against the
drought, though more properly it should be said to be dependent on the
moisture. A plant which annually produces a thousand seeds, of which
only one of an average comes to maturity, may be more truly said to
struggle with the plants of the same and other kinds which already
clothe the ground. The mistletoe is dependent on the apple and a few
other trees, but can only in a far-fetched sense be said to struggle
with these trees, for, if too many of these parasites grow on the same
tree, it languishes and dies. But several seedling mistletoes, growing
close together on the same branch, may more truly be said to struggle
with each other. As the mistletoe is disseminated by birds, its
existence depends on them; and it may metaphorically be said to struggle
with other fruit-bearing plants, in tempting the birds to devour and
thus disseminate its seeds. In these several senses, which pass into
each other, I use for convenience sake the general term of Struggle for
Existence.
 
GEOMETRICAL RATIO OF INCREASE.
 
A struggle for existence inevitably follows from the high rate at which
all organic beings tend to increase. Every being, which during its
natural lifetime produces several eggs or seeds, must suffer destruction
during some period of its life, and during some season or occasional
year, otherwise, on the principle of geometrical increase, its numbers
would quickly become so inordinately great that no country could support
the product. Hence, as more individuals are produced than can possibly
survive, there must in every case be a struggle for existence, either
one individual with another of the same species, or with the individuals
of distinct species, or with the physical conditions of life. It is the
doctrine of Malthus applied with manifold force to the whole animal and
vegetable kingdoms; for in this case there can be no artificial increase
of food, and no prudential restraint from marriage. Although some
species may be now increasing, more or less rapidly, in numbers, all
cannot do so, for the world would not hold them.
 
There is no exception to the rule that every organic being naturally
increases at so high a rate, that, if not destroyed, the earth would
soon be covered by the progeny of a single pair. Even slow-breeding
man has doubled in twenty-five years, and at this rate, in less than
a thousand years, there would literally not be standing room for his
progeny. Linnaeus has calculated that if an annual plant produced only
two seeds--and there is no plant so unproductive as this--and their
seedlings next year produced two, and so on, then in twenty years there
would be a million plants. The elephant is reckoned the slowest breeder
of all known animals, and I have taken some pains to estimate its
probable minimum rate of natural increase; it will be safest to assume
that it begins breeding when thirty years old, and goes on breeding
till ninety years old, bringing forth six young in the interval, and
surviving till one hundred years old; if this be so, after a period of
from 740 to 750 years there would be nearly nineteen million elephants
alive descended from the first pair.
 
But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three following
seasons. Still more striking is the evidence from our domestic animals
of many kinds which have run wild in several parts of the world; if the
statements of the rate of increase of slow-breeding cattle and horses
in South America, and latterly in Australia, had not been well
authenticated, they would have been incredible. So it is with plants;
cases could be given of introduced plants which have become common
throughout whole islands in a period of less than ten years. Several of
the plants, such as the cardoon and a tall thistle, which are now the
commonest over the wide plains of La Plata, clothing square leagues
of surface almost to the exclusion of every other plant, have been
introduced from Europe; and there are plants which now range in India,
as I hear from Dr. Falconer, from Cape Comorin to the Himalaya, which
have been imported from America since its discovery. In such cases, and
endless others could be given, no one supposes that the fertility of
the animals or plants has been suddenly and temporarily increased in any
sensible degree. The obvious explanation is that the conditions of life
have been highly favourable, and that there has consequently been less
destruction of the old and young and that nearly all the young have been
enabled to breed. Their geometrical ratio of increase, the result
of which never fails to be surprising, simply explains their
extraordinarily rapid increase and wide diffusion in their new homes.
 
In a state of nature almost every full-grown plant annually produces
seed, and among animals there are very few which do not annually pair.
Hence we may confidently assert that all plants and animals are tending
to increase at a geometrical ratio--that all would rapidly stock every
station in which they could any how exist, and that this geometrical
tendency to increase must be checked by destruction at some period of
life. Our familiarity with the larger domestic animals tends, I think,
to mislead us; we see no great destruction falling on them, and we do
not keep in mind that thousands are annually slaughtered for food,
and that in a state of nature an equal number would have somehow to be
disposed of.
 
The only difference between organisms which annually produce eggs or
seeds by the thousand, and those which produce extremely few, is,
that the slow breeders would require a few more years to people, under
favourable conditions, a whole district, let it be ever so large. The
condor lays a couple of eggs and the ostrich a score, and yet in the
same country the condor may be the more numerous of the two. The Fulmar
petrel lays but one egg, yet it is believed to be the most numerous bird
in the world. One fly deposits hundreds of eggs, and another, like the
hippobosca, a single one. But this difference does not determine how
many individuals of the two species can be supported in a district. A
large number of eggs is of some importance to those species which depend
on a fluctuating amount of food, for it allows them rapidly to increase
in number. But the real importance of a large number of eggs or seeds is
to make up for much destruction at some period of life; and this period
in the great majority of cases is an early one. If an animal can in any
way protect its own eggs or young, a small number may be produced, and
yet the average stock be fully kept up; but if many eggs or young are
destroyed, many must be produced or the species will become extinct. It
would suffice to keep up the full number of a tree, which lived on an
average for a thousand years, if a single seed were produced once in a
thousand years, supposing that this seed were never destroyed and could
be ensured to germinate in a fitting place; so that, in all cases, the
average number of any animal or plant depends only indirectly on the
number of its eggs or seeds.
 
In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind--never to forget that every single organic
being may be said to be striving to the utmost to increase in numbers;
that each lives by a struggle at some period of its life; that heavy
destruction inevitably falls either on the young or old during each
generation or at recurrent intervals. Lighten any check, mitigate the
destruction ever so little, and the number of the species will almost
instantaneously increase to any amount.
 
NATURE OF THE CHECKS TO INCREASE.
 
The causes which check the natural tendency of each species to increase
are most obscure. Look at the most vigorous species; by as much as it
swarms in numbers, by so much will it tend to increase still further. We
know not exactly what the checks are even in a single instance. Nor will
this surprise any one who reflects how ignorant we are on this head,
even in regard to mankind, although so incomparably better known than
any other animal. This subject of the checks to increase has been ably
treated by several authors, and I hope in a future work to discuss it at
considerable length, more especially in regard to the feral animals of
South America. Here I will make only a few remarks, just to recall to
the reader's mind some of the chief points. Eggs or very young animals
seem generally to suffer most, but this is not invariably the case. With
plants there is a vast destruction of seeds, but from some observations
which I have made it appears that the seedlings suffer most from
germinating in ground already thickly stocked with other plants.
Seedlings, also, are destroyed in vast numbers by various enemies; for
instance, on a piece of ground three feet long and two wide, dug and
cleared, and where there could be no choking from other plants, I marked
all the seedlings of our native weeds as they came up, and out of 357
no less than 295 were destroyed, chiefly by slugs and insects. If turf
which has long been mown, and the case would be the same with turf
closely browsed by quadrupeds, be let to grow, the more vigorous plants
gradually kill the less vigorous, though fully grown plants; thus out of
twenty species grown on a little plot of mown turf (three feet by four)
nine species perished, from the other species being allowed to grow up
freely.
 
The amount of food for each species, of course, gives the extreme limit
to which each can increase; but very frequently it is not the obtaining
food, but the serving as prey to other animals, which determines the
average number of a species. Thus, there seems to be little doubt that
the stock of partridges, grouse, and hares on any large estate depends
chiefly on the destruction of vermin. If not one head of game were shot
during the next twenty years in England, and, at the same time, if no
vermin were destroyed, there would, in all probability, be less game
than at present, although hundreds of thousands of game animals are now
annually shot. On the other hand, in some cases, as with the elephant,
none are destroyed by beasts of prey; for even the tiger in India most
rarely dares to attack a young elephant protected by its dam.
 
Climate plays an important part in determining the average numbers of
a species, and periodical seasons of extreme cold or drought seem to be
the most effective of all checks. I estimated (chiefly from the greatly
reduced numbers of nests in the spring) that the winter of 1854-5
destroyed four-fifths of the birds in my own grounds; and this is
a tremendous destruction, when we remember that ten per cent. is an
extraordinarily severe mortality from epidemics with man. The action of
climate seems at first sight to be quite independent of the struggle for
existence; but in so far as climate chiefly acts in reducing food, it
brings on the most severe struggle between the individuals, whether of
the same or of distinct species, which subsist on the same kind of food.
Even when climate, for instance, extreme cold, acts directly, it will
be the least vigorous individuals, or those which have got least food
through the advancing winter, which will suffer the most. When we travel
from south to north, or from a damp region to a dry, we invariably
see some species gradually getting rarer and rarer, and finally
disappearing; and the change of climate being conspicuous, we are
tempted to attribute the whole effect to its direct action. But this is
a false view; we forget that each species, even where it most abounds,
is constantly suffering enormous destruction at some period of its life,
from enemies or from competitors for the same place and food; and if
these enemies or competitors be in the least degree favoured by any
slight change of climate, they will increase in numbers; and as each
area is already fully stocked with inhabitants, the other species must
decrease. When we travel southward and see a species decreasing in
numbers, we may feel sure that the cause lies quite as much in other
species being favoured, as in this one being hurt. So it is when we
travel northward, but in a somewhat lesser degree, for the number of
species of all kinds, and therefore of competitors, decreases northward;
hence in going northward, or in ascending a mountain, we far oftener
meet with stunted forms, due to the DIRECTLY injurious action of
climate, than we do in proceeding southward or in descending a mountain.
When we reach the Arctic regions, or snow-capped summits, or absolute
deserts, the struggle for life is almost exclusively with the elements.
 
That climate acts in main part indirectly by favouring other species we
clearly see in the prodigious number of plants which in our gardens can
perfectly well endure our climate, but which never become naturalised,
for they cannot compete with our native plants nor resist destruction by
our native animals.
 
When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics--at least, this
seems generally to occur with our game animals--often ensue; and here
we have a limiting check independent of the struggle for life. But even
some of these so-called epidemics appear to be due to parasitic worms,
which have from some cause, possibly in part through facility of
diffusion among the crowded animals, been disproportionally favoured:
and here comes in a sort of struggle between the parasite and its prey.
 
On the other hand, in many cases, a large stock of individuals of the
same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of
corn and rape-seed, etc., in our fields, because the seeds are in great
excess compared with the number of birds which feed on them; nor can
the birds, though having a superabundance of food at this one season,
increase in number proportionally to the supply of seed, as their
numbers are checked during the winter; but any one who has tried knows
how troublesome it is to get seed from a few wheat or other such plants
in a garden; I have in this case lost every single seed. This view of
the necessity of a large stock of the same species for its preservation,
explains, I believe, some singular facts in nature such as that of very
rare plants being sometimes extremely abundant, in the few spots where
they do exist; and that of some social plants being social, that is
abounding in individuals, even on the extreme verge of their range. For
in such cases, we may believe, that a plant could exist only where
the conditions of its life were so favourable that many could exist
together, and thus save the species from utter destruction. I should
add that the good effects of intercrossing, and the ill effects of close
interbreeding, no doubt come into play in many of these cases; but I
will not here enlarge on this subject.
 
COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER IN THE
STRUGGLE FOR EXISTENCE.
 
Many cases are on record showing how complex and unexpected are the
checks and relations between organic beings, which have to struggle
together in the same country. I will give only a single instance, which,
though a simple one, interested me. In Staffordshire, on the estate of
a relation, where I had ample means of investigation, there was a large
and extremely barren heath, which had never been touched by the hand
of man; but several hundred acres of exactly the same nature had been
enclosed twenty-five years previously and planted with Scotch fir. The
change in the native vegetation of the planted part of the heath was
most remarkable, more than is generally seen in passing from one quite
different soil to another: not only the proportional numbers of the
heath-plants were wholly changed, but twelve species of plants (not
counting grasses and carices) flourished in the plantations, which could
not be found on the heath. The effect on the insects must have been
still greater, for six insectivorous birds were very common in the
plantations, which were not to be seen on the heath; and the heath was
frequented by two or three distinct insectivorous birds. Here we see how
potent has been the effect of the introduction of a single tree, nothing
whatever else having been done, with the exception of the land having
been enclosed, so that cattle could not enter. But how important an
element enclosure is, I plainly saw near Farnham, in Surrey. Here
there are extensive heaths, with a few clumps of old Scotch firs on
the distant hill-tops: within the last ten years large spaces have been
enclosed, and self-sown firs are now springing up in multitudes, so
close together that all cannot live. When I ascertained that these young
trees had not been sown or planted I was so much surprised at their
numbers that I went to several points of view, whence I could examine
hundreds of acres of the unenclosed heath, and literally I could not
see a single Scotch fir, except the old planted clumps. But on looking
closely between the stems of the heath, I found a multitude of seedlings
and little trees, which had been perpetually browsed down by the cattle.
In one square yard, at a point some hundred yards distant from one of
the old clumps, I counted thirty-two little trees; and one of them, with
twenty-six rings of growth, had, during many years tried to raise its
head above the stems of the heath, and had failed. No wonder that, as
soon as the land was enclosed, it became thickly clothed with vigorously
growing young firs. Yet the heath was so extremely barren and so
extensive that no one would ever have imagined that cattle would have so
closely and effectually searched it for food.
 
Here we see that cattle absolutely determine the existence of the Scotch
fir; but in several parts of the world insects determine the existence
of cattle. Perhaps Paraguay offers the most curious instance of this;
for here neither cattle nor horses nor dogs have ever run wild, though
they swarm southward and northward in a feral state; and Azara and
Rengger have shown that this is caused by the greater number in Paraguay
of a certain fly, which lays its eggs in the navels of these animals
when first born. The increase of these flies, numerous as they are,
must be habitually checked by some means, probably by other parasitic
insects. Hence, if certain insectivorous birds were to decrease in
Paraguay, the parasitic insects would probably increase; and this would
lessen the number of the navel-frequenting flies--then cattle and horses
would become feral, and this would certainly greatly alter (as indeed
I have observed in parts of South America) the vegetation: this again
would largely affect the insects; and this, as we have just seen
in Staffordshire, the insectivorous birds, and so onwards in
ever-increasing circles of complexity. Not that under nature the
relations will ever be as simple as this. Battle within battle must be
continually recurring with varying success; and yet in the long-run the
forces are so nicely balanced that the face of nature remains for long
periods of time uniform, though assuredly the merest trifle would give
the victory to one organic being over another. Nevertheless, so profound
is our ignorance, and so high our presumption, that we marvel when we
hear of the extinction of an organic being; and as we do not see the
cause, we invoke cataclysms to desolate the world, or invent laws on the
duration of the forms of life!
 
I am tempted to give one more instance showing how plants and animals,
remote in the scale of nature, are bound together by a web of complex
relations. I shall hereafter have occasion to show that the exotic
Lobelia fulgens is never visited in my garden by insects, and
consequently, from its peculiar structure, never sets a seed. Nearly
all our orchidaceous plants absolutely require the visits of insects
to remove their pollen-masses and thus to fertilise them. I find
from experiments that humble-bees are almost indispensable to the
fertilisation of the heartsease (Viola tricolor), for other bees do
not visit this flower. I have also found that the visits of bees are
necessary for the fertilisation of some kinds of clover; for instance
twenty heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but
twenty other heads, protected from bees, produced not one. Again, 100
heads of red clover (T. pratense) produced 2,700 seeds, but the same
number of protected heads produced not a single seed. Humble bees alone
visit red clover, as other bees cannot reach the nectar. It has been
suggested that moths may fertilise the clovers; but I doubt whether they
could do so in the case of the red clover, from their weight not being
sufficient to depress the wing petals. Hence we may infer as highly
probable that, if the whole genus of humble-bees became extinct or very
rare in England, the heartsease and red clover would become very rare,
or wholly disappear. The number of humble-bees in any district depends
in a great measure upon the number of field-mice, which destroy their
combs and nests; and Colonel Newman, who has long attended to the habits
of humble-bees, believes that "more than two-thirds of them are
thus destroyed all over England." Now the number of mice is largely
dependent, as every one knows, on the number of cats; and Colonel
Newman says, "Near villages and small towns I have found the nests
of humble-bees more numerous than elsewhere, which I attribute to the
number of cats that destroy the mice." Hence it is quite credible that
the presence of a feline animal in large numbers in a district might
determine, through the intervention first of mice and then of bees, the
frequency of certain flowers in that district!
 
In the case of every species, many different checks, acting at different
periods of life, and during different seasons or years, probably come
into play; some one check or some few being generally the most potent,
but all will concur in determining the average number, or even
the existence of the species. In some cases it can be shown that
widely-different checks act on the same species in different districts.
When we look at the plants and bushes clothing an entangled bank, we
are tempted to attribute their proportional numbers and kinds to what we
call chance. But how false a view is this! Every one has heard that when
an American forest is cut down, a very different vegetation springs
up; but it has been observed that ancient Indian ruins in the Southern
United States, which must formerly have been cleared of trees, now
display the same beautiful diversity and proportion of kinds as in the
surrounding virgin forests. What a struggle must have gone on during
long centuries between the several kinds of trees, each annually
scattering its seeds by the thousand; what war between insect and
insect--between insects, snails, and other animals with birds and beasts
of prey--all striving to increase, all feeding on each other, or on the
trees, their seeds and seedlings, or on the other plants which first
clothed the ground and thus checked the growth of the trees. Throw up
a handful of feathers, and all fall to the ground according to definite
laws; but how simple is the problem where each shall fall compared to
that of the action and reaction of the innumerable plants and animals
which have determined, in the course of centuries, the proportional
numbers and kinds of trees now growing on the old Indian ruins!
 
The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in the scale of nature. This
is likewise sometimes the case with those which may strictly be said to
struggle with each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle will almost invariably
be most severe between the individuals of the same species, for they
frequent the same districts, require the same food, and are exposed
to the same dangers. In the case of varieties of the same species, the
struggle will generally be almost equally severe, and we sometimes see
the contest soon decided: for instance, if several varieties of wheat be
sown together, and the mixed seed be resown, some of the varieties which
best suit the soil or climate, or are naturally the most fertile, will
beat the others and so yield more seed, and will consequently in a few
years supplant the other varieties. To keep up a mixed stock of even
such extremely close varieties as the variously coloured sweet-peas,
they must be each year harvested separately, and the seed then mixed
in due proportion, otherwise the weaker kinds will steadily decrease in
number and disappear. So again with the varieties of sheep: it has
been asserted that certain mountain-varieties will starve out other
mountain-varieties, so that they cannot be kept together. The same
result has followed from keeping together different varieties of the
medicinal leech. It may even be doubted whether the varieties of any
of our domestic plants or animals have so exactly the same strength,
habits, and constitution, that the original proportions of a mixed
stock (crossing being prevented) could be kept up for half-a-dozen
generations, if they were allowed to struggle together, in the same
manner as beings in a state of nature, and if the seed or young were not
annually preserved in due proportion.
 
STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF THE
SAME SPECIES.
 
As the species of the same genus usually have, though by no means
invariably, much similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between them, if
they come into competition with each other, than between the species of
distinct genera. We see this in the recent extension over parts of the
United States of one species of swallow having caused the decrease of
another species. The recent increase of the missel-thrush in parts of
Scotland has caused the decrease of the song-thrush. How frequently we
hear of one species of rat taking the place of another species under
the most different climates! In Russia the small Asiatic cockroach
has everywhere driven before it its great congener. In Australia the
imported hive-bee is rapidly exterminating the small, stingless native
bee. One species of charlock has been known to supplant another species;
and so in other cases. We can dimly see why the competition should be
most severe between allied forms, which fill nearly the same place in
the economy of nature; but probably in no one case could we precisely
say why one species has been victorious over another in the great battle
of life.
 
A corollary of the highest importance may be deduced from the foregoing
remarks, namely, that the structure of every organic being is related,
in the most essential yet often hidden manner, to that of all other
organic beings, with which it comes into competition for food or
residence, or from which it has to escape, or on which it preys. This
is obvious in the structure of the teeth and talons of the tiger; and in
that of the legs and claws of the parasite which clings to the hair on
the tiger's body. But in the beautifully plumed seed of the dandelion,
and in the flattened and fringed legs of the water-beetle, the relation
seems at first confined to the elements of air and water. Yet the
advantage of the plumed seeds no doubt stands in the closest relation
to the land being already thickly clothed with other plants; so that the
seeds may be widely distributed and fall on unoccupied ground. In the
water-beetle, the structure of its legs, so well adapted for diving,
allows it to compete with other aquatic insects, to hunt for its own
prey, and to escape serving as prey to other animals.
 
The store of nutriment laid up within the seeds of many plants seems at
first sight to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds, as peas and
beans, when sown in the midst of long grass, it may be suspected that
the chief use of the nutriment in the seed is to favour the growth of
the seedlings, whilst struggling with other plants growing vigorously
all around.
 
Look at a plant in the midst of its range! Why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges
into slightly hotter or colder, damper or drier districts. In this case
we can clearly see that if we wish in imagination to give the plant the
power of increasing in numbers, we should have to give it some advantage
over its competitors, or over the animals which prey on it. On the
confines of its geographical range, a change of constitution with
respect to climate would clearly be an advantage to our plant; but we
have reason to believe that only a few plants or animals range so far,
that they are destroyed exclusively by the rigour of the climate. Not
until we reach the extreme confines of life, in the Arctic regions or on
the borders of an utter desert, will competition cease. The land may be
extremely cold or dry, yet there will be competition between some few
species, or between the individuals of the same species, for the warmest
or dampest spots.
 
Hence we can see that when a plant or animal is placed in a new country,
among new competitors, the conditions of its life will generally be
changed in an essential manner, although the climate may be exactly the
same as in its former home. If its average numbers are to increase in
its new home, we should have to modify it in a different way to what we
should have had to do in its native country; for we should have to give
it some advantage over a different set of competitors or enemies.
 
It is good thus to try in imagination to give any one species an
advantage over another. Probably in no single instance should we know
what to do. This ought to convince us of our ignorance on the mutual
relations of all organic beings; a conviction as necessary, as it is
difficult to acquire. All that we can do is to keep steadily in mind
that each organic being is striving to increase in a geometrical ratio;
that each, at some period of its life, during some season of the year,
during each generation, or at intervals, has to struggle for life and
to suffer great destruction. When we reflect on this struggle we may
console ourselves with the full belief that the war of nature is not
incessant, that no fear is felt, that death is generally prompt, and
that the vigorous, the healthy, and the happy survive and multiply.
 
 
 
 
CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
 
 Natural Selection--its power compared with man's selection--its power
 on characters of trifling importance--its power at all ages and on
 both sexes--Sexual Selection--On the generality of intercrosses
 between individuals of the same species--Circumstances favourable and
 unfavourable to the results of Natural Selection, namely, intercrossing,
 isolation, number of individuals--Slow action--Extinction caused by
 Natural Selection--Divergence of Character, related to the diversity of
 inhabitants of any small area and to naturalisation--Action of Natural
 Selection, through Divergence of Character and Extinction, on the
 descendants from a common parent--Explains the Grouping of all organic
 beings--Advance in organisation--Low forms preserved--Convergence of
 character--Indefinite multiplication of species--Summary.
 
 
How will the struggle for existence, briefly discussed in the last
chapter, act in regard to variation? Can the principle of selection,
which we have seen is so potent in the hands of man, apply under nature?
I think we shall see that it can act most efficiently. Let the endless
number of slight variations and individual differences occurring in our
domestic productions, and, in a lesser degree, in those under nature, be
borne in mind; as well as the strength of the hereditary tendency. Under
domestication, it may truly be said that the whole organisation becomes
in some degree plastic. But the variability, which we almost universally
meet with in our domestic productions is not directly produced, as
Hooker and Asa Gray have well remarked, by man; he can neither originate
varieties nor prevent their occurrence; he can only preserve and
accumulate such as do occur. Unintentionally he exposes organic beings
to new and changing conditions of life, and variability ensues; but
similar changes of conditions might and do occur under nature. Let it
also be borne in mind how infinitely complex and close-fitting are
the mutual relations of all organic beings to each other and to their
physical conditions of life; and consequently what infinitely varied
diversities of structure might be of use to each being under changing
conditions of life. Can it then be thought improbable, seeing that
variations useful to man have undoubtedly occurred, that other
variations useful in some way to each being in the great and complex
battle of life, should occur in the course of many successive
generations? If such do occur, can we doubt (remembering that many more
individuals are born than can possibly survive) that individuals having
any advantage, however slight, over others, would have the best chance
of surviving and procreating their kind? On the other hand, we may feel
sure that any variation in the least degree injurious would be rigidly
destroyed. This preservation of favourable individual differences and
variations, and the destruction of those which are injurious, I have
called Natural Selection, or the Survival of the Fittest. Variations
neither useful nor injurious would not be affected by natural selection,
and would be left either a fluctuating element, as perhaps we see in
certain polymorphic species, or would ultimately become fixed, owing to
the nature of the organism and the nature of the conditions.
 
Several writers have misapprehended or objected to the term Natural
Selection. Some have even imagined that natural selection induces
variability, whereas it implies only the preservation of such variations
as arise and are beneficial to the being under its conditions of life.
No one objects to agriculturists speaking of the potent effects of man's
selection; and in this case the individual differences given by nature,
which man for some object selects, must of necessity first occur. Others
have objected that the term selection implies conscious choice in the
animals which become modified; and it has even been urged that, as
plants have no volition, natural selection is not applicable to them!
In the literal sense of the word, no doubt, natural selection is a
false term; but who ever objected to chemists speaking of the elective
affinities of the various elements?--and yet an acid cannot strictly be
said to elect the base with which it in preference combines. It has been
said that I speak of natural selection as an active power or Deity; but
who objects to an author speaking of the attraction of gravity as ruling
the movements of the planets? Every one knows what is meant and is
implied by such metaphorical expressions; and they are almost necessary
for brevity. So again it is difficult to avoid personifying the word
Nature; but I mean by nature, only the aggregate action and product of
many natural laws, and by laws the sequence of events as ascertained
by us. With a little familiarity such superficial objections will be
forgotten.
 
We shall best understand the probable course of natural selection by
taking the case of a country undergoing some slight physical change, for
instance, of climate. The proportional numbers of its inhabitants will
almost immediately undergo a change, and some species will probably
become extinct. We may conclude, from what we have seen of the intimate
and complex manner in which the inhabitants of each country are
bound together, that any change in the numerical proportions of the
inhabitants, independently of the change of climate itself, would
seriously affect the others. If the country were open on its borders,
new forms would certainly immigrate, and this would likewise seriously
disturb the relations of some of the former inhabitants. Let it be
remembered how powerful the influence of a single introduced tree or
mammal has been shown to be. But in the case of an island, or of a
country partly surrounded by barriers, into which new and better adapted
forms could not freely enter, we should then have places in the economy
of nature which would assuredly be better filled up if some of the
original inhabitants were in some manner modified; for, had the area
been open to immigration, these same places would have been seized on
by intruders. In such cases, slight modifications, which in any way
favoured the individuals of any species, by better adapting them to
their altered conditions, would tend to be preserved; and natural
selection would have free scope for the work of improvement.
 
We have good reason to believe, as shown in the first chapter,
that changes in the conditions of life give a tendency to increased
variability; and in the foregoing cases the conditions the changed, and
this would manifestly be favourable to natural selection, by affording
a better chance of the occurrence of profitable variations. Unless such
occur, natural selection can do nothing. Under the term of "variations,"
it must never be forgotten that mere individual differences are
included. As man can produce a great result with his domestic animals
and plants by adding up in any given direction individual differences,
so could natural selection, but far more easily from having incomparably
longer time for action. Nor do I believe that any great physical change,
as of climate, or any unusual degree of isolation, to check immigration,
is necessary in order that new and unoccupied places should be left
for natural selection to fill up by improving some of the varying
inhabitants. For as all the inhabitants of each country are struggling
together with nicely balanced forces, extremely slight modifications in
the structure or habits of one species would often give it an advantage
over others; and still further modifications of the same kind would
often still further increase the advantage, as long as the species
continued under the same conditions of life and profited by similar
means of subsistence and defence. No country can be named in which all
the native inhabitants are now so perfectly adapted to each other and to
the physical conditions under which they live, that none of them could
be still better adapted or improved; for in all countries, the natives
have been so far conquered by naturalised productions that they have
allowed some foreigners to take firm possession of the land. And as
foreigners have thus in every country beaten some of the natives, we
may safely conclude that the natives might have been modified with
advantage, so as to have better resisted the intruders.
 
As man can produce, and certainly has produced, a great result by his
methodical and unconscious means of selection, what may not natural
selection effect? Man can act only on external and visible characters:
Nature, if I may be allowed to personify the natural preservation or
survival of the fittest, cares nothing for appearances, except in so far
as they are useful to any being. She can act on every internal organ,
on every shade of constitutional difference, on the whole machinery of
life. Man selects only for his own good; Nature only for that of the
being which she tends. Every selected character is fully exercised by
her, as is implied by the fact of their selection. Man keeps the natives
of many climates in the same country. He seldom exercises each selected
character in some peculiar and fitting manner; he feeds a long and a
short-beaked pigeon on the same food; he does not exercise a long-backed
or long-legged quadruped in any peculiar manner; he exposes sheep
with long and short wool to the same climate; does not allow the most
vigorous males to struggle for the females; he does not rigidly destroy
all inferior animals, but protects during each varying season, as far as
lies in his power, all his productions. He often begins his selection
by some half-monstrous form, or at least by some modification prominent
enough to catch the eye or to be plainly useful to him. Under nature,
the slightest differences of structure or constitution may well turn the
nicely-balanced scale in the struggle for life, and so be preserved.
How fleeting are the wishes and efforts of man! How short his time,
and consequently how poor will be his results, compared with those
accumulated by Nature during whole geological periods! Can we wonder,
then, that Nature's productions should be far "truer" in character than
man's productions; that they should be infinitely better adapted to the
most complex conditions of life, and should plainly bear the stamp of
far higher workmanship?
 
It may metaphorically be said that natural selection is daily and hourly
scrutinising, throughout the world, the slightest variations; rejecting
those that are bad, preserving and adding up all that are good; silently
and insensibly working, WHENEVER AND WHEREVER OPPORTUNITY OFFERS, at
the improvement of each organic being in relation to its organic and
inorganic conditions of life. We see nothing of these slow changes in
progress, until the hand of time has marked the long lapse of ages, and
then so imperfect is our view into long-past geological ages that we see
only that the forms of life are now different from what they formerly
were.
 
In order that any great amount of modification should be effected in a
species, a variety, when once formed must again, perhaps after a long
interval of time, vary or present individual differences of the same
favourable nature as before; and these must again be preserved, and so
onward, step by step. Seeing that individual differences of the
same kind perpetually recur, this can hardly be considered as an
unwarrantable assumption. But whether it is true, we can judge only
by seeing how far the hypothesis accords with and explains the general
phenomena of nature. On the other hand, the ordinary belief that the
amount of possible variation is a strictly limited quantity, is likewise
a simple assumption.
 
Although natural selection can act only through and for the good of each
being, yet characters and structures, which we are apt to consider as of
very trifling importance, may thus be acted on. When we see leaf-eating
insects green, and bark-feeders mottled-grey; the alpine ptarmigan white
in winter, the red-grouse the colour of heather, we must believe that
these tints are of service to these birds and insects in preserving them
from danger. Grouse, if not destroyed at some period of their lives,
would increase in countless numbers; they are known to suffer largely
from birds of prey; and hawks are guided by eyesight to their prey,--so
much so that on parts of the continent persons are warned not to keep
white pigeons, as being the most liable to destruction. Hence natural
selection might be effective in giving the proper colour to each kind
of grouse, and in keeping that colour, when once acquired, true and
constant. Nor ought we to think that the occasional destruction of an
animal of any particular colour would produce little effect; we should
remember how essential it is in a flock of white sheep to destroy a lamb
with the faintest trace of black. We have seen how the colour of hogs,
which feed on the "paint-root" in Virginia, determines whether they
shall live or die. In plants, the down on the fruit and the colour of
the flesh are considered by botanists as characters of the most trifling
importance; yet we hear from an excellent horticulturist, Downing,
that in the United States smooth-skinned fruits suffer far more from a
beetle, a Curculio, than those with down; that purple plums suffer far
more from a certain disease than yellow plums; whereas another disease
attacks yellow-fleshed peaches far more than those with other coloured
flesh. If, with all the aids of art, these slight differences make a
great difference in cultivating the several varieties, assuredly, in a
state of nature, where the trees would have to struggle with other trees
and with a host of enemies, such differences would effectually settle
which variety, whether a smooth or downy, a yellow or a purple-fleshed
fruit, should succeed.
 
In looking at many small points of difference between species, which,
as far as our ignorance permits us to judge, seem quite unimportant, we
must not forget that climate, food, etc., have no doubt produced some
direct effect. It is also necessary to bear in mind that, owing to
the law of correlation, when one part varies and the variations are
accumulated through natural selection, other modifications, often of the
most unexpected nature, will ensue.
 
As we see that those variations which, under domestication, appear at
any particular period of life, tend to reappear in the offspring at the
same period; for instance, in the shape, size and flavour of the seeds
of the many varieties of our culinary and agricultural plants; in the
caterpillar and cocoon stages of the varieties of the silkworm; in the
eggs of poultry, and in the colour of the down of their chickens; in the
horns of our sheep and cattle when nearly adult; so in a state of nature
natural selection will be enabled to act on and modify organic beings at
any age, by the accumulation of variations profitable at that age, and
by their inheritance at a corresponding age. If it profit a plant to
have its seeds more and more widely disseminated by the wind, I can see
no greater difficulty in this being effected through natural selection,
than in the cotton-planter increasing and improving by selection the
down in the pods on his cotton-trees. Natural selection may modify
and adapt the larva of an insect to a score of contingencies, wholly
different from those which concern the mature insect; and these
modifications may affect, through correlation, the structure of the
adult. So, conversely, modifications in the adult may affect the
structure of the larva; but in all cases natural selection will ensure
that they shall not be injurious: for if they were so, the species would
become extinct.
 
Natural selection will modify the structure of the young in relation to
the parent and of the parent in relation to the young. In social animals
it will adapt the structure of each individual for the benefit of the
whole community; if the community profits by the selected change. What
natural selection cannot do, is to modify the structure of one species,
without giving it any advantage, for the good of another species;
and though statements to this effect may be found in works of natural
history, I cannot find one case which will bear investigation. A
structure used only once in an animal's life, if of high importance to
it, might be modified to any extent by natural selection; for instance,
the great jaws possessed by certain insects, used exclusively for
opening the cocoon--or the hard tip to the beak of unhatched birds,
used for breaking the eggs. It has been asserted, that of the best
short-beaked tumbler-pigeons a greater number perish in the egg than are
able to get out of it; so that fanciers assist in the act of hatching.
Now, if nature had to make the beak of a full-grown pigeon very short
for the bird's own advantage, the process of modification would be very
slow, and there would be simultaneously the most rigorous selection
of all the young birds within the egg, which had the most powerful and
hardest beaks, for all with weak beaks would inevitably perish: or, more
delicate and more easily broken shells might be selected, the thickness
of the shell being known to vary like every other structure.
 
It may be well here to remark that with all beings there must be much
fortuitous destruction, which can have little or no influence on the
course of natural selection. For instance, a vast number of eggs or
seeds are annually devoured, and these could be modified through natural
selection only if they varied in some manner which protected them from
their enemies. Yet many of these eggs or seeds would perhaps, if not
destroyed, have yielded individuals better adapted to their conditions
of life than any of those which happened to survive. So again a vast
number of mature animals and plants, whether or not they be the best
adapted to their conditions, must be annually destroyed by accidental
causes, which would not be in the least degree mitigated by certain
changes of structure or constitution which would in other ways be
beneficial to the species. But let the destruction of the adults be ever
so heavy, if the number which can exist in any district be not wholly
kept down by such causes--or again let the destruction of eggs or
seeds be so great that only a hundredth or a thousandth part are
developed--yet of those which do survive, the best adapted individuals,
supposing that there is any variability in a favourable direction,
will tend to propagate their kind in larger numbers than the less
well adapted. If the numbers be wholly kept down by the causes just
indicated, as will often have been the case, natural selection will
be powerless in certain beneficial directions; but this is no valid
objection to its efficiency at other times and in other ways; for we
are far from having any reason to suppose that many species ever undergo
modification and improvement at the same time in the same area.
 
SEXUAL SELECTION.
 
Inasmuch as peculiarities often appear under domestication in one sex
and become hereditarily attached to that sex, so no doubt it will be
under nature. Thus it is rendered possible for the two sexes to be
modified through natural selection in relation to different habits
of life, as is sometimes the case; or for one sex to be modified in
relation to the other sex, as commonly occurs. This leads me to say a
few words on what I have called sexual selection. This form of selection
depends, not on a struggle for existence in relation to other organic
beings or to external conditions, but on a struggle between the
individuals of one sex, generally the males, for the possession of the
other sex. The result is not death to the unsuccessful competitor, but
few or no offspring. Sexual selection is, therefore, less rigorous than
natural selection. Generally, the most vigorous males, those which are
best fitted for their places in nature, will leave most progeny. But in
many cases victory depends not so much on general vigour, but on having
special weapons, confined to the male sex. A hornless stag or spurless
cock would have a poor chance of leaving numerous offspring. Sexual
selection, by always allowing the victor to breed, might surely give
indomitable courage, length of spur, and strength to the wing to
strike in the spurred leg, in nearly the same manner as does the brutal
cockfighter by the careful selection of his best cocks. How low in the
scale of nature the law of battle descends I know not; male alligators
have been described as fighting, bellowing, and whirling round, like
Indians in a war-dance, for the possession of the females; male salmons
have been observed fighting all day long; male stag-beetles sometimes
bear wounds from the huge mandibles of other males; the males of certain
hymenopterous insects have been frequently seen by that inimitable
observer M. Fabre, fighting for a particular female who sits by, an
apparently unconcerned beholder of the struggle, and then retires
with the conqueror. The war is, perhaps, severest between the males
of polygamous animals, and these seem oftenest provided with special
weapons. The males of carnivorous animals are already well armed; though
to them and to others, special means of defence may be given through
means of sexual selection, as the mane of the lion, and the hooked jaw
to the male salmon; for the shield may be as important for victory as
the sword or spear.
 
Among birds, the contest is often of a more peaceful character. All
those who have attended to the subject, believe that there is the
severest rivalry between the males of many species to attract, by
singing, the females. The rock-thrush of Guiana, birds of paradise,
and some others, congregate, and successive males display with the most
elaborate care, and show off in the best manner, their gorgeous plumage;
they likewise perform strange antics before the females, which, standing
by as spectators, at last choose the most attractive partner. Those who
have closely attended to birds in confinement well know that they
often take individual preferences and dislikes: thus Sir R. Heron has
described how a pied peacock was eminently attractive to all his hen
birds. I cannot here enter on the necessary details; but if man can in a
short time give beauty and an elegant carriage to his bantams, according
to his standard of beauty, I can see no good reason to doubt that female
birds, by selecting, during thousands of generations, the most melodious
or beautiful males, according to their standard of beauty, might produce
a marked effect. Some well-known laws, with respect to the plumage of
male and female birds, in comparison with the plumage of the young, can
partly be explained through the action of sexual selection on variations
occurring at different ages, and transmitted to the males alone or to
both sexes at corresponding ages; but I have not space here to enter on
this subject.
 
Thus it is, as I believe, that when the males and females of any animal
have the same general habits of life, but differ in structure, colour,
or ornament, such differences have been mainly caused by sexual
selection: that is, by individual males having had, in successive
generations, some slight advantage over other males, in their weapons,
means of defence, or charms; which they have transmitted to their
male offspring alone. Yet, I would not wish to attribute all sexual
differences to this agency: for we see in our domestic animals
peculiarities arising and becoming attached to the male sex, which
apparently have not been augmented through selection by man. The tuft of
hair on the breast of the wild turkey-cock cannot be of any use, and it
is doubtful whether it can be ornamental in the eyes of the female bird;
indeed, had the tuft appeared under domestication it would have been
called a monstrosity.
 
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE SURVIVAL OF THE
FITTEST.
 
In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some
by craft, some by strength, and some by fleetness; and let us suppose
that the fleetest prey, a deer for instance, had from any change in
the country increased in numbers, or that other prey had decreased
in numbers, during that season of the year when the wolf was hardest
pressed for food. Under such circumstances the swiftest and slimmest
wolves have the best chance of surviving, and so be preserved or
selected, provided always that they retained strength to master their
prey at this or some other period of the year, when they were compelled
to prey on other animals. I can see no more reason to doubt that
this would be the result, than that man should be able to improve the
fleetness of his greyhounds by careful and methodical selection, or by
that kind of unconscious selection which follows from each man trying to
keep the best dogs without any thought of modifying the breed. I may
add that, according to Mr. Pierce, there are two varieties of the wolf
inhabiting the Catskill Mountains, in the United States, one with a
light greyhound-like form, which pursues deer, and the other more bulky,
with shorter legs, which more frequently attacks the shepherd's flocks.
 
Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable;
for we often observe great differences in the natural tendencies of our
domestic animals; one cat, for instance, taking to catch rats, another
mice; one cat, according to Mr. St. John, bringing home winged game,
another hares or rabbits, and another hunting on marshy ground and
almost nightly catching woodcocks or snipes. The tendency to catch rats
rather than mice is known to be inherited. Now, if any slight innate
change of habit or of structure benefited an individual wolf, it would
have the best chance of surviving and of leaving offspring. Some of its
young would probably inherit the same habits or structure, and by the
repetition of this process, a new variety might be formed which would
either supplant or coexist with the parent-form of wolf. Or, again,
the wolves inhabiting a mountainous district, and those frequenting the
lowlands, would naturally be forced to hunt different prey; and from the
continued preservation of the individuals best fitted for the two sites,
two varieties might slowly be formed. These varieties would cross and
blend where they met; but to this subject of intercrossing we shall soon
have to return. I may add, that, according to Mr. Pierce, there are two
varieties of the wolf inhabiting the Catskill Mountains in the United
States, one with a light greyhound-like form, which pursues deer, and
the other more bulky, with shorter legs, which more frequently attacks
the shepherd's flocks.
 
It should be observed that in the above illustration, I speak of the
slimmest individual wolves, and not of any single strongly marked
variation having been preserved. In former editions of this work I
sometimes spoke as if this latter alternative had frequently occurred.
I saw the great importance of individual differences, and this led me
fully to discuss the results of unconscious selection by man,
which depends on the preservation of all the more or less valuable
individuals, and on the destruction of the worst. I saw, also, that
the preservation in a state of nature of any occasional deviation of
structure, such as a monstrosity, would be a rare event; and that, if at
first preserved, it would generally be lost by subsequent intercrossing
with ordinary individuals. Nevertheless, until reading an able and
valuable article in the "North British Review" (1867), I did not
appreciate how rarely single variations, whether slight or strongly
marked, could be perpetuated. The author takes the case of a pair of
animals, producing during their lifetime two hundred offspring, of
which, from various causes of destruction, only two on an average
survive to pro-create their kind. This is rather an extreme estimate
for most of the higher animals, but by no means so for many of the lower
organisms. He then shows that if a single individual were born, which
varied in some manner, giving it twice as good a chance of life as that
of the other individuals, yet the chances would be strongly against its
survival. Supposing it to survive and to breed, and that half its young
inherited the favourable variation; still, as the Reviewer goes onto
show, the young would have only a slightly better chance of surviving
and breeding; and this chance would go on decreasing in the succeeding
generations. The justice of these remarks cannot, I think, be disputed.
If, for instance, a bird of some kind could procure its food more easily
by having its beak curved, and if one were born with its beak strongly
curved, and which consequently flourished, nevertheless there would be
a very poor chance of this one individual perpetuating its kind to the
exclusion of the common form; but there can hardly be a doubt, judging
by what we see taking place under domestication, that this result would
follow from the preservation during many generations of a large number
of individuals with more or less strongly curved beaks, and from the
destruction of a still larger number with the straightest beaks.
 
It should not, however, be overlooked that certain rather strongly
marked variations, which no one would rank as mere individual
differences, frequently recur owing to a similar organisation being
similarly acted on--of which fact numerous instances could be given with
our domestic productions. In such cases, if the varying individual did
not actually transmit to its offspring its newly-acquired character, it
would undoubtedly transmit to them, as long as the existing conditions
remained the same, a still stronger tendency to vary in the same manner.
There can also be little doubt that the tendency to vary in the same
manner has often been so strong that all the individuals of the same
species have been similarly modified without the aid of any form of
selection. Or only a third, fifth, or tenth part of the individuals may
have been thus affected, of which fact several instances could be given.
Thus Graba estimates that about one-fifth of the guillemots in the Faroe
Islands consist of a variety so well marked, that it was formerly ranked
as a distinct species under the name of Uria lacrymans. In cases of this
kind, if the variation were of a beneficial nature, the original form
would soon be supplanted by the modified form, through the survival of
the fittest.
 
To the effects of intercrossing in eliminating variations of all kinds,
I shall have to recur; but it may be here remarked that most animals and
plants keep to their proper homes, and do not needlessly wander about;
we see this even with migratory birds, which almost always return to the
same spot. Consequently each newly-formed variety would generally be at
first local, as seems to be the common rule with varieties in a state
of nature; so that similarly modified individuals would soon exist in a
small body together, and would often breed together. If the new variety
were successful in its battle for life, it would slowly spread from
a central district, competing with and conquering the unchanged
individuals on the margins of an ever-increasing circle.
 
It may be worth while to give another and more complex illustration of
the action of natural selection. Certain plants excrete sweet juice,
apparently for the sake of eliminating something injurious from the sap:
this is effected, for instance, by glands at the base of the stipules in
some Leguminosae, and at the backs of the leaves of the common laurel.
This juice, though small in quantity, is greedily sought by insects; but
their visits do not in any way benefit the plant. Now, let us suppose
that the juice or nectar was excreted from the inside of the flowers of
a certain number of plants of any species. Insects in seeking the nectar
would get dusted with pollen, and would often transport it from one
flower to another. The flowers of two distinct individuals of the same
species would thus get crossed; and the act of crossing, as can be fully
proved, gives rise to vigorous seedlings, which consequently would have
the best chance of flourishing and surviving. The plants which produced
flowers with the largest glands or nectaries, excreting most nectar,
would oftenest be visited by insects, and would oftenest be crossed; and
so in the long-run would gain the upper hand and form a local variety.
The flowers, also, which had their stamens and pistils placed, in
relation to the size and habits of the particular insect which visited
them, so as to favour in any degree the transportal of the pollen, would
likewise be favoured. We might have taken the case of insects visiting
flowers for the sake of collecting pollen instead of nectar; and as
pollen is formed for the sole purpose of fertilisation, its destruction
appears to be a simple loss to the plant; yet if a little pollen
were carried, at first occasionally and then habitually, by the
pollen-devouring insects from flower to flower, and a cross thus
effected, although nine-tenths of the pollen were destroyed it
might still be a great gain to the plant to be thus robbed; and the
individuals which produced more and more pollen, and had larger anthers,
would be selected.
 
When our plant, by the above process long continued, had been rendered
highly attractive to insects, they would, unintentionally on their part,
regularly carry pollen from flower to flower; and that they do this
effectually I could easily show by many striking facts. I will give only
one, as likewise illustrating one step in the separation of the sexes of
plants. Some holly-trees bear only male flowers, which have four stamens
producing a rather small quantity of pollen, and a rudimentary pistil;
other holly-trees bear only female flowers; these have a full-sized
pistil, and four stamens with shrivelled anthers, in which not a grain
of pollen can be detected. Having found a female tree exactly sixty
yards from a male tree, I put the stigmas of twenty flowers, taken from
different branches, under the microscope, and on all, without exception,
there were a few pollen-grains, and on some a profusion. As the wind had
set for several days from the female to the male tree, the pollen could
not thus have been carried. The weather had been cold and boisterous and
therefore not favourable to bees, nevertheless every female flower which
I examined had been effectually fertilised by the bees, which had flown
from tree to tree in search of nectar. But to return to our imaginary
case; as soon as the plant had been rendered so highly attractive to
insects that pollen was regularly carried from flower to flower, another
process might commence. No naturalist doubts the advantage of what has
been called the "physiological division of labour;" hence we may believe
that it would be advantageous to a plant to produce stamens alone in one
flower or on one whole plant, and pistils alone in another flower or on
another plant. In plants under culture and placed under new conditions
of life, sometimes the male organs and sometimes the female organs
become more or less impotent; now if we suppose this to occur in ever
so slight a degree under nature, then, as pollen is already carried
regularly from flower to flower, and as a more complete separation of
the sexes of our plant would be advantageous on the principle of
the division of labour, individuals with this tendency more and more
increased, would be continually favoured or selected, until at last a
complete separation of the sexes might be effected. It would take up
too much space to show the various steps, through dimorphism and other
means, by which the separation of the sexes in plants of various kinds
is apparently now in progress; but I may add that some of the species
of holly in North America are, according to Asa Gray, in an exactly
intermediate condition, or, as he expresses it, are more or less
dioeciously polygamous.
 
Let us now turn to the nectar-feeding insects; we may suppose the
plant of which we have been slowly increasing the nectar by continued
selection, to be a common plant; and that certain insects depended in
main part on its nectar for food. I could give many facts showing how
anxious bees are to save time: for instance, their habit of cutting
holes and sucking the nectar at the bases of certain flowers, which with
a very little more trouble they can enter by the mouth. Bearing such
facts in mind, it may be believed that under certain circumstances
individual differences in the curvature or length of the proboscis,
etc., too slight to be appreciated by us, might profit a bee or other
insect, so that certain individuals would be able to obtain their
food more quickly than others; and thus the communities to which they
belonged would flourish and throw off many swarms inheriting the same
peculiarities. The tubes of the corolla of the common red or incarnate
clovers (Trifolium pratense and incarnatum) do not on a hasty glance
appear to differ in length; yet the hive-bee can easily suck the nectar
out of the incarnate clover, but not out of the common red clover, which
is visited by humble-bees alone; so that whole fields of the red clover
offer in vain an abundant supply of precious nectar to the hive-bee.
That this nectar is much liked by the hive-bee is certain; for I have
repeatedly seen, but only in the autumn, many hive-bees sucking the
flowers through holes bitten in the base of the tube by humble bees.
The difference in the length of the corolla in the two kinds of clover,
which determines the visits of the hive-bee, must be very trifling; for
I have been assured that when red clover has been mown, the flowers of
the second crop are somewhat smaller, and that these are visited by many
hive-bees. I do not know whether this statement is accurate; nor whether
another published statement can be trusted, namely, that the Ligurian
bee, which is generally considered a mere variety of the common
hive-bee, and which freely crosses with it, is able to reach and suck
the nectar of the red clover. Thus, in a country where this kind of
clover abounded, it might be a great advantage to the hive-bee to have a
slightly longer or differently constructed proboscis. On the other hand,
as the fertility of this clover absolutely depends on bees visiting the
flowers, if humble-bees were to become rare in any country, it might be
a great advantage to the plant to have a shorter or more deeply divided
corolla, so that the hive-bees should be enabled to suck its flowers.
Thus I can understand how a flower and a bee might slowly become, either
simultaneously or one after the other, modified and adapted to each
other in the most perfect manner, by the continued preservation of all
the individuals which presented slight deviations of structure mutually
favourable to each other.
 
I am well aware that this doctrine of natural selection, exemplified in
the above imaginary instances, is open to the same objections which
were first urged against Sir Charles Lyell's noble views on "the modern
changes of the earth, as illustrative of geology;" but we now seldom
hear the agencies which we see still at work, spoken of as trifling and
insignificant, when used in explaining the excavation of the deepest
valleys or the formation of long lines of inland cliffs. Natural
selection acts only by the preservation and accumulation of small
inherited modifications, each profitable to the preserved being; and
as modern geology has almost banished such views as the excavation of a
great valley by a single diluvial wave, so will natural selection banish
the belief of the continued creation of new organic beings, or of any
great and sudden modification in their structure.
 
ON THE INTERCROSSING OF INDIVIDUALS.
 
I must here introduce a short digression. In the case of animals
and plants with separated sexes, it is of course obvious that two
individuals must always (with the exception of the curious and not well
understood cases of parthenogenesis) unite for each birth; but in the
case of hermaphrodites this is far from obvious. Nevertheless there is
reason to believe that with all hermaphrodites two individuals, either
occasionally or habitually, concur for the reproduction of their kind.
This view was long ago doubtfully suggested by Sprengel, Knight and
Kolreuter. We shall presently see its importance; but I must here treat
the subject with extreme brevity, though I have the materials prepared
for an ample discussion. All vertebrate animals, all insects and some
other large groups of animals, pair for each birth. Modern research
has much diminished the number of supposed hermaphrodites and of real
hermaphrodites a large number pair; that is, two individuals regularly
unite for reproduction, which is all that concerns us. But still there
are many hermaphrodite animals which certainly do not habitually pair,
and a vast majority of plants are hermaphrodites. What reason, it may be
asked, is there for supposing in these cases that two individuals ever
concur in reproduction? As it is impossible here to enter on details, I
must trust to some general considerations alone.
 
In the first place, I have collected so large a body of facts, and made
so many experiments, showing, in accordance with the almost universal
belief of breeders, that with animals and plants a cross between
different varieties, or between individuals of the same variety but of
another strain, gives vigour and fertility to the offspring; and on the
other hand, that CLOSE interbreeding diminishes vigour and fertility;
that these facts alone incline me to believe that it is a general law
of nature that no organic being fertilises itself for a perpetuity
of generations; but that a cross with another individual is
occasionally--perhaps at long intervals of time--indispensable.
 
On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any
other view are inexplicable. Every hybridizer knows how unfavourable
exposure to wet is to the fertilisation of a flower, yet what a
multitude of flowers have their anthers and stigmas fully exposed to the
weather! If an occasional cross be indispensable, notwithstanding that
the plant's own anthers and pistil stand so near each other as almost
to ensure self-fertilisation, the fullest freedom for the entrance of
pollen from another individual will explain the above state of exposure
of the organs. Many flowers, on the other hand, have their organs of
fructification closely enclosed, as in the great papilionaceous or
pea-family; but these almost invariably present beautiful and curious
adaptations in relation to the visits of insects. So necessary are the
visits of bees to many papilionaceous flowers, that their fertility is
greatly diminished if these visits be prevented. Now, it is scarcely
possible for insects to fly from flower to flower, and not to carry
pollen from one to the other, to the great good of the plant.
Insects act like a camel-hair pencil, and it is sufficient, to ensure
fertilisation, just to touch with the same brush the anthers of one
flower and then the stigma of another; but it must not be supposed that
bees would thus produce a multitude of hybrids between distinct species;
for if a plant's own pollen and that from another species are placed
on the same stigma, the former is so prepotent that it invariably and
completely destroys, as has been shown by Gartner, the influence of the
foreign pollen.
 
When the stamens of a flower suddenly spring towards the pistil, or
slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but the agency of insects is often required to cause the
stamens to spring forward, as Kolreuter has shown to be the case with
the barberry; and in this very genus, which seems to have a special
contrivance for self-fertilisation, it is well known that, if
closely-allied forms or varieties are planted near each other, it is
hardly possible to raise pure seedlings, so largely do they naturally
cross. In numerous other cases, far from self-fertilisation being
favoured, there are special contrivances which effectually prevent the
stigma receiving pollen from its own flower, as I could show from the
works of Sprengel and others, as well as from my own observations: for
instance, in Lobelia fulgens, there is a really beautiful and elaborate
contrivance by which all the infinitely numerous pollen-granules are
swept out of the conjoined anthers of each flower, before the stigma of
that individual flower is ready to receive them; and as this flower is
never visited, at least in my garden, by insects, it never sets a seed,
though by placing pollen from one flower on the stigma of another, I
raise plenty of seedlings. Another species of Lobelia, which is visited
by bees, seeds freely in my garden. In very many other cases, though
there is no special mechanical contrivance to prevent the stigma
receiving pollen from the same flower, yet, as Sprengel, and more
recently Hildebrand and others have shown, and as I can confirm, either
the anthers burst before the stigma is ready for fertilisation, or the
stigma is ready before the pollen of that flower is ready, so that
these so-named dichogamous plants have in fact separated sexes, and
must habitually be crossed. So it is with the reciprocally dimorphic and
trimorphic plants previously alluded to. How strange are these facts!
How strange that the pollen and stigmatic surface of the same flower,
though placed so close together, as if for the very purpose of
self-fertilisation, should be in so many cases mutually useless to each
other! How simply are these facts explained on the view of an occasional
cross with a distinct individual being advantageous or indispensable!
 
If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority of the
seedlings thus raised turn out, as I found, mongrels: for instance, I
raised 233 seedling cabbages from some plants of different varieties
growing near each other, and of these only 78 were true to their kind,
and some even of these were not perfectly true. Yet the pistil of each
cabbage-flower is surrounded not only by its own six stamens but by
those of the many other flowers on the same plant; and the pollen of
each flower readily gets on its stigma without insect agency; for I
have found that plants carefully protected from insects produce the
full number of pods. How, then, comes it that such a vast number of the
seedlings are mongrelized? It must arise from the pollen of a distinct
VARIETY having a prepotent effect over the flower's own pollen; and
that this is part of the general law of good being derived from the
intercrossing of distinct individuals of the same species. When distinct
SPECIES are crossed the case is reversed, for a plant's own pollen
is always prepotent over foreign pollen; but to this subject we shall
return in a future chapter.
 
In the case of a large tree covered with innumerable flowers, it may be
objected that pollen could seldom be carried from tree to tree, and at
most only from flower to flower on the same tree; and flowers on the
same tree can be considered as distinct individuals only in a limited
sense. I believe this objection to be valid, but that nature has largely
provided against it by giving to trees a strong tendency to bear flowers
with separated sexes. When the sexes are separated, although the male
and female flowers may be produced on the same tree, pollen must be
regularly carried from flower to flower; and this will give a better
chance of pollen being occasionally carried from tree to tree. That
trees belonging to all orders have their sexes more often separated than
other plants, I find to be the case in this country; and at my request
Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those
of the United States, and the result was as I anticipated. On the
other hand, Dr. Hooker informs me that the rule does not hold good in
Australia: but if most of the Australian trees are dichogamous, the same
result would follow as if they bore flowers with separated sexes. I have
made these few remarks on trees simply to call attention to the subject.
 
Turning for a brief space to animals: various terrestrial species are
hermaphrodites, such as the land-mollusca and earth-worms; but these
all pair. As yet I have not found a single terrestrial animal which
can fertilise itself. This remarkable fact, which offers so strong a
contrast with terrestrial plants, is intelligible on the view of an
occasional cross being indispensable; for owing to the nature of the
fertilising element there are no means, analogous to the action of
insects and of the wind with plants, by which an occasional cross could
be effected with terrestrial animals without the concurrence of two
individuals. Of aquatic animals, there are many self-fertilising
hermaphrodites; but here the currents of water offer an obvious means
for an occasional cross. As in the case of flowers, I have as yet
failed, after consultation with one of the highest authorities, namely,
Professor Huxley, to discover a single hermaphrodite animal with the
organs of reproduction so perfectly enclosed that access from without,
and the occasional influence of a distinct individual, can be shown to
be physically impossible. Cirripedes long appeared to me to present,
under this point of view, a case of great difficulty; but I have been
enabled, by a fortunate chance, to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.
 
It must have struck most naturalists as a strange anomaly that, both
with animals and plants, some species of the same family and even of
the same genus, though agreeing closely with each other in their whole
organisation, are hermaphrodites, and some unisexual. But if, in fact,
all hermaphrodites do occasionally intercross, the difference between
them and unisexual species is, as far as function is concerned, very
small.
 
From these several considerations and from the many special facts which
I have collected, but which I am unable here to give, it appears that
with animals and plants an occasional intercross between distinct
individuals is a very general, if not universal, law of nature.
 
CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW FORMS THROUGH NATURAL
SELECTION.
 
This is an extremely intricate subject. A great amount of variability,
under which term individual differences are always included, will
evidently be favourable. A large number of individuals, by giving a
better chance within any given period for the appearance of profitable
variations, will compensate for a lesser amount of variability in each
individual, and is, I believe, a highly important element of success.
Though nature grants long periods of time for the work of natural
selection, she does not grant an indefinite period; for as all organic
beings are striving to seize on each place in the economy of nature, if
any one species does not become modified and improved in a corresponding
degree with its competitors it will be exterminated. Unless favourable
variations be inherited by some at least of the offspring, nothing can
be effected by natural selection. The tendency to reversion may often
check or prevent the work; but as this tendency has not prevented man
from forming by selection numerous domestic races, why should it prevail
against natural selection?
 
In the case of methodical selection, a breeder selects for some definite
object, and if the individuals be allowed freely to intercross, his work
will completely fail. But when many men, without intending to alter
the breed, have a nearly common standard of perfection, and all try to
procure and breed from the best animals, improvement surely but slowly
follows from this unconscious process of selection, notwithstanding that
there is no separation of selected individuals. Thus it will be under
nature; for within a confined area, with some place in the natural
polity not perfectly occupied, all the individuals varying in the right
direction, though in different degrees, will tend to be preserved.
But if the area be large, its several districts will almost certainly
present different conditions of life; and then, if the same species
undergoes modification in different districts, the newly formed
varieties will intercross on the confines of each. But we shall see in
the sixth chapter that intermediate varieties, inhabiting intermediate
districts, will in the long run generally be supplanted by one of the
adjoining varieties. Intercrossing will chiefly affect those animals
which unite for each birth and wander much, and which do not breed at a
very quick rate. Hence with animals of this nature, for instance birds,
varieties will generally be confined to separated countries; and this
I find to be the case. With hermaphrodite organisms which cross only
occasionally, and likewise with animals which unite for each birth, but
which wander little and can increase at a rapid rate, a new and improved
variety might be quickly formed on any one spot, and might there
maintain itself in a body and afterward spread, so that the individuals
of the new variety would chiefly cross together. On this principle
nurserymen always prefer saving seed from a large body of plants, as the
chance of intercrossing is thus lessened.
 
Even with animals which unite for each birth, and which do not propagate
rapidly, we must not assume that free intercrossing would always
eliminate the effects of natural selection; for I can bring forward
a considerable body of facts showing that within the same area two
varieties of the same animal may long remain distinct, from haunting
different stations, from breeding at slightly different seasons, or from
the individuals of each variety preferring to pair together.
 
Intercrossing plays a very important part in nature by keeping the
individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals which unite for each birth; but, as already stated,
we have reason to believe that occasional intercrosses take place with
all animals and plants. Even if these take place only at long intervals
of time, the young thus produced will gain so much in vigour and
fertility over the offspring from long-continued self-fertilisation,
that they will have a better chance of surviving and propagating their
kind; and thus in the long run the influence of crosses, even at rare
intervals, will be great. With respect to organic beings extremely low
in the scale, which do not propagate sexually, nor conjugate, and which
cannot possibly intercross, uniformity of character can be retained by
them under the same conditions of life, only through the principle
of inheritance, and through natural selection which will destroy any
individuals departing from the proper type. If the conditions of life
change and the form undergoes modification, uniformity of character
can be given to the modified offspring, solely by natural selection
preserving similar favourable variations.
 
Isolation also is an important element in the modification of species
through natural selection. In a confined or isolated area, if not very
large, the organic and inorganic conditions of life will generally be
almost uniform; so that natural selection will tend to modify all
the varying individuals of the same species in the same manner.
Intercrossing with the inhabitants of the surrounding districts,
will also be thus prevented. Moritz Wagner has lately published an
interesting essay on this subject, and has shown that the service
rendered by isolation in preventing crosses between newly-formed
varieties is probably greater even than I supposed. But from reasons
already assigned I can by no means agree with this naturalist, that
migration and isolation are necessary elements for the formation of new
species. The importance of isolation is likewise great in preventing,
after any physical change in the conditions, such as of climate,
elevation of the land, etc., the immigration of better adapted
organisms; and thus new places in the natural economy of the district
will be left open to be filled up by the modification of the old
inhabitants. Lastly, isolation will give time for a new variety to be
improved at a slow rate; and this may sometimes be of much importance.
If, however, an isolated area be very small, either from being
surrounded by barriers, or from having very peculiar physical
conditions, the total number of the inhabitants will be small; and this
will retard the production of new species through natural selection, by
decreasing the chances of favourable variations arising.
 
The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously asserted
that the element of time has been assumed by me to play an all-important
part in modifying species, as if all the forms of life were necessarily
undergoing change through some innate law. Lapse of time is only so far
important, and its importance in this respect is great, that it gives
a better chance of beneficial variations arising and of their being
selected, accumulated, and fixed. It likewise tends to increase the
direct action of the physical conditions of life, in relation to the
constitution of each organism.
 
If we turn to nature to test the truth of these remarks, and look at any
small isolated area, such as an oceanic island, although the number of
the species inhabiting it is small, as we shall see in our chapter on
Geographical Distribution; yet of these species a very large proportion
are endemic,--that is, have been produced there and nowhere else in the
world. Hence an oceanic island at first sight seems to have been highly
favourable for the production of new species. But we may thus deceive
ourselves, for to ascertain whether a small isolated area, or a large
open area like a continent, has been most favourable for the production
of new organic forms, we ought to make the comparison within equal
times; and this we are incapable of doing.
 
Although isolation is of great importance in the production of new
species, on the whole I am inclined to believe that largeness of area
is still more important, especially for the production of species which
shall prove capable of enduring for a long period, and of spreading
widely. Throughout a great and open area, not only will there be a
better chance of favourable variations, arising from the large number of
individuals of the same species there supported, but the conditions of
life are much more complex from the large number of already existing
species; and if some of these many species become modified and improved,
others will have to be improved in a corresponding degree, or they
will be exterminated. Each new form, also, as soon as it has been much
improved, will be able to spread over the open and continuous area, and
will thus come into competition with many other forms. Moreover, great
areas, though now continuous, will often, owing to former oscillations
of level, have existed in a broken condition, so that the good effects
of isolation will generally, to a certain extent, have concurred.
Finally, I conclude that, although small isolated areas have been in
some respects highly favourable for the production of new species, yet
that the course of modification will generally have been more rapid on
large areas; and what is more important, that the new forms produced on
large areas, which already have been victorious over many competitors,
will be those that will spread most widely, and will give rise to the
greatest number of new varieties and species. They will thus play a more
important part in the changing history of the organic world.
 
In accordance with this view, we can, perhaps, understand some
facts which will be again alluded to in our chapter on Geographical
Distribution; for instance, the fact of the productions of the
smaller continent of Australia now yielding before those of the larger
Europaeo-Asiatic area. Thus, also, it is that continental productions
have everywhere become so largely naturalised on islands. On a small
island, the race for life will have been less severe, and there will
have been less modification and less extermination. Hence, we can
understand how it is that the flora of Madeira, according to Oswald
Heer, resembles to a certain extent the extinct tertiary flora of
Europe. All fresh water basins, taken together, make a small area
compared with that of the sea or of the land. Consequently, the
competition between fresh water productions will have been less severe
than elsewhere; new forms will have been more slowly produced, and old
forms more slowly exterminated. And it is in fresh water basins that
we find seven genera of Ganoid fishes, remnants of a once preponderant
order: and in fresh water we find some of the most anomalous forms now
known in the world, as the Ornithorhynchus and Lepidosiren, which, like
fossils, connect to a certain extent orders at present widely separated
in the natural scale. These anomalous forms may be called living
fossils; they have endured to the present day, from having inhabited
a confined area, and from having been exposed to less varied, and
therefore less severe, competition.
 
To sum up, as far as the extreme intricacy of the subject permits, the
circumstances favourable and unfavourable for the production of new
species through natural selection. I conclude that for terrestrial
productions a large continental area, which has undergone many
oscillations of level, will have been the most favourable for the
production of many new forms of life, fitted to endure for a long
time and to spread widely. While the area existed as a continent the
inhabitants will have been numerous in individuals and kinds, and will
have been subjected to severe competition. When converted by subsidence
into large separate islands there will still have existed many
individuals of the same species on each island: intercrossing on the
confines of the range of each new species will have been checked: after
physical changes of any kind immigration will have been prevented, so
that new places in the polity of each island will have had to be filled
up by the modification of the old inhabitants; and time will have been
allowed for the varieties in each to become well modified and perfected.
When, by renewed elevation, the islands were reconverted into a
continental area, there will again have been very severe competition;
the most favoured or improved varieties will have been enabled to
spread; there will have been much extinction of the less improved forms,
and the relative proportional numbers of the various inhabitants of the
reunited continent will again have been changed; and again there will
have been a fair field for natural selection to improve still further
the inhabitants, and thus to produce new species.
 
That natural selection generally act with extreme slowness I fully
admit. It can act only when there are places in the natural polity of a
district which can be better occupied by the modification of some of its
existing inhabitants. The occurrence of such places will often depend
on physical changes, which generally take place very slowly, and on the
immigration of better adapted forms being prevented. As some few of
the old inhabitants become modified the mutual relations of others will
often be disturbed; and this will create new places, ready to be filled
up by better adapted forms; but all this will take place very slowly.
Although all the individuals of the same species differ in some slight
degree from each other, it would often be long before differences of
the right nature in various parts of the organisation might occur. The
result would often be greatly retarded by free intercrossing. Many will
exclaim that these several causes are amply sufficient to neutralise the
power of natural selection. I do not believe so. But I do believe that
natural selection will generally act very slowly, only at long intervals
of time, and only on a few of the inhabitants of the same region. I
further believe that these slow, intermittent results accord well with
what geology tells us of the rate and manner at which the inhabitants of
the world have changed.
 
Slow though the process of selection may be, if feeble man can do much
by artificial selection, I can see no limit to the amount of change,
to the beauty and complexity of the coadaptations between all organic
beings, one with another and with their physical conditions of life,
which may have been effected in the long course of time through nature's
power of selection, that is by the survival of the fittest.
 
EXTINCTION CAUSED BY NATURAL SELECTION.
 
This subject will be more fully discussed in our chapter on Geology; but
it must here be alluded to from being intimately connected with natural
selection. Natural selection acts solely through the preservation of
variations in some way advantageous, which consequently endure. Owing to
the high geometrical rate of increase of all organic beings, each area
is already fully stocked with inhabitants, and it follows from this,
that as the favoured forms increase in number, so, generally, will the
less favoured decrease and become rare. Rarity, as geology tells us,
is the precursor to extinction. We can see that any form which
is represented by few individuals will run a good chance of utter
extinction, during great fluctuations in the nature or the seasons, or
from a temporary increase in the number of its enemies. But we may go
further than this; for as new forms are produced, unless we admit that
specific forms can go on indefinitely increasing in number, many old
forms must become extinct. That the number of specific forms has not
indefinitely increased, geology plainly tells us; and we shall presently
attempt to show why it is that the number of species throughout the
world has not become immeasurably great.
 
We have seen that the species which are most numerous in individuals
have the best chance of producing favourable variations within any given
period. We have evidence of this, in the facts stated in the second
chapter, showing that it is the common and diffused or dominant species
which offer the greatest number of recorded varieties. Hence, rare
species will be less quickly modified or improved within any given
period; they will consequently be beaten in the race for life by the
modified and improved descendants of the commoner species.
 
From these several considerations I think it inevitably follows,
that as new species in the course of time are formed through natural
selection, others will become rarer and rarer, and finally extinct.
The forms which stand in closest competition with those undergoing
modification and improvement, will naturally suffer most. And we have
seen in the chapter on the Struggle for Existence that it is the most
closely-allied forms,--varieties of the same species, and species of
the same genus or related genera,--which, from having nearly the same
structure, constitution and habits, generally come into the severest
competition with each other. Consequently, each new variety or species,
during the progress of its formation, will generally press hardest
on its nearest kindred, and tend to exterminate them. We see the same
process of extermination among our domesticated productions, through
the selection of improved forms by man. Many curious instances could be
given showing how quickly new breeds of cattle, sheep and other animals,
and varieties of flowers, take the place of older and inferior kinds. In
Yorkshire, it is historically known that the ancient black cattle were
displaced by the long-horns, and that these "were swept away by the
short-horns" (I quote the words of an agricultural writer) "as if by
some murderous pestilence."
 
DIVERGENCE OF CHARACTER.
 
The principle, which I have designated by this term, is of high
importance, and explains, as I believe, several important facts. In
the first place, varieties, even strongly-marked ones, though having
somewhat of the character of species--as is shown by the hopeless doubts
in many cases how to rank them--yet certainly differ far less from each
other than do good and distinct species. Nevertheless according to my
view, varieties are species in the process of formation, or are, as
I have called them, incipient species. How, then, does the lesser
difference between varieties become augmented into the greater
difference between species? That this does habitually happen, we must
infer from most of the innumerable species throughout nature presenting
well-marked differences; whereas varieties, the supposed prototypes and
parents of future well-marked species, present slight and ill-defined
differences. Mere chance, as we may call it, might cause one variety
to differ in some character from its parents, and the offspring of this
variety again to differ from its parent in the very same character and
in a greater degree; but this alone would never account for so habitual
and large a degree of difference as that between the species of the same
genus.
 
As has always been my practice, I have sought light on this head from
our domestic productions. We shall here find something analogous. It
will be admitted that the production of races so different as short-horn
and Hereford cattle, race and cart horses, the several breeds of
pigeons, etc., could never have been effected by the mere chance
accumulation of similar variations during many successive generations.
In practice, a fancier is, for instance, struck by a pigeon having a
slightly shorter beak; another fancier is struck by a pigeon having a
rather longer beak; and on the acknowledged principle that "fanciers do
not and will not admire a medium standard, but like extremes," they
both go on (as has actually occurred with the sub-breeds of the
tumbler-pigeon) choosing and breeding from birds with longer and longer
beaks, or with shorter and shorter beaks. Again, we may suppose that at
an early period of history, the men of one nation or district required
swifter horses, while those of another required stronger and bulkier
horses. The early differences would be very slight; but, in the course
of time, from the continued selection of swifter horses in the one case,
and of stronger ones in the other, the differences would become greater,
and would be noted as forming two sub-breeds. Ultimately after the
lapse of centuries, these sub-breeds would become converted into two
well-established and distinct breeds. As the differences became greater,
the inferior animals with intermediate characters, being neither very
swift nor very strong, would not have been used for breeding, and will
thus have tended to disappear. Here, then, we see in man's productions
the action of what may be called the principle of divergence, causing
differences, at first barely appreciable, steadily to increase, and
the breeds to diverge in character, both from each other and from their
common parent.
 
But how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently (though it was a long
time before I saw how), from the simple circumstance that the more
diversified the descendants from any one species become in structure,
constitution, and habits, by so much will they be better enabled to
seize on many and widely diversified places in the polity of nature, and
so be enabled to increase in numbers.
 
We can clearly discern this in the case of animals with simple habits.
Take the case of a carnivorous quadruped, of which the number that can
be supported in any country has long ago arrived at its full average.
If its natural power of increase be allowed to act, it can succeed in
increasing (the country not undergoing any change in conditions) only by
its varying descendants seizing on places at present occupied by other
animals: some of them, for instance, being enabled to feed on new kinds
of prey, either dead or alive; some inhabiting new stations, climbing
trees, frequenting water, and some perhaps becoming less carnivorous.
The more diversified in habits and structure the descendants of our
carnivorous animals become, the more places they will be enabled to
occupy. What applies to one animal will apply throughout all time to
all animals--that is, if they vary--for otherwise natural selection can
effect nothing. So it will be with plants. It has been experimentally
proved, that if a plot of ground be sown with one species of grass,
and a similar plot be sown with several distinct genera of grasses,
a greater number of plants and a greater weight of dry herbage can be
raised in the latter than in the former case. The same has been found
to hold good when one variety and several mixed varieties of wheat have
been sown on equal spaces of ground. Hence, if any one species of grass
were to go on varying, and the varieties were continually selected which
differed from each other in the same manner, though in a very slight
degree, as do the distinct species and genera of grasses, a greater
number of individual plants of this species, including its modified
descendants, would succeed in living on the same piece of ground. And
we know that each species and each variety of grass is annually sowing
almost countless seeds; and is thus striving, as it may be said, to
the utmost to increase in number. Consequently, in the course of many
thousand generations, the most distinct varieties of any one species
of grass would have the best chance of succeeding and of increasing
in numbers, and thus of supplanting the less distinct varieties; and
varieties, when rendered very distinct from each other, take the rank of
species.
 
The truth of the principle that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if freely
open to immigration, and where the contest between individual and
individual must be very severe, we always find great diversity in its
inhabitants. For instance, I found that a piece of turf, three feet by
four in size, which had been exposed for many years to exactly the same
conditions, supported twenty species of plants, and these belonged to
eighteen genera and to eight orders, which shows how much these plants
differed from each other. So it is with the plants and insects on small
and uniform islets: also in small ponds of fresh water. Farmers find
that they can raise more food by a rotation of plants belonging to the
most different orders: nature follows what may be called a simultaneous
rotation. Most of the animals and plants which live close round any
small piece of ground, could live on it (supposing its nature not to be
in any way peculiar), and may be said to be striving to the utmost
to live there; but, it is seen, that where they come into the closest
competition, the advantages of diversification of structure, with the
accompanying differences of habit and constitution, determine that the
inhabitants, which thus jostle each other most closely, shall, as a
general rule, belong to what we call different genera and orders.
 
The same principle is seen in the naturalisation of plants through man's
agency in foreign lands. It might have been expected that the plants
which would succeed in becoming naturalised in any land would generally
have been closely allied to the indigenes; for these are commonly looked
at as specially created and adapted for their own country. It might
also, perhaps, have been expected that naturalised plants would have
belonged to a few groups more especially adapted to certain stations in
their new homes. But the case is very different; and Alph. de Candolle
has well remarked, in his great and admirable work, that floras gain by
naturalisation, proportionally with the number of the native genera and
species, far more in new genera than in new species. To give a single
instance: in the last edition of Dr. Asa Gray's "Manual of the Flora of
the Northern United States," 260 naturalised plants are enumerated, and
these belong to 162 genera. We thus see that these naturalised plants
are of a highly diversified nature. They differ, moreover, to a large
extent, from the indigenes, for out of the 162 naturalised genera,
no less than 100 genera are not there indigenous, and thus a large
proportional addition is made to the genera now living in the United
States.
 
By considering the nature of the plants or animals which have in any
country struggled successfully with the indigenes, and have there become
naturalised, we may gain some crude idea in what manner some of the
natives would have had to be modified in order to gain an advantage over
their compatriots; and we may at least infer that diversification of
structure, amounting to new generic differences, would be profitable to
them.
 
The advantage of diversification of structure in the inhabitants of the
same region is, in fact, the same as that of the physiological division
of labour in the organs of the same individual body--a subject so well
elucidated by Milne Edwards. No physiologist doubts that a stomach by
being adapted to digest vegetable matter alone, or flesh alone, draws
most nutriment from these substances. So in the general economy of
any land, the more widely and perfectly the animals and plants are
diversified for different habits of life, so will a greater number of
individuals be capable of there supporting themselves. A set of animals,
with their organisation but little diversified, could hardly compete
with a set more perfectly diversified in structure. It may be doubted,
for instance, whether the Australian marsupials, which are divided into
groups differing but little from each other, and feebly representing, as
Mr. Waterhouse and others have remarked, our carnivorous, ruminant, and
rodent mammals, could successfully compete with these well-developed
orders. In the Australian mammals, we see the process of diversification
in an early and incomplete stage of development.
 
THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH
DIVERGENCE OF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF A COMMON
ANCESTOR.
 
After the foregoing discussion, which has been much compressed, we may
assume that the modified descendants of any one species will succeed so
much the better as they become more diversified in structure, and are
thus enabled to encroach on places occupied by other beings. Now let
us see how this principle of benefit being derived from divergence of
character, combined with the principles of natural selection and of
extinction, tends to act.
 
The accompanying diagram will aid us in understanding this rather
perplexing subject. Let A to L represent the species of a genus large
in its own country; these species are supposed to resemble each other
in unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal distances.
I have said a large genus, because as we saw in the second chapter, on
an average more species vary in large genera than in small genera; and
the varying species of the large genera present a greater number of
varieties. We have, also, seen that the species, which are the commonest
and most widely-diffused, vary more than do the rare and restricted
species. Let (A) be a common, widely-diffused, and varying species,
belonging to a genus large in its own country. The branching and
diverging dotted lines of unequal lengths proceeding from (A), may
represent its varying offspring. The variations are supposed to be
extremely slight, but of the most diversified nature; they are not
supposed all to appear simultaneously, but often after long intervals of
time; nor are they all supposed to endure for equal periods. Only
those variations which are in some way profitable will be preserved or
naturally selected. And here the importance of the principle of benefit
derived from divergence of character comes in; for this will generally
lead to the most different or divergent variations (represented by
the outer dotted lines) being preserved and accumulated by natural
selection. When a dotted line reaches one of the horizontal lines,
and is there marked by a small numbered letter, a sufficient amount of
variation is supposed to have been accumulated to form it into a fairly
well-marked variety, such as would be thought worthy of record in a
systematic work.
 
The intervals between the horizontal lines in the diagram, may represent
each a thousand or more generations. After a thousand generations,
species (A) is supposed to have produced two fairly well-marked
varieties, namely a1 and m1. These two varieties will generally still
be exposed to the same conditions which made their parents variable, and
the tendency to variability is in itself hereditary; consequently they
will likewise tend to vary, and commonly in nearly the same manner as
did their parents. Moreover, these two varieties, being only slightly
modified forms, will tend to inherit those advantages which made their
parent (A) more numerous than most of the other inhabitants of the same
country; they will also partake of those more general advantages which
made the genus to which the parent-species belonged, a large genus
in its own country. And all these circumstances are favourable to the
production of new varieties.
 
If, then, these two varieties be variable, the most divergent of
their variations will generally be preserved during the next thousand
generations. And after this interval, variety a1 is supposed in the
diagram to have produced variety a2, which will, owing to the principle
of divergence, differ more from (A) than did variety a1. Variety m1 is
supposed to have produced two varieties, namely m2 and s2, differing
from each other, and more considerably from their common parent (A). We
may continue the process by similar steps for any length of time; some
of the varieties, after each thousand generations, producing only
a single variety, but in a more and more modified condition, some
producing two or three varieties, and some failing to produce any. Thus
the varieties or modified descendants of the common parent (A), will
generally go on increasing in number and diverging in character. In the
diagram the process is represented up to the ten-thousandth generation,
and under a condensed and simplified form up to the fourteen-thousandth
generation.
 
But I must here remark that I do not suppose that the process ever goes
on so regularly as is represented in the diagram, though in itself made
somewhat irregular, nor that it goes on continuously; it is far more
probable that each form remains for long periods unaltered, and then
again undergoes modification. Nor do I suppose that the most divergent
varieties are invariably preserved: a medium form may often long endure,
and may or may not produce more than one modified descendant; for
natural selection will always act according to the nature of the places
which are either unoccupied or not perfectly occupied by other beings;
and this will depend on infinitely complex relations. But as a general
rule, the more diversified in structure the descendants from any one
species can be rendered, the more places they will be enabled to seize
on, and the more their modified progeny will increase. In our diagram
the line of succession is broken at regular intervals by small numbered
letters marking the successive forms which have become sufficiently
distinct to be recorded as varieties. But these breaks are imaginary,
and might have been inserted anywhere, after intervals long enough to
allow the accumulation of a considerable amount of divergent variation.
 
As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake of the
same advantages which made their parent successful in life, they will
generally go on multiplying in number as well as diverging in character:
this is represented in the diagram by the several divergent branches
proceeding from (A). The modified offspring from the later and more
highly improved branches in the lines of descent, will, it is probable,
often take the place of, and so destroy, the earlier and less improved
branches: this is represented in the diagram by some of the lower
branches not reaching to the upper horizontal lines. In some cases no
doubt the process of modification will be confined to a single line of
descent, and the number of modified descendants will not be increased;
although the amount of divergent modification may have been augmented.
This case would be represented in the diagram, if all the lines
proceeding from (A) were removed, excepting that from a1 to a10. In the
same way the English racehorse and English pointer have apparently
both gone on slowly diverging in character from their original stocks,
without either having given off any fresh branches or races.
 
After ten thousand generations, species (A) is supposed to have produced
three forms, a10, f10, and m10, which, from having diverged in character
during the successive generations, will have come to differ largely, but
perhaps unequally, from each other and from their common parent. If we
suppose the amount of change between each horizontal line in our diagram
to be excessively small, these three forms may still be only well-marked
varieties; but we have only to suppose the steps in the process of
modification to be more numerous or greater in amount, to convert these
three forms into doubtful or at least into well-defined species:
thus the diagram illustrates the steps by which the small differences
distinguishing varieties are increased into the larger differences
distinguishing species. By continuing the same process for a greater
number of generations (as shown in the diagram in a condensed and
simplified manner), we get eight species, marked by the letters between
a14 and m14, all descended from (A). Thus, as I believe, species are
multiplied and genera are formed.
 
In a large genus it is probable that more than one species would vary.
In the diagram I have assumed that a second species (I) has produced, by
analogous steps, after ten thousand generations, either two well-marked
varieties (w10 and z10) or two species, according to the amount of
change supposed to be represented between the horizontal lines. After
fourteen thousand generations, six new species, marked by the letters
n14 to z14, are supposed to have been produced. In any genus, the
species which are already very different in character from each
other, will generally tend to produce the greatest number of modified