Title: The Origin of Species by Means of Natural Selection
 - Or, the Preservation of Favoured Races in the Struggle for Life, 6th Edition
Author: Darwin, Charles
Language: English
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1228    1859, First Edition
22764   1860, Second Edition


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. 


"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

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

   CHAPTER I.   VARIATION UNDER
  DOMESTICATION 
  CHAPTER II.   VARIATION
  UNDER NATURE 
  CHAPTER III.   STRUGGLE
  FOR EXISTENCE 
  CHAPTER IV.   NATURAL
  SELECTION; OR THE SURVIVAL OF THE FITTEST 
  CHAPTER V.   LAWS OF VARIATION 

   CHAPTER VI.   DIFFICULTIES OF THE
  THEORY 
  CHAPTER VII.   MISCELLANEOUS
  OBJECTIONS TO THE THEORY OF NATURAL SELECTION 
  CHAPTER VIII.   INSTINCT 

   CHAPTER IX.   HYBRIDISM 

   CHAPTER X.   ON THE IMPERFECTION
  OF THE GEOLOGICAL RECORD 
  CHAPTER XI.
    ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 

   CHAPTER XII.   GEOGRAPHICAL
  DISTRIBUTION 
  CHAPTER XIII.     GEOGRAPHICAL
  DISTRIBUTION—continued 
  CHAPTER
  XIV.   MUTUAL AFFINITIES OF ORGANIC BEINGS 
  CHAPTER XV.   RECAPITULATION AND
  CONCLUSION 

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



DETAILED CONTENTS.


  INTRODUCTION 
 CHAPTER I. 
 VARIATION UNDER DOMESTICATION. 
  Causes of Variability—Effects of Habit and the use or disuse of
  
 Parts—Correlated Variation—Inheritance—Character
  of Domestic 
 Varieties—Difficulty of distinguishing between
  Varieties and 
 Species—Origin of Domestic Varieties from one
  or more Species—Domestic 
 Pigeons, their Differences and
  Origin—Principles of Selection, 
 anciently followed, their
  Effects—Methodical and Unconscious 
 Selection—Unknown
  Origin of our Domestic Productions—Circumstances 
 favourable
  to Man's power of Selection. 
 CHAPTER II. 
 VARIATION UNDER
  NATURE. 
 Variability—Individual Differences—Doubtful
  species—Wide ranging, 
 much diffused, and common species,
  vary most—Species of the larger 
 genera in each country vary
  more frequently than the species of the 
 smaller genera—Many
  of the species of the larger genera resemble 
 varieties in being
  very closely, but unequally, related to each other, 
 and in having
  restricted ranges. 
 CHAPTER III. 
 STRUGGLE FOR EXISTENCE.
  
 Its bearing on natural selection—The term used in a wide
  
 sense—Geometrical ratio of increase—Rapid increase of
  naturalised 
 animals and plants—Nature of the checks to
  increase—Competition 
 universal—Effects of climate—Protection
  from the number of 
 individuals—Complex relations of all
  animals and plants throughout 
 nature—Struggle for life most
  severe between individuals and varieties 
 of the same species;
  often severe between species of the same genus—The 
 relation
  of organism to organism the most important of all relations. 
  CHAPTER IV. 
 NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
  
 Natural Selection—its power compared with man's selection—its
  power 
 on characters of trifling importance—its power at all
  ages and on 
 both sexes—Sexual Selection—On the
  generality of intercrosses 
 between individuals of the same species—Circumstances
  favourable and 
 unfavourable to the results of Natural Selection,
  namely, intercrossing, 
 isolation, number of individuals—Slow
  action—Extinction caused by 
 Natural Selection—Divergence
  of Character, related to the diversity of 
 inhabitants of any small
  area and to naturalisation—Action of Natural 
 Selection,
  through Divergence of Character and Extinction, on the 
 descendants
  from a common parent—Explains the Grouping of all organic 
  beings—Advance in organisation—Low forms preserved—Convergence
  of 
 character—Indefinite multiplication of species—Summary.
  
 CHAPTER V. 
 LAWS OF VARIATION. 
 Effects of changed
  conditions—Use and disuse, combined with natural 
 selection;
  organs of flight and of vision—Acclimatisation—Correlated
  
 variation—Compensation and economy of growth—False
  
 correlations—Multiple, rudimentary, and lowly organised
  structures 
 variable—Parts developed in an unusual manner are
  highly variable; 
 specific characters more variable than generic;
  secondary sexual 
 characters variable—Species of the same
  genus vary in an analogous 
 manner—Reversions to long-lost
  characters—Summary. 
 CHAPTER VI. 
 DIFFICULTIES OF THE
  THEORY. 
 Difficulties of the theory of descent with modification—Absence
  
 or rarity of transitional varieties—Transitions in habits of
  
 life—Diversified habits in the same species—Species
  with habits 
 widely different from those of their allies—Organs
  of extreme 
 perfection—Modes of transition—Cases of
  difficulty—Natura non facit 
 saltum—Organs of small
  importance—Organs not in all cases absolutely 
 perfect—The
  law of Unity of Type and of the Conditions of Existence 
 embraced
  by the theory of Natural Selection. 
 CHAPTER VII. 
  MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION. 
  Longevity—Modifications not necessarily simultaneous—Modifications
  
 apparently of no direct service—Progressive development—Characters
  of 
 small functional importance, the most constant—Supposed
  incompetence 
 of natural selection to account for the incipient
  stages of useful 
 structures—Causes which interfere with the
  acquisition through natural 
 selection of useful structures—Gradations
  of structure with changed 
 functions—Widely different organs
  in members of the same class, 
 developed from one and the same
  source—Reasons for disbelieving in 
 great and abrupt
  modifications. 
 CHAPTER VIII. 
 INSTINCT. 
 Instincts
  comparable with habits, but different in their 
 origin—Instincts
  graduated—Aphides and ants—Instincts 
 variable—Domestic
  instincts, their origin—Natural instincts of 
 the cuckoo,
  molothrus, ostrich, and parasitic bees—Slave-making 
 ants—Hive-bee,
  its cell-making instinct—Changes of instinct and 
 structure
  not necessarily simultaneous—Difficulties on the theory of 
  the Natural Selection of instincts—Neuter or sterile insects—Summary.
  
 CHAPTER IX. 
 HYBRIDISM. 
 Distinction between the
  sterility of first crosses and of 
 hybrids—Sterility various
  in degree, not universal, affected by close 
 interbreeding, removed
  by domestication—Laws governing the sterility 
 of hybrids—Sterility
  not a special endowment, but incidental on 
 other differences, not
  accumulated by natural selection—Causes of 
 the sterility of
  first crosses and of hybrids—Parallelism between the 
 effects
  of changed conditions of life and of crossing—Dimorphism and 
  Trimorphism—Fertility of varieties when crossed and of their
  mongrel 
 offspring not universal—Hybrids and mongrels
  compared independently of 
 their fertility—Summary. 
  CHAPTER X. 
 ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. 
 On
  the absence of intermediate varieties at the present day—On the
  
 nature of extinct intermediate varieties; on their number—On
  the lapse 
 of time, as inferred from the rate of denudation and of
  deposition—On 
 the lapse of time as estimated in years—On
  the poorness of our 
 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.
Linnæus has calculated that if an annual plant produced only two seeds—and
there is no plant so unproductive as this—and their seedlings next
year produced two, and so on, then in twenty years there would be a
million plants. The elephant is reckoned the slowest breeder of all known
animals, and I have taken some pains to estimate its probable minimum rate
of natural increase; it will be safest to assume that it begins breeding
when thirty years old, and goes on breeding till ninety years old,
bringing forth six young in the interval, and surviving till one hundred
years old; if this be so, after a period of from 740 to 750 years there
would be nearly nineteen million elephants alive descended from the first
pair.

But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly
rapid increase of various animals in a state of nature, when circumstances
have been favourable to them during two or three following seasons. Still
more striking is the evidence from our domestic animals of many kinds
which have run wild in several parts of the world; if the statements of
the rate of increase of slow-breeding cattle and horses in South America,
and latterly in Australia, had not been well authenticated, they would
have been incredible. So it is with plants; cases could be given of
introduced plants which have become common throughout whole islands in a
period of less than ten years. Several of the plants, such as the cardoon
and a tall thistle, which are now the commonest over the wide plains of La
Plata, clothing square leagues of surface almost to the exclusion of every
other plant, have been introduced from Europe; and there are plants which
now range in India, as I hear from Dr. Falconer, from Cape Comorin to the
Himalaya, which have been imported from America since its discovery. In
such cases, and endless others could be given, no one supposes that the
fertility of the animals or plants has been suddenly and temporarily
increased in any sensible degree. The obvious explanation is that the
conditions of life have been highly favourable, and that there has
consequently been less destruction of the old and young and that nearly
all the young have been enabled to breed. Their geometrical ratio of
increase, the result of which never fails to be surprising, simply
explains their extraordinarily rapid increase and wide diffusion in their
new homes.

In a state of nature almost every full-grown plant annually produces seed,
and among animals there are very few which do not annually pair. Hence we
may confidently assert that all plants and animals are tending to increase
at a geometrical ratio—that all would rapidly stock every station in
which they could any how exist, and that this geometrical tendency to
increase must be checked by destruction at some period of life. Our
familiarity with the larger domestic animals tends, I think, to mislead
us; we see no great destruction falling on them, and we do not keep in
mind that thousands are annually slaughtered for food, and that in a state
of nature an equal number would have somehow to be disposed of.

The only difference between organisms which annually produce eggs or seeds
by the thousand, and those which produce extremely few, is, that the slow
breeders would require a few more years to people, under favourable
conditions, a whole district, let it be ever so large. The condor lays a
couple of eggs and the ostrich a score, and yet in the same country the
condor may be the more numerous of the two. The Fulmar petrel lays but one
egg, yet it is believed to be the most numerous bird in the world. One fly
deposits hundreds of eggs, and another, like the hippobosca, a single one.
But this difference does not determine how many individuals of the two
species can be supported in a district. A large number of eggs is of some
importance to those species which depend on a fluctuating amount of food,
for it allows them rapidly to increase in number. But the real importance
of a large number of eggs or seeds is to make up for much destruction at
some period of life; and this period in the great majority of cases is an
early one. If an animal can in any way protect its own eggs or young, a
small number may be produced, and yet the average stock be fully kept up;
but if many eggs or young are destroyed, many must be produced or the
species will become extinct. It would suffice to keep up the full number
of a tree, which lived on an average for a thousand years, if a single
seed were produced once in a thousand years, supposing that this seed were
never destroyed and could be ensured to germinate in a fitting place; so
that, in all cases, the average number of any animal or plant depends only
indirectly on the number of its eggs or seeds.

In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind—never to forget that every single
organic being may be said to be striving to the utmost to increase in
numbers; that each lives by a struggle at some period of its life; that
heavy destruction inevitably falls either on the young or old during each
generation or at recurrent intervals. Lighten any check, mitigate the
destruction ever so little, and the number of the species will almost
instantaneously increase to any amount.

NATURE OF THE CHECKS TO INCREASE.

The causes which check the natural tendency of each species to increase
are most obscure. Look at the most vigorous species; by as much as it
swarms in numbers, by so much will it tend to increase still further. We
know not exactly what the checks are even in a single instance. Nor will
this surprise any one who reflects how ignorant we are on this head, even
in regard to mankind, although so incomparably better known than any other
animal. This subject of the checks to increase has been ably treated by
several authors, and I hope in a future work to discuss it at considerable
length, more especially in regard to the feral animals of South America.
Here I will make only a few remarks, just to recall to the reader's mind
some of the chief points. Eggs or very young animals seem generally to
suffer most, but this is not invariably the case. With plants there is a
vast destruction of seeds, but from some observations which I have made it
appears that the seedlings suffer most from germinating in ground already
thickly stocked with other plants. Seedlings, also, are destroyed in vast
numbers by various enemies; for instance, on a piece of ground three feet
long and two wide, dug and cleared, and where there could be no choking
from other plants, I marked all the seedlings of our native weeds as they
came up, and out of 357 no less than 295 were destroyed, chiefly by slugs
and insects. If turf which has long been mown, and the case would be the
same with turf closely browsed by quadrupeds, be let to grow, the more
vigorous plants gradually kill the less vigorous, though fully grown
plants; thus out of twenty species grown on a little plot of mown turf
(three feet by four) nine species perished, from the other species being
allowed to grow up freely.

The amount of food for each species, of course, gives the extreme limit to
which each can increase; but very frequently it is not the obtaining food,
but the serving as prey to other animals, which determines the average
number of a species. Thus, there seems to be little doubt that the stock
of partridges, grouse, and hares on any large estate depends chiefly on
the destruction of vermin. If not one head of game were shot during the
next twenty years in England, and, at the same time, if no vermin were
destroyed, there would, in all probability, be less game than at present,
although hundreds of thousands of game animals are now annually shot. On
the other hand, in some cases, as with the elephant, none are destroyed by
beasts of prey; for even the tiger in India most rarely dares to attack a
young elephant protected by its dam.

Climate plays an important part in determining the average numbers of a
species, and periodical seasons of extreme cold or drought seem to be the
most effective of all checks. I estimated (chiefly from the greatly
reduced numbers of nests in the spring) that the winter of 1854-5
destroyed four-fifths of the birds in my own grounds; and this is a
tremendous destruction, when we remember that ten per cent. is an
extraordinarily severe mortality from epidemics with man. The action of
climate seems at first sight to be quite independent of the struggle for
existence; but in so far as climate chiefly acts in reducing food, it
brings on the most severe struggle between the individuals, whether of the
same or of distinct species, which subsist on the same kind of food. Even
when climate, for instance, extreme cold, acts directly, it will be the
least vigorous individuals, or those which have got least food through the
advancing winter, which will suffer the most. When we travel from south to
north, or from a damp region to a dry, we invariably see some species
gradually getting rarer and rarer, and finally disappearing; and the
change of climate being conspicuous, we are tempted to attribute the whole
effect to its direct action. But this is a false view; we forget that each
species, even where it most abounds, is constantly suffering enormous
destruction at some period of its life, from enemies or from competitors
for the same place and food; and if these enemies or competitors be in the
least degree favoured by any slight change of climate, they will increase
in numbers; and as each area is already fully stocked with inhabitants,
the other species must decrease. When we travel southward and see a
species decreasing in numbers, we may feel sure that the cause lies quite
as much in other species being favoured, as in this one being hurt. So it
is when we travel northward, but in a somewhat lesser degree, for the
number of species of all kinds, and therefore of competitors, decreases
northward; hence in going northward, or in ascending a mountain, we far
oftener meet with stunted forms, due to the DIRECTLY injurious action of
climate, than we do in proceeding southward or in descending a mountain.
When we reach the Arctic regions, or snow-capped summits, or absolute
deserts, the struggle for life is almost exclusively with the elements.

That climate acts in main part indirectly by favouring other species we
clearly see in the prodigious number of plants which in our gardens can
perfectly well endure our climate, but which never become naturalised, for
they cannot compete with our native plants nor resist destruction by our
native animals.

When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics—at least, this
seems generally to occur with our game animals—often ensue; and here
we have a limiting check independent of the struggle for life. But even
some of these so-called epidemics appear to be due to parasitic worms,
which have from some cause, possibly in part through facility of diffusion
among the crowded animals, been disproportionally favoured: and here comes
in a sort of struggle between the parasite and its prey.

On the other hand, in many cases, a large stock of individuals of the same
species, relatively to the numbers of its enemies, is absolutely necessary
for its preservation. Thus we can easily raise plenty of corn and
rape-seed, etc., in our fields, because the seeds are in great excess
compared with the number of birds which feed on them; nor can the birds,
though having a superabundance of food at this one season, increase in
number proportionally to the supply of seed, as their numbers are checked
during the winter; but any one who has tried knows how troublesome it is
to get seed from a few wheat or other such plants in a garden; I have in
this case lost every single seed. This view of the necessity of a large
stock of the same species for its preservation, explains, I believe, some
singular facts in nature such as that of very rare plants being sometimes
extremely abundant, in the few spots where they do exist; and that of some
social plants being social, that is abounding in individuals, even on the
extreme verge of their range. For in such cases, we may believe, that a
plant could exist only where the conditions of its life were so favourable
that many could exist together, and thus save the species from utter
destruction. I should add that the good effects of intercrossing, and the
ill effects of close interbreeding, no doubt come into play in many of
these cases; but I will not here enlarge on this subject.

COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER IN THE STRUGGLE
FOR EXISTENCE.

Many cases are on record showing how complex and unexpected are the checks
and relations between organic beings, which have to struggle together in
the same country. I will give only a single instance, which, though a
simple one, interested me. In Staffordshire, on the estate of a relation,
where I had ample means of investigation, there was a large and extremely
barren heath, which had never been touched by the hand of man; but several
hundred acres of exactly the same nature had been enclosed twenty-five
years previously and planted with Scotch fir. The change in the native
vegetation of the planted part of the heath was most remarkable, more than
is generally seen in passing from one quite different soil to another: not
only the proportional numbers of the heath-plants were wholly changed, but
twelve species of plants (not counting grasses and carices) flourished in
the plantations, which could not be found on the heath. The effect on the
insects must have been still greater, for six insectivorous birds were
very common in the plantations, which were not to be seen on the heath;
and the heath was frequented by two or three distinct insectivorous birds.
Here we see how potent has been the effect of the introduction of a single
tree, nothing whatever else having been done, with the exception of the
land having been enclosed, so that cattle could not enter. But how
important an element enclosure is, I plainly saw near Farnham, in Surrey.
Here there are extensive heaths, with a few clumps of old Scotch firs on
the distant hill-tops: within the last ten years large spaces have been
enclosed, and self-sown firs are now springing up in multitudes, so close
together that all cannot live. When I ascertained that these young trees
had not been sown or planted I was so much surprised at their numbers that
I went to several points of view, whence I could examine hundreds of acres
of the unenclosed heath, and literally I could not see a single Scotch
fir, except the old planted clumps. But on looking closely between the
stems of the heath, I found a multitude of seedlings and little trees,
which had been perpetually browsed down by the cattle. In one square yard,
at a point some hundred yards distant from one of the old clumps, I
counted thirty-two little trees; and one of them, with twenty-six rings of
growth, had, during many years tried to raise its head above the stems of
the heath, and had failed. No wonder that, as soon as the land was
enclosed, it became thickly clothed with vigorously growing young firs.
Yet the heath was so extremely barren and so extensive that no one would
ever have imagined that cattle would have so closely and effectually
searched it for food.

Here we see that cattle absolutely determine the existence of the Scotch
fir; but in several parts of the world insects determine the existence of
cattle. Perhaps Paraguay offers the most curious instance of this; for
here neither cattle nor horses nor dogs have ever run wild, though they
swarm southward and northward in a feral state; and Azara and Rengger have
shown that this is caused by the greater number in Paraguay of a certain
fly, which lays its eggs in the navels of these animals when first born.
The increase of these flies, numerous as they are, must be habitually
checked by some means, probably by other parasitic insects. Hence, if
certain insectivorous birds were to decrease in Paraguay, the parasitic
insects would probably increase; and this would lessen the number of the
navel-frequenting flies—then cattle and horses would become feral,
and this would certainly greatly alter (as indeed I have observed in parts
of South America) the vegetation: this again would largely affect the
insects; and this, as we have just seen in Staffordshire, the
insectivorous birds, and so onwards in ever-increasing circles of
complexity. Not that under nature the relations will ever be as simple as
this. Battle within battle must be continually recurring with varying
success; and yet in the long-run the forces are so nicely balanced that
the face of nature remains for long periods of time uniform, though
assuredly the merest trifle would give the victory to one organic being
over another. Nevertheless, so profound is our ignorance, and so high our
presumption, that we marvel when we hear of the extinction of an organic
being; and as we do not see the cause, we invoke cataclysms to desolate
the world, or invent laws on the duration of the forms of life!

I am tempted to give one more instance showing how plants and animals,
remote in the scale of nature, are bound together by a web of complex
relations. I shall hereafter have occasion to show that the exotic Lobelia
fulgens is never visited in my garden by insects, and consequently, from
its peculiar structure, never sets a seed. Nearly all our orchidaceous
plants absolutely require the visits of insects to remove their
pollen-masses and thus to fertilise them. I find from experiments that
humble-bees are almost indispensable to the fertilisation of the
heartsease (Viola tricolor), for other bees do not visit this flower. I
have also found that the visits of bees are necessary for the
fertilisation of some kinds of clover; for instance twenty heads of Dutch
clover (Trifolium repens) yielded 2,290 seeds, but twenty other heads,
protected from bees, produced not one. Again, 100 heads of red clover (T.
pratense) produced 2,700 seeds, but the same number of protected heads
produced not a single seed. Humble bees alone visit red clover, as other
bees cannot reach the nectar. It has been suggested that moths may
fertilise the clovers; but I doubt whether they could do so in the case of
the red clover, from their weight not being sufficient to depress the wing
petals. Hence we may infer as highly probable that, if the whole genus of
humble-bees became extinct or very rare in England, the heartsease and red
clover would become very rare, or wholly disappear. The number of
humble-bees in any district depends in a great measure upon the number of
field-mice, which destroy their combs and nests; and Colonel Newman, who
has long attended to the habits of humble-bees, believes that "more than
two-thirds of them are thus destroyed all over England." Now the number of
mice is largely dependent, as every one knows, on the number of cats; and
Colonel Newman says, "Near villages and small towns I have found the nests
of humble-bees more numerous than elsewhere, which I attribute to the
number of cats that destroy the mice." Hence it is quite credible that the
presence of a feline animal in large numbers in a district might
determine, through the intervention first of mice and then of bees, the
frequency of certain flowers in that district!

In the case of every species, many different checks, acting at different
periods of life, and during different seasons or years, probably come into
play; some one check or some few being generally the most potent, but all
will concur in determining the average number, or even the existence of
the species. In some cases it can be shown that widely-different checks
act on the same species in different districts. When we look at the plants
and bushes clothing an entangled bank, we are tempted to attribute their
proportional numbers and kinds to what we call chance. But how false a
view is this! Every one has heard that when an American forest is cut
down, a very different vegetation springs up; but it has been observed
that ancient Indian ruins in the Southern United States, which must
formerly have been cleared of trees, now display the same beautiful
diversity and proportion of kinds as in the surrounding virgin forests.
What a struggle must have gone on during long centuries between the
several kinds of trees, each annually scattering its seeds by the
thousand; what war between insect and insect—between insects,
snails, and other animals with birds and beasts of prey—all striving
to increase, all feeding on each other, or on the trees, their seeds and
seedlings, or on the other plants which first clothed the ground and thus
checked the growth of the trees. Throw up a handful of feathers, and all
fall to the ground according to definite laws; but how simple is the
problem where each shall fall compared to that of the action and reaction
of the innumerable plants and animals which have determined, in the course
of centuries, the proportional numbers and kinds of trees now growing on
the old Indian ruins!

The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in the scale of nature. This is
likewise sometimes the case with those which may strictly be said to
struggle with each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle will almost invariably be most
severe between the individuals of the same species, for they frequent the
same districts, require the same food, and are exposed to the same
dangers. In the case of varieties of the same species, the struggle will
generally be almost equally severe, and we sometimes see the contest soon
decided: for instance, if several varieties of wheat be sown together, and
the mixed seed be resown, some of the varieties which best suit the soil
or climate, or are naturally the most fertile, will beat the others and so
yield more seed, and will consequently in a few years supplant the other
varieties. To keep up a mixed stock of even such extremely close varieties
as the variously coloured sweet-peas, they must be each year harvested
separately, and the seed then mixed in due proportion, otherwise the
weaker kinds will steadily decrease in number and disappear. So again with
the varieties of sheep: it has been asserted that certain
mountain-varieties will starve out other mountain-varieties, so that they
cannot be kept together. The same result has followed from keeping
together different varieties of the medicinal leech. It may even be
doubted whether the varieties of any of our domestic plants or animals
have so exactly the same strength, habits, and constitution, that the
original proportions of a mixed stock (crossing being prevented) could be
kept up for half-a-dozen generations, if they were allowed to struggle
together, in the same manner as beings in a state of nature, and if the
seed or young were not annually preserved in due proportion.

STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF THE
SAME SPECIES.

As the species of the same genus usually have, though by no means
invariably, much similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between them, if
they come into competition with each other, than between the species of
distinct genera. We see this in the recent extension over parts of the
United States of one species of swallow having caused the decrease of
another species. The recent increase of the missel-thrush in parts of
Scotland has caused the decrease of the song-thrush. How frequently we
hear of one species of rat taking the place of another species under the
most different climates! In Russia the small Asiatic cockroach has
everywhere driven before it its great congener. In Australia the imported
hive-bee is rapidly exterminating the small, stingless native bee. One
species of charlock has been known to supplant another species; and so in
other cases. We can dimly see why the competition should be most severe
between allied forms, which fill nearly the same place in the economy of
nature; but probably in no one case could we precisely say why one species
has been victorious over another in the great battle of life.

A corollary of the highest importance may be deduced from the foregoing
remarks, namely, that the structure of every organic being is related, in
the most essential yet often hidden manner, to that of all other organic
beings, with which it comes into competition for food or residence, or
from which it has to escape, or on which it preys. This is obvious in the
structure of the teeth and talons of the tiger; and in that of the legs
and claws of the parasite which clings to the hair on the tiger's body.
But in the beautifully plumed seed of the dandelion, and in the flattened
and fringed legs of the water-beetle, the relation seems at first confined
to the elements of air and water. Yet the advantage of the plumed seeds no
doubt stands in the closest relation to the land being already thickly
clothed with other plants; so that the seeds may be widely distributed and
fall on unoccupied ground. In the water-beetle, the structure of its legs,
so well adapted for diving, allows it to compete with other aquatic
insects, to hunt for its own prey, and to escape serving as prey to other
animals.

The store of nutriment laid up within the seeds of many plants seems at
first sight to have no sort of relation to other plants. But from the
strong growth of young plants produced from such seeds, as peas and beans,
when sown in the midst of long grass, it may be suspected that the chief
use of the nutriment in the seed is to favour the growth of the seedlings,
whilst struggling with other plants growing vigorously all around.

Look at a plant in the midst of its range! Why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a
little more heat or cold, dampness or dryness, for elsewhere it ranges
into slightly hotter or colder, damper or drier districts. In this case we
can clearly see that if we wish in imagination to give the plant the power
of increasing in numbers, we should have to give it some advantage over
its competitors, or over the animals which prey on it. On the confines of
its geographical range, a change of constitution with respect to climate
would clearly be an advantage to our plant; but we have reason to believe
that only a few plants or animals range so far, that they are destroyed
exclusively by the rigour of the climate. Not until we reach the extreme
confines of life, in the Arctic regions or on the borders of an utter
desert, will competition cease. The land may be extremely cold or dry, yet
there will be competition between some few species, or between the
individuals of the same species, for the warmest or dampest spots.

Hence we can see that when a plant or animal is placed in a new country,
among new competitors, the conditions of its life will generally be
changed in an essential manner, although the climate may be exactly the
same as in its former home. If its average numbers are to increase in its
new home, we should have to modify it in a different way to what we should
have had to do in its native country; for we should have to give it some
advantage over a different set of competitors or enemies.

It is good thus to try in imagination to give any one species an advantage
over another. Probably in no single instance should we know what to do.
This ought to convince us of our ignorance on the mutual relations of all
organic beings; a conviction as necessary, as it is difficult to acquire.
All that we can do is to keep steadily in mind that each organic being is
striving to increase in a geometrical ratio; that each, at some period of
its life, during some season of the year, during each generation, or at
intervals, has to struggle for life and to suffer great destruction. When
we reflect on this struggle we may console ourselves with the full belief
that the war of nature is not incessant, that no fear is felt, that death
is generally prompt, and that the vigorous, the healthy, and the happy
survive and multiply.



CHAPTER IV. NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.


 Natural Selection—its power compared with man's selection—its power
 on characters of trifling importance—its power at all ages and on
 both sexes—Sexual Selection—On the generality of intercrosses
 between individuals of the same species—Circumstances favourable and
 unfavourable to the results of Natural Selection, namely, intercrossing,
 isolation, number of individuals—Slow action—Extinction caused by
 Natural Selection—Divergence of Character, related to the diversity of
 inhabitants of any small area and to naturalisation—Action of Natural
 Selection, through Divergence of Character and Extinction, on the
 descendants from a common parent—Explains the Grouping of all organic
 beings—Advance in organisation—Low forms preserved—Convergence of
 character—Indefinite multiplication of species—Summary.

How will the struggle for existence, briefly discussed in the last
chapter, act in regard to variation? Can the principle of selection, which
we have seen is so potent in the hands of man, apply under nature? I think
we shall see that it can act most efficiently. Let the endless number of
slight variations and individual differences occurring in our domestic
productions, and, in a lesser degree, in those under nature, be borne in
mind; as well as the strength of the hereditary tendency. Under
domestication, it may truly be said that the whole organisation becomes in
some degree plastic. But the variability, which we almost universally meet
with in our domestic productions is not directly produced, as Hooker and
Asa Gray have well remarked, by man; he can neither originate varieties
nor prevent their occurrence; he can only preserve and accumulate such as
do occur. Unintentionally he exposes organic beings to new and changing
conditions of life, and variability ensues; but similar changes of
conditions might and do occur under nature. Let it also be borne in mind
how infinitely complex and close-fitting are the mutual relations of all
organic beings to each other and to their physical conditions of life; and
consequently what infinitely varied diversities of structure might be of
use to each being under changing conditions of life. Can it then be
thought improbable, seeing that variations useful to man have undoubtedly
occurred, that other variations useful in some way to each being in the
great and complex battle of life, should occur in the course of many
successive generations? If such do occur, can we doubt (remembering that
many more individuals are born than can possibly survive) that individuals
having any advantage, however slight, over others, would have the best
chance of surviving and procreating their kind? On the other hand, we may
feel sure that any variation in the least degree injurious would be
rigidly destroyed. This preservation of favourable individual differences
and variations, and the destruction of those which are injurious, I have
called Natural Selection, or the Survival of the Fittest. Variations
neither useful nor injurious would not be affected by natural selection,
and would be left either a fluctuating element, as perhaps we see in
certain polymorphic species, or would ultimately become fixed, owing to
the nature of the organism and the nature of the conditions.

Several writers have misapprehended or objected to the term Natural
Selection. Some have even imagined that natural selection induces
variability, whereas it implies only the preservation of such variations
as arise and are beneficial to the being under its conditions of life. No
one objects to agriculturists speaking of the potent effects of man's
selection; and in this case the individual differences given by nature,
which man for some object selects, must of necessity first occur. Others
have objected that the term selection implies conscious choice in the
animals which become modified; and it has even been urged that, as plants
have no volition, natural selection is not applicable to them! In the
literal sense of the word, no doubt, natural selection is a false term;
but who ever objected to chemists speaking of the elective affinities of
the various elements?—and yet an acid cannot strictly be said to
elect the base with which it in preference combines. It has been said that
I speak of natural selection as an active power or Deity; but who objects
to an author speaking of the attraction of gravity as ruling the movements
of the planets? Every one knows what is meant and is implied by such
metaphorical expressions; and they are almost necessary for brevity. So
again it is difficult to avoid personifying the word Nature; but I mean by
nature, only the aggregate action and product of many natural laws, and by
laws the sequence of events as ascertained by us. With a little
familiarity such superficial objections will be forgotten.

We shall best understand the probable course of natural selection by
taking the case of a country undergoing some slight physical change, for
instance, of climate. The proportional numbers of its inhabitants will
almost immediately undergo a change, and some species will probably become
extinct. We may conclude, from what we have seen of the intimate and
complex manner in which the inhabitants of each country are bound
together, that any change in the numerical proportions of the inhabitants,
independently of the change of climate itself, would seriously affect the
others. If the country were open on its borders, new forms would certainly
immigrate, and this would likewise seriously disturb the relations of some
of the former inhabitants. Let it be remembered how powerful the influence
of a single introduced tree or mammal has been shown to be. But in the
case of an island, or of a country partly surrounded by barriers, into
which new and better adapted forms could not freely enter, we should then
have places in the economy of nature which would assuredly be better
filled up if some of the original inhabitants were in some manner
modified; for, had the area been open to immigration, these same places
would have been seized on by intruders. In such cases, slight
modifications, which in any way favoured the individuals of any species,
by better adapting them to their altered conditions, would tend to be
preserved; and natural selection would have free scope for the work of
improvement.

We have good reason to believe, as shown in the first chapter, that
changes in the conditions of life give a tendency to increased
variability; and in the foregoing cases the conditions the changed, and
this would manifestly be favourable to natural selection, by affording a
better chance of the occurrence of profitable variations. Unless such
occur, natural selection can do nothing. Under the term of "variations,"
it must never be forgotten that mere individual differences are included.
As man can produce a great result with his domestic animals and plants by
adding up in any given direction individual differences, so could natural
selection, but far more easily from having incomparably longer time for
action. Nor do I believe that any great physical change, as of climate, or
any unusual degree of isolation, to check immigration, is necessary in
order that new and unoccupied places should be left for natural selection
to fill up by improving some of the varying inhabitants. For as all the
inhabitants of each country are struggling together with nicely balanced
forces, extremely slight modifications in the structure or habits of one
species would often give it an advantage over others; and still further
modifications of the same kind would often still further increase the
advantage, as long as the species continued under the same conditions of
life and profited by similar means of subsistence and defence. No country
can be named in which all the native inhabitants are now so perfectly
adapted to each other and to the physical conditions under which they
live, that none of them could be still better adapted or improved; for in
all countries, the natives have been so far conquered by naturalised
productions that they have allowed some foreigners to take firm possession
of the land. And as foreigners have thus in every country beaten some of
the natives, we may safely conclude that the natives might have been
modified with advantage, so as to have better resisted the intruders.

As man can produce, and certainly has produced, a great result by his
methodical and unconscious means of selection, what may not natural
selection effect? Man can act only on external and visible characters:
Nature, if I may be allowed to personify the natural preservation or
survival of the fittest, cares nothing for appearances, except in so far
as they are useful to any being. She can act on every internal organ, on
every shade of constitutional difference, on the whole machinery of life.
Man selects only for his own good; Nature only for that of the being which
she tends. Every selected character is fully exercised by her, as is
implied by the fact of their selection. Man keeps the natives of many
climates in the same country. He seldom exercises each selected character
in some peculiar and fitting manner; he feeds a long and a short-beaked
pigeon on the same food; he does not exercise a long-backed or long-legged
quadruped in any peculiar manner; he exposes sheep with long and short
wool to the same climate; does not allow the most vigorous males to
struggle for the females; he does not rigidly destroy all inferior
animals, but protects during each varying season, as far as lies in his
power, all his productions. He often begins his selection by some
half-monstrous form, or at least by some modification prominent enough to
catch the eye or to be plainly useful to him. Under nature, the slightest
differences of structure or constitution may well turn the nicely-balanced
scale in the struggle for life, and so be preserved. How fleeting are the
wishes and efforts of man! How short his time, and consequently how poor
will be his results, compared with those accumulated by Nature during
whole geological periods! Can we wonder, then, that Nature's productions
should be far "truer" in character than man's productions; that they
should be infinitely better adapted to the most complex conditions of
life, and should plainly bear the stamp of far higher workmanship?

It may metaphorically be said that natural selection is daily and hourly
scrutinising, throughout the world, the slightest variations; rejecting
those that are bad, preserving and adding up all that are good; silently
and insensibly working, WHENEVER AND WHEREVER OPPORTUNITY OFFERS, at the
improvement of each organic being in relation to its organic and inorganic
conditions of life. We see nothing of these slow changes in progress,
until the hand of time has marked the long lapse of ages, and then so
imperfect is our view into long-past geological ages that we see only that
the forms of life are now different from what they formerly were.

In order that any great amount of modification should be effected in a
species, a variety, when once formed must again, perhaps after a long
interval of time, vary or present individual differences of the same
favourable nature as before; and these must again be preserved, and so
onward, step by step. Seeing that individual differences of the same kind
perpetually recur, this can hardly be considered as an unwarrantable
assumption. But whether it is true, we can judge only by seeing how far
the hypothesis accords with and explains the general phenomena of nature.
On the other hand, the ordinary belief that the amount of possible
variation is a strictly limited quantity, is likewise a simple assumption.

Although natural selection can act only through and for the good of each
being, yet characters and structures, which we are apt to consider as of
very trifling importance, may thus be acted on. When we see leaf-eating
insects green, and bark-feeders mottled-grey; the alpine ptarmigan white
in winter, the red-grouse the colour of heather, we must believe that
these tints are of service to these birds and insects in preserving them
from danger. Grouse, if not destroyed at some period of their lives, would
increase in countless numbers; they are known to suffer largely from birds
of prey; and hawks are guided by eyesight to their prey,—so much so
that on parts of the continent persons are warned not to keep white
pigeons, as being the most liable to destruction. Hence natural selection
might be effective in giving the proper colour to each kind of grouse, and
in keeping that colour, when once acquired, true and constant. Nor ought
we to think that the occasional destruction of an animal of any particular
colour would produce little effect; we should remember how essential it is
in a flock of white sheep to destroy a lamb with the faintest trace of
black. We have seen how the colour of hogs, which feed on the "paint-root"
in Virginia, determines whether they shall live or die. In plants, the
down on the fruit and the colour of the flesh are considered by botanists
as characters of the most trifling importance; yet we hear from an
excellent horticulturist, Downing, that in the United States
smooth-skinned fruits suffer far more from a beetle, a Curculio, than
those with down; that purple plums suffer far more from a certain disease
than yellow plums; whereas another disease attacks yellow-fleshed peaches
far more than those with other coloured flesh. If, with all the aids of
art, these slight differences make a great difference in cultivating the
several varieties, assuredly, in a state of nature, where the trees would
have to struggle with other trees and with a host of enemies, such
differences would effectually settle which variety, whether a smooth or
downy, a yellow or a purple-fleshed fruit, should succeed.

In looking at many small points of difference between species, which, as
far as our ignorance permits us to judge, seem quite unimportant, we must
not forget that climate, food, etc., have no doubt produced some direct
effect. It is also necessary to bear in mind that, owing to the law of
correlation, when one part varies and the variations are accumulated
through natural selection, other modifications, often of the most
unexpected nature, will ensue.

As we see that those variations which, under domestication, appear at any
particular period of life, tend to reappear in the offspring at the same
period; for instance, in the shape, size and flavour of the seeds of the
many varieties of our culinary and agricultural plants; in the caterpillar
and cocoon stages of the varieties of the silkworm; in the eggs of
poultry, and in the colour of the down of their chickens; in the horns of
our sheep and cattle when nearly adult; so in a state of nature natural
selection will be enabled to act on and modify organic beings at any age,
by the accumulation of variations profitable at that age, and by their
inheritance at a corresponding age. If it profit a plant to have its seeds
more and more widely disseminated by the wind, I can see no greater
difficulty in this being effected through natural selection, than in the
cotton-planter increasing and improving by selection the down in the pods
on his cotton-trees. Natural selection may modify and adapt the larva of
an insect to a score of contingencies, wholly different from those which
concern the mature insect; and these modifications may affect, through
correlation, the structure of the adult. So, conversely, modifications in
the adult may affect the structure of the larva; but in all cases natural
selection will ensure that they shall not be injurious: for if they were
so, the species would become extinct.

Natural selection will modify the structure of the young in relation to
the parent and of the parent in relation to the young. In social animals
it will adapt the structure of each individual for the benefit of the
whole community; if the community profits by the selected change. What
natural selection cannot do, is to modify the structure of one species,
without giving it any advantage, for the good of another species; and
though statements to this effect may be found in works of natural history,
I cannot find one case which will bear investigation. A structure used
only once in an animal's life, if of high importance to it, might be
modified to any extent by natural selection; for instance, the great jaws
possessed by certain insects, used exclusively for opening the cocoon—or
the hard tip to the beak of unhatched birds, used for breaking the eggs.
It has been asserted, that of the best short-beaked tumbler-pigeons a
greater number perish in the egg than are able to get out of it; so that
fanciers assist in the act of hatching. Now, if nature had to make the
beak of a full-grown pigeon very short for the bird's own advantage, the
process of modification would be very slow, and there would be
simultaneously the most rigorous selection of all the young birds within
the egg, which had the most powerful and hardest beaks, for all with weak
beaks would inevitably perish: or, more delicate and more easily broken
shells might be selected, the thickness of the shell being known to vary
like every other structure.

It may be well here to remark that with all beings there must be much
fortuitous destruction, which can have little or no influence on the
course of natural selection. For instance, a vast number of eggs or seeds
are annually devoured, and these could be modified through natural
selection only if they varied in some manner which protected them from
their enemies. Yet many of these eggs or seeds would perhaps, if not
destroyed, have yielded individuals better adapted to their conditions of
life than any of those which happened to survive. So again a vast number
of mature animals and plants, whether or not they be the best adapted to
their conditions, must be annually destroyed by accidental causes, which
would not be in the least degree mitigated by certain changes of structure
or constitution which would in other ways be beneficial to the species.
But let the destruction of the adults be ever so heavy, if the number
which can exist in any district be not wholly kept down by such causes—or
again let the destruction of eggs or seeds be so great that only a
hundredth or a thousandth part are developed—yet of those which do
survive, the best adapted individuals, supposing that there is any
variability in a favourable direction, will tend to propagate their kind
in larger numbers than the less well adapted. If the numbers be wholly
kept down by the causes just indicated, as will often have been the case,
natural selection will be powerless in certain beneficial directions; but
this is no valid objection to its efficiency at other times and in other
ways; for we are far from having any reason to suppose that many species
ever undergo modification and improvement at the same time in the same
area.

SEXUAL SELECTION.

Inasmuch as peculiarities often appear under domestication in one sex and
become hereditarily attached to that sex, so no doubt it will be under
nature. Thus it is rendered possible for the two sexes to be modified
through natural selection in relation to different habits of life, as is
sometimes the case; or for one sex to be modified in relation to the other
sex, as commonly occurs. This leads me to say a few words on what I have
called sexual selection. This form of selection depends, not on a struggle
for existence in relation to other organic beings or to external
conditions, but on a struggle between the individuals of one sex,
generally the males, for the possession of the other sex. The result is
not death to the unsuccessful competitor, but few or no offspring. Sexual
selection is, therefore, less rigorous than natural selection. Generally,
the most vigorous males, those which are best fitted for their places in
nature, will leave most progeny. But in many cases victory depends not so
much on general vigour, but on having special weapons, confined to the
male sex. A hornless stag or spurless cock would have a poor chance of
leaving numerous offspring. Sexual selection, by always allowing the
victor to breed, might surely give indomitable courage, length of spur,
and strength to the wing to strike in the spurred leg, in nearly the same
manner as does the brutal cockfighter by the careful selection of his best
cocks. How low in the scale of nature the law of battle descends I know
not; male alligators have been described as fighting, bellowing, and
whirling round, like Indians in a war-dance, for the possession of the
females; male salmons have been observed fighting all day long; male
stag-beetles sometimes bear wounds from the huge mandibles of other males;
the males of certain hymenopterous insects have been frequently seen by
that inimitable observer M. Fabre, fighting for a particular female who
sits by, an apparently unconcerned beholder of the struggle, and then
retires with the conqueror. The war is, perhaps, severest between the
males of polygamous animals, and these seem oftenest provided with special
weapons. The males of carnivorous animals are already well armed; though
to them and to others, special means of defence may be given through means
of sexual selection, as the mane of the lion, and the hooked jaw to the
male salmon; for the shield may be as important for victory as the sword
or spear.

Among birds, the contest is often of a more peaceful character. All those
who have attended to the subject, believe that there is the severest
rivalry between the males of many species to attract, by singing, the
females. The rock-thrush of Guiana, birds of paradise, and some others,
congregate, and successive males display with the most elaborate care, and
show off in the best manner, their gorgeous plumage; they likewise perform
strange antics before the females, which, standing by as spectators, at
last choose the most attractive partner. Those who have closely attended
to birds in confinement well know that they often take individual
preferences and dislikes: thus Sir R. Heron has described how a pied
peacock was eminently attractive to all his hen birds. I cannot here enter
on the necessary details; but if man can in a short time give beauty and
an elegant carriage to his bantams, according to his standard of beauty, I
can see no good reason to doubt that female birds, by selecting, during
thousands of generations, the most melodious or beautiful males, according
to their standard of beauty, might produce a marked effect. Some
well-known laws, with respect to the plumage of male and female birds, in
comparison with the plumage of the young, can partly be explained through
the action of sexual selection on variations occurring at different ages,
and transmitted to the males alone or to both sexes at corresponding ages;
but I have not space here to enter on this subject.

Thus it is, as I believe, that when the males and females of any animal
have the same general habits of life, but differ in structure, colour, or
ornament, such differences have been mainly caused by sexual selection:
that is, by individual males having had, in successive generations, some
slight advantage over other males, in their weapons, means of defence, or
charms; which they have transmitted to their male offspring alone. Yet, I
would not wish to attribute all sexual differences to this agency: for we
see in our domestic animals peculiarities arising and becoming attached to
the male sex, which apparently have not been augmented through selection
by man. The tuft of hair on the breast of the wild turkey-cock cannot be
of any use, and it is doubtful whether it can be ornamental in the eyes of
the female bird; indeed, had the tuft appeared under domestication it
would have been called a monstrosity.

ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE SURVIVAL OF THE
FITTEST.

In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some by
craft, some by strength, and some by fleetness; and let us suppose that
the fleetest prey, a deer for instance, had from any change in the country
increased in numbers, or that other prey had decreased in numbers, during
that season of the year when the wolf was hardest pressed for food. Under
such circumstances the swiftest and slimmest wolves have the best chance
of surviving, and so be preserved or selected, provided always that they
retained strength to master their prey at this or some other period of the
year, when they were compelled to prey on other animals. I can see no more
reason to doubt that this would be the result, than that man should be
able to improve the fleetness of his greyhounds by careful and methodical
selection, or by that kind of unconscious selection which follows from
each man trying to keep the best dogs without any thought of modifying the
breed. I may add that, according to Mr. Pierce, there are two varieties of
the wolf inhabiting the Catskill Mountains, in the United States, one with
a light greyhound-like form, which pursues deer, and the other more bulky,
with shorter legs, which more frequently attacks the shepherd's flocks.

Even without any change in the proportional numbers of the animals on
which our wolf preyed, a cub might be born with an innate tendency to
pursue certain kinds of prey. Nor can this be thought very improbable; for
we often observe great differences in the natural tendencies of our
domestic animals; one cat, for instance, taking to catch rats, another
mice; one cat, according to Mr. St. John, bringing home winged game,
another hares or rabbits, and another hunting on marshy ground and almost
nightly catching woodcocks or snipes. The tendency to catch rats rather
than mice is known to be inherited. Now, if any slight innate change of
habit or of structure benefited an individual wolf, it would have the best
chance of surviving and of leaving offspring. Some of its young would
probably inherit the same habits or structure, and by the repetition of
this process, a new variety might be formed which would either supplant or
coexist with the parent-form of wolf. Or, again, the wolves inhabiting a
mountainous district, and those frequenting the lowlands, would naturally
be forced to hunt different prey; and from the continued preservation of
the individuals best fitted for the two sites, two varieties might slowly
be formed. These varieties would cross and blend where they met; but to
this subject of intercrossing we shall soon have to return. I may add,
that, according to Mr. Pierce, there are two varieties of the wolf
inhabiting the Catskill Mountains in the United States, one with a light
greyhound-like form, which pursues deer, and the other more bulky, with
shorter legs, which more frequently attacks the shepherd's flocks.

It should be observed that in the above illustration, I speak of the
slimmest individual wolves, and not of any single strongly marked
variation having been preserved. In former editions of this work I
sometimes spoke as if this latter alternative had frequently occurred. I
saw the great importance of individual differences, and this led me fully
to discuss the results of unconscious selection by man, which depends on
the preservation of all the more or less valuable individuals, and on the
destruction of the worst. I saw, also, that the preservation in a state of
nature of any occasional deviation of structure, such as a monstrosity,
would be a rare event; and that, if at first preserved, it would generally
be lost by subsequent intercrossing with ordinary individuals.
Nevertheless, until reading an able and valuable article in the "North
British Review" (1867), I did not appreciate how rarely single variations,
whether slight or strongly marked, could be perpetuated. The author takes
the case of a pair of animals, producing during their lifetime two hundred
offspring, of which, from various causes of destruction, only two on an
average survive to pro-create their kind. This is rather an extreme
estimate for most of the higher animals, but by no means so for many of
the lower organisms. He then shows that if a single individual were born,
which varied in some manner, giving it twice as good a chance of life as
that of the other individuals, yet the chances would be strongly against
its survival. Supposing it to survive and to breed, and that half its
young inherited the favourable variation; still, as the Reviewer goes onto
show, the young would have only a slightly better chance of surviving and
breeding; and this chance would go on decreasing in the succeeding
generations. The justice of these remarks cannot, I think, be disputed.
If, for instance, a bird of some kind could procure its food more easily
by having its beak curved, and if one were born with its beak strongly
curved, and which consequently flourished, nevertheless there would be a
very poor chance of this one individual perpetuating its kind to the
exclusion of the common form; but there can hardly be a doubt, judging by
what we see taking place under domestication, that this result would
follow from the preservation during many generations of a large number of
individuals with more or less strongly curved beaks, and from the
destruction of a still larger number with the straightest beaks.

It should not, however, be overlooked that certain rather strongly marked
variations, which no one would rank as mere individual differences,
frequently recur owing to a similar organisation being similarly acted on—of
which fact numerous instances could be given with our domestic
productions. In such cases, if the varying individual did not actually
transmit to its offspring its newly-acquired character, it would
undoubtedly transmit to them, as long as the existing conditions remained
the same, a still stronger tendency to vary in the same manner. There can
also be little doubt that the tendency to vary in the same manner has
often been so strong that all the individuals of the same species have
been similarly modified without the aid of any form of selection. Or only
a third, fifth, or tenth part of the individuals may have been thus
affected, of which fact several instances could be given. Thus Graba
estimates that about one-fifth of the guillemots in the Faroe Islands
consist of a variety so well marked, that it was formerly ranked as a
distinct species under the name of Uria lacrymans. In cases of this kind,
if the variation were of a beneficial nature, the original form would soon
be supplanted by the modified form, through the survival of the fittest.

To the effects of intercrossing in eliminating variations of all kinds, I
shall have to recur; but it may be here remarked that most animals and
plants keep to their proper homes, and do not needlessly wander about; we
see this even with migratory birds, which almost always return to the same
spot. Consequently each newly-formed variety would generally be at first
local, as seems to be the common rule with varieties in a state of nature;
so that similarly modified individuals would soon exist in a small body
together, and would often breed together. If the new variety were
successful in its battle for life, it would slowly spread from a central
district, competing with and conquering the unchanged individuals on the
margins of an ever-increasing circle.

It may be worth while to give another and more complex illustration of the
action of natural selection. Certain plants excrete sweet juice,
apparently for the sake of eliminating something injurious from the sap:
this is effected, for instance, by glands at the base of the stipules in
some Leguminosae, and at the backs of the leaves of the common laurel.
This juice, though small in quantity, is greedily sought by insects; but
their visits do not in any way benefit the plant. Now, let us suppose that
the juice or nectar was excreted from the inside of the flowers of a
certain number of plants of any species. Insects in seeking the nectar
would get dusted with pollen, and would often transport it from one flower
to another. The flowers of two distinct individuals of the same species
would thus get crossed; and the act of crossing, as can be fully proved,
gives rise to vigorous seedlings, which consequently would have the best
chance of flourishing and surviving. The plants which produced flowers
with the largest glands or nectaries, excreting most nectar, would
oftenest be visited by insects, and would oftenest be crossed; and so in
the long-run would gain the upper hand and form a local variety. The
flowers, also, which had their stamens and pistils placed, in relation to
the size and habits of the particular insect which visited them, so as to
favour in any degree the transportal of the pollen, would likewise be
favoured. We might have taken the case of insects visiting flowers for the
sake of collecting pollen instead of nectar; and as pollen is formed for
the sole purpose of fertilisation, its destruction appears to be a simple
loss to the plant; yet if a little pollen were carried, at first
occasionally and then habitually, by the pollen-devouring insects from
flower to flower, and a cross thus effected, although nine-tenths of the
pollen were destroyed it might still be a great gain to the plant to be
thus robbed; and the individuals which produced more and more pollen, and
had larger anthers, would be selected.

When our plant, by the above process long continued, had been rendered
highly attractive to insects, they would, unintentionally on their part,
regularly carry pollen from flower to flower; and that they do this
effectually I could easily show by many striking facts. I will give only
one, as likewise illustrating one step in the separation of the sexes of
plants. Some holly-trees bear only male flowers, which have four stamens
producing a rather small quantity of pollen, and a rudimentary pistil;
other holly-trees bear only female flowers; these have a full-sized
pistil, and four stamens with shrivelled anthers, in which not a grain of
pollen can be detected. Having found a female tree exactly sixty yards
from a male tree, I put the stigmas of twenty flowers, taken from
different branches, under the microscope, and on all, without exception,
there were a few pollen-grains, and on some a profusion. As the wind had
set for several days from the female to the male tree, the pollen could
not thus have been carried. The weather had been cold and boisterous and
therefore not favourable to bees, nevertheless every female flower which I
examined had been effectually fertilised by the bees, which had flown from
tree to tree in search of nectar. But to return to our imaginary case; as
soon as the plant had been rendered so highly attractive to insects that
pollen was regularly carried from flower to flower, another process might
commence. No naturalist doubts the advantage of what has been called the
"physiological division of labour;" hence we may believe that it would be
advantageous to a plant to produce stamens alone in one flower or on one
whole plant, and pistils alone in another flower or on another plant. In
plants under culture and placed under new conditions of life, sometimes
the male organs and sometimes the female organs become more or less
impotent; now if we suppose this to occur in ever so slight a degree under
nature, then, as pollen is already carried regularly from flower to
flower, and as a more complete separation of the sexes of our plant would
be advantageous on the principle of the division of labour, individuals
with this tendency more and more increased, would be continually favoured
or selected, until at last a complete separation of the sexes might be
effected. It would take up too much space to show the various steps,
through dimorphism and other means, by which the separation of the sexes
in plants of various kinds is apparently now in progress; but I may add
that some of the species of holly in North America are, according to Asa
Gray, in an exactly intermediate condition, or, as he expresses it, are
more or less dioeciously polygamous.

Let us now turn to the nectar-feeding insects; we may suppose the plant of
which we have been slowly increasing the nectar by continued selection, to
be a common plant; and that certain insects depended in main part on its
nectar for food. I could give many facts showing how anxious bees are to
save time: for instance, their habit of cutting holes and sucking the
nectar at the bases of certain flowers, which with a very little more
trouble they can enter by the mouth. Bearing such facts in mind, it may be
believed that under certain circumstances individual differences in the
curvature or length of the proboscis, etc., too slight to be appreciated
by us, might profit a bee or other insect, so that certain individuals
would be able to obtain their food more quickly than others; and thus the
communities to which they belonged would flourish and throw off many
swarms inheriting the same peculiarities. The tubes of the corolla of the
common red or incarnate clovers (Trifolium pratense and incarnatum) do not
on a hasty glance appear to differ in length; yet the hive-bee can easily
suck the nectar out of the incarnate clover, but not out of the common red
clover, which is visited by humble-bees alone; so that whole fields of the
red clover offer in vain an abundant supply of precious nectar to the
hive-bee. That this nectar is much liked by the hive-bee is certain; for I
have repeatedly seen, but only in the autumn, many hive-bees sucking the
flowers through holes bitten in the base of the tube by humble bees. The
difference in the length of the corolla in the two kinds of clover, which
determines the visits of the hive-bee, must be very trifling; for I have
been assured that when red clover has been mown, the flowers of the second
crop are somewhat smaller, and that these are visited by many hive-bees. I
do not know whether this statement is accurate; nor whether another
published statement can be trusted, namely, that the Ligurian bee, which
is generally considered a mere variety of the common hive-bee, and which
freely crosses with it, is able to reach and suck the nectar of the red
clover. Thus, in a country where this kind of clover abounded, it might be
a great advantage to the hive-bee to have a slightly longer or differently
constructed proboscis. On the other hand, as the fertility of this clover
absolutely depends on bees visiting the flowers, if humble-bees were to
become rare in any country, it might be a great advantage to the plant to
have a shorter or more deeply divided corolla, so that the hive-bees
should be enabled to suck its flowers. Thus I can understand how a flower
and a bee might slowly become, either simultaneously or one after the
other, modified and adapted to each other in the most perfect manner, by
the continued preservation of all the individuals which presented slight
deviations of structure mutually favourable to each other.

I am well aware that this doctrine of natural selection, exemplified in
the above imaginary instances, is open to the same objections which were
first urged against Sir Charles Lyell's noble views on "the modern changes
of the earth, as illustrative of geology;" but we now seldom hear the
agencies which we see still at work, spoken of as trifling and
insignificant, when used in explaining the excavation of the deepest
valleys or the formation of long lines of inland cliffs. Natural selection
acts only by the preservation and accumulation of small inherited
modifications, each profitable to the preserved being; and as modern
geology has almost banished such views as the excavation of a great valley
by a single diluvial wave, so will natural selection banish the belief of
the continued creation of new organic beings, or of any great and sudden
modification in their structure.

ON THE INTERCROSSING OF INDIVIDUALS.

I must here introduce a short digression. In the case of animals and
plants with separated sexes, it is of course obvious that two individuals
must always (with the exception of the curious and not well understood
cases of parthenogenesis) unite for each birth; but in the case of
hermaphrodites this is far from obvious. Nevertheless there is reason to
believe that with all hermaphrodites two individuals, either occasionally
or habitually, concur for the reproduction of their kind. This view was
long ago doubtfully suggested by Sprengel, Knight and Kolreuter. We shall
presently see its importance; but I must here treat the subject with
extreme brevity, though I have the materials prepared for an ample
discussion. All vertebrate animals, all insects and some other large
groups of animals, pair for each birth. Modern research has much
diminished the number of supposed hermaphrodites and of real
hermaphrodites a large number pair; that is, two individuals regularly
unite for reproduction, which is all that concerns us. But still there are
many hermaphrodite animals which certainly do not habitually pair, and a
vast majority of plants are hermaphrodites. What reason, it may be asked,
is there for supposing in these cases that two individuals ever concur in
reproduction? As it is impossible here to enter on details, I must trust
to some general considerations alone.

In the first place, I have collected so large a body of facts, and made so
many experiments, showing, in accordance with the almost universal belief
of breeders, that with animals and plants a cross between different
varieties, or between individuals of the same variety but of another
strain, gives vigour and fertility to the offspring; and on the other
hand, that CLOSE interbreeding diminishes vigour and fertility; that these
facts alone incline me to believe that it is a general law of nature that
no organic being fertilises itself for a perpetuity of generations; but
that a cross with another individual is occasionally—perhaps at long
intervals of time—indispensable.

On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any other
view are inexplicable. Every hybridizer knows how unfavourable exposure to
wet is to the fertilisation of a flower, yet what a multitude of flowers
have their anthers and stigmas fully exposed to the weather! If an
occasional cross be indispensable, notwithstanding that the plant's own
anthers and pistil stand so near each other as almost to ensure
self-fertilisation, the fullest freedom for the entrance of pollen from
another individual will explain the above state of exposure of the organs.
Many flowers, on the other hand, have their organs of fructification
closely enclosed, as in the great papilionaceous or pea-family; but these
almost invariably present beautiful and curious adaptations in relation to
the visits of insects. So necessary are the visits of bees to many
papilionaceous flowers, that their fertility is greatly diminished if
these visits be prevented. Now, it is scarcely possible for insects to fly
from flower to flower, and not to carry pollen from one to the other, to
the great good of the plant. Insects act like a camel-hair pencil, and it
is sufficient, to ensure fertilisation, just to touch with the same brush
the anthers of one flower and then the stigma of another; but it must not
be supposed that bees would thus produce a multitude of hybrids between
distinct species; for if a plant's own pollen and that from another
species are placed on the same stigma, the former is so prepotent that it
invariably and completely destroys, as has been shown by Gartner, the
influence of the foreign pollen.

When the stamens of a flower suddenly spring towards the pistil, or slowly
move one after the other towards it, the contrivance seems adapted solely
to ensure self-fertilisation; and no doubt it is useful for this end: but
the agency of insects is often required to cause the stamens to spring
forward, as Kolreuter has shown to be the case with the barberry; and in
this very genus, which seems to have a special contrivance for
self-fertilisation, it is well known that, if closely-allied forms or
varieties are planted near each other, it is hardly possible to raise pure
seedlings, so largely do they naturally cross. In numerous other cases,
far from self-fertilisation being favoured, there are special contrivances
which effectually prevent the stigma receiving pollen from its own flower,
as I could show from the works of Sprengel and others, as well as from my
own observations: for instance, in Lobelia fulgens, there is a really
beautiful and elaborate contrivance by which all the infinitely numerous
pollen-granules are swept out of the conjoined anthers of each flower,
before the stigma of that individual flower is ready to receive them; and
as this flower is never visited, at least in my garden, by insects, it
never sets a seed, though by placing pollen from one flower on the stigma
of another, I raise plenty of seedlings. Another species of Lobelia, which
is visited by bees, seeds freely in my garden. In very many other cases,
though there is no special mechanical contrivance to prevent the stigma
receiving pollen from the same flower, yet, as Sprengel, and more recently
Hildebrand and others have shown, and as I can confirm, either the anthers
burst before the stigma is ready for fertilisation, or the stigma is ready
before the pollen of that flower is ready, so that these so-named
dichogamous plants have in fact separated sexes, and must habitually be
crossed. So it is with the reciprocally dimorphic and trimorphic plants
previously alluded to. How strange are these facts! How strange that the
pollen and stigmatic surface of the same flower, though placed so close
together, as if for the very purpose of self-fertilisation, should be in
so many cases mutually useless to each other! How simply are these facts
explained on the view of an occasional cross with a distinct individual
being advantageous or indispensable!

If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority of the
seedlings thus raised turn out, as I found, mongrels: for instance, I
raised 233 seedling cabbages from some plants of different varieties
growing near each other, and of these only 78 were true to their kind, and
some even of these were not perfectly true. Yet the pistil of each
cabbage-flower is surrounded not only by its own six stamens but by those
of the many other flowers on the same plant; and the pollen of each flower
readily gets on its stigma without insect agency; for I have found that
plants carefully protected from insects produce the full number of pods.
How, then, comes it that such a vast number of the seedlings are
mongrelized? It must arise from the pollen of a distinct VARIETY having a
prepotent effect over the flower's own pollen; and that this is part of
the general law of good being derived from the intercrossing of distinct
individuals of the same species. When distinct SPECIES are crossed the
case is reversed, for a plant's own pollen is always prepotent over
foreign pollen; but to this subject we shall return in a future chapter.

In the case of a large tree covered with innumerable flowers, it may be
objected that pollen could seldom be carried from tree to tree, and at
most only from flower to flower on the same tree; and flowers on the same
tree can be considered as distinct individuals only in a limited sense. I
believe this objection to be valid, but that nature has largely provided
against it by giving to trees a strong tendency to bear flowers with
separated sexes. When the sexes are separated, although the male and
female flowers may be produced on the same tree, pollen must be regularly
carried from flower to flower; and this will give a better chance of
pollen being occasionally carried from tree to tree. That trees belonging
to all orders have their sexes more often separated than other plants, I
find to be the case in this country; and at my request Dr. Hooker
tabulated the trees of New Zealand, and Dr. Asa Gray those of the United
States, and the result was as I anticipated. On the other hand, Dr. Hooker
informs me that the rule does not hold good in Australia: but if most of
the Australian trees are dichogamous, the same result would follow as if
they bore flowers with separated sexes. I have made these few remarks on
trees simply to call attention to the subject.

Turning for a brief space to animals: various terrestrial species are
hermaphrodites, such as the land-mollusca and earth-worms; but these all
pair. As yet I have not found a single terrestrial animal which can
fertilise itself. This remarkable fact, which offers so strong a contrast
with terrestrial plants, is intelligible on the view of an occasional
cross being indispensable; for owing to the nature of the fertilising
element there are no means, analogous to the action of insects and of the
wind with plants, by which an occasional cross could be effected with
terrestrial animals without the concurrence of two individuals. Of aquatic
animals, there are many self-fertilising hermaphrodites; but here the
currents of water offer an obvious means for an occasional cross. As in
the case of flowers, I have as yet failed, after consultation with one of
the highest authorities, namely, Professor Huxley, to discover a single
hermaphrodite animal with the organs of reproduction so perfectly enclosed
that access from without, and the occasional influence of a distinct
individual, can be shown to be physically impossible. Cirripedes long
appeared to me to present, under this point of view, a case of great
difficulty; but I have been enabled, by a fortunate chance, to prove that
two individuals, though both are self-fertilising hermaphrodites, do
sometimes cross.

It must have struck most naturalists as a strange anomaly that, both with
animals and plants, some species of the same family and even of the same
genus, though agreeing closely with each other in their whole
organisation, are hermaphrodites, and some unisexual. But if, in fact, all
hermaphrodites do occasionally intercross, the difference between them and
unisexual species is, as far as function is concerned, very small.

From these several considerations and from the many special facts which I
have collected, but which I am unable here to give, it appears that with
animals and plants an occasional intercross between distinct individuals
is a very general, if not universal, law of nature.

CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW FORMS THROUGH NATURAL
SELECTION.

This is an extremely intricate subject. A great amount of variability,
under which term individual differences are always included, will
evidently be favourable. A large number of individuals, by giving a better
chance within any given period for the appearance of profitable
variations, will compensate for a lesser amount of variability in each
individual, and is, I believe, a highly important element of success.
Though nature grants long periods of time for the work of natural
selection, she does not grant an indefinite period; for as all organic
beings are striving to seize on each place in the economy of nature, if
any one species does not become modified and improved in a corresponding
degree with its competitors it will be exterminated. Unless favourable
variations be inherited by some at least of the offspring, nothing can be
effected by natural selection. The tendency to reversion may often check
or prevent the work; but as this tendency has not prevented man from
forming by selection numerous domestic races, why should it prevail
against natural selection?

In the case of methodical selection, a breeder selects for some definite
object, and if the individuals be allowed freely to intercross, his work
will completely fail. But when many men, without intending to alter the
breed, have a nearly common standard of perfection, and all try to procure
and breed from the best animals, improvement surely but slowly follows
from this unconscious process of selection, notwithstanding that there is
no separation of selected individuals. Thus it will be under nature; for
within a confined area, with some place in the natural polity not
perfectly occupied, all the individuals varying in the right direction,
though in different degrees, will tend to be preserved. But if the area be
large, its several districts will almost certainly present different
conditions of life; and then, if the same species undergoes modification
in different districts, the newly formed varieties will intercross on the
confines of each. But we shall see in the sixth chapter that intermediate
varieties, inhabiting intermediate districts, will in the long run
generally be supplanted by one of the adjoining varieties. Intercrossing
will chiefly affect those animals which unite for each birth and wander
much, and which do not breed at a very quick rate. Hence with animals of
this nature, for instance birds, varieties will generally be confined to
separated countries; and this I find to be the case. With hermaphrodite
organisms which cross only occasionally, and likewise with animals which
unite for each birth, but which wander little and can increase at a rapid
rate, a new and improved variety might be quickly formed on any one spot,
and might there maintain itself in a body and afterward spread, so that
the individuals of the new variety would chiefly cross together. On this
principle nurserymen always prefer saving seed from a large body of
plants, as the chance of intercrossing is thus lessened.

Even with animals which unite for each birth, and which do not propagate
rapidly, we must not assume that free intercrossing would always eliminate
the effects of natural selection; for I can bring forward a considerable
body of facts showing that within the same area two varieties of the same
animal may long remain distinct, from haunting different stations, from
breeding at slightly different seasons, or from the individuals of each
variety preferring to pair together.

Intercrossing plays a very important part in nature by keeping the
individuals of the same species, or of the same variety, true and uniform
in character. It will obviously thus act far more efficiently with those
animals which unite for each birth; but, as already stated, we have reason
to believe that occasional intercrosses take place with all animals and
plants. Even if these take place only at long intervals of time, the young
thus produced will gain so much in vigour and fertility over the offspring
from long-continued self-fertilisation, that they will have a better
chance of surviving and propagating their kind; and thus in the long run
the influence of crosses, even at rare intervals, will be great. With
respect to organic beings extremely low in the scale, which do not
propagate sexually, nor conjugate, and which cannot possibly intercross,
uniformity of character can be retained by them under the same conditions
of life, only through the principle of inheritance, and through natural
selection which will destroy any individuals departing from the proper
type. If the conditions of life change and the form undergoes
modification, uniformity of character can be given to the modified
offspring, solely by natural selection preserving similar favourable
variations.

Isolation also is an important element in the modification of species
through natural selection. In a confined or isolated area, if not very
large, the organic and inorganic conditions of life will generally be
almost uniform; so that natural selection will tend to modify all the
varying individuals of the same species in the same manner. Intercrossing
with the inhabitants of the surrounding districts, will also be thus
prevented. Moritz Wagner has lately published an interesting essay on this
subject, and has shown that the service rendered by isolation in
preventing crosses between newly-formed varieties is probably greater even
than I supposed. But from reasons already assigned I can by no means agree
with this naturalist, that migration and isolation are necessary elements
for the formation of new species. The importance of isolation is likewise
great in preventing, after any physical change in the conditions, such as
of climate, elevation of the land, etc., the immigration of better adapted
organisms; and thus new places in the natural economy of the district will
be left open to be filled up by the modification of the old inhabitants.
Lastly, isolation will give time for a new variety to be improved at a
slow rate; and this may sometimes be of much importance. If, however, an
isolated area be very small, either from being surrounded by barriers, or
from having very peculiar physical conditions, the total number of the
inhabitants will be small; and this will retard the production of new
species through natural selection, by decreasing the chances of favourable
variations arising.

The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously asserted
that the element of time has been assumed by me to play an all-important
part in modifying species, as if all the forms of life were necessarily
undergoing change through some innate law. Lapse of time is only so far
important, and its importance in this respect is great, that it gives a
better chance of beneficial variations arising and of their being
selected, accumulated, and fixed. It likewise tends to increase the direct
action of the physical conditions of life, in relation to the constitution
of each organism.

If we turn to nature to test the truth of these remarks, and look at any
small isolated area, such as an oceanic island, although the number of the
species inhabiting it is small, as we shall see in our chapter on
Geographical Distribution; yet of these species a very large proportion
are endemic,—that is, have been produced there and nowhere else in
the world. Hence an oceanic island at first sight seems to have been
highly favourable for the production of new species. But we may thus
deceive ourselves, for to ascertain whether a small isolated area, or a
large open area like a continent, has been most favourable for the
production of new organic forms, we ought to make the comparison within
equal times; and this we are incapable of doing.

Although isolation is of great importance in the production of new
species, on the whole I am inclined to believe that largeness of area is
still more important, especially for the production of species which shall
prove capable of enduring for a long period, and of spreading widely.
Throughout a great and open area, not only will there be a better chance
of favourable variations, arising from the large number of individuals of
the same species there supported, but the conditions of life are much more
complex from the large number of already existing species; and if some of
these many species become modified and improved, others will have to be
improved in a corresponding degree, or they will be exterminated. Each new
form, also, as soon as it has been much improved, will be able to spread
over the open and continuous area, and will thus come into competition
with many other forms. Moreover, great areas, though now continuous, will
often, owing to former oscillations of level, have existed in a broken
condition, so that the good effects of isolation will generally, to a
certain extent, have concurred. Finally, I conclude that, although small
isolated areas have been in some respects highly favourable for the
production of new species, yet that the course of modification will
generally have been more rapid on large areas; and what is more important,
that the new forms produced on large areas, which already have been
victorious over many competitors, will be those that will spread most
widely, and will give rise to the greatest number of new varieties and
species. They will thus play a more important part in the changing history
of the organic world.

In accordance with this view, we can, perhaps, understand some facts which
will be again alluded to in our chapter on Geographical Distribution; for
instance, the fact of the productions of the smaller continent of
Australia now yielding before those of the larger Europaeo-Asiatic area.
Thus, also, it is that continental productions have everywhere become so
largely naturalised on islands. On a small island, the race for life will
have been less severe, and there will have been less modification and less
extermination. Hence, we can understand how it is that the flora of
Madeira, according to Oswald Heer, resembles to a certain extent the
extinct tertiary flora of Europe. All fresh water basins, taken together,
make a small area compared with that of the sea or of the land.
Consequently, the competition between fresh water productions will have
been less severe than elsewhere; new forms will have been more slowly
produced, and old forms more slowly exterminated. And it is in fresh water
basins that we find seven genera of Ganoid fishes, remnants of a once
preponderant order: and in fresh water we find some of the most anomalous
forms now known in the world, as the Ornithorhynchus and Lepidosiren,
which, like fossils, connect to a certain extent orders at present widely
separated in the natural scale. These anomalous forms may be called living
fossils; they have endured to the present day, from having inhabited a
confined area, and from having been exposed to less varied, and therefore
less severe, competition.

To sum up, as far as the extreme intricacy of the subject permits, the
circumstances favourable and unfavourable for the production of new
species through natural selection. I conclude that for terrestrial
productions a large continental area, which has undergone many
oscillations of level, will have been the most favourable for the
production of many new forms of life, fitted to endure for a long time and
to spread widely. While the area existed as a continent the inhabitants
will have been numerous in individuals and kinds, and will have been
subjected to severe competition. When converted by subsidence into large
separate islands there will still have existed many individuals of the
same species on each island: intercrossing on the confines of the range of
each new species will have been checked: after physical changes of any
kind immigration will have been prevented, so that new places in the
polity of each island will have had to be filled up by the modification of
the old inhabitants; and time will have been allowed for the varieties in
each to become well modified and perfected. When, by renewed elevation,
the islands were reconverted into a continental area, there will again
have been very severe competition; the most favoured or improved varieties
will have been enabled to spread; there will have been much extinction of
the less improved forms, and the relative proportional numbers of the
various inhabitants of the reunited continent will again have been
changed; and again there will have been a fair field for natural selection
to improve still further the inhabitants, and thus to produce new species.

That natural selection generally act with extreme slowness I fully admit.
It can act only when there are places in the natural polity of a district
which can be better occupied by the modification of some of its existing
inhabitants. The occurrence of such places will often depend on physical
changes, which generally take place very slowly, and on the immigration of
better adapted forms being prevented. As some few of the old inhabitants
become modified the mutual relations of others will often be disturbed;
and this will create new places, ready to be filled up by better adapted
forms; but all this will take place very slowly. Although all the
individuals of the same species differ in some slight degree from each
other, it would often be long before differences of the right nature in
various parts of the organisation might occur. The result would often be
greatly retarded by free intercrossing. Many will exclaim that these
several causes are amply sufficient to neutralise the power of natural
selection. I do not believe so. But I do believe that natural selection
will generally act very slowly, only at long intervals of time, and only
on a few of the inhabitants of the same region. I further believe that
these slow, intermittent results accord well with what geology tells us of
the rate and manner at which the inhabitants of the world have changed.

Slow though the process of selection may be, if feeble man can do much by
artificial selection, I can see no limit to the amount of change, to the
beauty and complexity of the coadaptations between all organic beings, one
with another and with their physical conditions of life, which may have
been effected in the long course of time through nature's power of
selection, that is by the survival of the fittest.

EXTINCTION CAUSED BY NATURAL SELECTION.

This subject will be more fully discussed in our chapter on Geology; but
it must here be alluded to from being intimately connected with natural
selection. Natural selection acts solely through the preservation of
variations in some way advantageous, which consequently endure. Owing to
the high geometrical rate of increase of all organic beings, each area is
already fully stocked with inhabitants, and it follows from this, that as
the favoured forms increase in number, so, generally, will the less
favoured decrease and become rare. Rarity, as geology tells us, is the
precursor to extinction. We can see that any form which is represented by
few individuals will run a good chance of utter extinction, during great
fluctuations in the nature or the seasons, or from a temporary increase in
the number of its enemies. But we may go further than this; for as new
forms are produced, unless we admit that specific forms can go on
indefinitely increasing in number, many old forms must become extinct.
That the number of specific forms has not indefinitely increased, geology
plainly tells us; and we shall presently attempt to show why it is that
the number of species throughout the world has not become immeasurably
great.

We have seen that the species which are most numerous in individuals have
the best chance of producing favourable variations within any given
period. We have evidence of this, in the facts stated in the second
chapter, showing that it is the common and diffused or dominant species
which offer the greatest number of recorded varieties. Hence, rare species
will be less quickly modified or improved within any given period; they
will consequently be beaten in the race for life by the modified and
improved descendants of the commoner species.

From these several considerations I think it inevitably follows, that as
new species in the course of time are formed through natural selection,
others will become rarer and rarer, and finally extinct. The forms which
stand in closest competition with those undergoing modification and
improvement, will naturally suffer most. And we have seen in the chapter
on the Struggle for Existence that it is the most closely-allied forms,—varieties
of the same species, and species of the same genus or related genera,—which,
from having nearly the same structure, constitution and habits, generally
come into the severest competition with each other. Consequently, each new
variety or species, during the progress of its formation, will generally
press hardest on its nearest kindred, and tend to exterminate them. We see
the same process of extermination among our domesticated productions,
through the selection of improved forms by man. Many curious instances
could be given showing how quickly new breeds of cattle, sheep and other
animals, and varieties of flowers, take the place of older and inferior
kinds. In Yorkshire, it is historically known that the ancient black
cattle were displaced by the long-horns, and that these "were swept away
by the short-horns" (I quote the words of an agricultural writer) "as if
by some murderous pestilence."

DIVERGENCE OF CHARACTER.

The principle, which I have designated by this term, is of high
importance, and explains, as I believe, several important facts. In the
first place, varieties, even strongly-marked ones, though having somewhat
of the character of species—as is shown by the hopeless doubts in
many cases how to rank them—yet certainly differ far less from each
other than do good and distinct species. Nevertheless according to my
view, varieties are species in the process of formation, or are, as I have
called them, incipient species. How, then, does the lesser difference
between varieties become augmented into the greater difference between
species? That this does habitually happen, we must infer from most of the
innumerable species throughout nature presenting well-marked differences;
whereas varieties, the supposed prototypes and parents of future
well-marked species, present slight and ill-defined differences. Mere
chance, as we may call it, might cause one variety to differ in some
character from its parents, and the offspring of this variety again to
differ from its parent in the very same character and in a greater degree;
but this alone would never account for so habitual and large a degree of
difference as that between the species of the same genus.

As has always been my practice, I have sought light on this head from our
domestic productions. We shall here find something analogous. It will be
admitted that the production of races so different as short-horn and
Hereford cattle, race and cart horses, the several breeds of pigeons,
etc., could never have been effected by the mere chance accumulation of
similar variations during many successive generations. In practice, a
fancier is, for instance, struck by a pigeon having a slightly shorter
beak; another fancier is struck by a pigeon having a rather longer beak;
and on the acknowledged principle that "fanciers do not and will not
admire a medium standard, but like extremes," they both go on (as has
actually occurred with the sub-breeds of the tumbler-pigeon) choosing and
breeding from birds with longer and longer beaks, or with shorter and
shorter beaks. Again, we may suppose that at an early period of history,
the men of one nation or district required swifter horses, while those of
another required stronger and bulkier horses. The early differences would
be very slight; but, in the course of time, from the continued selection
of swifter horses in the one case, and of stronger ones in the other, the
differences would become greater, and would be noted as forming two
sub-breeds. Ultimately after the lapse of centuries, these sub-breeds
would become converted into two well-established and distinct breeds. As
the differences became greater, the inferior animals with intermediate
characters, being neither very swift nor very strong, would not have been
used for breeding, and will thus have tended to disappear. Here, then, we
see in man's productions the action of what may be called the principle of
divergence, causing differences, at first barely appreciable, steadily to
increase, and the breeds to diverge in character, both from each other and
from their common parent.

But how, it may be asked, can any analogous principle apply in nature? I
believe it can and does apply most efficiently (though it was a long time
before I saw how), from the simple circumstance that the more diversified
the descendants from any one species become in structure, constitution,
and habits, by so much will they be better enabled to seize on many and
widely diversified places in the polity of nature, and so be enabled to
increase in numbers.

We can clearly discern this in the case of animals with simple habits.
Take the case of a carnivorous quadruped, of which the number that can be
supported in any country has long ago arrived at its full average. If its
natural power of increase be allowed to act, it can succeed in increasing
(the country not undergoing any change in conditions) only by its varying
descendants seizing on places at present occupied by other animals: some
of them, for instance, being enabled to feed on new kinds of prey, either
dead or alive; some inhabiting new stations, climbing trees, frequenting
water, and some perhaps becoming less carnivorous. The more diversified in
habits and structure the descendants of our carnivorous animals become,
the more places they will be enabled to occupy. What applies to one animal
will apply throughout all time to all animals—that is, if they vary—for
otherwise natural selection can effect nothing. So it will be with plants.
It has been experimentally proved, that if a plot of ground be sown with
one species of grass, and a similar plot be sown with several distinct
genera of grasses, a greater number of plants and a greater weight of dry
herbage can be raised in the latter than in the former case. The same has
been found to hold good when one variety and several mixed varieties of
wheat have been sown on equal spaces of ground. Hence, if any one species
of grass were to go on varying, and the varieties were continually
selected which differed from each other in the same manner, though in a
very slight degree, as do the distinct species and genera of grasses, a
greater number of individual plants of this species, including its
modified descendants, would succeed in living on the same piece of ground.
And we know that each species and each variety of grass is annually sowing
almost countless seeds; and is thus striving, as it may be said, to the
utmost to increase in number. Consequently, in the course of many thousand
generations, the most distinct varieties of any one species of grass would
have the best chance of succeeding and of increasing in numbers, and thus
of supplanting the less distinct varieties; and varieties, when rendered
very distinct from each other, take the rank of species.

The truth of the principle that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if freely
open to immigration, and where the contest between individual and
individual must be very severe, we always find great diversity in its
inhabitants. For instance, I found that a piece of turf, three feet by
four in size, which had been exposed for many years to exactly the same
conditions, supported twenty species of plants, and these belonged to
eighteen genera and to eight orders, which shows how much these plants
differed from each other. So it is with the plants and insects on small
and uniform islets: also in small ponds of fresh water. Farmers find that
they can raise more food by a rotation of plants belonging to the most
different orders: nature follows what may be called a simultaneous
rotation. Most of the animals and plants which live close round any small
piece of ground, could live on it (supposing its nature not to be in any
way peculiar), and may be said to be striving to the utmost to live there;
but, it is seen, that where they come into the closest competition, the
advantages of diversification of structure, with the accompanying
differences of habit and constitution, determine that the inhabitants,
which thus jostle each other most closely, shall, as a general rule,
belong to what we call different genera and orders.

The same principle is seen in the naturalisation of plants through man's
agency in foreign lands. It might have been expected that the plants which
would succeed in becoming naturalised in any land would generally have
been closely allied to the indigenes; for these are commonly looked at as
specially created and adapted for their own country. It might also,
perhaps, have been expected that naturalised plants would have belonged to
a few groups more especially adapted to certain stations in their new
homes. But the case is very different; and Alph. de Candolle has well
remarked, in his great and admirable work, that floras gain by
naturalisation, proportionally with the number of the native genera and
species, far more in new genera than in new species. To give a single
instance: in the last edition of Dr. Asa Gray's "Manual of the Flora of
the Northern United States," 260 naturalised plants are enumerated, and
these belong to 162 genera. We thus see that these naturalised plants are
of a highly diversified nature. They differ, moreover, to a large extent,
from the indigenes, for out of the 162 naturalised genera, no less than
100 genera are not there indigenous, and thus a large proportional
addition is made to the genera now living in the United States.

By considering the nature of the plants or animals which have in any
country struggled successfully with the indigenes, and have there become
naturalised, we may gain some crude idea in what manner some of the
natives would have had to be modified in order to gain an advantage over
their compatriots; and we may at least infer that diversification of
structure, amounting to new generic differences, would be profitable to
them.

The advantage of diversification of structure in the inhabitants of the
same region is, in fact, the same as that of the physiological division of
labour in the organs of the same individual body—a subject so well
elucidated by Milne Edwards. No physiologist doubts that a stomach by
being adapted to digest vegetable matter alone, or flesh alone, draws most
nutriment from these substances. So in the general economy of any land,
the more widely and perfectly the animals and plants are diversified for
different habits of life, so will a greater number of individuals be
capable of there supporting themselves. A set of animals, with their
organisation but little diversified, could hardly compete with a set more
perfectly diversified in structure. It may be doubted, for instance,
whether the Australian marsupials, which are divided into groups differing
but little from each other, and feebly representing, as Mr. Waterhouse and
others have remarked, our carnivorous, ruminant, and rodent mammals, could
successfully compete with these well-developed orders. In the Australian
mammals, we see the process of diversification in an early and incomplete
stage of development.

THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH DIVERGENCE
OF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF A COMMON ANCESTOR.

After the foregoing discussion, which has been much compressed, we may
assume that the modified descendants of any one species will succeed so
much the better as they become more diversified in structure, and are thus
enabled to encroach on places occupied by other beings. Now let us see how
this principle of benefit being derived from divergence of character,
combined with the principles of natural selection and of extinction, tends
to act.

The accompanying diagram will aid us in understanding this rather
perplexing subject. Let A to L represent the species of a genus large in
its own country; these species are supposed to resemble each other in
unequal degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at unequal distances. I
have said a large genus, because as we saw in the second chapter, on an
average more species vary in large genera than in small genera; and the
varying species of the large genera present a greater number of varieties.
We have, also, seen that the species, which are the commonest and most
widely-diffused, vary more than do the rare and restricted species. Let
(A) be a common, widely-diffused, and varying species, belonging to a
genus large in its own country. The branching and diverging dotted lines
of unequal lengths proceeding from (A), may represent its varying
offspring. The variations are supposed to be extremely slight, but of the
most diversified nature; they are not supposed all to appear
simultaneously, but often after long intervals of time; nor are they all
supposed to endure for equal periods. Only those variations which are in
some way profitable will be preserved or naturally selected. And here the
importance of the principle of benefit derived from divergence of
character comes in; for this will generally lead to the most different or
divergent variations (represented by the outer dotted lines) being
preserved and accumulated by natural selection. When a dotted line reaches
one of the horizontal lines, and is there marked by a small numbered
letter, a sufficient amount of variation is supposed to have been
accumulated to form it into a fairly well-marked variety, such as would be
thought worthy of record in a systematic work.

The intervals between the horizontal lines in the diagram, may represent
each a thousand or more generations. After a thousand generations, species
(A) is supposed to have produced two fairly well-marked varieties, namely
a1 and m1. These two varieties will generally still be exposed to the same
conditions which made their parents variable, and the tendency to
variability is in itself hereditary; consequently they will likewise tend
to vary, and commonly in nearly the same manner as did their parents.
Moreover, these two varieties, being only slightly modified forms, will
tend to inherit those advantages which made their parent (A) more numerous
than most of the other inhabitants of the same country; they will also
partake of those more general advantages which made the genus to which the
parent-species belonged, a large genus in its own country. And all these
circumstances are favourable to the production of new varieties.

If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety a1 is supposed in the
diagram to have produced variety a2, which will, owing to the principle of
divergence, differ more from (A) than did variety a1. Variety m1 is
supposed to have produced two varieties, namely m2 and s2, differing from
each other, and more considerably from their common parent (A). We may
continue the process by similar steps for any length of time; some of the
varieties, after each thousand generations, producing only a single
variety, but in a more and more modified condition, some producing two or
three varieties, and some failing to produce any. Thus the varieties or
modified descendants of the common parent (A), will generally go on
increasing in number and diverging in character. In the diagram the
process is represented up to the ten-thousandth generation, and under a
condensed and simplified form up to the fourteen-thousandth generation.

But I must here remark that I do not suppose that the process ever goes on
so regularly as is represented in the diagram, though in itself made
somewhat irregular, nor that it goes on continuously; it is far more
probable that each form remains for long periods unaltered, and then again
undergoes modification. Nor do I suppose that the most divergent varieties
are invariably preserved: a medium form may often long endure, and may or
may not produce more than one modified descendant; for natural selection
will always act according to the nature of the places which are either
unoccupied or not perfectly occupied by other beings; and this will depend
on infinitely complex relations. But as a general rule, the more
diversified in structure the descendants from any one species can be
rendered, the more places they will be enabled to seize on, and the more
their modified progeny will increase. In our diagram the line of
succession is broken at regular intervals by small numbered letters
marking the successive forms which have become sufficiently distinct to be
recorded as varieties. But these breaks are imaginary, and might have been
inserted anywhere, after intervals long enough to allow the accumulation
of a considerable amount of divergent variation.

As all the modified descendants from a common and widely-diffused species,
belonging to a large genus, will tend to partake of the same advantages
which made their parent successful in life, they will generally go on
multiplying in number as well as diverging in character: this is
represented in the diagram by the several divergent branches proceeding
from (A). The modified offspring from the later and more highly improved
branches in the lines of descent, will, it is probable, often take the
place of, and so destroy, the earlier and less improved branches: this is
represented in the diagram by some of the lower branches not reaching to
the upper horizontal lines. In some cases no doubt the process of
modification will be confined to a single line of descent, and the number
of modified descendants will not be increased; although the amount of
divergent modification may have been augmented. This case would be
represented in the diagram, if all the lines proceeding from (A) were
removed, excepting that from a1 to a10. In the same way the English
racehorse and English pointer have apparently both gone on slowly
diverging in character from their original stocks, without either having
given off any fresh branches or races.

After ten thousand generations, species (A) is supposed to have produced
three forms, a10, f10, and m10, which, from having diverged in character
during the successive generations, will have come to differ largely, but
perhaps unequally, from each other and from their common parent. If we
suppose the amount of change between each horizontal line in our diagram
to be excessively small, these three forms may still be only well-marked
varieties; but we have only to suppose the steps in the process of
modification to be more numerous or greater in amount, to convert these
three forms into doubtful or at least into well-defined species: thus the
diagram illustrates the steps by which the small differences
distinguishing varieties are increased into the larger differences
distinguishing species. By continuing the same process for a greater
number of generations (as shown in the diagram in a condensed and
simplified manner), we get eight species, marked by the letters between
a14 and m14, all descended from (A). Thus, as I believe, species are
multiplied and genera are formed.

In a large genus it is probable that more than one species would vary. In
the diagram I have assumed that a second species (I) has produced, by
analogous steps, after ten thousand generations, either two well-marked
varieties (w10 and z10) or two species, according to the amount of change
supposed to be represented between the horizontal lines. After fourteen
thousand generations, six new species, marked by the letters n14 to z14,
are supposed to have been produced. In any genus, the species which are
already very different in character from each other, will generally tend
to produce the greatest number of modified descendants; for these will
have the best chance of seizing on new and widely different places in the
polity of nature: hence in the diagram I have chosen the extreme species
(A), and the nearly extreme species (I), as those which have largely
varied, and have given rise to new varieties and species. The other nine
species (marked by capital letters) of our original genus, may for long
but unequal periods continue to transmit unaltered descendants; and this
is shown in the diagram by the dotted lines unequally prolonged upwards.

But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played an
important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency in
the improved descendants of any one species to supplant and exterminate in
each stage of descent their predecessors and their original progenitor.
For it should be remembered that the competition will generally be most
severe between those forms which are most nearly related to each other in
habits, constitution and structure. Hence all the intermediate forms
between the earlier and later states, that is between the less and more
improved states of a the same species, as well as the original
parent-species itself, will generally tend to become extinct. So it
probably will be with many whole collateral lines of descent, which will
be conquered by later and improved lines. If, however, the modified
offspring of a species get into some distinct country, or become quickly
adapted to some quite new station, in which offspring and progenitor do
not come into competition, both may continue to exist.

If, then, our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have become
extinct, being replaced by eight new species (a14 to m14); and species (I)
will be replaced by six (n14 to z14) new species.

But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally the
case in nature; species (A) being more nearly related to B, C, and D than
to the other species; and species (I) more to G, H, K, L, than to the
others. These two species (A and I), were also supposed to be very common
and widely diffused species, so that they must originally have had some
advantage over most of the other species of the genus. Their modified
descendants, fourteen in number at the fourteen-thousandth generation,
will probably have inherited some of the same advantages: they have also
been modified and improved in a diversified manner at each stage of
descent, so as to have become adapted to many related places in the
natural economy of their country. It seems, therefore, extremely probable
that they will have taken the places of, and thus exterminated, not only
their parents (A) and (I), but likewise some of the original species which
were most nearly related to their parents. Hence very few of the original
species will have transmitted offspring to the fourteen-thousandth
generation. We may suppose that only one (F) of the two species (E and F)
which were least closely related to the other nine original species, has
transmitted descendants to this late stage of descent.

The new species in our diagram, descended from the original eleven
species, will now be fifteen in number. Owing to the divergent tendency of
natural selection, the extreme amount of difference in character between
species a14 and z14 will be much greater than that between the most
distinct of the original eleven species. The new species, moreover, will
be allied to each other in a widely different manner. Of the eight
descendants from (A) the three marked a14, q14, p14, will be nearly
related from having recently branched off from a10; b14 and f14, from
having diverged at an earlier period from a5, will be in some degree
distinct from the three first-named species; and lastly, o14, e14, and
m14, will be nearly related one to the other, but, from having diverged at
the first commencement of the process of modification, will be widely
different from the other five species, and may constitute a sub-genus or a
distinct genus.

The six descendants from (I) will form two sub-genera or genera. But as
the original species (I) differed largely from (A), standing nearly at the
extreme end of the original genus, the six descendants from (I) will,
owing to inheritance alone, differ considerably from the eight descendants
from (A); the two groups, moreover, are supposed to have gone on diverging
in different directions. The intermediate species, also (and this is a
very important consideration), which connected the original species (A)
and (I), have all become, except (F), extinct, and have left no
descendants. Hence the six new species descended from (I), and the eight
descendants from (A), will have to be ranked as very distinct genera, or
even as distinct sub-families.

Thus it is, as I believe, that two or more genera are produced by descent
with modification, from two or more species of the same genus. And the two
or more parent-species are supposed to be descended from some one species
of an earlier genus. In our diagram this is indicated by the broken lines
beneath the capital letters, converging in sub-branches downwards towards
a single point; this point represents a species, the supposed progenitor
of our several new sub-genera and genera.

It is worth while to reflect for a moment on the character of the new
species F14, which is supposed not to have diverged much in character, but
to have retained the form of (F), either unaltered or altered only in a
slight degree. In this case its affinities to the other fourteen new
species will be of a curious and circuitous nature. Being descended from a
form that stood between the parent-species (A) and (I), now supposed to be
extinct and unknown, it will be in some degree intermediate in character
between the two groups descended from these two species. But as these two
groups have gone on diverging in character from the type of their parents,
the new species (F14) will not be directly intermediate between them, but
rather between types of the two groups; and every naturalist will be able
to call such cases before his mind.

In the diagram each horizontal line has hitherto been supposed to
represent a thousand generations, but each may represent a million or more
generations; it may also represent a section of the successive strata of
the earth's crust including extinct remains. We shall, when we come to our
chapter on geology, have to refer again to this subject, and I think we
shall then see that the diagram throws light on the affinities of extinct
beings, which, though generally belonging to the same orders, families, or
genera, with those now living, yet are often, in some degree, intermediate
in character between existing groups; and we can understand this fact, for
the extinct species lived at various remote epochs when the branching
lines of descent had diverged less.

I see no reason to limit the process of modification, as now explained, to
the formation of genera alone. If, in the diagram, we suppose the amount
of change represented by each successive group of diverging dotted lines
to be great, the forms marked a14 to p14, those marked b14 and f14, and
those marked o14 to m14, will form three very distinct genera. We shall
also have two very distinct genera descended from (I), differing widely
from the descendants of (A). These two groups of genera will thus form two
distinct families, or orders, according to the amount of divergent
modification supposed to be represented in the diagram. And the two new
families, or orders, are descended from two species of the original genus;
and these are supposed to be descended from some still more ancient and
unknown form.

We have seen that in each country it is the species belonging to the
larger genera which oftenest present varieties or incipient species. This,
indeed, might have been expected; for as natural selection acts through
one form having some advantage over other forms in the struggle for
existence, it will chiefly act on those which already have some advantage;
and the largeness of any group shows that its species have inherited from
a common ancestor some advantage in common. Hence, the struggle for the
production of new and modified descendants will mainly lie between the
larger groups, which are all trying to increase in number. One large group
will slowly conquer another large group, reduce its number, and thus
lessen its chance of further variation and improvement. Within the same
large group, the later and more highly perfected sub-groups, from
branching out and seizing on many new places in the polity of nature, will
constantly tend to supplant and destroy the earlier and less improved
sub-groups. Small and broken groups and sub-groups will finally disappear.
Looking to the future, we can predict that the groups of organic beings
which are now large and triumphant, and which are least broken up, that
is, which have as yet suffered least extinction, will, for a long period,
continue to increase. But which groups will ultimately prevail, no man can
predict; for we know that many groups, formerly most extensively
developed, have now become extinct. Looking still more remotely to the
future, we may predict that, owing to the continued and steady increase of
the larger groups, a multitude of smaller groups will become utterly
extinct, and leave no modified descendants; and consequently that, of the
species living at any one period, extremely few will transmit descendants
to a remote futurity. I shall have to return to this subject in the
chapter on classification, but I may add that as, according to this view,
extremely few of the more ancient species have transmitted descendants to
the present day, and, as all the descendants of the same species form a
class, we can understand how it is that there exist so few classes in each
main division of the animal and vegetable kingdoms. Although few of the
most ancient species have left modified descendants, yet, at remote
geological periods, the earth may have been almost as well peopled with
species of many genera, families, orders and classes, as at the present
day.

ON THE DEGREE TO WHICH ORGANISATION TENDS TO ADVANCE.

Natural selection acts exclusively by the preservation and accumulation of
variations, which are beneficial under the organic and inorganic
conditions to which each creature is exposed at all periods of life. The
ultimate result is that each creature tends to become more and more
improved in relation to its conditions. This improvement inevitably leads
to the gradual advancement of the organisation of the greater number of
living beings throughout the world. But here we enter on a very intricate
subject, for naturalists have not defined to each other's satisfaction
what is meant by an advance in organisation. Among the vertebrata the
degree of intellect and an approach in structure to man clearly come into
play. It might be thought that the amount of change which the various
parts and organs pass through in their development from embryo to maturity
would suffice as a standard of comparison; but there are cases, as with
certain parasitic crustaceans, in which several parts of the structure
become less perfect, so that the mature animal cannot be called higher
than its larva. Von Baer's standard seems the most widely applicable and
the best, namely, the amount of differentiation of the parts of the same
organic being, in the adult state, as I should be inclined to add, and
their specialisation for different functions; or, as Milne Edwards would
express it, the completeness of the division of physiological labour. But
we shall see how obscure this subject is if we look, for instance, to
fishes, among which some naturalists rank those as highest which, like the
sharks, approach nearest to amphibians; while other naturalists rank the
common bony or teleostean fishes as the highest, inasmuch as they are most
strictly fish-like, and differ most from the other vertebrate classes. We
see still more plainly the obscurity of the subject by turning to plants,
among which the standard of intellect is of course quite excluded; and
here some botanists rank those plants as highest which have every organ,
as sepals, petals, stamens and pistils, fully developed in each flower;
whereas other botanists, probably with more truth, look at the plants
which have their several organs much modified and reduced in number as the
highest.

If we take as the standard of high organisation, the amount of
differentiation and specialisation of the several organs in each being
when adult (and this will include the advancement of the brain for
intellectual purposes), natural selection clearly leads towards this
standard: for all physiologists admit that the specialisation of organs,
inasmuch as in this state they perform their functions better, is an
advantage to each being; and hence the accumulation of variations tending
towards specialisation is within the scope of natural selection. On the
other hand, we can see, bearing in mind that all organic beings are
striving to increase at a high ratio and to seize on every unoccupied or
less well occupied place in the economy of nature, that it is quite
possible for natural selection gradually to fit a being to a situation in
which several organs would be superfluous or useless: in such cases there
would be retrogression in the scale of organisation. Whether organisation
on the whole has actually advanced from the remotest geological periods to
the present day will be more conveniently discussed in our chapter on
Geological Succession.

But it may be objected that if all organic beings thus tend to rise in the
scale, how is it that throughout the world a multitude of the lowest forms
still exist; and how is it that in each great class some forms are far
more highly developed than others? Why have not the more highly developed
forms every where supplanted and exterminated the lower? Lamarck, who
believed in an innate and inevitable tendency towards perfection in all
organic beings, seems to have felt this difficulty so strongly that he was
led to suppose that new and simple forms are continually being produced by
spontaneous generation. Science has not as yet proved the truth of this
belief, whatever the future may reveal. On our theory the continued
existence of lowly organisms offers no difficulty; for natural selection,
or the survival of the fittest, does not necessarily include progressive
development—it only takes advantage of such variations as arise and
are beneficial to each creature under its complex relations of life. And
it may be asked what advantage, as far as we can see, would it be to an
infusorian animalcule—to an intestinal worm—or even to an
earth-worm, to be highly organised. If it were no advantage, these forms
would be left, by natural selection, unimproved or but little improved,
and might remain for indefinite ages in their present lowly condition. And
geology tells us that some of the lowest forms, as the infusoria and
rhizopods, have remained for an enormous period in nearly their present
state. But to suppose that most of the many now existing low forms have
not in the least advanced since the first dawn of life would be extremely
rash; for every naturalist who has dissected some of the beings now ranked
as very low in the scale, must have been struck with their really wondrous
and beautiful organisation.

Nearly the same remarks are applicable, if we look to the different grades
of organisation within the same great group; for instance, in the
vertebrata, to the co-existence of mammals and fish—among mammalia,
to the co-existence of man and the ornithorhynchus—among fishes, to
the co-existence of the shark and the lancelet (Amphioxus), which latter
fish in the extreme simplicity of its structure approaches the
invertebrate classes. But mammals and fish hardly come into competition
with each other; the advancement of the whole class of mammals, or of
certain members in this class, to the highest grade would not lead to
their taking the place of fishes. Physiologists believe that the brain
must be bathed by warm blood to be highly active, and this requires aerial
respiration; so that warm-blooded mammals when inhabiting the water lie
under a disadvantage in having to come continually to the surface to
breathe. With fishes, members of the shark family would not tend to
supplant the lancelet; for the lancelet, as I hear from Fritz Muller, has
as sole companion and competitor on the barren sandy shore of South
Brazil, an anomalous annelid. The three lowest orders of mammals, namely,
marsupials, edentata, and rodents, co-exist in South America in the same
region with numerous monkeys, and probably interfere little with each
other. Although organisation, on the whole, may have advanced and be still
advancing throughout the world, yet the scale will always present many
degrees of perfection; for the high advancement of certain whole classes,
or of certain members of each class, does not at all necessarily lead to
the extinction of those groups with which they do not enter into close
competition. In some cases, as we shall hereafter see, lowly organised
forms appear to have been preserved to the present day, from inhabiting
confined or peculiar stations, where they have been subjected to less
severe competition, and where their scanty numbers have retarded the
chance of favourable variations arising.

Finally, I believe that many lowly organised forms now exist throughout
the world, from various causes. In some cases variations or individual
differences of a favourable nature may never have arisen for natural
selection to act on and accumulate. In no case, probably, has time
sufficed for the utmost possible amount of development. In some few cases
there has been what we must call retrogression or organisation. But the
main cause lies in the fact that under very simple conditions of life a
high organisation would be of no service—possibly would be of actual
disservice, as being of a more delicate nature, and more liable to be put
out of order and injured.

Looking to the first dawn of life, when all organic beings, as we may
believe, presented the simplest structure, how, it has been asked, could
the first step in the advancement or differentiation of parts have arisen?
Mr. Herbert Spencer would probably answer that, as soon as simple
unicellular organisms came by growth or division to be compounded of
several cells, or became attached to any supporting surface, his law "that
homologous units of any order become differentiated in proportion as their
relations to incident forces become different" would come into action. But
as we have no facts to guide us, speculation on the subject is almost
useless. It is, however, an error to suppose that there would be no
struggle for existence, and, consequently, no natural selection, until
many forms had been produced: variations in a single species inhabiting an
isolated station might be beneficial, and thus the whole mass of
individuals might be modified, or two distinct forms might arise. But, as
I remarked towards the close of the introduction, no one ought to feel
surprise at much remaining as yet unexplained on the origin of species, if
we make due allowance for our profound ignorance on the mutual relations
of the inhabitants of the world at the present time, and still more so
during past ages.

CONVERGENCE OF CHARACTER.

Mr. H.C. Watson thinks that I have overrated the importance of divergence
of character (in which, however, he apparently believes), and that
convergence, as it may be called, has likewise played a part. If two
species belonging to two distinct though allied genera, had both produced
a large number of new and divergent forms, it is conceivable that these
might approach each other so closely that they would have all to be
classed under the same genus; and thus the descendants of two distinct
genera would converge into one. But it would in most cases be extremely
rash to attribute to convergence a close and general similarity of
structure in the modified descendants of widely distinct forms. The shape
of a crystal is determined solely by the molecular forces, and it is not
surprising that dissimilar substances should sometimes assume the same
form; but with organic beings we should bear in mind that the form of each
depends on an infinitude of complex relations, namely on the variations
which have arisen, these being due to causes far too intricate to be
followed out—on the nature of the variations which have been
preserved or selected, and this depends on the surrounding physical
conditions, and in a still higher degree on the surrounding organisms with
which each being has come into competition—and lastly, on
inheritance (in itself a fluctuating element) from innumerable
progenitors, all of which have had their forms determined through equally
complex relations. It is incredible that the descendants of two organisms,
which had originally differed in a marked manner, should ever afterwards
converge so closely as to lead to a near approach to identity throughout
their whole organisation. If this had occurred, we should meet with the
same form, independently of genetic connection, recurring in widely
separated geological formations; and the balance of evidence is opposed to
any such an admission.

Mr. Watson has also objected that the continued action of natural
selection, together with divergence of character, would tend to make an
indefinite number of specific forms. As far as mere inorganic conditions
are concerned, it seems probable that a sufficient number of species would
soon become adapted to all considerable diversities of heat, moisture,
etc.; but I fully admit that the mutual relations of organic beings are
more important; and as the number of species in any country goes on
increasing, the organic conditions of life must become more and more
complex. Consequently there seems at first no limit to the amount of
profitable diversification of structure, and therefore no limit to the
number of species which might be produced. We do not know that even the
most prolific area is fully stocked with specific forms: at the Cape of
Good Hope and in Australia, which support such an astonishing number of
species, many European plants have become naturalised. But geology shows
us, that from an early part of the tertiary period the number of species
of shells, and that from the middle part of this same period, the number
of mammals has not greatly or at all increased. What then checks an
indefinite increase in the number of species? The amount of life (I do not
mean the number of specific forms) supported on an area must have a limit,
depending so largely as it does on physical conditions; therefore, if an
area be inhabited by very many species, each or nearly each species will
be represented by few individuals; and such species will be liable to
extermination from accidental fluctuations in the nature of the seasons or
in the number of their enemies. The process of extermination in such cases
would be rapid, whereas the production of new species must always be slow.
Imagine the extreme case of as many species as individuals in England, and
the first severe winter or very dry summer would exterminate thousands on
thousands of species. Rare species, and each species will become rare if
the number of species in any country becomes indefinitely increased, will,
on the principal often explained, present within a given period few
favourable variations; consequently, the process of giving birth to new
specific forms would thus be retarded. When any species becomes very rare,
close interbreeding will help to exterminate it; authors have thought that
this comes into play in accounting for the deterioration of the aurochs in
Lithuania, of red deer in Scotland and of bears in Norway, etc. Lastly,
and this I am inclined to think is the most important element, a dominant
species, which has already beaten many competitors in its own home, will
tend to spread and supplant many others. Alph. de Candolle has shown that
those species which spread widely tend generally to spread VERY widely,
consequently they will tend to supplant and exterminate several species in
several areas, and thus check the inordinate increase of specific forms
throughout the world. Dr. Hooker has recently shown that in the southeast
corner of Australia, where, apparently, there are many invaders from
different quarters of the globe, the endemic Australian species have been
greatly reduced in number. How much weight to attribute to these several
considerations I will not pretend to say; but conjointly they must limit
in each country the tendency to an indefinite augmentation of specific
forms.

SUMMARY OF CHAPTER.

If under changing conditions of life organic beings present individual
differences in almost every part of their structure, and this cannot be
disputed; if there be, owing to their geometrical rate of increase, a
severe struggle for life at some age, season or year, and this certainly
cannot be disputed; then, considering the infinite complexity of the
relations of all organic beings to each other and to their conditions of
life, causing an infinite diversity in structure, constitution, and
habits, to be advantageous to them, it would be a most extraordinary fact
if no variations had ever occurred useful to each being's own welfare, in
the same manner as so many variations have occurred useful to man. But if
variations useful to any organic being ever do occur, assuredly
individuals thus characterised will have the best chance of being
preserved in the struggle for life; and from the strong principle of
inheritance, these will tend to produce offspring similarly characterised.
This principle of preservation, or the survival of the fittest, I have
called natural selection. It leads to the improvement of each creature in
relation to its organic and inorganic conditions of life; and
consequently, in most cases, to what must be regarded as an advance in
organisation. Nevertheless, low and simple forms will long endure if well
fitted for their simple conditions of life.

Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young as easily as the
adult. Among many animals sexual selection will have given its aid to
ordinary selection by assuring to the most vigorous and best adapted males
the greatest number of offspring. Sexual selection will also give
characters useful to the males alone in their struggles or rivalry with
other males; and these characters will be transmitted to one sex or to
both sexes, according to the form of inheritance which prevails.

Whether natural selection has really thus acted in adapting the various
forms of life to their several conditions and stations, must be judged by
the general tenour and balance of evidence given in the following
chapters. But we have already seen how it entails extinction; and how
largely extinction has acted in the world's history, geology plainly
declares. Natural selection, also, leads to divergence of character; for
the more organic beings diverge in structure, habits and constitution, by
so much the more can a large number be supported on the area, of which we
see proof by looking to the inhabitants of any small spot, and to the
productions naturalised in foreign lands. Therefore, during the
modification of the descendants of any one species, and during the
incessant struggle of all species to increase in numbers, the more
diversified the descendants become, the better will be their chance of
success in the battle for life. Thus the small differences distinguishing
varieties of the same species, steadily tend to increase, till they equal
the greater differences between species of the same genus, or even of
distinct genera.

We have seen that it is the common, the widely diffused, and widely
ranging species, belonging to the larger genera within each class, which
vary most; and these tend to transmit to their modified offspring that
superiority which now makes them dominant in their own countries. Natural
selection, as has just been remarked, leads to divergence of character and
to much extinction of the less improved and intermediate forms of life. On
these principles, the nature of the affinities, and the generally well
defined distinctions between the innumerable organic beings in each class
throughout the world, may be explained. It is a truly wonderful fact—the
wonder of which we are apt to overlook from familiarity—that all
animals and all plants throughout all time and space should be related to
each other in groups, subordinate to groups, in the manner which we
everywhere behold—namely, varieties of the same species most closely
related, species of the same genus less closely and unequally related,
forming sections and sub-genera, species of distinct genera much less
closely related, and genera related in different degrees, forming
sub-families, families, orders, sub-classes, and classes. The several
subordinate groups in any class cannot be ranked in a single file, but
seem clustered round points, and these round other points, and so on in
almost endless cycles. If species had been independently created, no
explanation would have been possible of this kind of classification; but
it is explained through inheritance and the complex action of natural
selection, entailing extinction and divergence of character, as we have
seen illustrated in the diagram.

The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the
truth. The green and budding twigs may represent existing species; and
those produced during former years may represent the long succession of
extinct species. At each period of growth all the growing twigs have tried
to branch out on all sides, and to overtop and kill the surrounding twigs
and branches, in the same manner as species and groups of species have at
all times overmastered other species in the great battle for life. The
limbs divided into great branches, and these into lesser and lesser
branches, were themselves once, when the tree was young, budding twigs;
and this connexion of the former and present buds by ramifying branches
may well represent the classification of all extinct and living species in
groups subordinate to groups. Of the many twigs which flourished when the
tree was a mere bush, only two or three, now grown into great branches,
yet survive and bear the other branches; so with the species which lived
during long-past geological periods, very few have left living and
modified descendants. From the first growth of the tree, many a limb and
branch has decayed and dropped off; and these fallen branches of various
sizes may represent those whole orders, families, and genera which have
now no living representatives, and which are known to us only in a fossil
state. As we here and there see a thin, straggling branch springing from a
fork low down in a tree, and which by some chance has been favoured and is
still alive on its summit, so we occasionally see an animal like the
Ornithorhynchus or Lepidosiren, which in some small degree connects by its
affinities two large branches of life, and which has apparently been saved
from fatal competition by having inhabited a protected station. As buds
give rise by growth to fresh buds, and these, if vigorous, branch out and
overtop on all sides many a feebler branch, so by generation I believe it
has been with the great Tree of Life, which fills with its dead and broken
branches the crust of the earth, and covers the surface with its
ever-branching and beautiful ramifications.



CHAPTER V. LAWS OF VARIATION.


 Effects of changed conditions—Use and disuse, combined with natural
 selection; organs of flight and of vision—Acclimatisation—Correlated
 variation—Compensation and economy of growth—False
 correlations—Multiple, rudimentary, and lowly organised structures
 variable—Parts developed in an unusual manner are highly variable:
 specific characters more variable than generic: secondary sexual
 characters variable—Species of the same genus vary in an analogous
 manner—Reversions to long-lost characters—Summary.

I have hitherto sometimes spoken as if the variations—so common and
multiform with organic beings under domestication, and in a lesser degree
with those under nature—were due to chance. This, of course is a
wholly incorrect expression, but it serves to acknowledge plainly our
ignorance of the cause of each particular variation. Some authors believe
it to be as much the function of the reproductive system to produce
individual differences, or slight deviations of structure, as to make the
child like its parents. But the fact of variations and monstrosities
occurring much more frequently under domestication than under nature, and
the greater variability of species having wide ranges than of those with
restricted ranges, lead to the conclusion that variability is generally
related to the conditions of life to which each species has been exposed
during several successive generations. In the first chapter I attempted to
show that changed conditions act in two ways, directly on the whole
organisation or on certain parts alone, and indirectly through the
reproductive system. In all cases there are two factors, the nature of the
organism, which is much the most important of the two, and the nature of
the conditions. The direct action of changed conditions leads to definite
or indefinite results. In the latter case the organisation seems to become
plastic, and we have much fluctuating variability. In the former case the
nature of the organism is such that it yields readily, when subjected to
certain conditions, and all, or nearly all, the individuals become
modified in the same way.

It is very difficult to decide how far changed conditions, such as of
climate, food, etc., have acted in a definite manner. There is reason to
believe that in the course of time the effects have been greater than can
be proved by clear evidence. But we may safely conclude that the
innumerable complex co-adaptations of structure, which we see throughout
nature between various organic beings, cannot be attributed simply to such
action. In the following cases the conditions seem to have produced some
slight definite effect: E. Forbes asserts that shells at their southern
limit, and when living in shallow water, are more brightly coloured than
those of the same species from further north or from a greater depth; but
this certainly does not always hold good. Mr. Gould believes that birds of
the same species are more brightly coloured under a clear atmosphere, than
when living near the coast or on islands; and Wollaston is convinced that
residence near the sea affects the colours of insects. Moquin-Tandon gives
a list of plants which, when growing near the sea-shore, have their leaves
in some degree fleshy, though not elsewhere fleshy. These slightly varying
organisms are interesting in as far as they present characters analogous
to those possessed by the species which are confined to similar
conditions.

When a variation is of the slightest use to any being, we cannot tell how
much to attribute to the accumulative action of natural selection, and how
much to the definite action of the conditions of life. Thus, it is well
known to furriers that animals of the same species have thicker and better
fur the further north they live; but who can tell how much of this
difference may be due to the warmest-clad individuals having been favoured
and preserved during many generations, and how much to the action of the
severe climate? For it would appear that climate has some direct action on
the hair of our domestic quadrupeds.

Instances could be given of similar varieties being produced from the same
species under external conditions of life as different as can well be
conceived; and, on the other hand, of dissimilar varieties being produced
under apparently the same external conditions. Again, innumerable
instances are known to every naturalist, of species keeping true, or not
varying at all, although living under the most opposite climates. Such
considerations as these incline me to lay less weight on the direct action
of the surrounding conditions, than on a tendency to vary, due to causes
of which we are quite ignorant.

In one sense the conditions of life may be said, not only to cause
variability, either directly or indirectly, but likewise to include
natural selection, for the conditions determine whether this or that
variety shall survive. But when man is the selecting agent, we clearly see
that the two elements of change are distinct; variability is in some
manner excited, but it is the will of man which accumulates the variations
in certain direction; and it is this latter agency which answers to the
survival of the fittest under nature.

EFFECTS OF THE INCREASED USE AND DISUSE OF PARTS, AS CONTROLLED BY NATURAL
SELECTION.

From the facts alluded to in the first chapter, I think there can be no
doubt that use in our domestic animals has strengthened and enlarged
certain parts, and disuse diminished them; and that such modifications are
inherited. Under free nature we have no standard of comparison by which to
judge of the effects of long-continued use or disuse, for we know not the
parent-forms; but many animals possess structures which can be best
explained by the effects of disuse. As Professor Owen has remarked, there
is no greater anomaly in nature than a bird that cannot fly; yet there are
several in this state. The logger-headed duck of South America can only
flap along the surface of the water, and has its wings in nearly the same
condition as the domestic Aylesbury duck: it is a remarkable fact that the
young birds, according to Mr. Cunningham, can fly, while the adults have
lost this power. As the larger ground-feeding birds seldom take flight
except to escape danger, it is probable that the nearly wingless condition
of several birds, now inhabiting or which lately inhabited several oceanic
islands, tenanted by no beasts of prey, has been caused by disuse. The
ostrich indeed inhabits continents, and is exposed to danger from which it
cannot escape by flight, but it can defend itself, by kicking its enemies,
as efficiently as many quadrupeds. We may believe that the progenitor of
the ostrich genus had habits like those of the bustard, and that, as the
size and weight of its body were increased during successive generations,
its legs were used more and its wings less, until they became incapable of
flight.

Kirby has remarked (and I have observed the same fact) that the anterior
tarsi, or feet, of many male dung-feeding beetles are often broken off; he
examined seventeen specimens in his own collection, and not one had even a
relic left. In the Onites apelles the tarsi are so habitually lost that
the insect has been described as not having them. In some other genera
they are present, but in a rudimentary condition. In the Ateuchus or
sacred beetle of the Egyptians, they are totally deficient. The evidence
that accidental mutilations can be inherited is at present not decisive;
but the remarkable cases observed by Brown-Sequard in guinea-pigs, of the
inherited effects of operations, should make us cautious in denying this
tendency. Hence, it will perhaps be safest to look at the entire absence
of the anterior tarsi in Ateuchus, and their rudimentary condition in some
other genera, not as cases of inherited mutilations, but as due to the
effects of long-continued disuse; for as many dung-feeding beetles are
generally found with their tarsi lost, this must happen early in life;
therefore the tarsi cannot be of much importance or be much used by these
insects.

In some cases we might easily put down to disuse modifications of
structure which are wholly, or mainly due to natural selection. Mr.
Wollaston has discovered the remarkable fact that 200 beetles, out of the
550 species (but more are now known) inhabiting Madeira, are so far
deficient in wings that they cannot fly; and that, of the twenty-nine
endemic genera, no less than twenty-three have all their species in this
condition! Several facts, namely, that beetles in many parts of the world
are very frequently blown to sea and perish; that the beetles in Madeira,
as observed by Mr. Wollaston, lie much concealed, until the wind lulls and
the sun shines; that the proportion of wingless beetles is larger on the
exposed Desertas than in Madeira itself; and especially the extraordinary
fact, so strongly insisted on by Mr. Wollaston, that certain large groups
of beetles, elsewhere excessively numerous, which absolutely require the
use of their wings, are here almost entirely absent. These several
considerations make me believe that the wingless condition of so many
Madeira beetles is mainly due to the action of natural selection, combined
probably with disuse. For during many successive generations each
individual beetle which flew least, either from its wings having been ever
so little less perfectly developed or from indolent habit, will have had
the best chance of surviving from not being blown out to sea; and, on the
other hand, those beetles which most readily took to flight would oftenest
have been blown to sea, and thus destroyed.

The insects in Madeira which are not ground-feeders, and which, as certain
flower-feeding coleoptera and lepidoptera, must habitually use their wings
to gain their subsistence, have, as Mr. Wollaston suspects, their wings
not at all reduced, but even enlarged. This is quite compatible with the
action of natural selection. For when a new insect first arrived on the
island, the tendency of natural selection to enlarge or to reduce the
wings, would depend on whether a greater number of individuals were saved
by successfully battling with the winds, or by giving up the attempt and
rarely or never flying. As with mariners shipwrecked near a coast, it
would have been better for the good swimmers if they had been able to swim
still further, whereas it would have been better for the bad swimmers if
they had not been able to swim at all and had stuck to the wreck.

The eyes of moles and of some burrowing rodents are rudimentary in size,
and in some cases are quite covered by skin and fur. This state of the
eyes is probably due to gradual reduction from disuse, but aided perhaps
by natural selection. In South America, a burrowing rodent, the tuco-tuco,
or Ctenomys, is even more subterranean in its habits than the mole; and I
was assured by a Spaniard, who had often caught them, that they were
frequently blind. One which I kept alive was certainly in this condition,
the cause, as appeared on dissection, having been inflammation of the
nictitating membrane. As frequent inflammation of the eyes must be
injurious to any animal, and as eyes are certainly not necessary to
animals having subterranean habits, a reduction in their size, with the
adhesion of the eyelids and growth of fur over them, might in such case be
an advantage; and if so, natural selection would aid the effects of
disuse.

It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Carniola and Kentucky, are blind. In
some of the crabs the foot-stalk for the eye remains, though the eye is
gone; the stand for the telescope is there, though the telescope with its
glasses has been lost. As it is difficult to imagine that eyes, though
useless, could be in any way injurious to animals living in darkness,
their loss may be attributed to disuse. In one of the blind animals,
namely, the cave-rat (Neotoma), two of which were captured by Professor
Silliman at above half a mile distance from the mouth of the cave, and
therefore not in the profoundest depths, the eyes were lustrous and of
large size; and these animals, as I am informed by Professor Silliman,
after having been exposed for about a month to a graduated light, acquired
a dim perception of objects.

It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that, in accordance
with the old view of the blind animals having been separately created for
the American and European caverns, very close similarity in their
organisation and affinities might have been expected. This is certainly
not the case if we look at the two whole faunas; with respect to the
insects alone, Schiodte has remarked: "We are accordingly prevented from
considering the entire phenomenon in any other light than something purely
local, and the similarity which is exhibited in a few forms between the
Mammoth Cave (in Kentucky) and the caves in Carniola, otherwise than as a
very plain expression of that analogy which subsists generally between the
fauna of Europe and of North America." On my view we must suppose that
American animals, having in most cases ordinary powers of vision, slowly
migrated by successive generations from the outer world into the deeper
and deeper recesses of the Kentucky caves, as did European animals into
the caves of Europe. We have some evidence of this gradation of habit;
for, as Schiodte remarks: "We accordingly look upon the subterranean
faunas as small ramifications which have penetrated into the earth from
the geographically limited faunas of the adjacent tracts, and which, as
they extended themselves into darkness, have been accommodated to
surrounding circumstances. Animals not far remote from ordinary forms,
prepare the transition from light to darkness. Next follow those that are
constructed for twilight; and, last of all, those destined for total
darkness, and whose formation is quite peculiar." These remarks of
Schiodte's it should be understood, apply not to the same, but to distinct
species. By the time that an animal had reached, after numberless
generations, the deepest recesses, disuse will on this view have more or
less perfectly obliterated its eyes, and natural selection will often have
effected other changes, such as an increase in the length of the antennae
or palpi, as a compensation for blindness. Notwithstanding such
modifications, we might expect still to see in the cave-animals of
America, affinities to the other inhabitants of that continent, and in
those of Europe to the inhabitants of the European continent. And this is
the case with some of the American cave-animals, as I hear from Professor
Dana; and some of the European cave-insects are very closely allied to
those of the surrounding country. It would be difficult to give any
rational explanation of the affinities of the blind cave-animals to the
other inhabitants of the two continents on the ordinary view of their
independent creation. That several of the inhabitants of the caves of the
Old and New Worlds should be closely related, we might expect from the
well-known relationship of most of their other productions. As a blind
species of Bathyscia is found in abundance on shady rocks far from caves,
the loss of vision in the cave species of this one genus has probably had
no relation to its dark habitation; for it is natural that an insect
already deprived of vision should readily become adapted to dark caverns.
Another blind genus (Anophthalmus) offers this remarkable peculiarity,
that the species, as Mr. Murray observes, have not as yet been found
anywhere except in caves; yet those which inhabit the several caves of
Europe and America are distinct; but it is possible that the progenitors
of these several species, while they were furnished with eyes, may
formerly have ranged over both continents, and then have become extinct,
excepting in their present secluded abodes. Far from feeling surprise that
some of the cave-animals should be very anomalous, as Agassiz has remarked
in regard to the blind fish, the Amblyopsis, and as is the case with the
blind Proteus, with reference to the reptiles of Europe, I am only
surprised that more wrecks of ancient life have not been preserved, owing
to the less severe competition to which the scanty inhabitants of these
dark abodes will have been exposed.

ACCLIMATISATION.

Habit is hereditary with plants, as in the period of flowering, in the
time of sleep, in the amount of rain requisite for seeds to germinate,
etc., and this leads me to say a few words on acclimatisation. As it is
extremely common for distinct species belonging to the same genus to
inhabit hot and cold countries, if it be true that all the species of the
same genus are descended from a single parent-form, acclimatisation must
be readily effected during a long course of descent. It is notorious that
each species is adapted to the climate of its own home: species from an
arctic or even from a temperate region cannot endure a tropical climate,
or conversely. So again, many succulent plants cannot endure a damp
climate. But the degree of adaptation of species to the climates under
which they live is often overrated. We may infer this from our frequent
inability to predict whether or not an imported plant will endure our
climate, and from the number of plants and animals brought from different
countries which are here perfectly healthy. We have reason to believe that
species in a state of nature are closely limited in their ranges by the
competition of other organic beings quite as much as, or more than, by
adaptation to particular climates. But whether or not this adaptation is
in most cases very close, we have evidence with some few plants, of their
becoming, to a certain extent, naturally habituated to different
temperatures; that is, they become acclimatised: thus the pines and
rhododendrons, raised from seed collected by Dr. Hooker from the same
species growing at different heights on the Himalayas, were found to
possess in this country different constitutional powers of resisting cold.
Mr. Thwaites informs me that he has observed similar facts in Ceylon;
analogous observations have been made by Mr. H.C. Watson on European
species of plants brought from the Azores to England; and I could give
other cases. In regard to animals, several authentic instances could be
adduced of species having largely extended, within historical times, their
range from warmer to colder latitudes, and conversely; but we do not
positively know that these animals were strictly adapted to their native
climate, though in all ordinary cases we assume such to be the case; nor
do we know that they have subsequently become specially acclimatised to
their new homes, so as to be better fitted for them than they were at
first.

As we may infer that our domestic animals were originally chosen by
uncivilised man because they were useful, and because they bred readily
under confinement, and not because they were subsequently found capable of
far-extended transportation, the common and extraordinary capacity in our
domestic animals of not only withstanding the most different climates, but
of being perfectly fertile (a far severer test) under them, may be used as
an argument that a large proportion of other animals now in a state of
nature could easily be brought to bear widely different climates. We must
not, however, push the foregoing argument too far, on account of the
probable origin of some of our domestic animals from several wild stocks:
the blood, for instance, of a tropical and arctic wolf may perhaps be
mingled in our domestic breeds. The rat and mouse cannot be considered as
domestic animals, but they have been transported by man to many parts of
the world, and now have a far wider range than any other rodent; for they
live under the cold climate of Faroe in the north and of the Falklands in
the south, and on many an island in the torrid zones. Hence adaptation to
any special climate may be looked at as a quality readily grafted on an
innate wide flexibility of constitution, common to most animals. On this
view, the capacity of enduring the most different climates by man himself
and by his domestic animals, and the fact of the extinct elephant and
rhinoceros having formerly endured a glacial climate, whereas the living
species are now all tropical or sub-tropical in their habits, ought not to
be looked at as anomalies, but as examples of a very common flexibility of
constitution, brought, under peculiar circumstances, into action.

How much of the acclimatisation of species to any peculiar climate is due
to mere habit, and how much to the natural selection of varieties having
different innate constitutions, and how much to both means combined, is an
obscure question. That habit or custom has some influence, I must believe,
both from analogy and from the incessant advice given in agricultural
works, even in the ancient Encyclopaedias of China, to be very cautious in
transporting animals from one district to another. And as it is not likely
that man should have succeeded in selecting so many breeds and sub-breeds
with constitutions specially fitted for their own districts, the result
must, I think, be due to habit. On the other hand, natural selection would
inevitably tend to preserve those individuals which were born with
constitutions best adapted to any country which they inhabited. In
treatises on many kinds of cultivated plants, certain varieties are said
to withstand certain climates better than others; this is strikingly shown
in works on fruit-trees published in the United States, in which certain
varieties are habitually recommended for the northern and others for the
southern states; and as most of these varieties are of recent origin, they
cannot owe their constitutional differences to habit. The case of the
Jerusalem artichoke, which is never propagated in England by seed, and of
which, consequently, new varieties have not been produced, has even been
advanced, as proving that acclimatisation cannot be effected, for it is
now as tender as ever it was! The case, also, of the kidney-bean has been
often cited for a similar purpose, and with much greater weight; but until
some one will sow, during a score of generations, his kidney-beans so
early that a very large proportion are destroyed by frost, and then
collect seed from the few survivors, with care to prevent accidental
crosses, and then again get seed from these seedlings, with the same
precautions, the experiment cannot be said to have been even tried. Nor
let it be supposed that differences in the constitution of seedling
kidney-beans never appear, for an account has been published how much more
hardy some seedlings are than others; and of this fact I have myself
observed striking instances.

On the whole, we may conclude that habit, or use and disuse, have, in some
cases, played a considerable part in the modification of the constitution
and structure; but that the effects have often been largely combined with,
and sometimes overmastered by, the natural selection of innate variations.

CORRELATED VARIATION.

I mean by this expression that the whole organisation is so tied together,
during its growth and development, that when slight variations in any one
part occur and are accumulated through natural selection, other parts
become modified. This is a very important subject, most imperfectly
understood, and no doubt wholly different classes of facts may be here
easily confounded together. We shall presently see that simple inheritance
often gives the false appearance of correlation. One of the most obvious
real cases is, that variations of structure arising in the young or larvae
naturally tend to affect the structure of the mature animal. The several
parts which are homologous, and which, at an early embryonic period, are
identical in structure, and which are necessarily exposed to similar
conditions, seem eminently liable to vary in a like manner: we see this in
the right and left sides of the body varying in the same manner; in the
front and hind legs, and even in the jaws and limbs, varying together, for
the lower jaw is believed by some anatomists to be homologous with the
limbs. These tendencies, I do not doubt, may be mastered more or less
completely by natural selection: thus a family of stags once existed with
an antler only on one side; and if this had been of any great use to the
breed, it might probably have been rendered permanent by natural
selection.

Homologous parts, as has been remarked by some authors, tend to cohere;
this is often seen in monstrous plants: and nothing is more common than
the union of homologous parts in normal structures, as in the union of the
petals into a tube. Hard parts seem to affect the form of adjoining soft
parts; it is believed by some authors that with birds the diversity in the
shape of the pelvis causes the remarkable diversity in the shape of the
kidneys. Others believe that the shape of the pelvis in the human mother
influences by pressure the shape of the head of the child. In snakes,
according to Schlegel, the shape of the body and the manner of swallowing
determine the position and form of several of the most important viscera.

The nature of the bond is frequently quite obscure. M. Is. Geoffroy St.
Hilaire has forcibly remarked that certain malconformations frequently,
and that others rarely, coexist without our being able to assign any
reason. What can be more singular than the relation in cats between
complete whiteness and blue eyes with deafness, or between the
tortoise-shell colour and the female sex; or in pigeons, between their
feathered feet and skin betwixt the outer toes, or between the presence of
more or less down on the young pigeon when first hatched, with the future
colour of its plumage; or, again, the relation between the hair and the
teeth in the naked Turkish dog, though here no doubt homology comes into
play? With respect to this latter case of correlation, I think it can
hardly be accidental that the two orders of mammals which are most
abnormal in their dermal covering, viz., Cetacea (whales) and Edentata
(armadilloes, scaly ant-eaters, etc.), are likewise on the whole the most
abnormal in their teeth, but there are so many exceptions to this rule, as
Mr. Mivart has remarked, that it has little value.

I know of no case better adapted to show the importance of the laws of
correlation and variation, independently of utility, and therefore of
natural selection, than that of the difference between the outer and inner
flowers in some Compositous and Umbelliferous plants. Everyone is familiar
with the difference between the ray and central florets of, for instance,
the daisy, and this difference is often accompanied with the partial or
complete abortion of the reproductive organs. But in some of these plants
the seeds also differ in shape and sculpture. These differences have
sometimes been attributed to the pressure of the involucra on the florets,
or to their mutual pressure, and the shape of the seeds in the ray-florets
of some Compositae countenances this idea; but with the Umbelliferae it is
by no means, as Dr. Hooker informs me, the species with the densest heads
which most frequently differ in their inner and outer flowers. It might
have been thought that the development of the ray-petals, by drawing
nourishment from the reproductive organs causes their abortion; but this
can hardly be the sole case, for in some Compositae the seeds of the outer
and inner florets differ, without any difference in the corolla. Possibly
these several differences may be connected with the different flow of
nutriment towards the central and external flowers. We know, at least,
that with irregular flowers those nearest to the axis are most subject to
peloria, that is to become abnormally symmetrical. I may add, as an
instance of this fact, and as a striking case of correlation, that in many
pelargoniums the two upper petals in the central flower of the truss often
lose their patches of darker colour; and when this occurs, the adherent
nectary is quite aborted, the central flower thus becoming peloric or
regular. When the colour is absent from only one of the two upper petals,
the nectary is not quite aborted but is much shortened.

With respect to the development of the corolla, Sprengel's idea that the
ray-florets serve to attract insects, whose agency is highly advantageous,
or necessary for the fertilisation of these plants, is highly probable;
and if so, natural selection may have come into play. But with respect to
the seeds, it seems impossible that their differences in shape, which are
not always correlated with any difference in the corolla, can be in any
way beneficial; yet in the Umbelliferae these differences are of such
apparent importance—the seeds being sometimes orthospermous in the
exterior flowers and coelospermous in the central flowers—that the
elder De Candolle founded his main divisions in the order on such
characters. Hence modifications of structure, viewed by systematists as of
high value, may be wholly due to the laws of variation and correlation,
without being, as far as we can judge, of the slightest service to the
species.

We may often falsely attribute to correlated variation structures which
are common to whole groups of species, and which in truth are simply due
to inheritance; for an ancient progenitor may have acquired through
natural selection some one modification in structure, and, after thousands
of generations, some other and independent modification; and these two
modifications, having been transmitted to a whole group of descendants
with diverse habits, would naturally be thought to be in some necessary
manner correlated. Some other correlations are apparently due to the
manner in which natural selection can alone act. For instance, Alph. De
Candolle has remarked that winged seeds are never found in fruits which do
not open; I should explain this rule by the impossibility of seeds
gradually becoming winged through natural selection, unless the capsules
were open; for in this case alone could the seeds, which were a little
better adapted to be wafted by the wind, gain an advantage over others
less well fitted for wide dispersal.

COMPENSATION AND ECONOMY OF GROWTH.

The elder Geoffroy and Goethe propounded, at about the same time, their
law of compensation or balancement of growth; or, as Goethe expressed it,
"in order to spend on one side, nature is forced to economise on the other
side." I think this holds true to a certain extent with our domestic
productions: if nourishment flows to one part or organ in excess, it
rarely flows, at least in excess, to another part; thus it is difficult to
get a cow to give much milk and to fatten readily. The same varieties of
the cabbage do not yield abundant and nutritious foliage and a copious
supply of oil-bearing seeds. When the seeds in our fruits become
atrophied, the fruit itself gains largely in size and quality. In our
poultry, a large tuft of feathers on the head is generally accompanied by
a diminished comb, and a large beard by diminished wattles. With species
in a state of nature it can hardly be maintained that the law is of
universal application; but many good observers, more especially botanists,
believe in its truth. I will not, however, here give any instances, for I
see hardly any way of distinguishing between the effects, on the one hand,
of a part being largely developed through natural selection and another
and adjoining part being reduced by the same process or by disuse, and, on
the other hand, the actual withdrawal of nutriment from one part owing to
the excess of growth in another and adjoining part.

I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more
general principle, namely, that natural selection is continually trying to
economise in every part of the organisation. If under changed conditions
of life a structure, before useful, becomes less useful, its diminution
will be favoured, for it will profit the individual not to have its
nutriment wasted in building up a useless structure. I can thus only
understand a fact with which I was much struck when examining cirripedes,
and of which many other instances could be given: namely, that when a
cirripede is parasitic within another cirripede and is thus protected, it
loses more or less completely its own shell or carapace. This is the case
with the male Ibla, and in a truly extraordinary manner with the
Proteolepas: for the carapace in all other cirripedes consists of the
three highly important anterior segments of the head enormously developed,
and furnished with great nerves and muscles; but in the parasitic and
protected Proteolepas, the whole anterior part of the head is reduced to
the merest rudiment attached to the bases of the prehensile antennae. Now
the saving of a large and complex structure, when rendered superfluous,
would be a decided advantage to each successive individual of the species;
for in the struggle for life to which every animal is exposed, each would
have a better chance of supporting itself, by less nutriment being wasted.

Thus, as I believe, natural selection will tend in the long run to reduce
any part of the organisation, as soon as it becomes, through changed
habits, superfluous, without by any means causing some other part to be
largely developed in a corresponding degree. And conversely, that natural
selection may perfectly well succeed in largely developing an organ
without requiring as a necessary compensation the reduction of some
adjoining part.

MULTIPLE, RUDIMENTARY, AND LOWLY-ORGANISED STRUCTURES ARE VARIABLE.

It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both with
varieties and species, that when any part or organ is repeated many times
in the same individual (as the vertebrae in snakes, and the stamens in
polyandrous flowers) the number is variable; whereas the number of the
same part or organ, when it occurs in lesser numbers, is constant. The
same author as well as some botanists, have further remarked that multiple
parts are extremely liable to vary in structure. As "vegetative
repetition," to use Professor Owen's expression, is a sign of low
organisation; the foregoing statements accord with the common opinion of
naturalists, that beings which stand low in the scale of nature are more
variable than those which are higher. I presume that lowness here means
that the several parts of the organisation have been but little
specialised for particular functions; and as long as the same part has to
perform diversified work, we can perhaps see why it should remain
variable, that is, why natural selection should not have preserved or
rejected each little deviation of form so carefully as when the part has
to serve for some one special purpose. In the same way that a knife which
has to cut all sorts of things may be of almost any shape; whilst a tool
for some particular purpose must be of some particular shape. Natural
selection, it should never be forgotten, can act solely through and for
the advantage of each being.

Rudimentary parts, as is generally admitted, are apt to be highly
variable. We shall have to recur to this subject; and I will here only add
that their variability seems to result from their uselessness, and
consequently from natural selection having had no power to check
deviations in their structure.

A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER, IN
COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY
VARIABLE.

Several years ago I was much struck by a remark to the above effect made
by Mr. Waterhouse. Professor Owen, also, seems to have come to a nearly
similar conclusion. It is hopeless to attempt to convince any one of the
truth of the above proposition without giving the long array of facts
which I have collected, and which cannot possibly be here introduced. I
can only state my conviction that it is a rule of high generality. I am
aware of several causes of error, but I hope that I have made due
allowances for them. It should be understood that the rule by no means
applies to any part, however unusually developed, unless it be unusually
developed in one species or in a few species in comparison with the same
part in many closely allied species. Thus, the wing of the bat is a most
abnormal structure in the class of mammals; but the rule would not apply
here, because the whole group of bats possesses wings; it would apply only
if some one species had wings developed in a remarkable manner in
comparison with the other species of the same genus. The rule applies very
strongly in the case of secondary sexual characters, when displayed in any
unusual manner. The term, secondary sexual characters, used by Hunter,
relates to characters which are attached to one sex, but are not directly
connected with the act of reproduction. The rule applies to males and
females; but more rarely to females, as they seldom offer remarkable
secondary sexual characters. The rule being so plainly applicable in the
case of secondary sexual characters, may be due to the great variability
of these characters, whether or not displayed in any unusual manner—of
which fact I think there can be little doubt. But that our rule is not
confined to secondary sexual characters is clearly shown in the case of
hermaphrodite cirripedes; I particularly attended to Mr. Waterhouse's
remark, whilst investigating this order, and I am fully convinced that the
rule almost always holds good. I shall, in a future work, give a list of
all the more remarkable cases. I will here give only one, as it
illustrates the rule in its largest application. The opercular valves of
sessile cirripedes (rock barnacles) are, in every sense of the word, very
important structures, and they differ extremely little even in distinct
genera; but in the several species of one genus, Pyrgoma, these valves
present a marvellous amount of diversification; the homologous valves in
the different species being sometimes wholly unlike in shape; and the
amount of variation in the individuals of the same species is so great
that it is no exaggeration to state that the varieties of the same species
differ more from each other in the characters derived from these important
organs, than do the species belonging to other distinct genera.

As with birds the individuals of the same species, inhabiting the same
country, vary extremely little, I have particularly attended to them; and
the rule certainly seems to hold good in this class. I cannot make out
that it applies to plants, and this would have seriously shaken my belief
in its truth, had not the great variability in plants made it particularly
difficult to compare their relative degrees of variability.

When we see any part or organ developed in a remarkable degree or manner
in a species, the fair presumption is that it is of high importance to
that species: nevertheless it is in this case eminently liable to
variation. Why should this be so? On the view that each species has been
independently created, with all its parts as we now see them, I can see no
explanation. But on the view that groups of species are descended from
some other species, and have been modified through natural selection, I
think we can obtain some light. First let me make some preliminary
remarks. If, in our domestic animals, any part or the whole animal be
neglected, and no selection be applied, that part (for instance, the comb
in the Dorking fowl) or the whole breed will cease to have a uniform
character: and the breed may be said to be degenerating. In rudimentary
organs, and in those which have been but little specialised for any
particular purpose, and perhaps in polymorphic groups, we see a nearly
parallel case; for in such cases natural selection either has not or
cannot come into full play, and thus the organisation is left in a
fluctuating condition. But what here more particularly concerns us is,
that those points in our domestic animals, which at the present time are
undergoing rapid change by continued selection, are also eminently liable
to variation. Look at the individuals of the same breed of the pigeon; and
see what a prodigious amount of difference there is in the beak of
tumblers, in the beak and wattle of carriers, in the carriage and tail of
fantails, etc., these being the points now mainly attended to by English
fanciers. Even in the same sub-breed, as in that of the short-faced
tumbler, it is notoriously difficult to breed nearly perfect birds, many
departing widely from the standard. There may truly be said to be a
constant struggle going on between, on the one hand, the tendency to
reversion to a less perfect state, as well as an innate tendency to new
variations, and, on the other hand, the power of steady selection to keep
the breed true. In the long run selection gains the day, and we do not
expect to fail so completely as to breed a bird as coarse as a common
tumbler pigeon from a good short-faced strain. But as long as selection is
rapidly going on, much variability in the parts undergoing modification
may always be expected.

Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other species
of the same genus, we may conclude that this part has undergone an
extraordinary amount of modification since the period when the several
species branched off from the common progenitor of the genus. This period
will seldom be remote in any extreme degree, as species rarely endure for
more than one geological period. An extraordinary amount of modification
implies an unusually large and long-continued amount of variability, which
has continually been accumulated by natural selection for the benefit of
the species. But as the variability of the extraordinarily developed part
or organ has been so great and long-continued within a period not
excessively remote, we might, as a general rule, still expect to find more
variability in such parts than in other parts of the organisation which
have remained for a much longer period nearly constant. And this, I am
convinced, is the case. That the struggle between natural selection on the
one hand, and the tendency to reversion and variability on the other hand,
will in the course of time cease; and that the most abnormally developed
organs may be made constant, I see no reason to doubt. Hence, when an
organ, however abnormal it may be, has been transmitted in approximately
the same condition to many modified descendants, as in the case of the
wing of the bat, it must have existed, according to our theory, for an
immense period in nearly the same state; and thus it has come not to be
more variable than any other structure. It is only in those cases in which
the modification has been comparatively recent and extraordinarily great
that we ought to find the GENERATIVE VARIABILITY, as it may be called,
still present in a high degree. For in this case the variability will
seldom as yet have been fixed by the continued selection of the
individuals varying in the required manner and degree, and by the
continued rejection of those tending to revert to a former and less
modified condition.

SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC CHARACTERS.

The principle discussed under the last heading may be applied to our
present subject. It is notorious that specific characters are more
variable than generic. To explain by a simple example what is meant: if in
a large genus of plants some species had blue flowers and some had red,
the colour would be only a specific character, and no one would be
surprised at one of the blue species varying into red, or conversely; but
if all the species had blue flowers, the colour would become a generic
character, and its variation would be a more unusual circumstance. I have
chosen this example because the explanation which most naturalists would
advance is not here applicable, namely, that specific characters are more
variable than generic, because they are taken from parts of less
physiological importance than those commonly used for classing genera. I
believe this explanation is partly, yet only indirectly, true; I shall,
however, have to return to this point in the chapter on Classification. It
would be almost superfluous to adduce evidence in support of the
statement, that ordinary specific characters are more variable than
generic; but with respect to important characters, I have repeatedly
noticed in works on natural history, that when an author remarks with
surprise that some important organ or part, which is generally very
constant throughout a large group of species, DIFFERS considerably in
closely-allied species, it is often VARIABLE in the individuals of the
same species. And this fact shows that a character, which is generally of
generic value, when it sinks in value and becomes only of specific value,
often becomes variable, though its physiological importance may remain the
same. Something of the same kind applies to monstrosities: at least Is.
Geoffroy St. Hilaire apparently entertains no doubt, that the more an
organ normally differs in the different species of the same group, the
more subject it is to anomalies in the individuals.

On the ordinary view of each species having been independently created,
why should that part of the structure, which differs from the same part in
other independently created species of the same genus, be more variable
than those parts which are closely alike in the several species? I do not
see that any explanation can be given. But on the view that species are
only strongly marked and fixed varieties, we might expect often to find
them still continuing to vary in those parts of their structure which have
varied within a moderately recent period, and which have thus come to
differ. Or to state the case in another manner: the points in which all
the species of a genus resemble each other, and in which they differ from
allied genera, are called generic characters; and these characters may be
attributed to inheritance from a common progenitor, for it can rarely have
happened that natural selection will have modified several distinct
species, fitted to more or less widely different habits, in exactly the
same manner: and as these so-called generic characters have been inherited
from before the period when the several species first branched off from
their common progenitor, and subsequently have not varied or come to
differ in any degree, or only in a slight degree, it is not probable that
they should vary at the present day. On the other hand, the points in
which species differ from other species of the same genus are called
specific characters; and as these specific characters have varied and come
to differ since the period when the species branched off from a common
progenitor, it is probable that they should still often be in some degree
variable—at least more variable than those parts of the organisation
which have for a very long period remained constant.

SECONDARY SEXUAL CHARACTERS VARIABLE.

I think it will be admitted by naturalists, without my entering on
details, that secondary sexual characters are highly variable. It will
also be admitted that species of the same group differ from each other
more widely in their secondary sexual characters, than in other parts of
their organisation; compare, for instance, the amount of difference
between the males of gallinaceous birds, in which secondary sexual
characters are strongly displayed, with the amount of difference between
the females. The cause of the original variability of these characters is
not manifest; but we can see why they should not have been rendered as
constant and uniform as others, for they are accumulated by sexual
selection, which is less rigid in its action than ordinary selection, as
it does not entail death, but only gives fewer offspring to the less
favoured males. Whatever the cause may be of the variability of secondary
sexual characters, as they are highly variable, sexual selection will have
had a wide scope for action, and may thus have succeeded in giving to the
species of the same group a greater amount of difference in these than in
other respects.

It is a remarkable fact, that the secondary differences between the two
sexes of the same species are generally displayed in the very same parts
of the organisation in which the species of the same genus differ from
each other. Of this fact I will give in illustration the first two
instances which happen to stand on my list; and as the differences in
these cases are of a very unusual nature, the relation can hardly be
accidental. The same number of joints in the tarsi is a character common
to very large groups of beetles, but in the Engidae, as Westwood has
remarked, the number varies greatly and the number likewise differs in the
two sexes of the same species. Again in the fossorial hymenoptera, the
neuration of the wings is a character of the highest importance, because
common to large groups; but in certain genera the neuration differs in the
different species, and likewise in the two sexes of the same species. Sir
J. Lubbock has recently remarked, that several minute crustaceans offer
excellent illustrations of this law. "In Pontella, for instance, the
sexual characters are afforded mainly by the anterior antennae and by the
fifth pair of legs: the specific differences also are principally given by
these organs." This relation has a clear meaning on my view: I look at all
the species of the same genus as having as certainly descended from the
same progenitor, as have the two sexes of any one species. Consequently,
whatever part of the structure of the common progenitor, or of its early
descendants, became variable; variations of this part would, it is highly
probable, be taken advantage of by natural and sexual selection, in order
to fit the several places in the economy of nature, and likewise to fit
the two sexes of the same species to each other, or to fit the males to
struggle with other males for the possession of the females.

Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which are possessed by all the species; that
the frequent extreme variability of any part which is developed in a
species in an extraordinary manner in comparison with the same part in its
congeners; and the slight degree of variability in a part, however
extraordinarily it may be developed, if it be common to a whole group of
species; that the great variability of secondary sexual characters and
their great difference in closely allied species; that secondary sexual
and ordinary specific differences are generally displayed in the same
parts of the organisation, are all principles closely connected together.
All being mainly due to the species of the same group being the
descendants of a common progenitor, from whom they have inherited much in
common, to parts which have recently and largely varied being more likely
still to go on varying than parts which have long been inherited and have
not varied, to natural selection having more or less completely, according
to the lapse of time, overmastered the tendency to reversion and to
further variability, to sexual selection being less rigid than ordinary
selection, and to variations in the same parts having been accumulated by
natural and sexual selection, and thus having been adapted for secondary
sexual, and for ordinary purposes.

DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS, SO THAT A VARIETY OF ONE
SPECIES OFTEN ASSUMES A CHARACTER PROPER TO AN ALLIED SPECIES, OR REVERTS
TO SOME OF THE CHARACTERS OF AN EARLY PROGENITOR.

These propositions will be most readily understood by looking to our
domestic races. The most distinct breeds of the pigeon, in countries
widely apart, present sub-varieties with reversed feathers on the head,
and with feathers on the feet, characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more distinct
races. The frequent presence of fourteen or even sixteen tail-feathers in
the pouter may be considered as a variation representing the normal
structure of another race, the fantail. I presume that no one will doubt
that all such analogous variations are due to the several races of the
pigeon having inherited from a common parent the same constitution and
tendency to variation, when acted on by similar unknown influences. In the
vegetable kingdom we have a case of analogous variation, in the enlarged
stems, or as commonly called roots, of the Swedish turnip and ruta-baga,
plants which several botanists rank as varieties produced by cultivation
from a common parent: if this be not so, the case will then be one of
analogous variation in two so-called distinct species; and to these a
third may be added, namely, the common turnip. According to the ordinary
view of each species having been independently created, we should have to
attribute this similarity in the enlarged stems of these three plants, not
to the vera causa of community of descent, and a consequent tendency to
vary in a like manner, but to three separate yet closely related acts of
creation. Many similar cases of analogous variation have been observed by
Naudin in the great gourd family, and by various authors in our cereals.
Similar cases occurring with insects under natural conditions have lately
been discussed with much ability by Mr. Walsh, who has grouped them under
his law of equable variability.

With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars on
the wings, white loins, a bar at the end of the tail, with the outer
feathers externally edged near their bases with white. As all these marks
are characteristic of the parent rock-pigeon, I presume that no one will
doubt that this is a case of reversion, and not of a new yet analogous
variation appearing in the several breeds. We may, I think, confidently
come to this conclusion, because, as we have seen, these coloured marks
are eminently liable to appear in the crossed offspring of two distinct
and differently coloured breeds; and in this case there is nothing in the
external conditions of life to cause the reappearance of the slaty-blue,
with the several marks, beyond the influence of the mere act of crossing
on the laws of inheritance.

No doubt it is a very surprising fact that characters should reappear
after having been lost for many, probably for hundreds of generations. But
when a breed has been crossed only once by some other breed, the offspring
occasionally show for many generations a tendency to revert in character
to the foreign breed—some say, for a dozen or even a score of
generations. After twelve generations, the proportion of blood, to use a
common expression, from one ancestor, is only 1 in 2048; and yet, as we
see, it is generally believed that a tendency to reversion is retained by
this remnant of foreign blood. In a breed which has not been crossed, but
in which BOTH parents have lost some character which their progenitor
possessed, the tendency, whether strong or weak, to reproduce the lost
character might, as was formerly remarked, for all that we can see to the
contrary, be transmitted for almost any number of generations. When a
character which has been lost in a breed, reappears after a great number
of generations, the most probable hypothesis is, not that one individual
suddenly takes after an ancestor removed by some hundred generations, but
that in each successive generation the character in question has been
lying latent, and at last, under unknown favourable conditions, is
developed. With the barb-pigeon, for instance, which very rarely produces
a blue bird, it is probable that there is a latent tendency in each
generation to produce blue plumage. The abstract improbability of such a
tendency being transmitted through a vast number of generations, is not
greater than that of quite useless or rudimentary organs being similarly
transmitted. A mere tendency to produce a rudiment is indeed sometimes
thus inherited.

As all the species of the same genus are supposed to be descended from a
common progenitor, it might be expected that they would occasionally vary
in an analogous manner; so that the varieties of two or more species would
resemble each other, or that a variety of one species would resemble in
certain characters another and distinct species, this other species being,
according to our view, only a well-marked and permanent variety. But
characters exclusively due to analogous variation would probably be of an
unimportant nature, for the preservation of all functionally important
characters will have been determined through natural selection, in
accordance with the different habits of the species. It might further be
expected that the species of the same genus would occasionally exhibit
reversions to long-lost characters. As, however, we do not know the common
ancestor of any natural group, we cannot distinguish between reversionary
and analogous characters. If, for instance, we did not know that the
parent rock-pigeon was not feather-footed or turn-crowned, we could not
have told, whether such characters in our domestic breeds were reversions
or only analogous variations; but we might have inferred that the blue
colour was a case of reversion from the number of the markings, which are
correlated with this tint, and which would not probably have all appeared
together from simple variation. More especially we might have inferred
this from the blue colour and the several marks so often appearing when
differently coloured breeds are crossed. Hence, although under nature it
must generally be left doubtful, what cases are reversions to formerly
existing characters, and what are new but analogous variations, yet we
ought, on our theory, sometimes to find the varying offspring of a species
assuming characters which are already present in other members of the same
group. And this undoubtedly is the case.

The difficulty in distinguishing variable species is largely due to the
varieties mocking, as it were, other species of the same genus. A
considerable catalogue, also, could be given of forms intermediate between
two other forms, which themselves can only doubtfully be ranked as
species; and this shows, unless all these closely allied forms be
considered as independently created species, that they have in varying
assumed some of the characters of the others. But the best evidence of
analogous variations is afforded by parts or organs which are generally
constant in character, but which occasionally vary so as to resemble, in
some degree, the same part or organ in an allied species. I have collected
a long list of such cases; but here, as before, I lie under the great
disadvantage of not being able to give them. I can only repeat that such
cases certainly occur, and seem to me very remarkable.

I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several species
of the same genus, partly under domestication and partly under nature. It
is a case almost certainly of reversion. The ass sometimes has very
distinct transverse bars on its legs, like those on the legs of a zebra.
It has been asserted that these are plainest in the foal, and from
inquiries which I have made, I believe this to be true. The stripe on the
shoulder is sometimes double, and is very variable in length and outline.
A white ass, but NOT an albino, has been described without either spinal
or shoulder stripe; and these stripes are sometimes very obscure, or
actually quite lost, in dark-coloured asses. The koulan of Pallas is said
to have been seen with a double shoulder-stripe. Mr. Blyth has seen a
specimen of the hemionus with a distinct shoulder-stripe, though it
properly has none; and I have been informed by Colonel Poole that foals of
this species are generally striped on the legs and faintly on the
shoulder. The quagga, though so plainly barred like a zebra over the body,
is without bars on the legs; but Dr. Gray has figured one specimen with
very distinct zebra-like bars on the hocks.

With respect to the horse, I have collected cases in England of the spinal
stripe in horses of the most distinct breeds, and of ALL colours;
transverse bars on the legs are not rare in duns, mouse-duns, and in one
instance in a chestnut; a faint shoulder-stripe may sometimes be seen in
duns, and I have seen a trace in a bay horse. My son made a careful
examination and sketch for me of a dun Belgian cart-horse with a double
stripe on each shoulder and with leg-stripes. I have myself seen a dun
Devonshire pony, and a small dun Welsh pony has been carefully described
to me, both with THREE parallel stripes on each shoulder.

In the northwest part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined this
breed for the Indian Government, a horse without stripes is not considered
as purely bred. The spine is always striped; the legs are generally
barred; and the shoulder-stripe, which is sometimes double and sometimes
treble, is common; the side of the face, moreover, is sometimes striped.
The stripes are often plainest in the foal; and sometimes quite disappear
in old horses. Colonel Poole has seen both gray and bay Kattywar horses
striped when first foaled. I have also reason to suspect, from information
given me by Mr. W.W. Edwards, that with the English race-horse the spinal
stripe is much commoner in the foal than in the full-grown animal. I have
myself recently bred a foal from a bay mare (offspring of a Turkoman horse
and a Flemish mare) by a bay English race-horse. This foal, when a week
old, was marked on its hinder quarters and on its forehead with numerous
very narrow, dark, zebra-like bars, and its legs were feebly striped. All
the stripes soon disappeared completely. Without here entering on further
details I may state that I have collected cases of leg and shoulder
stripes in horses of very different breeds in various countries from
Britain to Eastern China; and from Norway in the north to the Malay
Archipelago in the south. In all parts of the world these stripes occur
far oftenest in duns and mouse-duns; by the term dun a large range of
colour is included, from one between brown and black to a close approach
to cream colour.

I am aware that Colonel Hamilton Smith, who has written on this subject,
believes that the several breeds of the horse are descended from several
aboriginal species, one of which, the dun, was striped; and that the
above-described appearances are all due to ancient crosses with the dun
stock. But this view may be safely rejected, for it is highly improbable
that the heavy Belgian cart-horse, Welsh ponies, Norwegian cobs, the lanky
Kattywar race, etc., inhabiting the most distant parts of the world,
should have all have been crossed with one supposed aboriginal stock.

Now let us turn to the effects of crossing the several species of the
horse genus. Rollin asserts that the common mule from the ass and horse is
particularly apt to have bars on its legs; according to Mr. Gosse, in
certain parts of the United States, about nine out of ten mules have
striped legs. I once saw a mule with its legs so much striped that any one
might have thought that it was a hybrid zebra; and Mr. W.C. Martin, in his
excellent treatise on the horse, has given a figure of a similar mule. In
four coloured drawings, which I have seen, of hybrids between the ass and
zebra, the legs were much more plainly barred than the rest of the body;
and in one of them there was a double shoulder-stripe. In Lord Morton's
famous hybrid, from a chestnut mare and male quagga, the hybrid and even
the pure offspring subsequently produced from the same mare by a black
Arabian sire, were much more plainly barred across the legs than is even
the pure quagga. Lastly, and this is another most remarkable case, a
hybrid has been figured by Dr. Gray (and he informs me that he knows of a
second case) from the ass and the hemionus; and this hybrid, though the
ass only occasionally has stripes on his legs and the hemionus has none
and has not even a shoulder-stripe, nevertheless had all four legs barred,
and had three short shoulder-stripes, like those on the dun Devonshire and
Welsh ponies, and even had some zebra-like stripes on the sides of its
face. With respect to this last fact, I was so convinced that not even a
stripe of colour appears from what is commonly called chance, that I was
led solely from the occurrence of the face-stripes on this hybrid from the
ass and hemionus to ask Colonel Poole whether such face-stripes ever
occurred in the eminently striped Kattywar breed of horses, and was, as we
have seen, answered in the affirmative.

What now are we to say to these several facts? We see several distinct
species of the horse genus becoming, by simple variation, striped on the
legs like a zebra, or striped on the shoulders like an ass. In the horse
we see this tendency strong whenever a dun tint appears—a tint which
approaches to that of the general colouring of the other species of the
genus. The appearance of the stripes is not accompanied by any change of
form, or by any other new character. We see this tendency to become
striped most strongly displayed in hybrids from between several of the
most distinct species. Now observe the case of the several breeds of
pigeons: they are descended from a pigeon (including two or three
sub-species or geographical races) of a bluish colour, with certain bars
and other marks; and when any breed assumes by simple variation a bluish
tint, these bars and other marks invariably reappear; but without any
other change of form or character. When the oldest and truest breeds of
various colours are crossed, we see a strong tendency for the blue tint
and bars and marks to reappear in the mongrels. I have stated that the
most probable hypothesis to account for the reappearance of very ancient
characters, is—that there is a TENDENCY in the young of each
successive generation to produce the long-lost character, and that this
tendency, from unknown causes, sometimes prevails. And we have just seen
that in several species of the horse genus the stripes are either plainer
or appear more commonly in the young than in the old. Call the breeds of
pigeons, some of which have bred true for centuries, species; and how
exactly parallel is the case with that of the species of the horse genus!
For myself, I venture confidently to look back thousands on thousands of
generations, and I see an animal striped like a zebra, but perhaps
otherwise very differently constructed, the common parent of our domestic
horse (whether or not it be descended from one or more wild stocks) of the
ass, the hemionus, quagga, and zebra.

He who believes that each equine species was independently created, will,
I presume, assert that each species has been created with a tendency to
vary, both under nature and under domestication, in this particular
manner, so as often to become striped like the other species of the genus;
and that each has been created with a strong tendency, when crossed with
species inhabiting distant quarters of the world, to produce hybrids
resembling in their stripes, not their own parents, but other species of
the genus. To admit this view is, as it seems to me, to reject a real for
an unreal, or at least for an unknown cause. It makes the works of God a
mere mockery and deception; I would almost as soon believe with the old
and ignorant cosmogonists, that fossil shells had never lived, but had
been created in stone so as to mock the shells now living on the
sea-shore.

SUMMARY.

Our ignorance of the laws of variation is profound. Not in one case out of
a hundred can we pretend to assign any reason why this or that part has
varied. But whenever we have the means of instituting a comparison, the
same laws appear to have acted in producing the lesser differences between
varieties of the same species, and the greater differences between species
of the same genus. Changed conditions generally induce mere fluctuating
variability, but sometimes they cause direct and definite effects; and
these may become strongly marked in the course of time, though we have not
sufficient evidence on this head. Habit in producing constitutional
peculiarities, and use in strengthening, and disuse in weakening and
diminishing organs, appear in many cases to have been potent in their
effects. Homologous parts tend to vary in the same manner, and homologous
parts tend to cohere. Modifications in hard parts and in external parts
sometimes affect softer and internal parts. When one part is largely
developed, perhaps it tends to draw nourishment from the adjoining parts;
and every part of the structure which can be saved without detriment will
be saved. Changes of structure at an early age may affect parts
subsequently developed; and many cases of correlated variation, the nature
of which we are unable to understand, undoubtedly occur. Multiple parts
are variable in number and in structure, perhaps arising from such parts
not having been closely specialised for any particular function, so that
their modifications have not been closely checked by natural selection. It
follows probably from this same cause, that organic beings low in the
scale are more variable than those standing higher in the scale, and which
have their whole organisation more specialised. Rudimentary organs, from
being useless, are not regulated by natural selection, and hence are
variable. Specific characters—that is, the characters which have
come to differ since the several species of the same genus branched off
from a common parent—are more variable than generic characters, or
those which have long been inherited, and have not differed within this
same period. In these remarks we have referred to special parts or organs
being still variable, because they have recently varied and thus come to
differ; but we have also seen in the second chapter that the same
principle applies to the whole individual; for in a district where many
species of a genus are found—that is, where there has been much
former variation and differentiation, or where the manufactory of new
specific forms has been actively at work—in that district and among
these species, we now find, on an average, most varieties. Secondary
sexual characters are highly variable, and such characters differ much in
the species of the same group. Variability in the same parts of the
organisation has generally been taken advantage of in giving secondary
sexual differences to the two sexes of the same species, and specific
differences to the several species of the same genus. Any part or organ
developed to an extraordinary size or in an extraordinary manner, in
comparison with the same part or organ in the allied species, must have
gone through an extraordinary amount of modification since the genus
arose; and thus we can understand why it should often still be variable in
a much higher degree than other parts; for variation is a long-continued
and slow process, and natural selection will in such cases not as yet have
had time to overcome the tendency to further variability and to reversion
to a less modified state. But when a species with an extraordinarily
developed organ has become the parent of many modified descendants—which
on our view must be a very slow process, requiring a long lapse of time—in
this case, natural selection has succeeded in giving a fixed character to
the organ, in however extraordinary a manner it may have been developed.
Species inheriting nearly the same constitution from a common parent, and
exposed to similar influences, naturally tend to present analogous
variations, or these same species may occasionally revert to some of the
characters of their ancient progenitors. Although new and important
modifications may not arise from reversion and analogous variation, such
modifications will add to the beautiful and harmonious diversity of
nature.

Whatever the cause may be of each slight difference between the offspring
and their parents—and a cause for each must exist—we have
reason to believe that it is the steady accumulation of beneficial
differences which has given rise to all the more important modifications
of structure in relation to the habits of each species.



CHAPTER VI. DIFFICULTIES OF THE THEORY.


 Difficulties of the theory of descent with modification—Absence
 or rarity of transitional varieties—Transitions in habits of
 life—Diversified habits in the same species—Species with habits
 widely different from those of their allies—Organs of extreme
 perfection—Modes of transition—Cases of difficulty—Natura non facit
 saltum—Organs of small importance—Organs not in all cases absolutely
 perfect—The law of Unity of Type and of the Conditions of Existence
 embraced by the theory of Natural Selection.

Long before the reader has arrived at this part of my work, a crowd of
difficulties will have occurred to him. Some of them are so serious that
to this day I can hardly reflect on them without being in some degree
staggered; but, to the best of my judgment, the greater number are only
apparent, and those that are real are not, I think, fatal to the theory.

These difficulties and objections may be classed under the following
heads: First, why, if species have descended from other species by fine
gradations, do we not everywhere see innumerable transitional forms? Why
is not all nature in confusion, instead of the species being, as we see
them, well defined?

Secondly, is it possible that an animal having, for instance, the
structure and habits of a bat, could have been formed by the modification
of some other animal with widely different habits and structure? Can we
believe that natural selection could produce, on the one hand, an organ of
trifling importance, such as the tail of a giraffe, which serves as a
fly-flapper, and, on the other hand, an organ so wonderful as the eye?

Thirdly, can instincts be acquired and modified through natural selection?
What shall we say to the instinct which leads the bee to make cells, and
which has practically anticipated the discoveries of profound
mathematicians?

Fourthly, how can we account for species, when crossed, being sterile and
producing sterile offspring, whereas, when varieties are crossed, their
fertility is unimpaired?

The two first heads will be here discussed; some miscellaneous objections
in the following chapter; Instinct and Hybridism in the two succeeding
chapters.

ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.

As natural selection acts solely by the preservation of profitable
modifications, each new form will tend in a fully-stocked country to take
the place of, and finally to exterminate, its own less improved
parent-form and other less-favoured forms with which it comes into
competition. Thus extinction and natural selection go hand in hand. Hence,
if we look at each species as descended from some unknown form, both the
parent and all the transitional varieties will generally have been
exterminated by the very process of the formation and perfection of the
new form.

But, as by this theory innumerable transitional forms must have existed,
why do we not find them embedded in countless numbers in the crust of the
earth? It will be more convenient to discuss this question in the chapter
on the imperfection of the geological record; and I will here only state
that I believe the answer mainly lies in the record being incomparably
less perfect than is generally supposed. The crust of the earth is a vast
museum; but the natural collections have been imperfectly made, and only
at long intervals of time.

But it may be urged that when several closely allied species inhabit the
same territory, we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from north to
south over a continent, we generally meet at successive intervals with
closely allied or representative species, evidently filling nearly the
same place in the natural economy of the land. These representative
species often meet and interlock; and as the one becomes rarer and rarer,
the other becomes more and more frequent, till the one replaces the other.
But if we compare these species where they intermingle, they are generally
as absolutely distinct from each other in every detail of structure as are
specimens taken from the metropolis inhabited by each. By my theory these
allied species are descended from a common parent; and during the process
of modification, each has become adapted to the conditions of life of its
own region, and has supplanted and exterminated its original parent-form
and all the transitional varieties between its past and present states.
Hence we ought not to expect at the present time to meet with numerous
transitional varieties in each region, though they must have existed
there, and may be embedded there in a fossil condition. But in the
intermediate region, having intermediate conditions of life, why do we not
now find closely-linking intermediate varieties? This difficulty for a
long time quite confounded me. But I think it can be in large part
explained.

In the first place we should be extremely cautious in inferring, because
an area is now continuous, that it has been continuous during a long
period. Geology would lead us to believe that most continents have been
broken up into islands even during the later tertiary periods; and in such
islands distinct species might have been separately formed without the
possibility of intermediate varieties existing in the intermediate zones.
By changes in the form of the land and of climate, marine areas now
continuous must often have existed within recent times in a far less
continuous and uniform condition than at present. But I will pass over
this way of escaping from the difficulty; for I believe that many
perfectly defined species have been formed on strictly continuous areas;
though I do not doubt that the formerly broken condition of areas now
continuous, has played an important part in the formation of new species,
more especially with freely-crossing and wandering animals.

In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then
becoming somewhat abruptly rarer and rarer on the confines, and finally
disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to
each. We see the same fact in ascending mountains, and sometimes it is
quite remarkable how abruptly, as Alph. De Candolle has observed, a common
alpine species disappears. The same fact has been noticed by E. Forbes in
sounding the depths of the sea with the dredge. To those who look at
climate and the physical conditions of life as the all-important elements
of distribution, these facts ought to cause surprise, as climate and
height or depth graduate away insensibly. But when we bear in mind that
almost every species, even in its metropolis, would increase immensely in
numbers, were it not for other competing species; that nearly all either
prey on or serve as prey for others; in short, that each organic being is
either directly or indirectly related in the most important manner to
other organic beings—we see that the range of the inhabitants of any
country by no means exclusively depends on insensibly changing physical
conditions, but in large part on the presence of other species, on which
it lives, or by which it is destroyed, or with which it comes into
competition; and as these species are already defined objects, not
blending one into another by insensible gradations, the range of any one
species, depending as it does on the range of others, will tend to be
sharply defined. Moreover, each species on the confines of its range,
where it exists in lessened numbers, will, during fluctuations in the
number of its enemies or of its prey, or in the nature of the seasons, be
extremely liable to utter extermination; and thus its geographical range
will come to be still more sharply defined.

As allied or representative species, when inhabiting a continuous area,
are generally distributed in such a manner that each has a wide range,
with a comparatively narrow neutral territory between them, in which they
become rather suddenly rarer and rarer; then, as varieties do not
essentially differ from species, the same rule will probably apply to
both; and if we take a varying species inhabiting a very large area, we
shall have to adapt two varieties to two large areas, and a third variety
to a narrow intermediate zone. The intermediate variety, consequently,
will exist in lesser numbers from inhabiting a narrow and lesser area; and
practically, as far as I can make out, this rule holds good with varieties
in a state of nature. I have met with striking instances of the rule in
the case of varieties intermediate between well-marked varieties in the
genus Balanus. And it would appear from information given me by Mr.
Watson, Dr. Asa Gray, and Mr. Wollaston, that generally, when varieties
intermediate between two other forms occur, they are much rarer
numerically than the forms which they connect. Now, if we may trust these
facts and inferences, and conclude that varieties linking two other
varieties together generally have existed in lesser numbers than the forms
which they connect, then we can understand why intermediate varieties
should not endure for very long periods: why, as a general rule, they
should be exterminated and disappear, sooner than the forms which they
originally linked together.

For any form existing in lesser numbers would, as already remarked, run a
greater chance of being exterminated than one existing in large numbers;
and in this particular case the intermediate form would be eminently
liable to the inroads of closely allied forms existing on both sides of
it. But it is a far more important consideration, that during the process
of further modification, by which two varieties are supposed to be
converted and perfected into two distinct species, the two which exist in
larger numbers, from inhabiting larger areas, will have a great advantage
over the intermediate variety, which exists in smaller numbers in a narrow
and intermediate zone. For forms existing in larger numbers will have a
better chance, within any given period, of presenting further favourable
variations for natural selection to seize on, than will the rarer forms
which exist in lesser numbers. Hence, the more common forms, in the race
for life, will tend to beat and supplant the less common forms, for these
will be more slowly modified and improved. It is the same principle which,
as I believe, accounts for the common species in each country, as shown in
the second chapter, presenting on an average a greater number of
well-marked varieties than do the rarer species. I may illustrate what I
mean by supposing three varieties of sheep to be kept, one adapted to an
extensive mountainous region; a second to a comparatively narrow, hilly
tract; and a third to the wide plains at the base; and that the
inhabitants are all trying with equal steadiness and skill to improve
their stocks by selection; the chances in this case will be strongly in
favour of the great holders on the mountains or on the plains improving
their breeds more quickly than the small holders on the intermediate
narrow, hilly tract; and consequently the improved mountain or plain breed
will soon take the place of the less improved hill breed; and thus the two
breeds, which originally existed in greater numbers, will come into close
contact with each other, without the interposition of the supplanted,
intermediate hill variety.

To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: first, because new varieties are very
slowly formed, for variation is a slow process, and natural selection can
do nothing until favourable individual differences or variations occur,
and until a place in the natural polity of the country can be better
filled by some modification of some one or more of its inhabitants. And
such new places will depend on slow changes of climate, or on the
occasional immigration of new inhabitants, and, probably, in a still more
important degree, on some of the old inhabitants becoming slowly modified,
with the new forms thus produced and the old ones acting and reacting on
each other. So that, in any one region and at any one time, we ought to
see only a few species presenting slight modifications of structure in
some degree permanent; and this assuredly we do see.

Secondly, areas now continuous must often have existed within the recent
period as isolated portions, in which many forms, more especially among
the classes which unite for each birth and wander much, may have
separately been rendered sufficiently distinct to rank as representative
species. In this case, intermediate varieties between the several
representative species and their common parent, must formerly have existed
within each isolated portion of the land, but these links during the
process of natural selection will have been supplanted and exterminated,
so that they will no longer be found in a living state.

Thirdly, when two or more varieties have been formed in different portions
of a strictly continuous area, intermediate varieties will, it is
probable, at first have been formed in the intermediate zones, but they
will generally have had a short duration. For these intermediate varieties
will, from reasons already assigned (namely from what we know of the
actual distribution of closely allied or representative species, and
likewise of acknowledged varieties), exist in the intermediate zones in
lesser numbers than the varieties which they tend to connect. From this
cause alone the intermediate varieties will be liable to accidental
extermination; and during the process of further modification through
natural selection, they will almost certainly be beaten and supplanted by
the forms which they connect; for these, from existing in greater numbers
will, in the aggregate, present more varieties, and thus be further
improved through natural selection and gain further advantages.

Lastly, looking not to any one time, but at all time, if my theory be
true, numberless intermediate varieties, linking closely together all the
species of the same group, must assuredly have existed; but the very
process of natural selection constantly tends, as has been so often
remarked, to exterminate the parent forms and the intermediate links.
Consequently evidence of their former existence could be found only among
fossil remains, which are preserved, as we shall attempt to show in a
future chapter, in an extremely imperfect and intermittent record.

ON THE ORIGIN AND TRANSITION OF ORGANIC BEINGS WITH PECULIAR HABITS AND
STRUCTURE.

It has been asked by the opponents of such views as I hold, how, for
instance, could a land carnivorous animal have been converted into one
with aquatic habits; for how could the animal in its transitional state
have subsisted? It would be easy to show that there now exist carnivorous
animals presenting close intermediate grades from strictly terrestrial to
aquatic habits; and as each exists by a struggle for life, it is clear
that each must be well adapted to its place in nature. Look at the Mustela
vison of North America, which has webbed feet, and which resembles an
otter in its fur, short legs, and form of tail; during summer this animal
dives for and preys on fish, but during the long winter it leaves the
frozen waters, and preys, like other polecats on mice and land animals. If
a different case had been taken, and it had been asked how an
insectivorous quadruped could possibly have been converted into a flying
bat, the question would have been far more difficult to answer. Yet I
think such difficulties have little weight.

Here, as on other occasions, I lie under a heavy disadvantage, for, out of
the many striking cases which I have collected, I can give only one or two
instances of transitional habits and structures in allied species; and of
diversified habits, either constant or occasional, in the same species.
And it seems to me that nothing less than a long list of such cases is
sufficient to lessen the difficulty in any particular case like that of
the bat.

Look at the family of squirrels; here we have the finest gradation from
animals with their tails only slightly flattened, and from others, as Sir
J. Richardson has remarked, with the posterior part of their bodies rather
wide and with the skin on their flanks rather full, to the so-called
flying squirrels; and flying squirrels have their limbs and even the base
of the tail united by a broad expanse of skin, which serves as a parachute
and allows them to glide through the air to an astonishing distance from
tree to tree. We cannot doubt that each structure is of use to each kind
of squirrel in its own country, by enabling it to escape birds or beasts
of prey, or to collect food more quickly, or, as there is reason to
believe, to lessen the danger from occasional falls. But it does not
follow from this fact that the structure of each squirrel is the best that
it is possible to conceive under all possible conditions. Let the climate
and vegetation change, let other competing rodents or new beasts of prey
immigrate, or old ones become modified, and all analogy would lead us to
believe that some, at least, of the squirrels would decrease in numbers or
become exterminated, unless they also become modified and improved in
structure in a corresponding manner. Therefore, I can see no difficulty,
more especially under changing conditions of life, in the continued
preservation of individuals with fuller and fuller flank-membranes, each
modification being useful, each being propagated, until, by the
accumulated effects of this process of natural selection, a perfect
so-called flying squirrel was produced.

Now look at the Galeopithecus or so-called flying lemur, which was
formerly ranked among bats, but is now believed to belong to the
Insectivora. An extremely wide flank-membrane stretches from the corners
of the jaw to the tail, and includes the limbs with the elongated fingers.
This flank-membrane is furnished with an extensor muscle. Although no
graduated links of structure, fitted for gliding through the air, now
connect the Galeopithecus with the other Insectivora, yet there is no
difficulty in supposing that such links formerly existed, and that each
was developed in the same manner as with the less perfectly gliding
squirrels; each grade of structure having been useful to its possessor.
Nor can I see any insuperable difficulty in further believing it possible
that the membrane-connected fingers and fore-arm of the Galeopithecus
might have been greatly lengthened by natural selection; and this, as far
as the organs of flight are concerned, would have converted the animal
into a bat. In certain bats in which the wing-membrane extends from the
top of the shoulder to the tail and includes the hind-legs, we perhaps see
traces of an apparatus originally fitted for gliding through the air
rather than for flight.

If about a dozen genera of birds were to become extinct, who would have
ventured to surmise that birds might have existed which used their wings
solely as flappers, like the logger headed duck (Micropterus of Eyton); as
fins in the water and as front legs on the land, like the penguin; as
sails, like the ostrich; and functionally for no purpose, like the
apteryx? Yet the structure of each of these birds is good for it, under
the conditions of life to which it is exposed, for each has to live by a
struggle: but it is not necessarily the best possible under all possible
conditions. It must not be inferred from these remarks that any of the
grades of wing-structure here alluded to, which perhaps may all be the
result of disuse, indicate the steps by which birds actually acquired
their perfect power of flight; but they serve to show what diversified
means of transition are at least possible.

Seeing that a few members of such water-breathing classes as the Crustacea
and Mollusca are adapted to live on the land; and seeing that we have
flying birds and mammals, flying insects of the most diversified types,
and formerly had flying reptiles, it is conceivable that flying-fish,
which now glide far through the air, slightly rising and turning by the
aid of their fluttering fins, might have been modified into perfectly
winged animals. If this had been effected, who would have ever imagined
that in an early transitional state they had been inhabitants of the open
ocean, and had used their incipient organs of flight exclusively, so far
as we know, to escape being devoured by other fish?

When we see any structure highly perfected for any particular habit, as
the wings of a bird for flight, we should bear in mind that animals
displaying early transitional grades of the structure will seldom have
survived to the present day, for they will have been supplanted by their
successors, which were gradually rendered more perfect through natural
selection. Furthermore, we may conclude that transitional states between
structures fitted for very different habits of life will rarely have been
developed at an early period in great numbers and under many subordinate
forms. Thus, to return to our imaginary illustration of the flying-fish,
it does not seem probable that fishes capable of true flight would have
been developed under many subordinate forms, for taking prey of many kinds
in many ways, on the land and in the water, until their organs of flight
had come to a high stage of perfection, so as to have given them a decided
advantage over other animals in the battle for life. Hence the chance of
discovering species with transitional grades of structure in a fossil
condition will always be less, from their having existed in lesser
numbers, than in the case of species with fully developed structures.

I will now give two or three instances, both of diversified and of changed
habits, in the individuals of the same species. In either case it would be
easy for natural selection to adapt the structure of the animal to its
changed habits, or exclusively to one of its several habits. It is,
however, difficult to decide and immaterial for us, whether habits
generally change first and structure afterwards; or whether slight
modifications of structure lead to changed habits; both probably often
occurring almost simultaneously. Of cases of changed habits it will
suffice merely to allude to that of the many British insects which now
feed on exotic plants, or exclusively on artificial substances. Of
diversified habits innumerable instances could be given: I have often
watched a tyrant flycatcher (Saurophagus sulphuratus) in South America,
hovering over one spot and then proceeding to another, like a kestrel, and
at other times standing stationary on the margin of water, and then
dashing into it like a kingfisher at a fish. In our own country the larger
titmouse (Parus major) may be seen climbing branches, almost like a
creeper; it sometimes, like a shrike, kills small birds by blows on the
head; and I have many times seen and heard it hammering the seeds of the
yew on a branch, and thus breaking them like a nuthatch. In North America
the black bear was seen by Hearne swimming for hours with widely open
mouth, thus catching, almost like a whale, insects in the water.

As we sometimes see individuals following habits different from those
proper to their species and to the other species of the same genus, we
might expect that such individuals would occasionally give rise to new
species, having anomalous habits, and with their structure either slightly
or considerably modified from that of their type. And such instances occur
in nature. Can a more striking instance of adaptation be given than that
of a woodpecker for climbing trees and seizing insects in the chinks of
the bark? Yet in North America there are woodpeckers which feed largely on
fruit, and others with elongated wings which chase insects on the wing. On
the plains of La Plata, where hardly a tree grows, there is a woodpecker
(Colaptes campestris) which has two toes before and two behind, a
long-pointed tongue, pointed tail-feathers, sufficiently stiff to support
the bird in a vertical position on a post, but not so stiff as in the
typical wood-peckers, and a straight, strong beak. The beak, however, is
not so straight or so strong as in the typical woodpeckers but it is
strong enough to bore into wood. Hence this Colaptes, in all the essential
parts of its structure, is a woodpecker. Even in such trifling characters
as the colouring, the harsh tone of the voice, and undulatory flight, its
close blood-relationship to our common woodpecker is plainly declared;
yet, as I can assert, not only from my own observations, but from those of
the accurate Azara, in certain large districts it does not climb trees,
and it makes its nest in holes in banks! In certain other districts,
however, this same woodpecker, as Mr. Hudson states, frequents trees, and
bores holes in the trunk for its nest. I may mention as another
illustration of the varied habits of this genus, that a Mexican Colaptes
has been described by De Saussure as boring holes into hard wood in order
to lay up a store of acorns.

Petrels are the most aerial and oceanic of birds, but, in the quiet sounds
of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its
astonishing power of diving, in its manner of swimming and of flying when
made to take flight, would be mistaken by any one for an auk or a grebe;
nevertheless, it is essentially a petrel, but with many parts of its
organisation profoundly modified in relation to its new habits of life;
whereas the woodpecker of La Plata has had its structure only slightly
modified. In the case of the water-ouzel, the acutest observer, by
examining its dead body, would never have suspected its sub-aquatic
habits; yet this bird, which is allied to the thrush family, subsists by
diving,—using its wings under water and grasping stones with its
feet. All the members of the great order of Hymenopterous insects are
terrestrial, excepting the genus Proctotrupes, which Sir John Lubbock has
discovered to be aquatic in its habits; it often enters the water and
dives about by the use not of its legs but of its wings, and remains as
long as four hours beneath the surface; yet it exhibits no modification in
structure in accordance with its abnormal habits.

He who believes that each being has been created as we now see it, must
occasionally have felt surprise when he has met with an animal having
habits and structure not in agreement. What can be plainer than that the
webbed feet of ducks and geese are formed for swimming? Yet there are
upland geese with webbed feet which rarely go near the water; and no one
except Audubon, has seen the frigate-bird, which has all its four toes
webbed, alight on the surface of the ocean. On the other hand, grebes and
coots are eminently aquatic, although their toes are only bordered by
membrane. What seems plainer than that the long toes, not furnished with
membrane, of the Grallatores, are formed for walking over swamps and
floating plants. The water-hen and landrail are members of this order, yet
the first is nearly as aquatic as the coot, and the second is nearly as
terrestrial as the quail or partridge. In such cases, and many others
could be given, habits have changed without a corresponding change of
structure. The webbed feet of the upland goose may be said to have become
almost rudimentary in function, though not in structure. In the
frigate-bird, the deeply scooped membrane between the toes shows that
structure has begun to change.

He who believes in separate and innumerable acts of creation may say, that
in these cases it has pleased the Creator to cause a being of one type to
take the place of one belonging to another type; but this seems to me only
restating the fact in dignified language. He who believes in the struggle
for existence and in the principle of natural selection, will acknowledge
that every organic being is constantly endeavouring to increase in
numbers; and that if any one being varies ever so little, either in habits
or structure, and thus gains an advantage over some other inhabitant of
the same country, it will seize on the place of that inhabitant, however
different that may be from its own place. Hence it will cause him no
surprise that there should be geese and frigate-birds with webbed feet,
living on the dry land and rarely alighting on the water, that there
should be long-toed corncrakes, living in meadows instead of in swamps;
that there should be woodpeckers where hardly a tree grows; that there
should be diving thrushes and diving Hymenoptera, and petrels with the
habits of auks.

ORGANS OF EXTREME PERFECTION AND COMPLICATION.

To suppose that the eye with all its inimitable contrivances for adjusting
the focus to different distances, for admitting different amounts of
light, and for the correction of spherical and chromatic aberration, could
have been formed by natural selection, seems, I freely confess, absurd in
the highest degree. When it was first said that the sun stood still and
the world turned round, the common sense of mankind declared the doctrine
false; but the old saying of Vox populi, vox Dei, as every philosopher
knows, cannot be trusted in science. Reason tells me, that if numerous
gradations from a simple and imperfect eye to one complex and perfect can
be shown to exist, each grade being useful to its possessor, as is
certainly the case; if further, the eye ever varies and the variations be
inherited, as is likewise certainly the case; and if such variations
should be useful to any animal under changing conditions of life, then the
difficulty of believing that a perfect and complex eye could be formed by
natural selection, though insuperable by our imagination, should not be
considered as subversive of the theory. How a nerve comes to be sensitive
to light, hardly concerns us more than how life itself originated; but I
may remark that, as some of the lowest organisms in which nerves cannot be
detected, are capable of perceiving light, it does not seem impossible
that certain sensitive elements in their sarcode should become aggregated
and developed into nerves, endowed with this special sensibility.

In searching for the gradations through which an organ in any species has
been perfected, we ought to look exclusively to its lineal progenitors;
but this is scarcely ever possible, and we are forced to look to other
species and genera of the same group, that is to the collateral
descendants from the same parent-form, in order to see what gradations are
possible, and for the chance of some gradations having been transmitted in
an unaltered or little altered condition. But the state of the same organ
in distinct classes may incidentally throw light on the steps by which it
has been perfected.

The simplest organ which can be called an eye consists of an optic nerve,
surrounded by pigment-cells and covered by translucent skin, but without
any lens or other refractive body. We may, however, according to M.
Jourdain, descend even a step lower and find aggregates of pigment-cells,
apparently serving as organs of vision, without any nerves, and resting
merely on sarcodic tissue. Eyes of the above simple nature are not capable
of distinct vision, and serve only to distinguish light from darkness. In
certain star-fishes, small depressions in the layer of pigment which
surrounds the nerve are filled, as described by the author just quoted,
with transparent gelatinous matter, projecting with a convex surface, like
the cornea in the higher animals. He suggests that this serves not to form
an image, but only to concentrate the luminous rays and render their
perception more easy. In this concentration of the rays we gain the first
and by far the most important step towards the formation of a true,
picture-forming eye; for we have only to place the naked extremity of the
optic nerve, which in some of the lower animals lies deeply buried in the
body, and in some near the surface, at the right distance from the
concentrating apparatus, and an image will be formed on it.

In the great class of the Articulata, we may start from an optic nerve
simply coated with pigment, the latter sometimes forming a sort of pupil,
but destitute of lens or other optical contrivance. With insects it is now
known that the numerous facets on the cornea of their great compound eyes
form true lenses, and that the cones include curiously modified nervous
filaments. But these organs in the Articulata are so much diversified that
Muller formerly made three main classes with seven subdivisions, besides a
fourth main class of aggregated simple eyes.

When we reflect on these facts, here given much too briefly, with respect
to the wide, diversified, and graduated range of structure in the eyes of
the lower animals; and when we bear in mind how small the number of all
living forms must be in comparison with those which have become extinct,
the difficulty ceases to be very great in believing that natural selection
may have converted the simple apparatus of an optic nerve, coated with
pigment and invested by transparent membrane, into an optical instrument
as perfect as is possessed by any member of the Articulata class.

He who will go thus far, ought not to hesitate to go one step further, if
he finds on finishing this volume that large bodies of facts, otherwise
inexplicable, can be explained by the theory of modification through
natural selection; he ought to admit that a structure even as perfect as
an eagle's eye might thus be formed, although in this case he does not
know the transitional states. It has been objected that in order to modify
the eye and still preserve it as a perfect instrument, many changes would
have to be effected simultaneously, which, it is assumed, could not be
done through natural selection; but as I have attempted to show in my work
on the variation of domestic animals, it is not necessary to suppose that
the modifications were all simultaneous, if they were extremely slight and
gradual. Different kinds of modification would, also, serve for the same
general purpose: as Mr. Wallace has remarked, "If a lens has too short or
too long a focus, it may be amended either by an alteration of curvature,
or an alteration of density; if the curvature be irregular, and the rays
do not converge to a point, then any increased regularity of curvature
will be an improvement. So the contraction of the iris and the muscular
movements of the eye are neither of them essential to vision, but only
improvements which might have been added and perfected at any stage of the
construction of the instrument." Within the highest division of the animal
kingdom, namely, the Vertebrata, we can start from an eye so simple, that
it consists, as in the lancelet, of a little sack of transparent skin,
furnished with a nerve and lined with pigment, but destitute of any other
apparatus. In fishes and reptiles, as Owen has remarked, "The range of
gradation of dioptric structures is very great." It is a significant fact
that even in man, according to the high authority of Virchow, the
beautiful crystalline lens is formed in the embryo by an accumulation of
epidermic cells, lying in a sack-like fold of the skin; and the vitreous
body is formed from embryonic subcutaneous tissue. To arrive, however, at
a just conclusion regarding the formation of the eye, with all its
marvellous yet not absolutely perfect characters, it is indispensable that
the reason should conquer the imagination; but I have felt the difficulty
far to keenly to be surprised at others hesitating to extend the principle
of natural selection to so startling a length.

It is scarcely possible to avoid comparing the eye with a telescope. We
know that this instrument has been perfected by the long-continued efforts
of the highest human intellects; and we naturally infer that the eye has
been formed by a somewhat analogous process. But may not this inference be
presumptuous? Have we any right to assume that the Creator works by
intellectual powers like those of man? If we must compare the eye to an
optical instrument, we ought in imagination to take a thick layer of
transparent tissue, with spaces filled with fluid, and with a nerve
sensitive to light beneath, and then suppose every part of this layer to
be continually changing slowly in density, so as to separate into layers
of different densities and thicknesses, placed at different distances from
each other, and with the surfaces of each layer slowly changing in form.
Further we must suppose that there is a power, represented by natural
selection or the survival of the fittest, always intently watching each
slight alteration in the transparent layers; and carefully preserving each
which, under varied circumstances, in any way or degree, tends to produce
a distincter image. We must suppose each new state of the instrument to be
multiplied by the million; each to be preserved until a better is
produced, and then the old ones to be all destroyed. In living bodies,
variation will cause the slight alteration, generation will multiply them
almost infinitely, and natural selection will pick out with unerring skill
each improvement. Let this process go on for millions of years; and during
each year on millions of individuals of many kinds; and may we not believe
that a living optical instrument might thus be formed as superior to one
of glass, as the works of the Creator are to those of man?

MODES Of TRANSITION.

If it could be demonstrated that any complex organ existed, which could
not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find out
no such case. No doubt many organs exist of which we do not know the
transitional grades, more especially if we look to much-isolated species,
around which, according to the theory, there has been much extinction. Or
again, if we take an organ common to all the members of a class, for in
this latter case the organ must have been originally formed at a remote
period, since which all the many members of the class have been developed;
and in order to discover the early transitional grades through which the
organ has passed, we should have to look to very ancient ancestral forms,
long since become extinct.

We should be extremely cautious in concluding that an organ could not have
been formed by transitional gradations of some kind. Numerous cases could
be given among the lower animals of the same organ performing at the same
time wholly distinct functions; thus in the larva of the dragon-fly and in
the fish Cobites the alimentary canal respires, digests, and excretes. In
the Hydra, the animal may be turned inside out, and the exterior surface
will then digest and the stomach respire. In such cases natural selection
might specialise, if any advantage were thus gained, the whole or part of
an organ, which had previously performed two functions, for one function
alone, and thus by insensible steps greatly change its nature. Many plants
are known which regularly produce at the same time differently constructed
flowers; and if such plants were to produce one kind alone, a great change
would be effected with comparative suddenness in the character of the
species. It is, however, probable that the two sorts of flowers borne by
the same plant were originally differentiated by finely graduated steps,
which may still be followed in some few cases.

Again, two distinct organs, or the same organ under two very different
forms, may simultaneously perform in the same individual the same
function, and this is an extremely important means of transition: to give
one instance—there are fish with gills or branchiae that breathe the
air dissolved in the water, at the same time that they breathe free air in
their swim-bladders, this latter organ being divided by highly vascular
partitions and having a ductus pneumaticus for the supply of air. To give
another instance from the vegetable kingdom: plants climb by three
distinct means, by spirally twining, by clasping a support with their
sensitive tendrils, and by the emission of aerial rootlets; these three
means are usually found in distinct groups, but some few species exhibit
two of the means, or even all three, combined in the same individual. In
all such cases one of the two organs might readily be modified and
perfected so as to perform all the work, being aided during the progress
of modification by the other organ; and then this other organ might be
modified for some other and quite distinct purpose, or be wholly
obliterated.

The illustration of the swim-bladder in fishes is a good one, because it
shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into one
for a widely different purpose, namely respiration. The swim-bladder has,
also, been worked in as an accessory to the auditory organs of certain
fishes. All physiologists admit that the swim-bladder is homologous, or
"ideally similar" in position and structure with the lungs of the higher
vertebrate animals: hence there is no reason to doubt that the
swim-bladder has actually been converted into lungs, or an organ used
exclusively for respiration.

According to this view it may be inferred that all vertebrate animals with
true lungs are descended by ordinary generation from an ancient and
unknown prototype which was furnished with a floating apparatus or
swim-bladder. We can thus, as I infer from Professor Owen's interesting
description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the orifice
of the trachea, with some risk of falling into the lungs, notwithstanding
the beautiful contrivance by which the glottis is closed. In the higher
Vertebrata the branchiae have wholly disappeared—but in the embryo
the slits on the sides of the neck and the loop-like course of the
arteries still mark their former position. But it is conceivable that the
now utterly lost branchiae might have been gradually worked in by natural
selection for some distinct purpose: for instance, Landois has shown that
the wings of insects are developed from the trachea; it is therefore
highly probable that in this great class organs which once served for
respiration have been actually converted into organs for flight.

In considering transitions of organs, it is so important to bear in mind
the probability of conversion from one function to another, that I will
give another instance. Pedunculated cirripedes have two minute folds of
skin, called by me the ovigerous frena, which serve, through the means of
a sticky secretion, to retain the eggs until they are hatched within the
sack. These cirripedes have no branchiae, the whole surface of the body
and of the sack, together with the small frena, serving for respiration.
The Balanidae or sessile cirripedes, on the other hand, have no ovigerous
frena, the eggs lying loose at the bottom of the sack, within the
well-enclosed shell; but they have, in the same relative position with the
frena, large, much-folded membranes, which freely communicate with the
circulatory lacunae of the sack and body, and which have been considered
by all naturalists to act as branchiae. Now I think no one will dispute
that the ovigerous frena in the one family are strictly homologous with
the branchiae of the other family; indeed, they graduate into each other.
Therefore it need not be doubted that the two little folds of skin, which
originally served as ovigerous frena, but which, likewise, very slightly
aided in the act of respiration, have been gradually converted by natural
selection into branchiae, simply through an increase in their size and the
obliteration of their adhesive glands. If all pedunculated cirripedes had
become extinct, and they have suffered far more extinction than have
sessile cirripedes, who would ever have imagined that the branchiae in
this latter family had originally existed as organs for preventing the ova
from being washed out of the sack?

There is another possible mode of transition, namely, through the
acceleration or retardation of the period of reproduction. This has lately
been insisted on by Professor Cope and others in the United States. It is
now known that some animals are capable of reproduction at a very early
age, before they have acquired their perfect characters; and if this power
became thoroughly well developed in a species, it seems probable that the
adult stage of development would sooner or later be lost; and in this
case, especially if the larva differed much from the mature form, the
character of the species would be greatly changed and degraded. Again, not
a few animals, after arriving at maturity, go on changing in character
during nearly their whole lives. With mammals, for instance, the form of
the skull is often much altered with age, of which Dr. Murie has given
some striking instances with seals. Every one knows how the horns of stags
become more and more branched, and the plumes of some birds become more
finely developed, as they grow older. Professor Cope states that the teeth
of certain lizards change much in shape with advancing years. With
crustaceans not only many trivial, but some important parts assume a new
character, as recorded by Fritz Muller, after maturity. In all such cases—and
many could be given—if the age for reproduction were retarded, the
character of the species, at least in its adult state, would be modified;
nor is it improbable that the previous and earlier stages of development
would in some cases be hurried through and finally lost. Whether species
have often or ever been modified through this comparatively sudden mode of
transition, I can form no opinion; but if this has occurred, it is
probable that the differences between the young and the mature, and
between the mature and the old, were primordially acquired by graduated
steps.

SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL SELECTION.

Although we must be extremely cautious in concluding that any organ could
not have been produced by successive, small, transitional gradations, yet
undoubtedly serious cases of difficulty occur.

One of the most serious is that of neuter insects, which are often
differently constructed from either the males or fertile females; but this
case will be treated of in the next chapter. The electric organs of fishes
offer another case of special difficulty; for it is impossible to conceive
by what steps these wondrous organs have been produced. But this is not
surprising, for we do not even know of what use they are. In the gymnotus
and torpedo they no doubt serve as powerful means of defence, and perhaps
for securing prey; yet in the ray, as observed by Matteucci, an analogous
organ in the tail manifests but little electricity, even when the animal
is greatly irritated; so little that it can hardly be of any use for the
above purposes. Moreover, in the ray, besides the organ just referred to,
there is, as Dr. R. McDonnell has shown, another organ near the head, not
known to be electrical, but which appears to be the real homologue of the
electric battery in the torpedo. It is generally admitted that there
exists between these organs and ordinary muscle a close analogy, in
intimate structure, in the distribution of the nerves, and in the manner
in which they are acted on by various reagents. It should, also, be
especially observed that muscular contraction is accompanied by an
electrical discharge; and, as Dr. Radcliffe insists, "in the electrical
apparatus of the torpedo during rest, there would seem to be a charge in
every respect like that which is met with in muscle and nerve during the
rest, and the discharge of the torpedo, instead of being peculiar, may be
only another form of the discharge which attends upon the action of muscle
and motor nerve." Beyond this we cannot at present go in the way of
explanation; but as we know so little about the uses of these organs, and
as we know nothing about the habits and structure of the progenitors of
the existing electric fishes, it would be extremely bold to maintain that
no serviceable transitions are possible by which these organs might have
been gradually developed.

These organs appear at first to offer another and far more serious
difficulty; for they occur in about a dozen kinds of fish, of which
several are widely remote in their affinities. When the same organ is
found in several members of the same class, especially if in members
having very different habits of life, we may generally attribute its
presence to inheritance from a common ancestor; and its absence in some of
the members to loss through disuse or natural selection. So that, if the
electric organs had been inherited from some one ancient progenitor, we
might have expected that all electric fishes would have been specially
related to each other; but this is far from the case. Nor does geology at
all lead to the belief that most fishes formerly possessed electric
organs, which their modified descendants have now lost. But when we look
at the subject more closely, we find in the several fishes provided with
electric organs, that these are situated in different parts of the body,
that they differ in construction, as in the arrangement of the plates,
and, according to Pacini, in the process or means by which the electricity
is excited—and lastly, in being supplied with nerves proceeding from
different sources, and this is perhaps the most important of all the
differences. Hence in the several fishes furnished with electric organs,
these cannot be considered as homologous, but only as analogous in
function. Consequently there is no reason to suppose that they have been
inherited from a common progenitor; for had this been the case they would
have closely resembled each other in all respects. Thus the difficulty of
an organ, apparently the same, arising in several remotely allied species,
disappears, leaving only the lesser yet still great difficulty: namely, by
what graduated steps these organs have been developed in each separate
group of fishes.

The luminous organs which occur in a few insects, belonging to widely
different families, and which are situated in different parts of the body,
offer, under our present state of ignorance, a difficulty almost exactly
parallel with that of the electric organs. Other similar cases could be
given; for instance in plants, the very curious contrivance of a mass of
pollen-grains, borne on a foot-stalk with an adhesive gland, is apparently
the same in Orchis and Asclepias, genera almost as remote as is possible
among flowering plants; but here again the parts are not homologous. In
all cases of beings, far removed from each other in the scale of
organisation, which are furnished with similar and peculiar organs, it
will be found that although the general appearance and function of the
organs may be the same, yet fundamental differences between them can
always be detected. For instance, the eyes of Cephalopods or cuttle-fish
and of vertebrate animals appear wonderfully alike; and in such widely
sundered groups no part of this resemblance can be due to inheritance from
a common progenitor. Mr. Mivart has advanced this case as one of special
difficulty, but I am unable to see the force of his argument. An organ for
vision must be formed of transparent tissue, and must include some sort of
lens for throwing an image at the back of a darkened chamber. Beyond this
superficial resemblance, there is hardly any real similarity between the
eyes of cuttle-fish and vertebrates, as may be seen by consulting Hensen's
admirable memoir on these organs in the Cephalopoda. It is impossible for
me here to enter on details, but I may specify a few of the points of
difference. The crystalline lens in the higher cuttle-fish consists of two
parts, placed one behind the other like two lenses, both having a very
different structure and disposition to what occurs in the vertebrata. The
retina is wholly different, with an actual inversion of the elemental
parts, and with a large nervous ganglion included within the membranes of
the eye. The relations of the muscles are as different as it is possible
to conceive, and so in other points. Hence it is not a little difficult to
decide how far even the same terms ought to be employed in describing the
eyes of the Cephalopoda and Vertebrata. It is, of course, open to any one
to deny that the eye in either case could have been developed through the
natural selection of successive slight variations; but if this be admitted
in the one case it is clearly possible in the other; and fundamental
differences of structure in the visual organs of two groups might have
been anticipated, in accordance with this view of their manner of
formation. As two men have sometimes independently hit on the same
invention, so in the several foregoing cases it appears that natural
selection, working for the good of each being, and taking advantage of all
favourable variations, has produced similar organs, as far as function is
concerned, in distinct organic beings, which owe none of their structure
in common to inheritance from a common progenitor.

Fritz Muller, in order to test the conclusions arrived at in this volume,
has followed out with much care a nearly similar line of argument. Several
families of crustaceans include a few species, possessing an air-breathing
apparatus and fitted to live out of the water. In two of these families,
which were more especially examined by Muller, and which are nearly
related to each other, the species agree most closely in all important
characters: namely in their sense organs, circulating systems, in the
position of the tufts of hair within their complex stomachs, and lastly in
the whole structure of the water-breathing branchiae, even to the
microscopical hooks by which they are cleansed. Hence it might have been
expected that in the few species belonging to both families which live on
the land, the equally important air-breathing apparatus would have been
the same; for why should this one apparatus, given for the same purpose,
have been made to differ, while all the other important organs were
closely similar, or rather, identical.

Fritz Muller argues that this close similarity in so many points of
structure must, in accordance with the views advanced by me, be accounted
for by inheritance from a common progenitor. But as the vast majority of
the species in the above two families, as well as most other crustaceans,
are aquatic in their habits, it is improbable in the highest degree that
their common progenitor should have been adapted for breathing air. Muller
was thus led carefully to examine the apparatus in the air-breathing
species; and he found it to differ in each in several important points, as
in the position of the orifices, in the manner in which they are opened
and closed, and in some accessory details. Now such differences are
intelligible, and might even have been expected, on the supposition that
species belonging to distinct families had slowly become adapted to live
more and more out of water, and to breathe the air. For these species,
from belonging to distinct families, would have differed to a certain
extent, and in accordance with the principle that the nature of each
variation depends on two factors, viz., the nature of the organism and
that of the surrounding conditions, their variability assuredly would not
have been exactly the same. Consequently natural selection would have had
different materials or variations to work on, in order to arrive at the
same functional result; and the structures thus acquired would almost
necessarily have differed. On the hypothesis of separate acts of creation
the whole case remains unintelligible. This line of argument seems to have
had great weight in leading Fritz Muller to accept the views maintained by
me in this volume.

Another distinguished zoologist, the late Professor Claparede, has argued
in the same manner, and has arrived at the same result. He shows that
there are parasitic mites (Acaridae), belonging to distinct sub-families
and families, which are furnished with hair-claspers. These organs must
have been independently developed, as they could not have been inherited
from a common progenitor; and in the several groups they are formed by the
modification of the fore legs, of the hind legs, of the maxillae or lips,
and of appendages on the under side of the hind part of the body.

In the foregoing cases, we see the same end gained and the same function
performed, in beings not at all or only remotely allied, by organs in
appearance, though not in development, closely similar. On the other hand,
it is a common rule throughout nature that the same end should be gained,
even sometimes in the case of closely related beings, by the most
diversified means. How differently constructed is the feathered wing of a
bird and the membrane-covered wing of a bat; and still more so the four
wings of a butterfly, the two wings of a fly, and the two wings with the
elytra of a beetle. Bivalve shells are made to open and shut, but on what
a number of patterns is the hinge constructed, from the long row of neatly
interlocking teeth in a Nucula to the simple ligament of a Mussel! Seeds
are disseminated by their minuteness, by their capsule being converted
into a light balloon-like envelope, by being embedded in pulp or flesh,
formed of the most diverse parts, and rendered nutritious, as well as
conspicuously coloured, so as to attract and be devoured by birds, by
having hooks and grapnels of many kinds and serrated awns, so as to adhere
to the fur of quadrupeds, and by being furnished with wings and plumes, as
different in shape as they are elegant in structure, so as to be wafted by
every breeze. I will give one other instance: for this subject of the same
end being gained by the most diversified means well deserves attention.
Some authors maintain that organic beings have been formed in many ways
for the sake of mere variety, almost like toys in a shop, but such a view
of nature is incredible. With plants having separated sexes, and with
those in which, though hermaphrodites, the pollen does not spontaneously
fall on the stigma, some aid is necessary for their fertilisation. With
several kinds this is effected by the pollen-grains, which are light and
incoherent, being blown by the wind through mere chance on to the stigma;
and this is the simplest plan which can well be conceived. An almost
equally simple, though very different plan occurs in many plants in which
a symmetrical flower secretes a few drops of nectar, and is consequently
visited by insects; and these carry the pollen from the anthers to the
stigma.

From this simple stage we may pass through an inexhaustible number of
contrivances, all for the same purpose and effected in essentially the
same manner, but entailing changes in every part of the flower. The nectar
may be stored in variously shaped receptacles, with the stamens and
pistils modified in many ways, sometimes forming trap-like contrivances,
and sometimes capable of neatly adapted movements through irritability or
elasticity. From such structures we may advance till we come to such a
case of extraordinary adaptation as that lately described by Dr. Cruger in
the Coryanthes. This orchid has part of its labellum or lower lip hollowed
out into a great bucket, into which drops of almost pure water continually
fall from two secreting horns which stand above it; and when the bucket is
half-full, the water overflows by a spout on one side. The basal part of
the labellum stands over the bucket, and is itself hollowed out into a
sort of chamber with two lateral entrances; within this chamber there are
curious fleshy ridges. The most ingenious man, if he had not witnessed
what takes place, could never have imagined what purpose all these parts
serve. But Dr. Cruger saw crowds of large humble-bees visiting the
gigantic flowers of this orchid, not in order to suck nectar, but to gnaw
off the ridges within the chamber above the bucket; in doing this they
frequently pushed each other into the bucket, and their wings being thus
wetted they could not fly away, but were compelled to crawl out through
the passage formed by the spout or overflow. Dr. Cruger saw a "continual
procession" of bees thus crawling out of their involuntary bath. The
passage is narrow, and is roofed over by the column, so that a bee, in
forcing its way out, first rubs its back against the viscid stigma and
then against the viscid glands of the pollen-masses. The pollen-masses are
thus glued to the back of the bee which first happens to crawl out through
the passage of a lately expanded flower, and are thus carried away. Dr.
Cruger sent me a flower in spirits of wine, with a bee which he had killed
before it had quite crawled out, with a pollen-mass still fastened to its
back. When the bee, thus provided, flies to another flower, or to the same
flower a second time, and is pushed by its comrades into the bucket and
then crawls out by the passage, the pollen-mass necessarily comes first
into contact with the viscid stigma, and adheres to it, and the flower is
fertilised. Now at last we see the full use of every part of the flower,
of the water-secreting horns of the bucket half-full of water, which
prevents the bees from flying away, and forces them to crawl out through
the spout, and rub against the properly placed viscid pollen-masses and
the viscid stigma.

The construction of the flower in another closely allied orchid, namely,
the Catasetum, is widely different, though serving the same end; and is
equally curious. Bees visit these flowers, like those of the Coryanthes,
in order to gnaw the labellum; in doing this they inevitably touch a long,
tapering, sensitive projection, or, as I have called it, the antenna. This
antenna, when touched, transmits a sensation or vibration to a certain
membrane which is instantly ruptured; this sets free a spring by which the
pollen-mass is shot forth, like an arrow, in the right direction, and
adheres by its viscid extremity to the back of the bee. The pollen-mass of
the male plant (for the sexes are separate in this orchid) is thus carried
to the flower of the female plant, where it is brought into contact with
the stigma, which is viscid enough to break certain elastic threads, and
retain the pollen, thus effecting fertilisation.

How, it may be asked, in the foregoing and in innumerable other instances,
can we understand the graduated scale of complexity and the multifarious
means for gaining the same end. The answer no doubt is, as already
remarked, that when two forms vary, which already differ from each other
in some slight degree, the variability will not be of the same exact
nature, and consequently the results obtained through natural selection
for the same general purpose will not be the same. We should also bear in
mind that every highly developed organism has passed through many changes;
and that each modified structure tends to be inherited, so that each
modification will not readily be quite lost, but may be again and again
further altered. Hence, the structure of each part of each species, for
whatever purpose it may serve, is the sum of many inherited changes,
through which the species has passed during its successive adaptations to
changed habits and conditions of life.

Finally, then, although in many cases it is most difficult even to
conjecture by what transitions organs could have arrived at their present
state; yet, considering how small the proportion of living and known forms
is to the extinct and unknown, I have been astonished how rarely an organ
can be named, towards which no transitional grade is known to lead. It is
certainly true, that new organs appearing as if created for some special
purpose rarely or never appear in any being; as indeed is shown by that
old, but somewhat exaggerated, canon in natural history of "Natura non
facit saltum." We meet with this admission in the writings of almost every
experienced naturalist; or, as Milne Edwards has well expressed it,
"Nature is prodigal in variety, but niggard in innovation." Why, on the
theory of Creation, should there be so much variety and so little real
novelty? Why should all the parts and organs of many independent beings,
each supposed to have been separately created for its own proper place in
nature, be so commonly linked together by graduated steps? Why should not
Nature take a sudden leap from structure to structure? On the theory of
natural selection, we can clearly understand why she should not; for
natural selection acts only by taking advantage of slight successive
variations; she can never take a great and sudden leap, but must advance
by the short and sure, though slow steps.

ORGANS OF LITTLE APPARENT IMPORTANCE, AS AFFECTED BY NATURAL SELECTION.

As natural selection acts by life and death, by the survival of the
fittest, and by the destruction of the less well-fitted individuals, I
have sometimes felt great difficulty in understanding the origin or
formation of parts of little importance; almost as great, though of a very
different kind, as in the case of the most perfect and complex organs.

In the first place, we are much too ignorant in regard to the whole
economy of any one organic being to say what slight modifications would be
of importance or not. In a former chapter I have given instances of very
trifling characters, such as the down on fruit and the colour of its
flesh, the colour of the skin and hair of quadrupeds, which, from being
correlated with constitutional differences, or from determining the
attacks of insects, might assuredly be acted on by natural selection. The
tail of the giraffe looks like an artificially constructed fly-flapper;
and it seems at first incredible that this could have been adapted for its
present purpose by successive slight modifications, each better and better
fitted, for so trifling an object as to drive away flies; yet we should
pause before being too positive even in this case, for we know that the
distribution and existence of cattle and other animals in South America
absolutely depend on their power of resisting the attacks of insects: so
that individuals which could by any means defend themselves from these
small enemies, would be able to range into new pastures and thus gain a
great advantage. It is not that the larger quadrupeds are actually
destroyed (except in some rare cases) by flies, but they are incessantly
harassed and their strength reduced, so that they are more subject to
disease, or not so well enabled in a coming dearth to search for food, or
to escape from beasts of prey.

Organs now of trifling importance have probably in some cases been of high
importance to an early progenitor, and, after having been slowly perfected
at a former period, have been transmitted to existing species in nearly
the same state, although now of very slight use; but any actually
injurious deviations in their structure would of course have been checked
by natural selection. Seeing how important an organ of locomotion the tail
is in most aquatic animals, its general presence and use for many purposes
in so many land animals, which in their lungs or modified swim-bladders
betray their aquatic origin, may perhaps be thus accounted for. A
well-developed tail having been formed in an aquatic animal, it might
subsequently come to be worked in for all sorts of purposes, as a
fly-flapper, an organ of prehension, or as an aid in turning, as in the
case of the dog, though the aid in this latter respect must be slight, for
the hare, with hardly any tail, can double still more quickly.

In the second place, we may easily err in attributing importance to
characters, and in believing that they have been developed through natural
selection. We must by no means overlook the effects of the definite action
of changed conditions of life, of so-called spontaneous variations, which
seem to depend in a quite subordinate degree on the nature of the
conditions, of the tendency to reversion to long-lost characters, of the
complex laws of growth, such as of correlation, comprehension, of the
pressure of one part on another, etc., and finally of sexual selection, by
which characters of use to one sex are often gained and then transmitted
more or less perfectly to the other sex, though of no use to the sex. But
structures thus indirectly gained, although at first of no advantage to a
species, may subsequently have been taken advantage of by its modified
descendants, under new conditions of life and newly acquired habits.

If green woodpeckers alone had existed, and we did not know that there
were many black and pied kinds, I dare say that we should have thought
that the green colour was a beautiful adaptation to conceal this
tree-frequenting bird from its enemies; and consequently that it was a
character of importance, and had been acquired through natural selection;
as it is, the colour is probably in chief part due to sexual selection. A
trailing palm in the Malay Archipelago climbs the loftiest trees by the
aid of exquisitely constructed hooks clustered around the ends of the
branches, and this contrivance, no doubt, is of the highest service to the
plant; but as we see nearly similar hooks on many trees which are not
climbers, and which, as there is reason to believe from the distribution
of the thorn-bearing species in Africa and South America, serve as a
defence against browsing quadrupeds, so the spikes on the palm may at
first have been developed for this object, and subsequently have been
improved and taken advantage of by the plant, as it underwent further
modification and became a climber. The naked skin on the head of a vulture
is generally considered as a direct adaptation for wallowing in putridity;
and so it may be, or it may possibly be due to the direct action of putrid
matter; but we should be very cautious in drawing any such inference, when
we see that the skin on the head of the clean-feeding male turkey is
likewise naked. The sutures in the skulls of young mammals have been
advanced as a beautiful adaptation for aiding parturition, and no doubt
they facilitate, or may be indispensable for this act; but as sutures
occur in the skulls of young birds and reptiles, which have only to escape
from a broken egg, we may infer that this structure has arisen from the
laws of growth, and has been taken advantage of in the parturition of the
higher animals.

We are profoundly ignorant of the cause of each slight variation or
individual difference; and we are immediately made conscious of this by
reflecting on the differences between the breeds of our domesticated
animals in different countries, more especially in the less civilized
countries, where there has been but little methodical selection. Animals
kept by savages in different countries often have to struggle for their
own subsistence, and are exposed to a certain extent to natural selection,
and individuals with slightly different constitutions would succeed best
under different climates. With cattle susceptibility to the attacks of
flies is correlated with colour, as is the liability to be poisoned by
certain plants; so that even colour would be thus subjected to the action
of natural selection. Some observers are convinced that a damp climate
affects the growth of the hair, and that with the hair the horns are
correlated. Mountain breeds always differ from lowland breeds; and a
mountainous country would probably affect the hind limbs from exercising
them more, and possibly even the form of the pelvis; and then by the law
of homologous variation, the front limbs and the head would probably be
affected. The shape, also, of the pelvis might affect by pressure the
shape of certain parts of the young in the womb. The laborious breathing
necessary in high regions tends, as we have good reason to believe, to
increase the size of the chest; and again correlation would come into
play. The effects of lessened exercise, together with abundant food, on
the whole organisation is probably still more important, and this, as H.
von Nathusius has lately shown in his excellent Treatise, is apparently
one chief cause of the great modification which the breeds of swine have
undergone. But we are far too ignorant to speculate on the relative
importance of the several known and unknown causes of variation; and I
have made these remarks only to show that, if we are unable to account for
the characteristic differences of our several domestic breeds, which
nevertheless are generally admitted to have arisen through ordinary
generation from one or a few parent-stocks, we ought not to lay too much
stress on our ignorance of the precise cause of the slight analogous
differences between true species.

UTILITARIAN DOCTRINE, HOW FAR TRUE: BEAUTY, HOW ACQUIRED.

The foregoing remarks lead me to say a few words on the protest lately
made by some naturalists against the utilitarian doctrine that every
detail of structure has been produced for the good of its possessor. They
believe that many structures have been created for the sake of beauty, to
delight man or the Creator (but this latter point is beyond the scope of
scientific discussion), or for the sake of mere variety, a view already
discussed. Such doctrines, if true, would be absolutely fatal to my
theory. I fully admit that many structures are now of no direct use to
their possessors, and may never have been of any use to their progenitors;
but this does not prove that they were formed solely for beauty or
variety. No doubt the definite action of changed conditions, and the
various causes of modifications, lately specified, have all produced an
effect, probably a great effect, independently of any advantage thus
gained. But a still more important consideration is that the chief part of
the organisation of every living creature is due to inheritance; and
consequently, though each being assuredly is well fitted for its place in
nature, many structures have now no very close and direct relation to
present habits of life. Thus, we can hardly believe that the webbed feet
of the upland goose, or of the frigate-bird, are of special use to these
birds; we cannot believe that the similar bones in the arm of the monkey,
in the fore leg of the horse, in the wing of the bat, and in the flipper
of the seal, are of special use to these animals. We may safely attribute
these structures to inheritance. But webbed feet no doubt were as useful
to the progenitor of the upland goose and of the frigate-bird, as they now
are to the most aquatic of living birds. So we may believe that the
progenitor of the seal did not possess a flipper, but a foot with five
toes fitted for walking or grasping; and we may further venture to believe
that the several bones in the limbs of the monkey, horse and bat, were
originally developed, on the principle of utility, probably through the
reduction of more numerous bones in the fin of some ancient fish-like
progenitor of the whole class. It is scarcely possible to decide how much
allowance ought to be made for such causes of change, as the definite
action of external conditions, so-called spontaneous variations, and the
complex laws of growth; but with these important exceptions, we may
conclude that the structure of every living creature either now is, or was
formerly, of some direct or indirect use to its possessor.

With respect to the belief that organic beings have been created beautiful
for the delight of man—a belief which it has been pronounced is
subversive of my whole theory—I may first remark that the sense of
beauty obviously depends on the nature of the mind, irrespective of any
real quality in the admired object; and that the idea of what is
beautiful, is not innate or unalterable. We see this, for instance, in the
men of different races admiring an entirely different standard of beauty
in their women. If beautiful objects had been created solely for man's
gratification, it ought to be shown that before man appeared there was
less beauty on the face of the earth than since he came on the stage. Were
the beautiful volute and cone shells of the Eocene epoch, and the
gracefully sculptured ammonites of the Secondary period, created that man
might ages afterwards admire them in his cabinet? Few objects are more
beautiful than the minute siliceous cases of the diatomaceae: were these
created that they might be examined and admired under the higher powers of
the microscope? The beauty in this latter case, and in many others, is
apparently wholly due to symmetry of growth. Flowers rank among the most
beautiful productions of nature; but they have been rendered conspicuous
in contrast with the green leaves, and in consequence at the same time
beautiful, so that they may be easily observed by insects. I have come to
this conclusion from finding it an invariable rule that when a flower is
fertilised by the wind it never has a gaily-coloured corolla. Several
plants habitually produce two kinds of flowers; one kind open and coloured
so as to attract insects; the other closed, not coloured, destitute of
nectar, and never visited by insects. Hence, we may conclude that, if
insects had not been developed on the face of the earth, our plants would
not have been decked with beautiful flowers, but would have produced only
such poor flowers as we see on our fir, oak, nut and ash trees, on
grasses, spinach, docks and nettles, which are all fertilised through the
agency of the wind. A similar line of argument holds good with fruits;
that a ripe strawberry or cherry is as pleasing to the eye as to the
palate—that the gaily-coloured fruit of the spindle-wood tree and
the scarlet berries of the holly are beautiful objects—will be
admitted by everyone. But this beauty serves merely as a guide to birds
and beasts, in order that the fruit may be devoured and the matured seeds
disseminated. I infer that this is the case from having as yet found no
exception to the rule that seeds are always thus disseminated when
embedded within a fruit of any kind (that is within a fleshy or pulpy
envelope), if it be coloured of any brilliant tint, or rendered
conspicuous by being white or black.

On the other hand, I willingly admit that a great number of male animals,
as all our most gorgeous birds, some fishes, reptiles, and mammals, and a
host of magnificently coloured butterflies, have been rendered beautiful
for beauty's sake. But this has been effected through sexual selection,
that is, by the more beautiful males having been continually preferred by
the females, and not for the delight of man. So it is with the music of
birds. We may infer from all this that a nearly similar taste for
beautiful colours and for musical sounds runs through a large part of the
animal kingdom. When the female is as beautifully coloured as the male,
which is not rarely the case with birds and butterflies, the cause
apparently lies in the colours acquired through sexual selection having
been transmitted to both sexes, instead of to the males alone. How the
sense of beauty in its simplest form—that is, the reception of a
peculiar kind of pleasure from certain colours, forms and sounds—was
first developed in the mind of man and of the lower animals, is a very
obscure subject. The same sort of difficulty is presented if we enquire
how it is that certain flavours and odours give pleasure, and others
displeasure. Habit in all these cases appears to have come to a certain
extent into play; but there must be some fundamental cause in the
constitution of the nervous system in each species.

Natural selection cannot possibly produce any modification in a species
exclusively for the good of another species; though throughout nature one
species incessantly takes advantage of, and profits by the structures of
others. But natural selection can and does often produce structures for
the direct injury of other animals, as we see in the fang of the adder,
and in the ovipositor of the ichneumon, by which its eggs are deposited in
the living bodies of other insects. If it could be proved that any part of
the structure of any one species had been formed for the exclusive good of
another species, it would annihilate my theory, for such could not have
been produced through natural selection. Although many statements may be
found in works on natural history to this effect, I cannot find even one
which seems to me of any weight. It is admitted that the rattlesnake has a
poison-fang for its own defence and for the destruction of its prey; but
some authors suppose that at the same time it is furnished with a rattle
for its own injury, namely, to warn its prey. I would almost as soon
believe that the cat curls the end of its tail when preparing to spring,
in order to warn the doomed mouse. It is a much more probable view that
the rattlesnake uses its rattle, the cobra expands its frill and the
puff-adder swells while hissing so loudly and harshly, in order to alarm
the many birds and beasts which are known to attack even the most venomous
species. Snakes act on the same principle which makes the hen ruffle her
feathers and expand her wings when a dog approaches her chickens. But I
have not space here to enlarge on the many ways by which animals endeavour
to frighten away their enemies.

Natural selection will never produce in a being any structure more
injurious than beneficial to that being, for natural selection acts solely
by and for the good of each. No organ will be formed, as Paley has
remarked, for the purpose of causing pain or for doing an injury to its
possessor. If a fair balance be struck between the good and evil caused by
each part, each will be found on the whole advantageous. After the lapse
of time, under changing conditions of life, if any part comes to be
injurious, it will be modified; or if it be not so, the being will become
extinct, as myriads have become extinct.

Natural selection tends only to make each organic being as perfect as, or
slightly more perfect than the other inhabitants of the same country with
which it comes into competition. And we see that this is the standard of
perfection attained under nature. The endemic productions of New Zealand,
for instance, are perfect, one compared with another; but they are now
rapidly yielding before the advancing legions of plants and animals
introduced from Europe. Natural selection will not produce absolute
perfection, nor do we always meet, as far as we can judge, with this high
standard under nature. The correction for the aberration of light is said
by Muller not to be perfect even in that most perfect organ, the human
eye. Helmholtz, whose judgment no one will dispute, after describing in
the strongest terms the wonderful powers of the human eye, adds these
remarkable words: "That which we have discovered in the way of inexactness
and imperfection in the optical machine and in the image on the retina, is
as nothing in comparison with the incongruities which we have just come
across in the domain of the sensations. One might say that nature has
taken delight in accumulating contradictions in order to remove all
foundation from the theory of a pre-existing harmony between the external
and internal worlds." If our reason leads us to admire with enthusiasm a
multitude of inimitable contrivances in nature, this same reason tells us,
though we may easily err on both sides, that some other contrivances are
less perfect. Can we consider the sting of the bee as perfect, which, when
used against many kinds of enemies, cannot be withdrawn, owing to the
backward serratures, and thus inevitably causes the death of the insect by
tearing out its viscera?

If we look at the sting of the bee, as having existed in a remote
progenitor, as a boring and serrated instrument, like that in so many
members of the same great order, and that it has since been modified but
not perfected for its present purpose, with the poison originally adapted
for some other object, such as to produce galls, since intensified, we can
perhaps understand how it is that the use of the sting should so often
cause the insect's own death: for if on the whole the power of stinging be
useful to the social community, it will fulfil all the requirements of
natural selection, though it may cause the death of some few members. If
we admire the truly wonderful power of scent by which the males of many
insects find their females, can we admire the production for this single
purpose of thousands of drones, which are utterly useless to the community
for any other purpose, and which are ultimately slaughtered by their
industrious and sterile sisters? It may be difficult, but we ought to
admire the savage instinctive hatred of the queen-bee, which urges her to
destroy the young queens, her daughters, as soon as they are born, or to
perish herself in the combat; for undoubtedly this is for the good of the
community; and maternal love or maternal hatred, though the latter
fortunately is most rare, is all the same to the inexorable principles of
natural selection. If we admire the several ingenious contrivances by
which orchids and many other plants are fertilised through insect agency,
can we consider as equally perfect the elaboration of dense clouds of
pollen by our fir-trees, so that a few granules may be wafted by chance on
to the ovules?

SUMMARY: THE LAW OF UNITY OF TYPE AND OF THE CONDITIONS
 OF EXISTENCE
EMBRACED BY THE THEORY OF NATURAL SELECTION.

We have in this chapter discussed some of the difficulties and objections
which may be urged against the theory. Many of them are serious; but I
think that in the discussion light has been thrown on several facts, which
on the belief of independent acts of creation are utterly obscure. We have
seen that species at any one period are not indefinitely variable, and are
not linked together by a multitude of intermediate gradations, partly
because the process of natural selection is always very slow, and at any
one time acts only on a few forms; and partly because the very process of
natural selection implies the continual supplanting and extinction of
preceding and intermediate gradations. Closely allied species, now living
on a continuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly graduate
away from one part to another. When two varieties are formed in two
districts of a continuous area, an intermediate variety will often be
formed, fitted for an intermediate zone; but from reasons assigned, the
intermediate variety will usually exist in lesser numbers than the two
forms which it connects; consequently the two latter, during the course of
further modification, from existing in greater numbers, will have a great
advantage over the less numerous intermediate variety, and will thus
generally succeed in supplanting and exterminating it.

We have seen in this chapter how cautious we should be in concluding that
the most different habits of life could not graduate into each other; that
a bat, for instance, could not have been formed by natural selection from
an animal which at first only glided through the air.

We have seen that a species under new conditions of life may change its
habits, or it may have diversified habits, with some very unlike those of
its nearest congeners. Hence we can understand, bearing in mind that each
organic being is trying to live wherever it can live, how it has arisen
that there are upland geese with webbed feet, ground woodpeckers, diving
thrushes, and petrels with the habits of auks.

Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is enough to stagger any one; yet in the case
of any organ, if we know of a long series of gradations in complexity,
each good for its possessor, then under changing conditions of life, there
is no logical impossibility in the acquirement of any conceivable degree
of perfection through natural selection. In the cases in which we know of
no intermediate or transitional states, we should be extremely cautious in
concluding that none can have existed, for the metamorphoses of many
organs show what wonderful changes in function are at least possible. For
instance, a swim-bladder has apparently been converted into an
air-breathing lung. The same organ having performed simultaneously very
different functions, and then having been in part or in whole specialised
for one function; and two distinct organs having performed at the same
time the same function, the one having been perfected whilst aided by the
other, must often have largely facilitated transitions.

We have seen that in two beings widely remote from each other in the
natural scale, organs serving for the same purpose and in external
appearance closely similar may have been separately and independently
formed; but when such organs are closely examined, essential differences
in their structure can almost always be detected; and this naturally
follows from the principle of natural selection. On the other hand, the
common rule throughout nature is infinite diversity of structure for
gaining the same end; and this again naturally follows from the same great
principle.

In many cases we are far too ignorant to be enabled to assert that a part
or organ is so unimportant for the welfare of a species, that
modifications in its structure could not have been slowly accumulated by
means of natural selection. In many other cases, modifications are
probably the direct result of the laws of variation or of growth,
independently of any good having been thus gained. But even such
structures have often, as we may feel assured, been subsequently taken
advantage of, and still further modified, for the good of species under
new conditions of life. We may, also, believe that a part formerly of high
importance has frequently been retained (as the tail of an aquatic animal
by its terrestrial descendants), though it has become of such small
importance that it could not, in its present state, have been acquired by
means of natural selection.

Natural selection can produce nothing in one species for the exclusive
good or injury of another; though it may well produce parts, organs, and
excretions highly useful or even indispensable, or highly injurious to
another species, but in all cases at the same time useful to the
possessor. In each well-stocked country natural selection acts through the
competition of the inhabitants and consequently leads to success in the
battle for life, only in accordance with the standard of that particular
country. Hence the inhabitants of one country, generally the smaller one,
often yield to the inhabitants of another and generally the larger
country. For in the larger country there will have existed more
individuals, and more diversified forms, and the competition will have
been severer, and thus the standard of perfection will have been rendered
higher. Natural selection will not necessarily lead to absolute
perfection; nor, as far as we can judge by our limited faculties, can
absolute perfection be everywhere predicated.

On the theory of natural selection we can clearly understand the full
meaning of that old canon in natural history, "Natura non facit saltum."
This canon, if we look to the present inhabitants alone of the world, is
not strictly correct; but if we include all those of past times, whether
known or unknown, it must on this theory be strictly true.

It is generally acknowledged that all organic beings have been formed on
two great laws—Unity of Type, and the Conditions of Existence. By
unity of type is meant that fundamental agreement in structure which we
see in organic beings of the same class, and which is quite independent of
their habits of life. On my theory, unity of type is explained by unity of
descent. The expression of conditions of existence, so often insisted on
by the illustrious Cuvier, is fully embraced by the principle of natural
selection. For natural selection acts by either now adapting the varying
parts of each being to its organic and inorganic conditions of life; or by
having adapted them during past periods of time: the adaptations being
aided in many cases by the increased use or disuse of parts, being
affected by the direct action of external conditions of life, and
subjected in all cases to the several laws of growth and variation. Hence,
in fact, the law of the Conditions of Existence is the higher law; as it
includes, through the inheritance of former variations and adaptations,
that of Unity of Type.



CHAPTER VII. MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.


 Longevity—Modifications not necessarily simultaneous—Modifications
 apparently of no direct service—Progressive development—Characters of
 small functional importance, the most constant—Supposed incompetence
 of natural selection to account for the incipient stages of useful
 structures—Causes which interfere with the acquisition through natural
 selection of useful structures—Gradations of structure with changed
 functions—Widely different organs in members of the same class,
 developed from one and the same source—Reasons for disbelieving in
 great and abrupt modifications.

I will devote this chapter to the consideration of various miscellaneous
objections which have been advanced against my views, as some of the
previous discussions may thus be made clearer; but it would be useless to
discuss all of them, as many have been made by writers who have not taken
the trouble to understand the subject. Thus a distinguished German
naturalist has asserted that the weakest part of my theory is, that I
consider all organic beings as imperfect: what I have really said is, that
all are not as perfect as they might have been in relation to their
conditions; and this is shown to be the case by so many native forms in
many quarters of the world having yielded their places to intruding
foreigners. Nor can organic beings, even if they were at any one time
perfectly adapted to their conditions of life, have remained so, when
their conditions changed, unless they themselves likewise changed; and no
one will dispute that the physical conditions of each country, as well as
the number and kinds of its inhabitants, have undergone many mutations.

A critic has lately insisted, with some parade of mathematical accuracy,
that longevity is a great advantage to all species, so that he who
believes in natural selection "must arrange his genealogical tree" in such
a manner that all the descendants have longer lives than their
progenitors! Cannot our critics conceive that a biennial plant or one of
the lower animals might range into a cold climate and perish there every
winter; and yet, owing to advantages gained through natural selection,
survive from year to year by means of its seeds or ova? Mr. E. Ray
Lankester has recently discussed this subject, and he concludes, as far as
its extreme complexity allows him to form a judgment, that longevity is
generally related to the standard of each species in the scale of
organisation, as well as to the amount of expenditure in reproduction and
in general activity. And these conditions have, it is probable, been
largely determined through natural selection.

It has been argued that, as none of the animals and plants of Egypt, of
which we know anything, have changed during the last three or four
thousand years, so probably have none in any part of the world. But, as
Mr. G.H. Lewes has remarked, this line of argument proves too much, for
the ancient domestic races figured on the Egyptian monuments, or embalmed,
are closely similar or even identical with those now living; yet all
naturalists admit that such races have been produced through the
modification of their original types. The many animals which have remained
unchanged since the commencement of the glacial period, would have been an
incomparably stronger case, for these have been exposed to great changes
of climate and have migrated over great distances; whereas, in Egypt,
during the last several thousand years, the conditions of life, as far as
we know, have remained absolutely uniform. The fact of little or no
modification having been effected since the glacial period, would have
been of some avail against those who believe in an innate and necessary
law of development, but is powerless against the doctrine of natural
selection or the survival of the fittest, which implies that when
variations or individual differences of a beneficial nature happen to
arise, these will be preserved; but this will be effected only under
certain favourable circumstances.

The celebrated palaeontologist, Bronn, at the close of his German
translation of this work, asks how, on the principle of natural selection,
can a variety live side by side with the parent species? If both have
become fitted for slightly different habits of life or conditions, they
might live together; and if we lay on one side polymorphic species, in
which the variability seems to be of a peculiar nature, and all mere
temporary variations, such as size, albinism, etc., the more permanent
varieties are generally found, as far as I can discover, inhabiting
distinct stations, such as high land or low land, dry or moist districts.
Moreover, in the case of animals which wander much about and cross freely,
their varieties seem to be generally confined to distinct regions.

Bronn also insists that distinct species never differ from each other in
single characters, but in many parts; and he asks, how it always comes
that many parts of the organisation should have been modified at the same
time through variation and natural selection? But there is no necessity
for supposing that all the parts of any being have been simultaneously
modified. The most striking modifications, excellently adapted for some
purpose, might, as was formerly remarked, be acquired by successive
variations, if slight, first in one part and then in another; and as they
would be transmitted all together, they would appear to us as if they had
been simultaneously developed. The best answer, however, to the above
objection is afforded by those domestic races which have been modified,
chiefly through man's power of selection, for some special purpose. Look
at the race and dray-horse, or at the greyhound and mastiff. Their whole
frames, and even their mental characteristics, have been modified; but if
we could trace each step in the history of their transformation—and
the latter steps can be traced—we should not see great and
simultaneous changes, but first one part and then another slightly
modified and improved. Even when selection has been applied by man to some
one character alone—of which our cultivated plants offer the best
instances—it will invariably be found that although this one part,
whether it be the flower, fruit, or leaves, has been greatly changed,
almost all the other parts have been slightly modified. This may be
attributed partly to the principle of correlated growth, and partly to
so-called spontaneous variation.

A much more serious objection has been urged by Bronn, and recently by
Broca, namely, that many characters appear to be of no service whatever to
their possessors, and therefore cannot have been influenced through
natural selection. Bronn adduces the length of the ears and tails in the
different species of hares and mice—the complex folds of enamel in
the teeth of many animals, and a multitude of analogous cases. With
respect to plants, this subject has been discussed by Nageli in an
admirable essay. He admits that natural selection has effected much, but
he insists that the families of plants differ chiefly from each other in
morphological characters, which appear to be quite unimportant for the
welfare of the species. He consequently believes in an innate tendency
towards progressive and more perfect development. He specifies the
arrangement of the cells in the tissues, and of the leaves on the axis, as
cases in which natural selection could not have acted. To these may be
added the numerical divisions in the parts of the flower, the position of
the ovules, the shape of the seed, when not of any use for dissemination,
etc.

There is much force in the above objection. Nevertheless, we ought, in the
first place, to be extremely cautious in pretending to decide what
structures now are, or have formerly been, of use to each species. In the
second place, it should always be borne in mind that when one part is
modified, so will be other parts, through certain dimly seen causes, such
as an increased or diminished flow of nutriment to a part, mutual
pressure, an early developed part affecting one subsequently developed,
and so forth—as well as through other causes which lead to the many
mysterious cases of correlation, which we do not in the least understand.
These agencies may be all grouped together, for the sake of brevity, under
the expression of the laws of growth. In the third place, we have to allow
for the direct and definite action of changed conditions of life, and for
so-called spontaneous variations, in which the nature of the conditions
apparently plays a quite subordinate part. Bud-variations, such as the
appearance of a moss-rose on a common rose, or of a nectarine on a
peach-tree, offer good instances of spontaneous variations; but even in
these cases, if we bear in mind the power of a minute drop of poison in
producing complex galls, we ought not to feel too sure that the above
variations are not the effect of some local change in the nature of the
sap, due to some change in the conditions. There must be some efficient
cause for each slight individual difference, as well as for more strongly
marked variations which occasionally arise; and if the unknown cause were
to act persistently, it is almost certain that all the individuals of the
species would be similarly modified.

In the earlier editions of this work I underrated, as it now seems
probable, the frequency and importance of modifications due to spontaneous
variability. But it is impossible to attribute to this cause the
innumerable structures which are so well adapted to the habits of life of
each species. I can no more believe in this than that the well-adapted
form of a race-horse or greyhound, which before the principle of selection
by man was well understood, excited so much surprise in the minds of the
older naturalists, can thus be explained.

It may be worth while to illustrate some of the foregoing remarks. With
respect to the assumed inutility of various parts and organs, it is hardly
necessary to observe that even in the higher and best-known animals many
structures exist, which are so highly developed that no one doubts that
they are of importance, yet their use has not been, or has only recently
been, ascertained. As Bronn gives the length of the ears and tail in the
several species of mice as instances, though trifling ones, of differences
in structure which can be of no special use, I may mention that, according
to Dr. Schobl, the external ears of the common mouse are supplied in an
extraordinary manner with nerves, so that they no doubt serve as tactile
organs; hence the length of the ears can hardly be quite unimportant. We
shall, also, presently see that the tail is a highly useful prehensile
organ to some of the species; and its use would be much influence by its
length.

With respect to plants, to which on account of Nageli's essay I shall
confine myself in the following remarks, it will be admitted that the
flowers of the orchids present a multitude of curious structures, which a
few years ago would have been considered as mere morphological differences
without any special function; but they are now known to be of the highest
importance for the fertilisation of the species through the aid of
insects, and have probably been gained through natural selection. No one
until lately would have imagined that in dimorphic and trimorphic plants
the different lengths of the stamens and pistils, and their arrangement,
could have been of any service, but now we know this to be the case.

In certain whole groups of plants the ovules stand erect, and in others
they are suspended; and within the same ovarium of some few plants, one
ovule holds the former and a second ovule the latter position. These
positions seem at first purely morphological, or of no physiological
signification; but Dr. Hooker informs me that within the same ovarium the
upper ovules alone in some cases, and in others the lower ones alone are
fertilised; and he suggests that this probably depends on the direction in
which the pollen-tubes enter the ovarium. If so, the position of the
ovules, even when one is erect and the other suspended within the same
ovarium, would follow the selection of any slight deviations in position
which favoured their fertilisation, and the production of seed.

Several plants belonging to distinct orders habitually produce flowers of
two kinds—the one open, of the ordinary structure, the other closed
and imperfect. These two kinds of flowers sometimes differ wonderfully in
structure, yet may be seen to graduate into each other on the same plant.
The ordinary and open flowers can be intercrossed; and the benefits which
certainly are derived from this process are thus secured. The closed and
imperfect flowers are, however, manifestly of high importance, as they
yield with the utmost safety a large stock of seed, with the expenditure
of wonderfully little pollen. The two kinds of flowers often differ much,
as just stated, in structure. The petals in the imperfect flowers almost
always consist of mere rudiments, and the pollen-grains are reduced in
diameter. In Ononis columnae five of the alternate stamens are
rudimentary; and in some species of Viola three stamens are in this state,
two retaining their proper function, but being of very small size. In six
out of thirty of the closed flowers in an Indian violet (name unknown, for
the plants have never produced with me perfect flowers), the sepals are
reduced from the normal number of five to three. In one section of the
Malpighiaceae the closed flowers, according to A. de Jussieu, are still
further modified, for the five stamens which stand opposite to the sepals
are all aborted, a sixth stamen standing opposite to a petal being alone
developed; and this stamen is not present in the ordinary flowers of this
species; the style is aborted; and the ovaria are reduced from three to
two. Now although natural selection may well have had the power to prevent
some of the flowers from expanding, and to reduce the amount of pollen,
when rendered by the closure of the flowers superfluous, yet hardly any of
the above special modifications can have been thus determined, but must
have followed from the laws of growth, including the functional inactivity
of parts, during the progress of the reduction of the pollen and the
closure of the flowers.

It is so necessary to appreciate the important effects of the laws of
growth, that I will give some additional cases of another kind, namely of
differences in the same part or organ, due to differences in relative
position on the same plant. In the Spanish chestnut, and in certain
fir-trees, the angles of divergence of the leaves differ, according to
Schacht, in the nearly horizontal and in the upright branches. In the
common rue and some other plants, one flower, usually the central or
terminal one, opens first, and has five sepals and petals, and five
divisions to the ovarium; while all the other flowers on the plant are
tetramerous. In the British Adoxa the uppermost flower generally has two
calyx-lobes with the other organs tetramerous, while the surrounding
flowers generally have three calyx-lobes with the other organs
pentamerous. In many Compositae and Umbelliferae (and in some other
plants) the circumferential flowers have their corollas much more
developed than those of the centre; and this seems often connected with
the abortion of the reproductive organs. It is a more curious fact,
previously referred to, that the achenes or seeds of the circumference and
centre sometimes differ greatly in form, colour and other characters. In
Carthamus and some other Compositae the central achenes alone are
furnished with a pappus; and in Hyoseris the same head yields achenes of
three different forms. In certain Umbelliferae the exterior seeds,
according to Tausch, are orthospermous, and the central one coelospermous,
and this is a character which was considered by De Candolle to be in other
species of the highest systematic importance. Professor Braun mentions a
Fumariaceous genus, in which the flowers in the lower part of the spike
bear oval, ribbed, one-seeded nutlets; and in the upper part of the spike,
lanceolate, two-valved and two-seeded siliques. In these several cases,
with the exception of that of the well-developed ray-florets, which are of
service in making the flowers conspicuous to insects, natural selection
cannot, as far as we can judge, have come into play, or only in a quite
subordinate manner. All these modifications follow from the relative
position and inter-action of the parts; and it can hardly be doubted that
if all the flowers and leaves on the same plant had been subjected to the
same external and internal condition, as are the flowers and leaves in
certain positions, all would have been modified in the same manner.

In numerous other cases we find modifications of structure, which are
considered by botanists to be generally of a highly important nature,
affecting only some of the flowers on the same plant, or occurring on
distinct plants, which grow close together under the same conditions. As
these variations seem of no special use to the plants, they cannot have
been influenced by natural selection. Of their cause we are quite
ignorant; we cannot even attribute them, as in the last class of cases, to
any proximate agency, such as relative position. I will give only a few
instances. It is so common to observe on the same plant, flowers
indifferently tetramerous, pentamerous, etc., that I need not give
examples; but as numerical variations are comparatively rare when the
parts are few, I may mention that, according to De Candolle, the flowers
of Papaver bracteatum offer either two sepals with four petals (which is
the common type with poppies), or three sepals with six petals. The manner
in which the petals are folded in the bud is in most groups a very
constant morphological character; but Professor Asa Gray states that with
some species of Mimulus, the aestivation is almost as frequently that of
the Rhinanthideae as of the Antirrhinideae, to which latter tribe the
genus belongs. Aug. St. Hilaire gives the following cases: the genus
Zanthoxylon belongs to a division of the Rutaceae with a single ovary, but
in some species flowers may be found on the same plant, and even in the
same panicle, with either one or two ovaries. In Helianthemum the capsule
has been described as unilocular or tri-locular; and in H. mutabile, "Une
lame PLUS OU MOINS LARGE, s'etend entre le pericarpe et le placenta." In
the flowers of Saponaria officinalis Dr. Masters has observed instances of
both marginal and free central placentation. Lastly, St. Hilaire found
towards the southern extreme of the range of Gomphia oleaeformis two forms
which he did not at first doubt were distinct species, but he subsequently
saw them growing on the same bush; and he then adds, "Voila donc dans un
meme individu des loges et un style qui se rattachent tantot a un axe
verticale et tantot a un gynobase."

We thus see that with plants many morphological changes may be attributed
to the laws of growth and the inter-action of parts, independently of
natural selection. But with respect to Nageli's doctrine of an innate
tendency towards perfection or progressive development, can it be said in
the case of these strongly pronounced variations, that the plants have
been caught in the act of progressing towards a higher state of
development? On the contrary, I should infer from the mere fact of the
parts in question differing or varying greatly on the same plant, that
such modifications were of extremely small importance to the plants
themselves, of whatever importance they may generally be to us for our
classifications. The acquisition of a useless part can hardly be said to
raise an organism in the natural scale; and in the case of the imperfect,
closed flowers, above described, if any new principle has to be invoked,
it must be one of retrogression rather than of progression; and so it must
be with many parasitic and degraded animals. We are ignorant of the
exciting cause of the above specified modifications; but if the unknown
cause were to act almost uniformly for a length of time, we may infer that
the result would be almost uniform; and in this case all the individuals
of the species would be modified in the same manner.

From the fact of the above characters being unimportant for the welfare of
the species, any slight variations which occurred in them would not have
been accumulated and augmented through natural selection. A structure
which has been developed through long-continued selection, when it ceases
to be of service to a species, generally becomes variable, as we see with
rudimentary organs; for it will no longer be regulated by this same power
of selection. But when, from the nature of the organism and of the
conditions, modifications have been induced which are unimportant for the
welfare of the species, they may be, and apparently often have been,
transmitted in nearly the same state to numerous, otherwise modified,
descendants. It cannot have been of much importance to the greater number
of mammals, birds, or reptiles, whether they were clothed with hair,
feathers or scales; yet hair has been transmitted to almost all mammals,
feathers to all birds, and scales to all true reptiles. A structure,
whatever it may be, which is common to many allied forms, is ranked by us
as of high systematic importance, and consequently is often assumed to be
of high vital importance to the species. Thus, as I am inclined to
believe, morphological differences, which we consider as important—such
as the arrangement of the leaves, the divisions of the flower or of the
ovarium, the position of the ovules, etc., first appeared in many cases as
fluctuating variations, which sooner or later became constant through the
nature of the organism and of the surrounding conditions, as well as
through the intercrossing of distinct individuals, but not through natural
selection; for as these morphological characters do not affect the welfare
of the species, any slight deviations in them could not have been governed
or accumulated through this latter agency. It is a strange result which we
thus arrive at, namely, that characters of slight vital importance to the
species, are the most important to the systematist; but, as we shall
hereafter see when we treat of the genetic principle of classification,
this is by no means so paradoxical as it may at first appear.

Although we have no good evidence of the existence in organic beings of an
innate tendency towards progressive development, yet this necessarily
follows, as I have attempted to show in the fourth chapter, through the
continued action of natural selection. For the best definition which has
ever been given of a high standard of organisation, is the degree to which
the parts have been specialised or differentiated; and natural selection
tends towards this end, inasmuch as the parts are thus enabled to perform
their functions more efficiently.

A distinguished zoologist, Mr. St. George Mivart, has recently collected
all the objections which have ever been advanced by myself and others
against the theory of natural selection, as propounded by Mr. Wallace and
myself, and has illustrated them with admirable art and force. When thus
marshalled, they make a formidable array; and as it forms no part of Mr.
Mivart's plan to give the various facts and considerations opposed to his
conclusions, no slight effort of reason and memory is left to the reader,
who may wish to weigh the evidence on both sides. When discussing special
cases, Mr. Mivart passes over the effects of the increased use and disuse
of parts, which I have always maintained to be highly important, and have
treated in my "Variation under Domestication" at greater length than, as I
believe, any other writer. He likewise often assumes that I attribute
nothing to variation, independently of natural selection, whereas in the
work just referred to I have collected a greater number of
well-established cases than can be found in any other work known to me. My
judgment may not be trustworthy, but after reading with care Mr. Mivart's
book, and comparing each section with what I have said on the same head, I
never before felt so strongly convinced of the general truth of the
conclusions here arrived at, subject, of course, in so intricate a
subject, to much partial error.

All Mr. Mivart's objections will be, or have been, considered in the
present volume. The one new point which appears to have struck many
readers is, "That natural selection is incompetent to account for the
incipient stages of useful structures." This subject is intimately
connected with that of the gradation of the characters, often accompanied
by a change of function, for instance, the conversion of a swim-bladder
into lungs, points which were discussed in the last chapter under two
headings. Nevertheless, I will here consider in some detail several of the
cases advanced by Mr. Mivart, selecting those which are the most
illustrative, as want of space prevents me from considering all.

The giraffe, by its lofty stature, much elongated neck, fore legs, head
and tongue, has its whole frame beautifully adapted for browsing on the
higher branches of trees. It can thus obtain food beyond the reach of the
other Ungulata or hoofed animals inhabiting the same country; and this
must be a great advantage to it during dearths. The Niata cattle in South
America show us how small a difference in structure may make, during such
periods, a great difference in preserving an animal's life. These cattle
can browse as well as others on grass, but from the projection of the
lower jaw they cannot, during the often recurrent droughts, browse on the
twigs of trees, reeds, etc., to which food the common cattle and horses
are then driven; so that at these times the Niatas perish, if not fed by
their owners. Before coming to Mr. Mivart's objections, it may be well to
explain once again how natural selection will act in all ordinary cases.
Man has modified some of his animals, without necessarily having attended
to special points of structure, by simply preserving and breeding from the
fleetest individuals, as with the race-horse and greyhound, or as with the
game-cock, by breeding from the victorious birds. So under nature with the
nascent giraffe, the individuals which were the highest browsers and were
able during dearths to reach even an inch or two above the others, will
often have been preserved; for they will have roamed over the whole
country in search of food. That the individuals of the same species often
differ slightly in the relative lengths of all their parts may be seen in
many works of natural history, in which careful measurements are given.
These slight proportional differences, due to the laws of growth and
variation, are not of the slightest use or importance to most species. But
it will have been otherwise with the nascent giraffe, considering its
probable habits of life; for those individuals which had some one part or
several parts of their bodies rather more elongated than usual, would
generally have survived. These will have intercrossed and left offspring,
either inheriting the same bodily peculiarities, or with a tendency to
vary again in the same manner; while the individuals less favoured in the
same respects will have been the most liable to perish.

We here see that there is no need to separate single pairs, as man does,
when he methodically improves a breed: natural selection will preserve and
thus separate all the superior individuals, allowing them freely to
intercross, and will destroy all the inferior individuals. By this process
long-continued, which exactly corresponds with what I have called
unconscious selection by man, combined, no doubt, in a most important
manner with the inherited effects of the increased use of parts, it seems
to me almost certain that an ordinary hoofed quadruped might be converted
into a giraffe.

To this conclusion Mr. Mivart brings forward two objections. One is that
the increased size of the body would obviously require an increased supply
of food, and he considers it as "very problematical whether the
disadvantages thence arising would not, in times of scarcity, more than
counterbalance the advantages." But as the giraffe does actually exist in
large numbers in Africa, and as some of the largest antelopes in the
world, taller than an ox, abound there, why should we doubt that, as far
as size is concerned, intermediate gradations could formerly have existed
there, subjected as now to severe dearths. Assuredly the being able to
reach, at each stage of increased size, to a supply of food, left
untouched by the other hoofed quadrupeds of the country, would have been
of some advantage to the nascent giraffe. Nor must we overlook the fact,
that increased bulk would act as a protection against almost all beasts of
prey excepting the lion; and against this animal, its tall neck—and
the taller the better—would, as Mr. Chauncey Wright has remarked,
serve as a watch-tower. It is from this cause, as Sir S. Baker remarks,
that no animal is more difficult to stalk than the giraffe. This animal
also uses its long neck as a means of offence or defence, by violently
swinging its head armed with stump-like horns. The preservation of each
species can rarely be determined by any one advantage, but by the union of
all, great and small.

Mr. Mivart then asks (and this is his second objection), if natural
selection be so potent, and if high browsing be so great an advantage, why
has not any other hoofed quadruped acquired a long neck and lofty stature,
besides the giraffe, and, in a lesser degree, the camel, guanaco and
macrauchenia? Or, again, why has not any member of the group acquired a
long proboscis? With respect to South Africa, which was formerly inhabited
by numerous herds of the giraffe, the answer is not difficult, and can
best be given by an illustration. In every meadow in England, in which
trees grow, we see the lower branches trimmed or planed to an exact level
by the browsing of the horses or cattle; and what advantage would it be,
for instance, to sheep, if kept there, to acquire slightly longer necks?
In every district some one kind of animal will almost certainly be able to
browse higher than the others; and it is almost equally certain that this
one kind alone could have its neck elongated for this purpose, through
natural selection and the effects of increased use. In South Africa the
competition for browsing on the higher branches of the acacias and other
trees must be between giraffe and giraffe, and not with the other ungulate
animals.

Why, in other quarters of the world, various animals belonging to this
same order have not acquired either an elongated neck or a proboscis,
cannot be distinctly answered; but it is as unreasonable to expect a
distinct answer to such a question as why some event in the history of
mankind did not occur in one country while it did in another. We are
ignorant with respect to the conditions which determine the numbers and
range of each species, and we cannot even conjecture what changes of
structure would be favourable to its increase in some new country. We can,
however, see in a general manner that various causes might have interfered
with the development of a long neck or proboscis. To reach the foliage at
a considerable height (without climbing, for which hoofed animals are
singularly ill-constructed) implies greatly increased bulk of body; and we
know that some areas support singularly few large quadrupeds, for instance
South America, though it is so luxuriant, while South Africa abounds with
them to an unparalleled degree. Why this should be so we do not know; nor
why the later tertiary periods should have been much more favourable for
their existence than the present time. Whatever the causes may have been,
we can see that certain districts and times would have been much more
favourable than others for the development of so large a quadruped as the
giraffe.

In order that an animal should acquire some structure specially and
largely developed, it is almost indispensable that several other parts
should be modified and coadapted. Although every part of the body varies
slightly, it does not follow that the necessary parts should always vary
in the right direction and to the right degree. With the different species
of our domesticated animals we know that the parts vary in a different
manner and degree, and that some species are much more variable than
others. Even if the fitting variations did arise, it does not follow that
natural selection would be able to act on them and produce a structure
which apparently would be beneficial to the species. For instance, if the
number of individuals existing in a country is determined chiefly through
destruction by beasts of prey—by external or internal parasites,
etc.—as seems often to be the case, then natural selection will be
able to do little, or will be greatly retarded, in modifying any
particular structure for obtaining food. Lastly, natural selection is a
slow process, and the same favourable conditions must long endure in order
that any marked effect should thus be produced. Except by assigning such
general and vague reasons, we cannot explain why, in many quarters of the
world, hoofed quadrupeds have not acquired much elongated necks or other
means for browsing on the higher branches of trees.

Objections of the same nature as the foregoing have been advanced by many
writers. In each case various causes, besides the general ones just
indicated, have probably interfered with the acquisition through natural
selection of structures, which it is thought would be beneficial to
certain species. One writer asks, why has not the ostrich acquired the
power of flight? But a moment's reflection will show what an enormous
supply of food would be necessary to give to this bird of the desert force
to move its huge body through the air. Oceanic islands are inhabited by
bats and seals, but by no terrestrial mammals; yet as some of these bats
are peculiar species, they must have long inhabited their present homes.
Therefore Sir C. Lyell asks, and assigns certain reasons in answer, why
have not seals and bats given birth on such islands to forms fitted to
live on the land? But seals would necessarily be first converted into
terrestrial carnivorous animals of considerable size, and bats into
terrestrial insectivorous animals; for the former there would be no prey;
for the bats ground-insects would serve as food, but these would already
be largely preyed on by the reptiles or birds, which first colonise and
abound on most oceanic islands. Gradations of structure, with each stage
beneficial to a changing species, will be favoured only under certain
peculiar conditions. A strictly terrestrial animal, by occasionally
hunting for food in shallow water, then in streams or lakes, might at last
be converted into an animal so thoroughly aquatic as to brave the open
ocean. But seals would not find on oceanic islands the conditions
favourable to their gradual reconversion into a terrestrial form. Bats, as
formerly shown, probably acquired their wings by at first gliding through
the air from tree to tree, like the so-called flying squirrels, for the
sake of escaping from their enemies, or for avoiding falls; but when the
power of true flight had once been acquired, it would never be reconverted
back, at least for the above purposes, into the less efficient power of
gliding through the air. Bats, might, indeed, like many birds, have had
their wings greatly reduced in size, or completely lost, through disuse;
but in this case it would be necessary that they should first have
acquired the power of running quickly on the ground, by the aid of their
hind legs alone, so as to compete with birds or other ground animals; and
for such a change a bat seems singularly ill-fitted. These conjectural
remarks have been made merely to show that a transition of structure, with
each step beneficial, is a highly complex affair; and that there is
nothing strange in a transition not having occurred in any particular
case.

Lastly, more than one writer has asked why have some animals had their
mental powers more highly developed than others, as such development would
be advantageous to all? Why have not apes acquired the intellectual powers
of man? Various causes could be assigned; but as they are conjectural, and
their relative probability cannot be weighed, it would be useless to give
them. A definite answer to the latter question ought not to be expected,
seeing that no one can solve the simpler problem, why, of two races of
savages, one has risen higher in the scale of civilisation than the other;
and this apparently implies increased brain power.

We will return to Mr. Mivart's other objections. Insects often resemble
for the sake of protection various objects, such as green or decayed
leaves, dead twigs, bits of lichen, flowers, spines, excrement of birds,
and living insects; but to this latter point I shall hereafter recur. The
resemblance is often wonderfully close, and is not confined to colour, but
extends to form, and even to the manner in which the insects hold
themselves. The caterpillars which project motionless like dead twigs from
the bushes on which they feed, offer an excellent instance of a
resemblance of this kind. The cases of the imitation of such objects as
the excrement of birds, are rare and exceptional. On this head, Mr. Mivart
remarks, "As, according to Mr. Darwin's theory, there is a constant
tendency to indefinite variation, and as the minute incipient variations
will be in ALL DIRECTIONS, they must tend to neutralize each other, and at
first to form such unstable modifications that it is difficult, if not
impossible, to see how such indefinite oscillations of infinitesimal
beginnings can ever build up a sufficiently appreciable resemblance to a
leaf, bamboo, or other object, for natural selection to seize upon and
perpetuate."

But in all the foregoing cases the insects in their original state no
doubt presented some rude and accidental resemblance to an object commonly
found in the stations frequented by them. Nor is this at all improbable,
considering the almost infinite number of surrounding objects and the
diversity in form and colour of the hosts of insects which exist. As some
rude resemblance is necessary for the first start, we can understand how
it is that the larger and higher animals do not (with the exception, as
far as I know, of one fish) resemble for the sake of protection special
objects, but only the surface which commonly surrounds them, and this
chiefly in colour. Assuming that an insect originally happened to resemble
in some degree a dead twig or a decayed leaf, and that it varied slightly
in many ways, then all the variations which rendered the insect at all
more like any such object, and thus favoured its escape, would be
preserved, while other variations would be neglected and ultimately lost;
or, if they rendered the insect at all less like the imitated object, they
would be eliminated. There would indeed be force in Mr. Mivart's
objection, if we were to attempt to account for the above resemblances,
independently of natural selection, through mere fluctuating variability;
but as the case stands there is none.

Nor can I see any force in Mr. Mivart's difficulty with respect to "the
last touches of perfection in the mimicry;" as in the case given by Mr.
Wallace, of a walking-stick insect (Ceroxylus laceratus), which resembles
"a stick grown over by a creeping moss or jungermannia." So close was this
resemblance, that a native Dyak maintained that the foliaceous
excrescences were really moss. Insects are preyed on by birds and other
enemies whose sight is probably sharper than ours, and every grade in
resemblance which aided an insect to escape notice or detection, would
tend towards its preservation; and the more perfect the resemblance so
much the better for the insect. Considering the nature of the differences
between the species in the group which includes the above Ceroxylus, there
is nothing improbable in this insect having varied in the irregularities
on its surface, and in these having become more or less green-coloured;
for in every group the characters which differ in the several species are
the most apt to vary, while the generic characters, or those common to all
the species, are the most constant.

The Greenland whale is one of the most wonderful animals in the world, and
the baleen, or whalebone, one of its greatest peculiarities. The baleen
consists of a row, on each side of the upper jaw, of about 300 plates or
laminae, which stand close together transversely to the longer axis of the
mouth. Within the main row there are some subsidiary rows. The extremities
and inner margins of all the plates are frayed into stiff bristles, which
clothe the whole gigantic palate, and serve to strain or sift the water,
and thus to secure the minute prey on which these great animals subsist.
The middle and longest lamina in the Greenland whale is ten, twelve, or
even fifteen feet in length; but in the different species of Cetaceans
there are gradations in length; the middle lamina being in one species,
according to Scoresby, four feet, in another three, in another eighteen
inches, and in the Balaenoptera rostrata only about nine inches in length.
The quality of the whalebone also differs in the different species.

With respect to the baleen, Mr. Mivart remarks that if it "had once
attained such a size and development as to be at all useful, then its
preservation and augmentation within serviceable limits would be promoted
by natural selection alone. But how to obtain the beginning of such useful
development?" In answer, it may be asked, why should not the early
progenitors of the whales with baleen have possessed a mouth constructed
something like the lamellated beak of a duck? Ducks, like whales, subsist
by sifting the mud and water; and the family has sometimes been called
Criblatores, or sifters. I hope that I may not be misconstrued into saying
that the progenitors of whales did actually possess mouths lamellated like
the beak of a duck. I wish only to show that this is not incredible, and
that the immense plates of baleen in the Greenland whale might have been
developed from such lamellae by finely graduated steps, each of service to
its possessor.

The beak of a shoveller-duck (Spatula clypeata) is a more beautiful and
complex structure than the mouth of a whale. The upper mandible is
furnished on each side (in the specimen examined by me) with a row or comb
formed of 188 thin, elastic lamellae, obliquely bevelled so as to be
pointed, and placed transversely to the longer axis of the mouth. They
arise from the palate, and are attached by flexible membrane to the sides
of the mandible. Those standing towards the middle are the longest, being
about one-third of an inch in length, and they project fourteen
one-hundredths of an inch beneath the edge. At their bases there is a
short subsidiary row of obliquely transverse lamellae. In these several
respects they resemble the plates of baleen in the mouth of a whale. But
towards the extremity of the beak they differ much, as they project
inward, instead of straight downward. The entire head of the shoveller,
though incomparably less bulky, is about one-eighteenth of the length of
the head of a moderately large Balaenoptera rostrata, in which species the
baleen is only nine inches long; so that if we were to make the head of
the shoveller as long as that of the Balaenoptera, the lamellae would be
six inches in length, that is, two-thirds of the length of the baleen in
this species of whale. The lower mandible of the shoveller-duck is
furnished with lamellae of equal length with these above, but finer; and
in being thus furnished it differs conspicuously from the lower jaw of a
whale, which is destitute of baleen. On the other hand, the extremities of
these lower lamellae are frayed into fine bristly points, so that they
thus curiously resemble the plates of baleen. In the genus Prion, a member
of the distinct family of the Petrels, the upper mandible alone is
furnished with lamellae, which are well developed and project beneath the
margin; so that the beak of this bird resembles in this respect the mouth
of a whale.

From the highly developed structure of the shoveller's beak we may proceed
(as I have learned from information and specimens sent to me by Mr.
Salvin), without any great break, as far as fitness for sifting is
concerned, through the beak of the Merganetta armata, and in some respects
through that of the Aix sponsa, to the beak of the common duck. In this
latter species the lamellae are much coarser than in the shoveller, and
are firmly attached to the sides of the mandible; they are only about
fifty in number on each side, and do not project at all beneath the
margin. They are square-topped, and are edged with translucent, hardish
tissue, as if for crushing food. The edges of the lower mandible are
crossed by numerous fine ridges, which project very little. Although the
beak is thus very inferior as a sifter to that of a shoveller, yet this
bird, as every one knows, constantly uses it for this purpose. There are
other species, as I hear from Mr. Salvin, in which the lamellae are
considerably less developed than in the common duck; but I do not know
whether they use their beaks for sifting the water.

Turning to another group of the same family. In the Egyptian goose
(Chenalopex) the beak closely resembles that of the common duck; but the
lamellae are not so numerous, nor so distinct from each other, nor do they
project so much inward; yet this goose, as I am informed by Mr. E.
Bartlett, "uses its bill like a duck by throwing the water out at the
corners." Its chief food, however, is grass, which it crops like the
common goose. In this latter bird the lamellae of the upper mandible are
much coarser than in the common duck, almost confluent, about twenty-seven
in number on each side, and terminating upward in teeth-like knobs. The
palate is also covered with hard rounded knobs. The edges of the lower
mandible are serrated with teeth much more prominent, coarser and sharper
than in the duck. The common goose does not sift the water, but uses its
beak exclusively for tearing or cutting herbage, for which purpose it is
so well fitted that it can crop grass closer than almost any other animal.
There are other species of geese, as I hear from Mr. Bartlett, in which
the lamellae are less developed than in the common goose.

We thus see that a member of the duck family, with a beak constructed like
that of a common goose and adapted solely for grazing, or even a member
with a beak having less well-developed lamellae, might be converted by
small changes into a species like the Egyptian goose—this into one
like the common duck—and, lastly, into one like the shoveller,
provided with a beak almost exclusively adapted for sifting the water; for
this bird could hardly use any part of its beak, except the hooked tip,
for seizing or tearing solid food. The beak of a goose, as I may add,
might also be converted by small changes into one provided with prominent,
recurved teeth, like those of the Merganser (a member of the same family),
serving for the widely different purpose of securing live fish.

Returning to the whales. The Hyperoodon bidens is destitute of true teeth
in an efficient condition, but its palate is roughened, according to
Lacepede, with small unequal, hard points of horn. There is, therefore,
nothing improbable in supposing that some early Cetacean form was provided
with similar points of horn on the palate, but rather more regularly
placed, and which, like the knobs on the beak of the goose, aided it in
seizing or tearing its food. If so, it will hardly be denied that the
points might have been converted through variation and natural selection
into lamellae as well-developed as those of the Egyptian goose, in which
case they would have been used both for seizing objects and for sifting
the water; then into lamellae like those of the domestic duck; and so
onward, until they became as well constructed as those of the shoveller,
in which case they would have served exclusively as a sifting apparatus.
From this stage, in which the lamellae would be two-thirds of the length
of the plates of baleen in the Balaenoptera rostrata, gradations, which
may be observed in still-existing Cetaceans, lead us onward to the
enormous plates of baleen in the Greenland whale. Nor is there the least
reason to doubt that each step in this scale might have been as
serviceable to certain ancient Cetaceans, with the functions of the parts
slowly changing during the progress of development, as are the gradations
in the beaks of the different existing members of the duck-family. We
should bear in mind that each species of duck is subjected to a severe
struggle for existence, and that the structure of every part of its frame
must be well adapted to its conditions of life.

The Pleuronectidae, or Flat-fish, are remarkable for their asymmetrical
bodies. They rest on one side—in the greater number of species on
the left, but in some on the right side; and occasionally reversed adult
specimens occur. The lower, or resting-surface, resembles at first sight
the ventral surface of an ordinary fish; it is of a white colour, less
developed in many ways than the upper side, with the lateral fins often of
smaller size. But the eyes offer the most remarkable peculiarity; for they
are both placed on the upper side of the head. During early youth,
however, they stand opposite to each other, and the whole body is then
symmetrical, with both sides equally coloured. Soon the eye proper to the
lower side begins to glide slowly round the head to the upper side; but
does not pass right through the skull, as was formerly thought to be the
case. It is obvious that unless the lower eye did thus travel round, it
could not be used by the fish while lying in its habitual position on one
side. The lower eye would, also, have been liable to be abraded by the
sandy bottom. That the Pleuronectidae are admirably adapted by their
flattened and asymmetrical structure for their habits of life, is manifest
from several species, such as soles, flounders, etc., being extremely
common. The chief advantages thus gained seem to be protection from their
enemies, and facility for feeding on the ground. The different members,
however, of the family present, as Schiodte remarks, "a long series of
forms exhibiting a gradual transition from Hippoglossus pinguis, which
does not in any considerable degree alter the shape in which it leaves the
ovum, to the soles, which are entirely thrown to one side."

Mr. Mivart has taken up this case, and remarks that a sudden spontaneous
transformation in the position of the eyes is hardly conceivable, in which
I quite agree with him. He then adds: "If the transit was gradual, then
how such transit of one eye a minute fraction of the journey towards the
other side of the head could benefit the individual is, indeed, far from
clear. It seems, even, that such an incipient transformation must rather
have been injurious." But he might have found an answer to this objection
in the excellent observations published in 1867 by Malm. The
Pleuronectidae, while very young and still symmetrical, with their eyes
standing on opposite sides of the head, cannot long retain a vertical
position, owing to the excessive depth of their bodies, the small size of
their lateral fins, and to their being destitute of a swim-bladder. Hence,
soon growing tired, they fall to the bottom on one side. While thus at
rest they often twist, as Malm observed, the lower eye upward, to see
above them; and they do this so vigorously that the eye is pressed hard
against the upper part of the orbit. The forehead between the eyes
consequently becomes, as could be plainly seen, temporarily contracted in
breadth. On one occasion Malm saw a young fish raise and depress the lower
eye through an angular distance of about seventy degrees.

We should remember that the skull at this early age is cartilaginous and
flexible, so that it readily yields to muscular action. It is also known
with the higher animals, even after early youth, that the skull yields and
is altered in shape, if the skin or muscles be permanently contracted
through disease or some accident. With long-eared rabbits, if one ear
flops forward and downward, its weight drags forward all the bones of the
skull on the same side, of which I have given a figure. Malm states that
the newly-hatched young of perches, salmon, and several other symmetrical
fishes, have the habit of occasionally resting on one side at the bottom;
and he has observed that they often then strain their lower eyes so as to
look upward; and their skulls are thus rendered rather crooked. These
fishes, however, are soon able to hold themselves in a vertical position,
and no permanent effect is thus produced. With the Pleuronectidae, on the
other hand, the older they grow the more habitually they rest on one side,
owing to the increasing flatness of their bodies, and a permanent effect
is thus produced on the form of the head, and on the position of the eyes.
Judging from analogy, the tendency to distortion would no doubt be
increased through the principle of inheritance. Schiodte believes, in
opposition to some other naturalists, that the Pleuronectidae are not
quite symmetrical even in the embryo; and if this be so, we could
understand how it is that certain species, while young, habitually fall
over and rest on the left side, and other species on the right side. Malm
adds, in confirmation of the above view, that the adult Trachypterus
arcticus, which is not a member of the Pleuronectidae, rests on its left
side at the bottom, and swims diagonally through the water; and in this
fish, the two sides of the head are said to be somewhat dissimilar. Our
great authority on Fishes, Dr. Gunther, concludes his abstract of Malm's
paper, by remarking that "the author gives a very simple explanation of
the abnormal condition of the Pleuronectoids."

We thus see that the first stages of the transit of the eye from one side
of the head to the other, which Mr. Mivart considers would be injurious,
may be attributed to the habit, no doubt beneficial to the individual and
to the species, of endeavouring to look upward with both eyes, while
resting on one side at the bottom. We may also attribute to the inherited
effects of use the fact of the mouth in several kinds of flat-fish being
bent towards the lower surface, with the jaw bones stronger and more
effective on this, the eyeless side of the head, than on the other, for
the sake, as Dr. Traquair supposes, of feeding with ease on the ground.
Disuse, on the other hand, will account for the less developed condition
of the whole inferior half of the body, including the lateral fins; though
Yarrel thinks that the reduced size of these fins is advantageous to the
fish, as "there is so much less room for their action than with the larger
fins above." Perhaps the lesser number of teeth in the proportion of four
to seven in the upper halves of the two jaws of the plaice, to twenty-five
to thirty in the lower halves, may likewise be accounted for by disuse.
From the colourless state of the ventral surface of most fishes and of
many other animals, we may reasonably suppose that the absence of colour
in flat-fish on the side, whether it be the right or left, which is
under-most, is due to the exclusion of light. But it cannot be supposed
that the peculiar speckled appearance of the upper side of the sole, so
like the sandy bed of the sea, or the power in some species, as recently
shown by Pouchet, of changing their colour in accordance with the
surrounding surface, or the presence of bony tubercles on the upper side
of the turbot, are due to the action of the light. Here natural selection
has probably come into play, as well as in adapting the general shape of
the body of these fishes, and many other peculiarities, to their habits of
life. We should keep in mind, as I have before insisted, that the
inherited effects of the increased use of parts, and perhaps of their
disuse, will be strengthened by natural selection. For all spontaneous
variations in the right direction will thus be preserved; as will those
individuals which inherit in the highest degree the effects of the
increased and beneficial use of any part. How much to attribute in each
particular case to the effects of use, and how much to natural selection,
it seems impossible to decide.

I may give another instance of a structure which apparently owes its
origin exclusively to use or habit. The extremity of the tail in some
American monkeys has been converted into a wonderfully perfect prehensile
organ, and serves as a fifth hand. A reviewer, who agrees with Mr. Mivart
in every detail, remarks on this structure: "It is impossible to believe
that in any number of ages the first slight incipient tendency to grasp
could preserve the lives of the individuals possessing it, or favour their
chance of having and of rearing offspring." But there is no necessity for
any such belief. Habit, and this almost implies that some benefit great or
small is thus derived, would in all probability suffice for the work.
Brehm saw the young of an African monkey (Cercopithecus) clinging to the
under surface of their mother by their hands, and at the same time they
hooked their little tails round that of their mother. Professor Henslow
kept in confinement some harvest mice (Mus messorius) which do not possess
a structurally prehensive tail; but he frequently observed that they
curled their tails round the branches of a bush placed in the cage, and
thus aided themselves in climbing. I have received an analogous account
from Dr. Gunther, who has seen a mouse thus suspend itself. If the harvest
mouse had been more strictly arboreal, it would perhaps have had its tail
rendered structurally prehensile, as is the case with some members of the
same order. Why Cercopithecus, considering its habits while young, has not
become thus provided, it would be difficult to say. It is, however,
possible that the long tail of this monkey may be of more service to it as
a balancing organ in making its prodigious leaps, than as a prehensile
organ.

The mammary glands are common to the whole class of mammals, and are
indispensable for their existence; they must, therefore, have been
developed at an extremely remote period, and we can know nothing
positively about their manner of development. Mr. Mivart asks: "Is it
conceivable that the young of any animal was ever saved from destruction
by accidentally sucking a drop of scarcely nutritious fluid from an
accidentally hypertrophied cutaneous gland of its mother? And even if one
was so, what chance was there of the perpetuation of such a variation?"
But the case is not here put fairly. It is admitted by most evolutionists
that mammals are descended from a marsupial form; and if so, the mammary
glands will have been at first developed within the marsupial sack. In the
case of the fish (Hippocampus) the eggs are hatched, and the young are
reared for a time, within a sack of this nature; and an American
naturalist, Mr. Lockwood, believes from what he has seen of the
development of the young, that they are nourished by a secretion from the
cutaneous glands of the sack. Now, with the early progenitors of mammals,
almost before they deserved to be thus designated, is it not at least
possible that the young might have been similarly nourished? And in this
case, the individuals which secreted a fluid, in some degree or manner the
most nutritious, so as to partake of the nature of milk, would in the long
run have reared a larger number of well-nourished offspring, than would
the individuals which secreted a poorer fluid; and thus the cutaneous
glands, which are the homologues of the mammary glands, would have been
improved or rendered more effective. It accords with the widely extended
principle of specialisation, that the glands over a certain space of the
sack should have become more highly developed than the remainder; and they
would then have formed a breast, but at first without a nipple, as we see
in the Ornithorhyncus, at the base of the mammalian series. Through what
agency the glands over a certain space became more highly specialised than
the others, I will not pretend to decide, whether in part through
compensation of growth, the effects of use, or of natural selection.

The development of the mammary glands would have been of no service, and
could not have been affected through natural selection, unless the young
at the same time were able to partake of the secretion. There is no
greater difficulty in understanding how young mammals have instinctively
learned to suck the breast, than in understanding how unhatched chickens
have learned to break the egg-shell by tapping against it with their
specially adapted beaks; or how a few hours after leaving the shell they
have learned to pick up grains of food. In such cases the most probable
solution seems to be, that the habit was at first acquired by practice at
a more advanced age, and afterwards transmitted to the offspring at an
earlier age. But the young kangaroo is said not to suck, only to cling to
the nipple of its mother, who has the power of injecting milk into the
mouth of her helpless, half-formed offspring. On this head Mr. Mivart
remarks: "Did no special provision exist, the young one must infallibly be
choked by the intrusion of the milk into the wind-pipe. But there IS a
special provision. The larynx is so elongated that it rises up into the
posterior end of the nasal passage, and is thus enabled to give free
entrance to the air for the lungs, while the milk passes harmlessly on
each side of this elongated larynx, and so safely attains the gullet
behind it." Mr. Mivart then asks how did natural selection remove in the
adult kangaroo (and in most other mammals, on the assumption that they are
descended from a marsupial form), "this at least perfectly innocent and
harmless structure?" It may be suggested in answer that the voice, which
is certainly of high importance to many animals, could hardly have been
used with full force as long as the larynx entered the nasal passage; and
Professor Flower has suggested to me that this structure would have
greatly interfered with an animal swallowing solid food.

We will now turn for a short space to the lower divisions of the animal
kingdom. The Echinodermata (star-fishes, sea-urchins, etc.) are furnished
with remarkable organs, called pedicellariae, which consist, when well
developed, of a tridactyle forceps—that is, of one formed of three
serrated arms, neatly fitting together and placed on the summit of a
flexible stem, moved by muscles. These forceps can seize firmly hold of
any object; and Alexander Agassiz has seen an Echinus or sea-urchin
rapidly passing particles of excrement from forceps to forceps down
certain lines of its body, in order that its shell should not be fouled.
But there is no doubt that besides removing dirt of all kinds, they
subserve other functions; and one of these apparently is defence.

With respect to these organs, Mr. Mivart, as on so many previous
occasions, asks: "What would be the utility of the FIRST RUDIMENTARY
BEGINNINGS of such structures, and how could such insipient buddings have
ever preserved the life of a single Echinus?" He adds, "not even the
SUDDEN development of the snapping action would have been beneficial
without the freely movable stalk, nor could the latter have been efficient
without the snapping jaws, yet no minute, nearly indefinite variations
could simultaneously evolve these complex co-ordinations of structure; to
deny this seems to do no less than to affirm a startling paradox."
Paradoxical as this may appear to Mr. Mivart, tridactyle forcepses,
immovably fixed at the base, but capable of a snapping action, certainly
exist on some star-fishes; and this is intelligible if they serve, at
least in part, as a means of defence. Mr. Agassiz, to whose great kindness
I am indebted for much information on the subject, informs me that there
are other star-fishes, in which one of the three arms of the forceps is
reduced to a support for the other two; and again, other genera in which
the third arm is completely lost. In Echinoneus, the shell is described by
M. Perrier as bearing two kinds of pedicellariae, one resembling those of
Echinus, and the other those of Spatangus; and such cases are always
interesting as affording the means of apparently sudden transitions,
through the abortion of one of the two states of an organ.

With respect to the steps by which these curious organs have been evolved,
Mr. Agassiz infers from his own researches and those of Mr. Muller, that
both in star-fishes and sea-urchins the pedicellariae must undoubtedly be
looked at as modified spines. This may be inferred from their manner of
development in the individual, as well as from a long and perfect series
of gradations in different species and genera, from simple granules to
ordinary spines, to perfect tridactyle pedicellariae. The gradation
extends even to the manner in which ordinary spines and the pedicellariae,
with their supporting calcareous rods, are articulated to the shell. In
certain genera of star-fishes, "the very combinations needed to show that
the pedicellariae are only modified branching spines" may be found. Thus
we have fixed spines, with three equi-distant, serrated, movable branches,
articulated to near their bases; and higher up, on the same spine, three
other movable branches. Now when the latter arise from the summit of a
spine they form, in fact, a rude tridactyle pedicellariae, and such may be
seen on the same spine together with the three lower branches. In this
case the identity in nature between the arms of the pedicellariae and the
movable branches of a spine, is unmistakable. It is generally admitted
that the ordinary spines serve as a protection; and if so, there can be no
reason to doubt that those furnished with serrated and movable branches
likewise serve for the same purpose; and they would thus serve still more
effectively as soon as by meeting together they acted as a prehensile or
snapping apparatus. Thus every gradation, from an ordinary fixed spine to
a fixed pedicellariae, would be of service.

In certain genera of star-fishes these organs, instead of being fixed or
borne on an immovable support, are placed on the summit of a flexible and
muscular, though short, stem; and in this case they probably subserve some
additional function besides defence. In the sea-urchins the steps can be
followed by which a fixed spine becomes articulated to the shell, and is
thus rendered movable. I wish I had space here to give a fuller abstract
of Mr. Agassiz's interesting observations on the development of the
pedicellariae. All possible gradations, as he adds, may likewise be found
between the pedicellariae of the star-fishes and the hooks of the
Ophiurians, another group of the Echinodermata; and again between the
pedicellariae of sea-urchins and the anchors of the Holothuriae, also
belonging to the same great class.

Certain compound animals, or zoophytes, as they have been termed, namely
the Polyzoa, are provided with curious organs called avicularia. These
differ much in structure in the different species. In their most perfect
condition they curiously resemble the head and beak of a vulture in
miniature, seated on a neck and capable of movement, as is likewise the
lower jaw or mandible. In one species observed by me, all the avicularia
on the same branch often moved simultaneously backwards and forwards, with
the lower jaw widely open, through an angle of about 90 degrees, in the
course of five seconds; and their movement caused the whole polyzoary to
tremble. When the jaws are touched with a needle they seize it so firmly
that the branch can thus be shaken.

Mr. Mivart adduces this case, chiefly on account of the supposed
difficulty of organs, namely the avicularia of the Polyzoa and the
pedicellariae of the Echinodermata, which he considers as "essentially
similar," having been developed through natural selection in widely
distinct divisions of the animal kingdom. But, as far as structure is
concerned, I can see no similarity between tridactyle pedicellariae and
avicularia. The latter resembles somewhat more closely the chelae or
pincers of Crustaceans; and Mr. Mivart might have adduced with equal
appropriateness this resemblance as a special difficulty, or even their
resemblance to the head and beak of a bird. The avicularia are believed by
Mr. Busk, Dr. Smitt and Dr. Nitsche—naturalists who have carefully
studied this group—to be homologous with the zooids and their cells
which compose the zoophyte, the movable lip or lid of the cell
corresponding with the lower and movable mandible of the avicularium. Mr.
Busk, however, does not know of any gradations now existing between a
zooid and an avicularium. It is therefore impossible to conjecture by what
serviceable gradations the one could have been converted into the other,
but it by no means follows from this that such gradations have not
existed.

As the chelae of Crustaceans resemble in some degree the avicularia of
Polyzoa, both serving as pincers, it may be worth while to show that with
the former a long series of serviceable gradations still exists. In the
first and simplest stage, the terminal segment of a limb shuts down either
on the square summit of the broad penultimate segment, or against one
whole side, and is thus enabled to catch hold of an object, but the limb
still serves as an organ of locomotion. We next find one corner of the
broad penultimate segment slightly prominent, sometimes furnished with
irregular teeth, and against these the terminal segment shuts down. By an
increase in the size of this projection, with its shape, as well as that
of the terminal segment, slightly modified and improved, the pincers are
rendered more and more perfect, until we have at last an instrument as
efficient as the chelae of a lobster. And all these gradations can be
actually traced.

Besides the avicularia, the polyzoa possess curious organs called
vibracula. These generally consist of long bristles, capable of movement
and easily excited. In one species examined by me the vibracula were
slightly curved and serrated along the outer margin, and all of them on
the same polyzoary often moved simultaneously; so that, acting like long
oars, they swept a branch rapidly across the object-glass of my
microscope. When a branch was placed on its face, the vibracula became
entangled, and they made violent efforts to free themselves. They are
supposed to serve as a defence, and may be seen, as Mr. Busk remarks, "to
sweep slowly and carefully over the surface of the polyzoary, removing
what might be noxious to the delicate inhabitants of the cells when their
tentacula are protruded." The avicularia, like the vibracula, probably
serve for defence, but they also catch and kill small living animals,
which, it is believed, are afterwards swept by the currents within reach
of the tentacula of the zooids. Some species are provided with avicularia
and vibracula, some with avicularia alone and a few with vibracula alone.

It is not easy to imagine two objects more widely different in appearance
than a bristle or vibraculum, and an avicularium like the head of a bird;
yet they are almost certainly homologous and have been developed from the
same common source, namely a zooid with its cell. Hence, we can understand
how it is that these organs graduate in some cases, as I am informed by
Mr. Busk, into each other. Thus, with the avicularia of several species of
Lepralia, the movable mandible is so much produced and is so like a
bristle that the presence of the upper or fixed beak alone serves to
determine its avicularian nature. The vibracula may have been directly
developed from the lips of the cells, without having passed through the
avicularian stage; but it seems more probable that they have passed
through this stage, as during the early stages of the transformation, the
other parts of the cell, with the included zooid, could hardly have
disappeared at once. In many cases the vibracula have a grooved support at
the base, which seems to represent the fixed beak; though this support in
some species is quite absent. This view of the development of the
vibracula, if trustworthy, is interesting; for supposing that all the
species provided with avicularia had become extinct, no one with the most
vivid imagination would ever have thought that the vibracula had
originally existed as part of an organ, resembling a bird's head, or an
irregular box or hood. It is interesting to see two such widely different
organs developed from a common origin; and as the movable lip of the cell
serves as a protection to the zooid, there is no difficulty in believing
that all the gradations, by which the lip became converted first into the
lower mandible of an avicularium, and then into an elongated bristle,
likewise served as a protection in different ways and under different
circumstances.

In the vegetable kingdom Mr. Mivart only alludes to two cases, namely the
structure of the flowers of orchids, and the movements of climbing plants.
With respect to the former, he says: "The explanation of their ORIGIN is
deemed thoroughly unsatisfactory—utterly insufficient to explain the
incipient, infinitesimal beginnings of structures which are of utility
only when they are considerably developed." As I have fully treated this
subject in another work, I will here give only a few details on one alone
of the most striking peculiarities of the flowers of orchids, namely,
their pollinia. A pollinium, when highly developed, consists of a mass of
pollen-grains, affixed to an elastic foot-stalk or caudicle, and this to a
little mass of extremely viscid matter. The pollinia are by this means
transported by insects from one flower to the stigma of another. In some
orchids there is no caudicle to the pollen-masses, and the grains are
merely tied together by fine threads; but as these are not confined to
orchids, they need not here be considered; yet I may mention that at the
base of the orchidaceous series, in Cypripedium, we can see how the
threads were probably first developed. In other orchids the threads cohere
at one end of the pollen-masses; and this forms the first or nascent trace
of a caudicle. That this is the origin of the caudicle, even when of
considerable length and highly developed, we have good evidence in the
aborted pollen-grains which can sometimes be detected embedded within the
central and solid parts.

With respect to the second chief peculiarity, namely, the little mass of
viscid matter attached to the end of the caudicle, a long series of
gradations can be specified, each of plain service to the plant. In most
flowers belonging to other orders the stigma secretes a little viscid
matter. Now, in certain orchids similar viscid matter is secreted, but in
much larger quantities by one alone of the three stigmas; and this stigma,
perhaps in consequence of the copious secretion, is rendered sterile. When
an insect visits a flower of this kind, it rubs off some of the viscid
matter, and thus at the same time drags away some of the pollen-grains.
From this simple condition, which differs but little from that of a
multitude of common flowers, there are endless gradations—to species
in which the pollen-mass terminates in a very short, free caudicle—to
others in which the caudicle becomes firmly attached to the viscid matter,
with the sterile stigma itself much modified. In this latter case we have
a pollinium in its most highly developed and perfect condition. He who
will carefully examine the flowers of orchids for himself will not deny
the existence of the above series of gradations—from a mass of
pollen-grains merely tied together by threads, with the stigma differing
but little from that of the ordinary flowers, to a highly complex
pollinium, admirably adapted for transportal by insects; nor will he deny
that all the gradations in the several species are admirably adapted in
relation to the general structure of each flower for its fertilisation by
different insects. In this, and in almost every other case, the enquiry
may be pushed further backwards; and it may be asked how did the stigma of
an ordinary flower become viscid, but as we do not know the full history
of any one group of beings, it is as useless to ask, as it is hopeless to
attempt answering, such questions.

We will now turn to climbing plants. These can be arranged in a long
series, from those which simply twine round a support, to those which I
have called leaf-climbers, and to those provided with tendrils. In these
two latter classes the stems have generally, but not always, lost the
power of twining, though they retain the power of revolving, which the
tendrils likewise possess. The gradations from leaf-climbers to tendril
bearers are wonderfully close, and certain plants may be differently
placed in either class. But in ascending the series from simple twiners to
leaf-climbers, an important quality is added, namely sensitiveness to a
touch, by which means the foot-stalks of the leaves or flowers, or these
modified and converted into tendrils, are excited to bend round and clasp
the touching object. He who will read my memoir on these plants will, I
think, admit that all the many gradations in function and structure
between simple twiners and tendril-bearers are in each case beneficial in
a high degree to the species. For instance, it is clearly a great
advantage to a twining plant to become a leaf-climber; and it is probable
that every twiner which possessed leaves with long foot-stalks would have
been developed into a leaf-climber, if the foot-stalks had possessed in
any slight degree the requisite sensitiveness to a touch.

As twining is the simplest means of ascending a support, and forms the
basis of our series, it may naturally be asked how did plants acquire this
power in an incipient degree, afterwards to be improved and increased
through natural selection. The power of twining depends, firstly, on the
stems while young being extremely flexible (but this is a character common
to many plants which are not climbers); and, secondly, on their
continually bending to all points of the compass, one after the other in
succession, in the same order. By this movement the stems are inclined to
all sides, and are made to move round and round. As soon as the lower part
of a stem strikes against any object and is stopped, the upper part still
goes on bending and revolving, and thus necessarily twines round and up
the support. The revolving movement ceases after the early growth of each
shoot. As in many widely separated families of plants, single species and
single genera possess the power of revolving, and have thus become
twiners, they must have independently acquired it, and cannot have
inherited it from a common progenitor. Hence, I was led to predict that
some slight tendency to a movement of this kind would be found to be far
from uncommon with plants which did not climb; and that this had afforded
the basis for natural selection to work on and improve. When I made this
prediction, I knew of only one imperfect case, namely, of the young
flower-peduncles of a Maurandia which revolved slightly and irregularly,
like the stems of twining plants, but without making any use of this
habit. Soon afterwards Fritz Muller discovered that the young stems of an
Alisma and of a Linum—plants which do not climb and are widely
separated in the natural system—revolved plainly, though
irregularly, and he states that he has reason to suspect that this occurs
with some other plants. These slight movements appear to be of no service
to the plants in question; anyhow, they are not of the least use in the
way of climbing, which is the point that concerns us. Nevertheless we can
see that if the stems of these plants had been flexible, and if under the
conditions to which they are exposed it had profited them to ascend to a
height, then the habit of slightly and irregularly revolving might have
been increased and utilised through natural selection, until they had
become converted into well-developed twining species.

With respect to the sensitiveness of the foot-stalks of the leaves and
flowers, and of tendrils, nearly the same remarks are applicable as in the
case of the revolving movements of twining plants. As a vast number of
species, belonging to widely distinct groups, are endowed with this kind
of sensitiveness, it ought to be found in a nascent condition in many
plants which have not become climbers. This is the case: I observed that
the young flower-peduncles of the above Maurandia curved themselves a
little towards the side which was touched. Morren found in several species
of Oxalis that the leaves and their foot-stalks moved, especially after
exposure to a hot sun, when they were gently and repeatedly touched, or
when the plant was shaken. I repeated these observations on some other
species of Oxalis with the same result; in some of them the movement was
distinct, but was best seen in the young leaves; in others it was
extremely slight. It is a more important fact that according to the high
authority of Hofmeister, the young shoots and leaves of all plants move
after being shaken; and with climbing plants it is, as we know, only
during the early stages of growth that the foot-stalks and tendrils are
sensitive.

It is scarcely possible that the above slight movements, due to a touch or
shake, in the young and growing organs of plants, can be of any functional
importance to them. But plants possess, in obedience to various stimuli,
powers of movement, which are of manifest importance to them; for
instance, towards and more rarely from the light—in opposition to,
and more rarely in the direction of, the attraction of gravity. When the
nerves and muscles of an animal are excited by galvanism or by the
absorption of strychnine, the consequent movements may be called an
incidental result, for the nerves and muscles have not been rendered
specially sensitive to these stimuli. So with plants it appears that, from
having the power of movement in obedience to certain stimuli, they are
excited in an incidental manner by a touch, or by being shaken. Hence
there is no great difficulty in admitting that in the case of
leaf-climbers and tendril-bearers, it is this tendency which has been
taken advantage of and increased through natural selection. It is,
however, probable, from reasons which I have assigned in my memoir, that
this will have occurred only with plants which had already acquired the
power of revolving, and had thus become twiners.

I have already endeavoured to explain how plants became twiners, namely,
by the increase of a tendency to slight and irregular revolving movements,
which were at first of no use to them; this movement, as well as that due
to a touch or shake, being the incidental result of the power of moving,
gained for other and beneficial purposes. Whether, during the gradual
development of climbing plants, natural selection has been aided by the
inherited effects of use, I will not pretend to decide; but we know that
certain periodical movements, for instance the so-called sleep of plants,
are governed by habit.

I have now considered enough, perhaps more than enough, of the cases,
selected with care by a skilful naturalist, to prove that natural
selection is incompetent to account for the incipient stages of useful
structures; and I have shown, as I hope, that there is no great difficulty
on this head. A good opportunity has thus been afforded for enlarging a
little on gradations of structure, often associated with strange functions—an
important subject, which was not treated at sufficient length in the
former editions of this work. I will now briefly recapitulate the
foregoing cases.

With the giraffe, the continued preservation of the individuals of some
extinct high-reaching ruminant, which had the longest necks, legs, etc.,
and could browse a little above the average height, and the continued
destruction of those which could not browse so high, would have sufficed
for the production of this remarkable quadruped; but the prolonged use of
all the parts, together with inheritance, will have aided in an important
manner in their co-ordination. With the many insects which imitate various
objects, there is no improbability in the belief that an accidental
resemblance to some common object was in each case the foundation for the
work of natural selection, since perfected through the occasional
preservation of slight variations which made the resemblance at all
closer; and this will have been carried on as long as the insect continued
to vary, and as long as a more and more perfect resemblance led to its
escape from sharp-sighted enemies. In certain species of whales there is a
tendency to the formation of irregular little points of horn on the
palate; and it seems to be quite within the scope of natural selection to
preserve all favourable variations, until the points were converted, first
into lamellated knobs or teeth, like those on the beak of a goose—then
into short lamellae, like those of the domestic ducks—and then into
lamellae, as perfect as those of the shoveller-duck—and finally into
the gigantic plates of baleen, as in the mouth of the Greenland whale. In
the family of the ducks, the lamellae are first used as teeth, then partly
as teeth and partly as a sifting apparatus, and at last almost exclusively
for this latter purpose.

With such structures as the above lamellae of horn or whalebone, habit or
use can have done little or nothing, as far as we can judge, towards their
development. On the other hand, the transportal of the lower eye of a
flat-fish to the upper side of the head, and the formation of a prehensile
tail, may be attributed almost wholly to continued use, together with
inheritance. With respect to the mammae of the higher animals, the most
probable conjecture is that primordially the cutaneous glands over the
whole surface of a marsupial sack secreted a nutritious fluid; and that
these glands were improved in function through natural selection, and
concentrated into a confined area, in which case they would have formed a
mamma. There is no more difficulty in understanding how the branched
spines of some ancient Echinoderm, which served as a defence, became
developed through natural selection into tridactyle pedicellariae, than in
understanding the development of the pincers of crustaceans, through
slight, serviceable modifications in the ultimate and penultimate segments
of a limb, which was at first used solely for locomotion. In the
avicularia and vibracula of the Polyzoa we have organs widely different in
appearance developed from the same source; and with the vibracula we can
understand how the successive gradations might have been of service. With
the pollinia of orchids, the threads which originally served to tie
together the pollen-grains, can be traced cohering into caudicles; and the
steps can likewise be followed by which viscid matter, such as that
secreted by the stigmas of ordinary flowers, and still subserving nearly
but not quite the same purpose, became attached to the free ends of the
caudicles—all these gradations being of manifest benefit to the
plants in question. With respect to climbing plants, I need not repeat
what has been so lately said.

It has often been asked, if natural selection be so potent, why has not
this or that structure been gained by certain species, to which it would
apparently have been advantageous? But it is unreasonable to expect a
precise answer to such questions, considering our ignorance of the past
history of each species, and of the conditions which at the present day
determine its numbers and range. In most cases only general reasons, but
in some few cases special reasons, can be assigned. Thus to adapt a
species to new habits of life, many co-ordinated modifications are almost
indispensable, and it may often have happened that the requisite parts did
not vary in the right manner or to the right degree. Many species must
have been prevented from increasing in numbers through destructive
agencies, which stood in no relation to certain structures, which we
imagine would have been gained through natural selection from appearing to
us advantageous to the species. In this case, as the struggle for life did
not depend on such structures, they could not have been acquired through
natural selection. In many cases complex and long-enduring conditions,
often of a peculiar nature, are necessary for the development of a
structure; and the requisite conditions may seldom have concurred. The
belief that any given structure, which we think, often erroneously, would
have been beneficial to a species, would have been gained under all
circumstances through natural selection, is opposed to what we can
understand of its manner of action. Mr. Mivart does not deny that natural
selection has effected something; but he considers it as "demonstrably
insufficient" to account for the phenomena which I explain by its agency.
His chief arguments have now been considered, and the others will
hereafter be considered. They seem to me to partake little of the
character of demonstration, and to have little weight in comparison with
those in favour of the power of natural selection, aided by the other
agencies often specified. I am bound to add, that some of the facts and
arguments here used by me, have been advanced for the same purpose in an
able article lately published in the "Medico-Chirurgical Review."

At the present day almost all naturalists admit evolution under some form.
Mr. Mivart believes that species change through "an internal force or
tendency," about which it is not pretended that anything is known. That
species have a capacity for change will be admitted by all evolutionists;
but there is no need, as it seems to me, to invoke any internal force
beyond the tendency to ordinary variability, which through the aid of
selection, by man has given rise to many well-adapted domestic races, and
which, through the aid of natural selection, would equally well give rise
by graduated steps to natural races or species. The final result will
generally have been, as already explained, an advance, but in some few
cases a retrogression, in organisation.

Mr. Mivart is further inclined to believe, and some naturalists agree with
him, that new species manifest themselves "with suddenness and by
modifications appearing at once." For instance, he supposes that the
differences between the extinct three-toed Hipparion and the horse arose
suddenly. He thinks it difficult to believe that the wing of a bird "was
developed in any other way than by a comparatively sudden modification of
a marked and important kind;" and apparently he would extend the same view
to the wings of bats and pterodactyles. This conclusion, which implies
great breaks or discontinuity in the series, appears to me improbable in
the highest degree.

Everyone who believes in slow and gradual evolution, will of course admit
that specific changes may have been as abrupt and as great as any single
variation which we meet with under nature, or even under domestication.
But as species are more variable when domesticated or cultivated than
under their natural conditions, it is not probable that such great and
abrupt variations have often occurred under nature, as are known
occasionally to arise under domestication. Of these latter variations
several may be attributed to reversion; and the characters which thus
reappear were, it is probable, in many cases at first gained in a gradual
manner. A still greater number must be called monstrosities, such as
six-fingered men, porcupine men, Ancon sheep, Niata cattle, etc.; and as
they are widely different in character from natural species, they throw
very little light on our subject. Excluding such cases of abrupt
variations, the few which remain would at best constitute, if found in a
state of nature, doubtful species, closely related to their parental
types.

My reasons for doubting whether natural species have changed as abruptly
as have occasionally domestic races, and for entirely disbelieving that
they have changed in the wonderful manner indicated by Mr. Mivart, are as
follows. According to our experience, abrupt and strongly marked
variations occur in our domesticated productions, singly and at rather
long intervals of time. If such occurred under nature, they would be
liable, as formerly explained, to be lost by accidental causes of
destruction and by subsequent intercrossing; and so it is known to be
under domestication, unless abrupt variations of this kind are specially
preserved and separated by the care of man. Hence, in order that a new
species should suddenly appear in the manner supposed by Mr. Mivart, it is
almost necessary to believe, in opposition to all analogy, that several
wonderfully changed individuals appeared simultaneously within the same
district. This difficulty, as in the case of unconscious selection by man,
is avoided on the theory of gradual evolution, through the preservation of
a large number of individuals, which varied more or less in any favourable
direction, and of the destruction of a large number which varied in an
opposite manner.

That many species have been evolved in an extremely gradual manner, there
can hardly be a doubt. The species and even the genera of many large
natural families are so closely allied together that it is difficult to
distinguish not a few of them. On every continent, in proceeding from
north to south, from lowland to upland, etc., we meet with a host of
closely related or representative species; as we likewise do on certain
distinct continents, which we have reason to believe were formerly
connected. But in making these and the following remarks, I am compelled
to allude to subjects hereafter to be discussed. Look at the many outlying
islands round a continent, and see how many of their inhabitants can be
raised only to the rank of doubtful species. So it is if we look to past
times, and compare the species which have just passed away with those
still living within the same areas; or if we compare the fossil species
embedded in the sub-stages of the same geological formation. It is indeed
manifest that multitudes of species are related in the closest manner to
other species that still exist, or have lately existed; and it will hardly
be maintained that such species have been developed in an abrupt or sudden
manner. Nor should it be forgotten, when we look to the special parts of
allied species, instead of to distinct species, that numerous and
wonderfully fine gradations can be traced, connecting together widely
different structures.

Many large groups of facts are intelligible only on the principle that
species have been evolved by very small steps. For instance, the fact that
the species included in the larger genera are more closely related to each
other, and present a greater number of varieties than do the species in
the smaller genera. The former are also grouped in little clusters, like
varieties round species; and they present other analogies with varieties,
as was shown in our second chapter. On this same principle we can
understand how it is that specific characters are more variable than
generic characters; and how the parts which are developed in an
extraordinary degree or manner are more variable than other parts of the
same species. Many analogous facts, all pointing in the same direction,
could be added.

Although very many species have almost certainly been produced by steps
not greater than those separating fine varieties; yet it may be maintained
that some have been developed in a different and abrupt manner. Such an
admission, however, ought not to be made without strong evidence being
assigned. The vague and in some respects false analogies, as they have
been shown to be by Mr. Chauncey Wright, which have been advanced in
favour of this view, such as the sudden crystallisation of inorganic
substances, or the falling of a facetted spheroid from one facet to
another, hardly deserve consideration. One class of facts, however,
namely, the sudden appearance of new and distinct forms of life in our
geological formations supports at first sight the belief in abrupt
development. But the value of this evidence depends entirely on the
perfection of the geological record, in relation to periods remote in the
history of the world. If the record is as fragmentary as many geologists
strenuously assert, there is nothing strange in new forms appearing as if
suddenly developed.

Unless we admit transformations as prodigious as those advocated by Mr.
Mivart, such as the sudden development of the wings of birds or bats, or
the sudden conversion of a Hipparion into a horse, hardly any light is
thrown by the belief in abrupt modifications on the deficiency of
connecting links in our geological formations. But against the belief in
such abrupt changes, embryology enters a strong protest. It is notorious
that the wings of birds and bats, and the legs of horses or other
quadrupeds, are undistinguishable at an early embryonic period, and that
they become differentiated by insensibly fine steps. Embryological
resemblances of all kinds can be accounted for, as we shall hereafter see,
by the progenitors of our existing species having varied after early
youth, and having transmitted their newly-acquired characters to their
offspring, at a corresponding age. The embryo is thus left almost
unaffected, and serves as a record of the past condition of the species.
Hence it is that existing species during the early stages of their
development so often resemble ancient and extinct forms belonging to the
same class. On this view of the meaning of embryological resemblances, and
indeed on any view, it is incredible that an animal should have undergone
such momentous and abrupt transformations as those above indicated, and
yet should not bear even a trace in its embryonic condition of any sudden
modification, every detail in its structure being developed by insensibly
fine steps.

He who believes that some ancient form was transformed suddenly through an
internal force or tendency into, for instance, one furnished with wings,
will be almost compelled to assume, in opposition to all analogy, that
many individuals varied simultaneously. It cannot be denied that such
abrupt and great changes of structure are widely different from those
which most species apparently have undergone. He will further be compelled
to believe that many structures beautifully adapted to all the other parts
of the same creature and to the surrounding conditions, have been suddenly
produced; and of such complex and wonderful co-adaptations, he will not be
able to assign a shadow of an explanation. He will be forced to admit that
these great and sudden transformations have left no trace of their action
on the embryo. To admit all this is, as it seems to me, to enter into the
realms of miracle, and to leave those of science.



CHAPTER VIII. INSTINCT.


 Instincts comparable with habits, but different in their
 origin—Instincts graduated—Aphides and ants—Instincts
 variable—Domestic instincts, their origin—Natural instincts of
 the cuckoo, molothrus, ostrich, and parasitic bees—Slave-making
 ants—Hive-bee, its cell-making instinct—Changes of instinct and
 structure not necessarily simultaneous—Difficulties of the theory of
 the Natural Selection of instincts—Neuter or sterile insects—Summary.

Many instincts are so wonderful that their development will probably
appear to the reader a difficulty sufficient to overthrow my whole theory.
I may here premise, that I have nothing to do with the origin of the
mental powers, any more than I have with that of life itself. We are
concerned only with the diversities of instinct and of the other mental
faculties in animals of the same class.

I will not attempt any definition of instinct. It would be easy to show
that several distinct mental actions are commonly embraced by this term;
but every one understands what is meant, when it is said that instinct
impels the cuckoo to migrate and to lay her eggs in other birds' nests. An
action, which we ourselves require experience to enable us to perform,
when performed by an animal, more especially by a very young one, without
experience, and when performed by many individuals in the same way,
without their knowing for what purpose it is performed, is usually said to
be instinctive. But I could show that none of these characters are
universal. A little dose of judgment or reason, as Pierre Huber expresses
it, often comes into play, even with animals low in the scale of nature.

Frederick Cuvier and several of the older metaphysicians have compared
instinct with habit. This comparison gives, I think, an accurate notion of
the frame of mind under which an instinctive action is performed, but not
necessarily of its origin. How unconsciously many habitual actions are
performed, indeed not rarely in direct opposition to our conscious will!
yet they may be modified by the will or reason. Habits easily become
associated with other habits, with certain periods of time and states of
the body. When once acquired, they often remain constant throughout life.
Several other points of resemblance between instincts and habits could be
pointed out. As in repeating a well-known song, so in instincts, one
action follows another by a sort of rhythm; if a person be interrupted in
a song, or in repeating anything by rote, he is generally forced to go
back to recover the habitual train of thought: so P. Huber found it was
with a caterpillar, which makes a very complicated hammock; for if he took
a caterpillar which had completed its hammock up to, say, the sixth stage
of construction, and put it into a hammock completed up only to the third
stage, the caterpillar simply re-performed the fourth, fifth, and sixth
stages of construction. If, however, a caterpillar were taken out of a
hammock made up, for instance, to the third stage, and were put into one
finished up to the sixth stage, so that much of its work was already done
for it, far from deriving any benefit from this, it was much embarrassed,
and, in order to complete its hammock, seemed forced to start from the
third stage, where it had left off, and thus tried to complete the already
finished work.

If we suppose any habitual action to become inherited—and it can be
shown that this does sometimes happen—then the resemblance between
what originally was a habit and an instinct becomes so close as not to be
distinguished. If Mozart, instead of playing the pianoforte at three years
old with wonderfully little practice, had played a tune with no practice
at all, be might truly be said to have done so instinctively. But it would
be a serious error to suppose that the greater number of instincts have
been acquired by habit in one generation, and then transmitted by
inheritance to succeeding generations. It can be clearly shown that the
most wonderful instincts with which we are acquainted, namely, those of
the hive-bee and of many ants, could not possibly have been acquired by
habit.

It will be universally admitted that instincts are as important as
corporeal structures for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a
species; and if it can be shown that instincts do vary ever so little,
then I can see no difficulty in natural selection preserving and
continually accumulating variations of instinct to any extent that was
profitable. It is thus, as I believe, that all the most complex and
wonderful instincts have originated. As modifications of corporeal
structure arise from, and are increased by, use or habit, and are
diminished or lost by disuse, so I do not doubt it has been with
instincts. But I believe that the effects of habit are in many cases of
subordinate importance to the effects of the natural selection of what may
be called spontaneous variations of instincts;—that is of variations
produced by the same unknown causes which produce slight deviations of
bodily structure.

No complex instinct can possibly be produced through natural selection,
except by the slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures, we
ought to find in nature, not the actual transitional gradations by which
each complex instinct has been acquired—for these could be found
only in the lineal ancestors of each species—but we ought to find in
the collateral lines of descent some evidence of such gradations; or we
ought at least to be able to show that gradations of some kind are
possible; and this we certainly can do. I have been surprised to find,
making allowance for the instincts of animals having been but little
observed, except in Europe and North America, and for no instinct being
known among extinct species, how very generally gradations, leading to the
most complex instincts, can be discovered. Changes of instinct may
sometimes be facilitated by the same species having different instincts at
different periods of life, or at different seasons of the year, or when
placed under different circumstances, etc.; in which case either the one
or the other instinct might be preserved by natural selection. And such
instances of diversity of instinct in the same species can be shown to
occur in nature.

Again, as in the case of corporeal structure, and conformably to my
theory, the instinct of each species is good for itself, but has never, as
far as we can judge, been produced for the exclusive good of others. One
of the strongest instances of an animal apparently performing an action
for the sole good of another, with which I am acquainted, is that of
aphides voluntarily yielding, as was first observed by Huber, their sweet
excretion to ants: that they do so voluntarily, the following facts show.
I removed all the ants from a group of about a dozen aphides on a
dock-plant, and prevented their attendance during several hours. After
this interval, I felt sure that the aphides would want to excrete. I
watched them for some time through a lens, but not one excreted; I then
tickled and stroked them with a hair in the same manner, as well as I
could, as the ants do with their antennae; but not one excreted.
Afterwards, I allowed an ant to visit them, and it immediately seemed, by
its eager way of running about to be well aware what a rich flock it had
discovered; it then began to play with its antennae on the abdomen first
of one aphis and then of another; and each, as soon as it felt the
antennae, immediately lifted up its abdomen and excreted a limpid drop of
sweet juice, which was eagerly devoured by the ant. Even the quite young
aphides behaved in this manner, showing that the action was instinctive,
and not the result of experience. It is certain, from the observations of
Huber, that the aphides show no dislike to the ants: if the latter be not
present they are at last compelled to eject their excretion. But as the
excretion is extremely viscid, it is no doubt a convenience to the aphides
to have it removed; therefore probably they do not excrete solely for the
good of the ants. Although there is no evidence that any animal performs
an action for the exclusive good of another species, yet each tries to
take advantage of the instincts of others, as each takes advantage of the
weaker bodily structure of other species. So again certain instincts
cannot be considered as absolutely perfect; but as details on this and
other such points are not indispensable, they may be here passed over.

As some degree of variation in instincts under a state of nature, and the
inheritance of such variations, are indispensable for the action of
natural selection, as many instances as possible ought to be given; but
want of space prevents me. I can only assert that instincts certainly do
vary—for instance, the migratory instinct, both in extent and
direction, and in its total loss. So it is with the nests of birds, which
vary partly in dependence on the situations chosen, and on the nature and
temperature of the country inhabited, but often from causes wholly unknown
to us. Audubon has given several remarkable cases of differences in the
nests of the same species in the northern and southern United States. Why,
it has been asked, if instinct be variable, has it not granted to the bee
"the ability to use some other material when wax was deficient?" But what
other natural material could bees use? They will work, as I have seen,
with wax hardened with vermilion or softened with lard. Andrew Knight
observed that his bees, instead of laboriously collecting propolis, used a
cement of wax and turpentine, with which he had covered decorticated
trees. It has lately been shown that bees, instead of searching for
pollen, will gladly use a very different substance, namely, oatmeal. Fear
of any particular enemy is certainly an instinctive quality, as may be
seen in nestling birds, though it is strengthened by experience, and by
the sight of fear of the same enemy in other animals. The fear of man is
slowly acquired, as I have elsewhere shown, by the various animals which
inhabit desert islands; and we see an instance of this, even in England,
in the greater wildness of all our large birds in comparison with our
small birds; for the large birds have been most persecuted by man. We may
safely attribute the greater wildness of our large birds to this cause;
for in uninhabited islands large birds are not more fearful than small;
and the magpie, so wary in England, is tame in Norway, as is the hooded
crow in Egypt.

That the mental qualities of animals of the same kind, born in a state of
nature, vary much, could be shown by many facts. Several cases could also
be adduced of occasional and strange habits in wild animals, which, if
advantageous to the species, might have given rise, through natural
selection, to new instincts. But I am well aware that these general
statements, without the facts in detail, can produce but a feeble effect
on the reader's mind. I can only repeat my assurance, that I do not speak
without good evidence.

INHERITED CHANGES OF HABIT OR INSTINCT IN DOMESTICATED ANIMALS.

The possibility, or even probability, of inherited variations of instinct
in a state of nature will be strengthened by briefly considering a few
cases under domestication. We shall thus be enabled to see the part which
habit and the selection of so-called spontaneous variations have played in
modifying the mental qualities of our domestic animals. It is notorious
how much domestic animals vary in their mental qualities. With cats, for
instance, one naturally takes to catching rats, and another mice, and
these tendencies are known to be inherited. One cat, according to Mr. St.
John, always brought home game birds, another hares or rabbits, and
another hunted on marshy ground and almost nightly caught woodcocks or
snipes. A number of curious and authentic instances could be given of
various shades of disposition and taste, and likewise of the oddest
tricks, associated with certain frames of mind or periods of time. But let
us look to the familiar case of the breeds of dogs: it cannot be doubted
that young pointers (I have myself seen striking instances) will sometimes
point and even back other dogs the very first time that they are taken
out; retrieving is certainly in some degree inherited by retrievers; and a
tendency to run round, instead of at, a flock of sheep, by shepherd-dogs.
I cannot see that these actions, performed without experience by the
young, and in nearly the same manner by each individual, performed with
eager delight by each breed, and without the end being known—for the
young pointer can no more know that he points to aid his master, than the
white butterfly knows why she lays her eggs on the leaf of the cabbage—I
cannot see that these actions differ essentially from true instincts. If
we were to behold one kind of wolf, when young and without any training,
as soon as it scented its prey, stand motionless like a statue, and then
slowly crawl forward with a peculiar gait; and another kind of wolf
rushing round, instead of at, a herd of deer, and driving them to a
distant point, we should assuredly call these actions instinctive.
Domestic instincts, as they may be called, are certainly far less fixed
than natural instincts; but they have been acted on by far less rigorous
selection, and have been transmitted for an incomparably shorter period,
under less fixed conditions of life.

How strongly these domestic instincts, habits, and dispositions are
inherited, and how curiously they become mingled, is well shown when
different breeds of dogs are crossed. Thus it is known that a cross with a
bull-dog has affected for many generations the courage and obstinacy of
greyhounds; and a cross with a greyhound has given to a whole family of
shepherd-dogs a tendency to hunt hares. These domestic instincts, when
thus tested by crossing, resemble natural instincts, which in a like
manner become curiously blended together, and for a long period exhibit
traces of the instincts of either parent: for example, Le Roy describes a
dog, whose great-grandfather was a wolf, and this dog showed a trace of
its wild parentage only in one way, by not coming in a straight line to
his master, when called.

Domestic instincts are sometimes spoken of as actions which have become
inherited solely from long-continued and compulsory habit, but this is not
true. No one would ever have thought of teaching, or probably could have
taught, the tumbler-pigeon to tumble—an action which, as I have
witnessed, is performed by young birds, that have never seen a pigeon
tumble. We may believe that some one pigeon showed a slight tendency to
this strange habit, and that the long-continued selection of the best
individuals in successive generations made tumblers what they now are; and
near Glasgow there are house-tumblers, as I hear from Mr. Brent, which
cannot fly eighteen inches high without going head over heels. It may be
doubted whether any one would have thought of training a dog to point, had
not some one dog naturally shown a tendency in this line; and this is
known occasionally to happen, as I once saw, in a pure terrier: the act of
pointing is probably, as many have thought, only the exaggerated pause of
an animal preparing to spring on its prey. When the first tendency to
point was once displayed, methodical selection and the inherited effects
of compulsory training in each successive generation would soon complete
the work; and unconscious selection is still in progress, as each man
tries to procure, without intending to improve the breed, dogs which stand
and hunt best. On the other hand, habit alone in some cases has sufficed;
hardly any animal is more difficult to tame than the young of the wild
rabbit; scarcely any animal is tamer than the young of the tame rabbit;
but I can hardly suppose that domestic rabbits have often been selected
for tameness alone; so that we must attribute at least the greater part of
the inherited change from extreme wildness to extreme tameness, to habit
and long-continued close confinement.

Natural instincts are lost under domestication: a remarkable instance of
this is seen in those breeds of fowls which very rarely or never become
"broody," that is, never wish to sit on their eggs. Familiarity alone
prevents our seeing how largely and how permanently the minds of our
domestic animals have been modified. It is scarcely possible to doubt that
the love of man has become instinctive in the dog. All wolves, foxes,
jackals and species of the cat genus, when kept tame, are most eager to
attack poultry, sheep and pigs; and this tendency has been found incurable
in dogs which have been brought home as puppies from countries such as
Tierra del Fuego and Australia, where the savages do not keep these
domestic animals. How rarely, on the other hand, do our civilised dogs,
even when quite young, require to be taught not to attack poultry, sheep,
and pigs! No doubt they occasionally do make an attack, and are then
beaten; and if not cured, they are destroyed; so that habit and some
degree of selection have probably concurred in civilising by inheritance
our dogs. On the other hand, young chickens have lost wholly by habit,
that fear of the dog and cat which no doubt was originally instinctive in
them, for I am informed by Captain Hutton that the young chickens of the
parent stock, the Gallus bankiva, when reared in India under a hen, are at
first excessively wild. So it is with young pheasants reared in England
under a hen. It is not that chickens have lost all fear, but fear only of
dogs and cats, for if the hen gives the danger chuckle they will run (more
especially young turkeys) from under her and conceal themselves in the
surrounding grass or thickets; and this is evidently done for the
instinctive purpose of allowing, as we see in wild ground-birds, their
mother to fly away. But this instinct retained by our chickens has become
useless under domestication, for the mother-hen has almost lost by disuse
the power of flight.

Hence, we may conclude that under domestication instincts have been
acquired and natural instincts have been lost, partly by habit and partly
by man selecting and accumulating, during successive generations, peculiar
mental habits and actions, which at first appeared from what we must in
our ignorance call an accident. In some cases compulsory habit alone has
sufficed to produce inherited mental changes; in other cases compulsory
habit has done nothing, and all has been the result of selection, pursued
both methodically and unconsciously; but in most cases habit and selection
have probably concurred.

SPECIAL INSTINCTS.

We shall, perhaps, best understand how instincts in a state of nature have
become modified by selection by considering a few cases. I will select
only three, namely, the instinct which leads the cuckoo to lay her eggs in
other birds' nests; the slave-making instinct of certain ants; and the
cell-making power of the hive-bee: these two latter instincts have
generally and justly been ranked by naturalists as the most wonderful of
all known instincts.

INSTINCTS OF THE CUCKOO.

It is supposed by some naturalists that the more immediate cause of the
instinct of the cuckoo is that she lays her eggs, not daily, but at
intervals of two or three days; so that, if she were to make her own nest
and sit on her own eggs, those first laid would have to be left for some
time unincubated or there would be eggs and young birds of different ages
in the same nest. If this were the case the process of laying and hatching
might be inconveniently long, more especially as she migrates at a very
early period; and the first hatched young would probably have to be fed by
the male alone. But the American cuckoo is in this predicament, for she
makes her own nest and has eggs and young successively hatched, all at the
same time. It has been both asserted and denied that the American cuckoo
occasionally lays her eggs in other birds' nests; but I have lately heard
from Dr. Merrill, of Iowa, that he once found in Illinois a young cuckoo,
together with a young jay in the nest of a blue jay (Garrulus cristatus);
and as both were nearly full feathered, there could be no mistake in their
identification. I could also give several instances of various birds which
have been known occasionally to lay their eggs in other birds' nests. Now
let us suppose that the ancient progenitor of our European cuckoo had the
habits of the American cuckoo, and that she occasionally laid an egg in
another bird's nest. If the old bird profited by this occasional habit
through being enabled to emigrate earlier or through any other cause; or
if the young were made more vigorous by advantage being taken of the
mistaken instinct of another species than when reared by their own mother,
encumbered as she could hardly fail to be by having eggs and young of
different ages at the same time, then the old birds or the fostered young
would gain an advantage. And analogy would lead us to believe that the
young thus reared would be apt to follow by inheritance the occasional and
aberrant habit of their mother, and in their turn would be apt to lay
their eggs in other birds' nests, and thus be more successful in rearing
their young. By a continued process of this nature, I believe that the
strange instinct of our cuckoo has been generated. It has, also recently
been ascertained on sufficient evidence, by Adolf Muller, that the cuckoo
occasionally lays her eggs on the bare ground, sits on them and feeds her
young. This rare event is probably a case of reversion to the long-lost,
aboriginal instinct of nidification.

It has been objected that I have not noticed other related instincts and
adaptations of structure in the cuckoo, which are spoken of as necessarily
co-ordinated. But in all cases, speculation on an instinct known to us
only in a single species, is useless, for we have hitherto had no facts to
guide us. Until recently the instincts of the European and of the
non-parasitic American cuckoo alone were known; now, owing to Mr. Ramsay's
observations, we have learned something about three Australian species,
which lay their eggs in other birds' nests. The chief points to be
referred to are three: first, that the common cuckoo, with rare
exceptions, lays only one egg in a nest, so that the large and voracious
young bird receives ample food. Secondly, that the eggs are remarkably
small, not exceeding those of the skylark—a bird about one-fourth as
large as the cuckoo. That the small size of the egg is a real case of
adaptation we may infer from the fact of the mon-parasitic American cuckoo
laying full-sized eggs. Thirdly, that the young cuckoo, soon after birth,
has the instinct, the strength and a properly shaped back for ejecting its
foster-brothers, which then perish from cold and hunger. This has been
boldly called a beneficent arrangement, in order that the young cuckoo may
get sufficient food, and that its foster-brothers may perish before they
had acquired much feeling!

Turning now to the Australian species: though these birds generally lay
only one egg in a nest, it is not rare to find two and even three eggs in
the same nest. In the bronze cuckoo the eggs vary greatly in size, from
eight to ten lines in length. Now, if it had been of an advantage to this
species to have laid eggs even smaller than those now laid, so as to have
deceived certain foster-parents, or, as is more probable, to have been
hatched within a shorter period (for it is asserted that there is a
relation between the size of eggs and the period of their incubation),
then there is no difficulty in believing that a race or species might have
been formed which would have laid smaller and smaller eggs; for these
would have been more safely hatched and reared. Mr. Ramsay remarks that
two of the Australian cuckoos, when they lay their eggs in an open nest,
manifest a decided preference for nests containing eggs similar in colour
to their own. The European species apparently manifests some tendency
towards a similar instinct, but not rarely departs from it, as is shown by
her laying her dull and pale-coloured eggs in the nest of the
hedge-warbler with bright greenish-blue eggs. Had our cuckoo invariably
displayed the above instinct, it would assuredly have been added to those
which it is assumed must all have been acquired together. The eggs of the
Australian bronze cuckoo vary, according to Mr. Ramsay, to an
extraordinary degree in colour; so that in this respect, as well as in
size, natural selection might have secured and fixed any advantageous
variation.

In the case of the European cuckoo, the offspring of the foster-parents
are commonly ejected from the nest within three days after the cuckoo is
hatched; and as the latter at this age is in a most helpless condition,
Mr. Gould was formerly inclined to believe that the act of ejection was
performed by the foster-parents themselves. But he has now received a
trustworthy account of a young cuckoo which was actually seen, while still
blind and not able even to hold up its own head, in the act of ejecting
its foster-brothers. One of these was replaced in the nest by the
observer, and was again thrown out. With respect to the means by which
this strange and odious instinct was acquired, if it were of great
importance for the young cuckoo, as is probably the case, to receive as
much food as possible soon after birth, I can see no special difficulty in
its having gradually acquired, during successive generations, the blind
desire, the strength, and structure necessary for the work of ejection;
for those cuckoos which had such habits and structure best developed would
be the most securely reared. The first step towards the acquisition of the
proper instinct might have been mere unintentional restlessness on the
part of the young bird, when somewhat advanced in age and strength; the
habit having been afterwards improved, and transmitted to an earlier age.
I can see no more difficulty in this than in the unhatched young of other
birds acquiring the instinct to break through their own shells; or than in
young snakes acquiring in their upper jaws, as Owen has remarked, a
transitory sharp tooth for cutting through the tough egg-shell. For if
each part is liable to individual variations at all ages, and the
variations tend to be inherited at a corresponding or earlier age—propositions
which cannot be disputed—then the instincts and structure of the
young could be slowly modified as surely as those of the adult; and both
cases must stand or fall together with the whole theory of natural
selection.

Some species of Molothrus, a widely distinct genus of American birds,
allied to our starlings, have parasitic habits like those of the cuckoo;
and the species present an interesting gradation in the perfection of
their instincts. The sexes of Molothrus badius are stated by an excellent
observer, Mr. Hudson, sometimes to live promiscuously together in flocks,
and sometimes to pair. They either build a nest of their own or seize on
one belonging to some other bird, occasionally throwing out the nestlings
of the stranger. They either lay their eggs in the nest thus appropriated,
or oddly enough build one for themselves on the top of it. They usually
sit on their own eggs and rear their own young; but Mr. Hudson says it is
probable that they are occasionally parasitic, for he has seen the young
of this species following old birds of a distinct kind and clamouring to
be fed by them. The parasitic habits of another species of Molothrus, the
M. bonariensis, are much more highly developed than those of the last, but
are still far from perfect. This bird, as far as it is known, invariably
lays its eggs in the nests of strangers; but it is remarkable that several
together sometimes commence to build an irregular untidy nest of their
own, placed in singular ill-adapted situations, as on the leaves of a
large thistle. They never, however, as far as Mr. Hudson has ascertained,
complete a nest for themselves. They often lay so many eggs—from
fifteen to twenty—in the same foster-nest, that few or none can
possibly be hatched. They have, moreover, the extraordinary habit of
pecking holes in the eggs, whether of their own species or of their foster
parents, which they find in the appropriated nests. They drop also many
eggs on the bare ground, which are thus wasted. A third species, the M.
pecoris of North America, has acquired instincts as perfect as those of
the cuckoo, for it never lays more than one egg in a foster-nest, so that
the young bird is securely reared. Mr. Hudson is a strong disbeliever in
evolution, but he appears to have been so much struck by the imperfect
instincts of the Molothrus bonariensis that he quotes my words, and asks,
"Must we consider these habits, not as especially endowed or created
instincts, but as small consequences of one general law, namely,
transition?"

Various birds, as has already been remarked, occasionally lay their eggs
in the nests of other birds. This habit is not very uncommon with the
Gallinaceae, and throws some light on the singular instinct of the
ostrich. In this family several hen birds unite and lay first a few eggs
in one nest and then in another; and these are hatched by the males. This
instinct may probably be accounted for by the fact of the hens laying a
large number of eggs, but, as with the cuckoo, at intervals of two or
three days. The instinct, however, of the American ostrich, as in the case
of the Molothrus bonariensis, has not as yet been perfected; for a
surprising number of eggs lie strewed over the plains, so that in one
day's hunting I picked up no less than twenty lost and wasted eggs.

Many bees are parasitic, and regularly lay their eggs in the nests of
other kinds of bees. This case is more remarkable than that of the cuckoo;
for these bees have not only had their instincts but their structure
modified in accordance with their parasitic habits; for they do not
possess the pollen-collecting apparatus which would have been
indispensable if they had stored up food for their own young. Some species
of Sphegidae (wasp-like insects) are likewise parasitic; and M. Fabre has
lately shown good reason for believing that, although the Tachytes nigra
generally makes its own burrow and stores it with paralysed prey for its
own larvae, yet that, when this insect finds a burrow already made and
stored by another sphex, it takes advantage of the prize, and becomes for
the occasion parasitic. In this case, as with that of the Molothrus or
cuckoo, I can see no difficulty in natural selection making an occasional
habit permanent, if of advantage to the species, and if the insect whose
nest and stored food are feloniously appropriated, be not thus
exterminated.

SLAVE-MAKING INSTINCT.

This remarkable instinct was first discovered in the Formica (Polyerges)
rufescens by Pierre Huber, a better observer even than his celebrated
father. This ant is absolutely dependent on its slaves; without their aid,
the species would certainly become extinct in a single year. The males and
fertile females do no work of any kind, and the workers or sterile
females, though most energetic and courageous in capturing slaves, do no
other work. They are incapable of making their own nests, or of feeding
their own larvae. When the old nest is found inconvenient, and they have
to migrate, it is the slaves which determine the migration, and actually
carry their masters in their jaws. So utterly helpless are the masters,
that when Huber shut up thirty of them without a slave, but with plenty of
the food which they like best, and with their larvae and pupae to
stimulate them to work, they did nothing; they could not even feed
themselves, and many perished of hunger. Huber then introduced a single
slave (F. fusca), and she instantly set to work, fed and saved the
survivors; made some cells and tended the larvae, and put all to rights.
What can be more extraordinary than these well-ascertained facts? If we
had not known of any other slave-making ant, it would have been hopeless
to speculate how so wonderful an instinct could have been perfected.

Another species, Formica sanguinea, was likewise first discovered by P.
Huber to be a slave-making ant. This species is found in the southern
parts of England, and its habits have been attended to by Mr. F. Smith, of
the British Museum, to whom I am much indebted for information on this and
other subjects. Although fully trusting to the statements of Huber and Mr.
Smith, I tried to approach the subject in a sceptical frame of mind, as
any one may well be excused for doubting the existence of so extraordinary
an instinct as that of making slaves. Hence, I will give the observations
which I made in some little detail. I opened fourteen nests of F.
sanguinea, and found a few slaves in all. Males and fertile females of the
slave-species (F. fusca) are found only in their own proper communities,
and have never been observed in the nests of F. sanguinea. The slaves are
black and not above half the size of their red masters, so that the
contrast in their appearance is great. When the nest is slightly
disturbed, the slaves occasionally come out, and like their masters are
much agitated and defend the nest: when the nest is much disturbed, and
the larvae and pupae are exposed, the slaves work energetically together
with their masters in carrying them away to a place of safety. Hence, it
is clear that the slaves feel quite at home. During the months of June and
July, on three successive years, I watched for many hours several nests in
Surrey and Sussex, and never saw a slave either leave or enter a nest. As,
during these months, the slaves are very few in number, I thought that
they might behave differently when more numerous; but Mr. Smith informs me
that he has watched the nests at various hours during May, June and
August, both in Surrey and Hampshire, and has never seen the slaves,
though present in large numbers in August, either leave or enter the nest.
Hence, he considers them as strictly household slaves. The masters, on the
other hand, may be constantly seen bringing in materials for the nest, and
food of all kinds. During the year 1860, however, in the month of July, I
came across a community with an unusually large stock of slaves, and I
observed a few slaves mingled with their masters leaving the nest, and
marching along the same road to a tall Scotch-fir tree, twenty-five yards
distant, which they ascended together, probably in search of aphides or
cocci. According to Huber, who had ample opportunities for observation,
the slaves in Switzerland habitually work with their masters in making the
nest, and they alone open and close the doors in the morning and evening;
and, as Huber expressly states, their principal office is to search for
aphides. This difference in the usual habits of the masters and slaves in
the two countries, probably depends merely on the slaves being captured in
greater numbers in Switzerland than in England.

One day I fortunately witnessed a migration of F. sanguinea from one nest
to another, and it was a most interesting spectacle to behold the masters
carefully carrying their slaves in their jaws instead of being carried by
them, as in the case of F. rufescens. Another day my attention was struck
by about a score of the slave-makers haunting the same spot, and evidently
not in search of food; they approached and were vigorously repulsed by an
independent community of the slave species (F. fusca); sometimes as many
as three of these ants clinging to the legs of the slave-making F.
sanguinea. The latter ruthlessly killed their small opponents and carried
their dead bodies as food to their nest, twenty-nine yards distant; but
they were prevented from getting any pupae to rear as slaves. I then dug
up a small parcel of the pupae of F. fusca from another nest, and put them
down on a bare spot near the place of combat; they were eagerly seized and
carried off by the tyrants, who perhaps fancied that, after all, they had
been victorious in their late combat.

At the same time I laid on the same place a small parcel of the pupae of
another species, F. flava, with a few of these little yellow ants still
clinging to the fragments of their nest. This species is sometimes, though
rarely, made into slaves, as has been described by Mr. Smith. Although so
small a species, it is very courageous, and I have seen it ferociously
attack other ants. In one instance I found to my surprise an independent
community of F. flava under a stone beneath a nest of the slave-making F.
sanguinea; and when I had accidentally disturbed both nests, the little
ants attacked their big neighbours with surprising courage. Now I was
curious to ascertain whether F. sanguinea could distinguish the pupae of
F. fusca, which they habitually make into slaves, from those of the little
and furious F. flava, which they rarely capture, and it was evident that
they did at once distinguish them; for we have seen that they eagerly and
instantly seized the pupae of F. fusca, whereas they were much terrified
when they came across the pupae, or even the earth from the nest, of F.
flava, and quickly ran away; but in about a quarter of an hour, shortly
after all the little yellow ants had crawled away, they took heart and
carried off the pupae.

One evening I visited another community of F. sanguinea, and found a
number of these ants returning home and entering their nests, carrying the
dead bodies of F. fusca (showing that it was not a migration) and numerous
pupae. I traced a long file of ants burdened with booty, for about forty
yards back, to a very thick clump of heath, whence I saw the last
individual of F. sanguinea emerge, carrying a pupa; but I was not able to
find the desolated nest in the thick heath. The nest, however, must have
been close at hand, for two or three individuals of F. fusca were rushing
about in the greatest agitation, and one was perched motionless with its
own pupa in its mouth on the top of a spray of heath, an image of despair
over its ravaged home.

Such are the facts, though they did not need confirmation by me, in regard
to the wonderful instinct of making slaves. Let it be observed what a
contrast the instinctive habits of F. sanguinea present with those of the
continental F. rufescens. The latter does not build its own nest, does not
determine its own migrations, does not collect food for itself or its
young, and cannot even feed itself: it is absolutely dependent on its
numerous slaves. Formica sanguinea, on the other hand, possesses much
fewer slaves, and in the early part of the summer extremely few. The
masters determine when and where a new nest shall be formed, and when they
migrate, the masters carry the slaves. Both in Switzerland and England the
slaves seem to have the exclusive care of the larvae, and the masters
alone go on slave-making expeditions. In Switzerland the slaves and
masters work together, making and bringing materials for the nest: both,
but chiefly the slaves, tend and milk as it may be called, their aphides;
and thus both collect food for the community. In England the masters alone
usually leave the nest to collect building materials and food for
themselves, their slaves and larvae. So that the masters in this country
receive much less service from their slaves than they do in Switzerland.

By what steps the instinct of F. sanguinea originated I will not pretend
to conjecture. But as ants which are not slave-makers, will, as I have
seen, carry off pupae of other species, if scattered near their nests, it
is possible that such pupae originally stored as food might become
developed; and the foreign ants thus unintentionally reared would then
follow their proper instincts, and do what work they could. If their
presence proved useful to the species which had seized them—if it
were more advantageous to this species, to capture workers than to
procreate them—the habit of collecting pupae, originally for food,
might by natural selection be strengthened and rendered permanent for the
very different purpose of raising slaves. When the instinct was once
acquired, if carried out to a much less extent even than in our British F.
sanguinea, which, as we have seen, is less aided by its slaves than the
same species in Switzerland, natural selection might increase and modify
the instinct—always supposing each modification to be of use to the
species—until an ant was formed as abjectly dependent on its slaves
as is the Formica rufescens.

CELL-MAKING INSTINCT OF THE HIVE-BEE.

I will not here enter on minute details on this subject, but will merely
give an outline of the conclusions at which I have arrived. He must be a
dull man who can examine the exquisite structure of a comb, so beautifully
adapted to its end, without enthusiastic admiration. We hear from
mathematicians that bees have practically solved a recondite problem, and
have made their cells of the proper shape to hold the greatest possible
amount of honey, with the least possible consumption of precious wax in
their construction. It has been remarked that a skilful workman, with
fitting tools and measures, would find it very difficult to make cells of
wax of the true form, though this is effected by a crowd of bees working
in a dark hive. Granting whatever instincts you please, it seems at first
quite inconceivable how they can make all the necessary angles and planes,
or even perceive when they are correctly made. But the difficulty is not
nearly so great as at first appears: all this beautiful work can be shown,
I think, to follow from a few simple instincts.

I was led to investigate this subject by Mr. Waterhouse, who has shown
that the form of the cell stands in close relation to the presence of
adjoining cells; and the following view may, perhaps, be considered only
as a modification of his theory. Let us look to the great principle of
gradation, and see whether Nature does not reveal to us her method of
work. At one end of a short series we have humble-bees, which use their
old cocoons to hold honey, sometimes adding to them short tubes of wax,
and likewise making separate and very irregular rounded cells of wax. At
the other end of the series we have the cells of the hive-bee, placed in a
double layer: each cell, as is well known, is an hexagonal prism, with the
basal edges of its six sides bevelled so as to join an inverted pyramid,
of three rhombs. These rhombs have certain angles, and the three which
form the pyramidal base of a single cell on one side of the comb, enter
into the composition of the bases of three adjoining cells on the opposite
side. In the series between the extreme perfection of the cells of the
hive-bee and the simplicity of those of the humble-bee, we have the cells
of the Mexican Melipona domestica, carefully described and figured by
Pierre Huber. The Melipona itself is intermediate in structure between the
hive and humble bee, but more nearly related to the latter: it forms a
nearly regular waxen comb of cylindrical cells, in which the young are
hatched, and, in addition, some large cells of wax for holding honey.
These latter cells are nearly spherical and of nearly equal sizes, and are
aggregated into an irregular mass. But the important point to notice is,
that these cells are always made at that degree of nearness to each other
that they would have intersected or broken into each other if the spheres
had been completed; but this is never permitted, the bees building
perfectly flat walls of wax between the spheres which thus tend to
intersect. Hence, each cell consists of an outer spherical portion, and of
two, three, or more flat surfaces, according as the cell adjoins two,
three or more other cells. When one cell rests on three other cells,
which, from the spheres being nearly of the same size, is very frequently
and necessarily the case, the three flat surfaces are united into a
pyramid; and this pyramid, as Huber has remarked, is manifestly a gross
imitation of the three-sided pyramidal base of the cell of the hive-bee.
As in the cells of the hive-bee, so here, the three plane surfaces in any
one cell necessarily enter into the construction of three adjoining cells.
It is obvious that the Melipona saves wax, and what is more important,
labour, by this manner of building; for the flat walls between the
adjoining cells are not double, but are of the same thickness as the outer
spherical portions, and yet each flat portion forms a part of two cells.

Reflecting on this case, it occurred to me that if the Melipona had made
its spheres at some given distance from each other, and had made them of
equal sizes and had arranged them symmetrically in a double layer, the
resulting structure would have been as perfect as the comb of the
hive-bee. Accordingly I wrote to Professor Miller, of Cambridge, and this
geometer has kindly read over the following statement, drawn up from his
information, and tells me that it is strictly correct:—

If a number of equal spheres be described with their centres placed in two
parallel layers; with the centre of each sphere at the distance of radius
x sqrt(2) or radius x 1.41421 (or at some lesser distance), from the
centres of the six surrounding spheres in the same layer; and at the same
distance from the centres of the adjoining spheres in the other and
parallel layer; then, if planes of intersection between the several
spheres in both layers be formed, there will result a double layer of
hexagonal prisms united together by pyramidal bases formed of three
rhombs; and the rhombs and the sides of the hexagonal prisms will have
every angle identically the same with the best measurements which have
been made of the cells of the hive-bee. But I hear from Professor Wyman,
who has made numerous careful measurements, that the accuracy of the
workmanship of the bee has been greatly exaggerated; so much so, that
whatever the typical form of the cell may be, it is rarely, if ever,
realised.

Hence we may safely conclude that, if we could slightly modify the
instincts already possessed by the Melipona, and in themselves not very
wonderful, this bee would make a structure as wonderfully perfect as that
of the hive-bee. We must suppose the Melipona to have the power of forming
her cells truly spherical, and of equal sizes; and this would not be very
surprising, seeing that she already does so to a certain extent, and
seeing what perfectly cylindrical burrows many insects make in wood,
apparently by turning round on a fixed point. We must suppose the Melipona
to arrange her cells in level layers, as she already does her cylindrical
cells; and we must further suppose, and this is the greatest difficulty,
that she can somehow judge accurately at what distance to stand from her
fellow-labourers when several are making their spheres; but she is already
so far enabled to judge of distance, that she always describes her spheres
so as to intersect to a certain extent; and then she unites the points of
intersection by perfectly flat surfaces. By such modifications of
instincts which in themselves are not very wonderful—hardly more
wonderful than those which guide a bird to make its nest—I believe
that the hive-bee has acquired, through natural selection, her inimitable
architectural powers.

But this theory can be tested by experiment. Following the example of Mr.
Tegetmeier, I separated two combs, and put between them a long, thick,
rectangular strip of wax: the bees instantly began to excavate minute
circular pits in it; and as they deepened these little pits, they made
them wider and wider until they were converted into shallow basins,
appearing to the eye perfectly true or parts of a sphere, and of about the
diameter of a cell. It was most interesting to observe that, wherever
several bees had begun to excavate these basins near together, they had
begun their work at such a distance from each other that by the time the
basins had acquired the above stated width (i.e. about the width of an
ordinary cell), and were in depth about one sixth of the diameter of the
sphere of which they formed a part, the rims of the basins intersected or
broke into each other. As soon as this occurred, the bees ceased to
excavate, and began to build up flat walls of wax on the lines of
intersection between the basins, so that each hexagonal prism was built
upon the scalloped edge of a smooth basin, instead of on the straight
edges of a three-sided pyramid as in the case of ordinary cells.

I then put into the hive, instead of a thick, rectangular piece of wax, a
thin and narrow, knife-edged ridge, coloured with vermilion. The bees
instantly began on both sides to excavate little basins near to each
other, in the same way as before; but the ridge of wax was so thin, that
the bottoms of the basins, if they had been excavated to the same depth as
in the former experiment, would have broken into each other from the
opposite sides. The bees, however, did not suffer this to happen, and they
stopped their excavations in due time; so that the basins, as soon as they
had been a little deepened, came to have flat bases; and these flat bases,
formed by thin little plates of the vermilion wax left ungnawed, were
situated, as far as the eye could judge, exactly along the planes of
imaginary intersection between the basins on the opposite side of the
ridge of wax. In some parts, only small portions, in other parts, large
portions of a rhombic plate were thus left between the opposed basins, but
the work, from the unnatural state of things, had not been neatly
performed. The bees must have worked at very nearly the same rate in
circularly gnawing away and deepening the basins on both sides of the
ridge of vermilion wax, in order to have thus succeeded in leaving flat
plates between the basins, by stopping work at the planes of intersection.

Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip of wax,
perceiving when they have gnawed the wax away to the proper thinness, and
then stopping their work. In ordinary combs it has appeared to me that the
bees do not always succeed in working at exactly the same rate from the
opposite sides; for I have noticed half-completed rhombs at the base of a
just-commenced cell, which were slightly concave on one side, where I
suppose that the bees had excavated too quickly, and convex on the opposed
side where the bees had worked less quickly. In one well-marked instance,
I put the comb back into the hive, and allowed the bees to go on working
for a short time, and again examined the cell, and I found that the
rhombic plate had been completed, and had become PERFECTLY FLAT: it was
absolutely impossible, from the extreme thinness of the little plate, that
they could have effected this by gnawing away the convex side; and I
suspect that the bees in such cases stand in the opposed cells and push
and bend the ductile and warm wax (which as I have tried is easily done)
into its proper intermediate plane, and thus flatten it.

From the experiment of the ridge of vermilion wax we can see that, if the
bees were to build for themselves a thin wall of wax, they could make
their cells of the proper shape, by standing at the proper distance from
each other, by excavating at the same rate, and by endeavouring to make
equal spherical hollows, but never allowing the spheres to break into each
other. Now bees, as may be clearly seen by examining the edge of a growing
comb, do make a rough, circumferential wall or rim all round the comb; and
they gnaw this away from the opposite sides, always working circularly as
they deepen each cell. They do not make the whole three-sided pyramidal
base of any one cell at the same time, but only that one rhombic plate
which stands on the extreme growing margin, or the two plates, as the case
may be; and they never complete the upper edges of the rhombic plates,
until the hexagonal walls are commenced. Some of these statements differ
from those made by the justly celebrated elder Huber, but I am convinced
of their accuracy; and if I had space, I could show that they are
conformable with my theory.

Huber's statement, that the very first cell is excavated out of a little
parallel-sided wall of wax, is not, as far as I have seen, strictly
correct; the first commencement having always been a little hood of wax;
but I will not here enter on details. We see how important a part
excavation plays in the construction of the cells; but it would be a great
error to suppose that the bees cannot build up a rough wall of wax in the
proper position—that is, along the plane of intersection between two
adjoining spheres. I have several specimens showing clearly that they can
do this. Even in the rude circumferential rim or wall of wax round a
growing comb, flexures may sometimes be observed, corresponding in
position to the planes of the rhombic basal plates of future cells. But
the rough wall of wax has in every case to be finished off, by being
largely gnawed away on both sides. The manner in which the bees build is
curious; they always make the first rough wall from ten to twenty times
thicker than the excessively thin finished wall of the cell, which will
ultimately be left. We shall understand how they work, by supposing masons
first to pile up a broad ridge of cement, and then to begin cutting it
away equally on both sides near the ground, till a smooth, very thin wall
is left in the middle; the masons always piling up the cut-away cement,
and adding fresh cement on the summit of the ridge. We shall thus have a
thin wall steadily growing upward but always crowned by a gigantic coping.
From all the cells, both those just commenced and those completed, being
thus crowned by a strong coping of wax, the bees can cluster and crawl
over the comb without injuring the delicate hexagonal walls. These walls,
as Professor Miller has kindly ascertained for me, vary greatly in
thickness; being, on an average of twelve measurements made near the
border of the comb, 1/352 of an inch in thickness; whereas the basal
rhomboidal plates are thicker, nearly in the proportion of three to two,
having a mean thickness, from twenty-one measurements, of 1/229 of an
inch. By the above singular manner of building, strength is continually
given to the comb, with the utmost ultimate economy of wax.

It seems at first to add to the difficulty of understanding how the cells
are made, that a multitude of bees all work together; one bee after
working a short time at one cell going to another, so that, as Huber has
stated, a score of individuals work even at the commencement of the first
cell. I was able practically to show this fact, by covering the edges of
the hexagonal walls of a single cell, or the extreme margin of the
circumferential rim of a growing comb, with an extremely thin layer of
melted vermilion wax; and I invariably found that the colour was most
delicately diffused by the bees—as delicately as a painter could
have done it with his brush—by atoms of the coloured wax having been
taken from the spot on which it had been placed, and worked into the
growing edges of the cells all round. The work of construction seems to be
a sort of balance struck between many bees, all instinctively standing at
the same relative distance from each other, all trying to sweep equal
spheres, and then building up, or leaving ungnawed, the planes of
intersection between these spheres. It was really curious to note in cases
of difficulty, as when two pieces of comb met at an angle, how often the
bees would pull down and rebuild in different ways the same cell,
sometimes recurring to a shape which they had at first rejected.

When bees have a place on which they can stand in their proper positions
for working—for instance, on a slip of wood, placed directly under
the middle of a comb growing downwards, so that the comb has to be built
over one face of the slip—in this case the bees can lay the
foundations of one wall of a new hexagon, in its strictly proper place,
projecting beyond the other completed cells. It suffices that the bees
should be enabled to stand at their proper relative distances from each
other and from the walls of the last completed cells, and then, by
striking imaginary spheres, they can build up a wall intermediate between
two adjoining spheres; but, as far as I have seen, they never gnaw away
and finish off the angles of a cell till a large part both of that cell
and of the adjoining cells has been built. This capacity in bees of laying
down under certain circumstances a rough wall in its proper place between
two just-commenced cells, is important, as it bears on a fact, which seems
at first subversive of the foregoing theory; namely, that the cells on the
extreme margin of wasp-combs are sometimes strictly hexagonal; but I have
not space here to enter on this subject. Nor does there seem to me any
great difficulty in a single insect (as in the case of a queen-wasp)
making hexagonal cells, if she were to work alternately on the inside and
outside of two or three cells commenced at the same time, always standing
at the proper relative distance from the parts of the cells just begun,
sweeping spheres or cylinders, and building up intermediate planes.

As natural selection acts only by the accumulation of slight modifications
of structure or instinct, each profitable to the individual under its
conditions of life, it may reasonably be asked, how a long and graduated
succession of modified architectural instincts, all tending towards the
present perfect plan of construction, could have profited the progenitors
of the hive-bee? I think the answer is not difficult: cells constructed
like those of the bee or the wasp gain in strength, and save much in
labour and space, and in the materials of which they are constructed. With
respect to the formation of wax, it is known that bees are often hard
pressed to get sufficient nectar; and I am informed by Mr. Tegetmeier that
it has been experimentally proved that from twelve to fifteen pounds of
dry sugar are consumed by a hive of bees for the secretion of a pound of
wax; so that a prodigious quantity of fluid nectar must be collected and
consumed by the bees in a hive for the secretion of the wax necessary for
the construction of their combs. Moreover, many bees have to remain idle
for many days during the process of secretion. A large store of honey is
indispensable to support a large stock of bees during the winter; and the
security of the hive is known mainly to depend on a large number of bees
being supported. Hence the saving of wax by largely saving honey, and the
time consumed in collecting the honey, must be an important element of
success any family of bees. Of course the success of the species may be
dependent on the number of its enemies, or parasites, or on quite distinct
causes, and so be altogether independent of the quantity of honey which
the bees can collect. But let us suppose that this latter circumstance
determined, as it probably often has determined, whether a bee allied to
our humble-bees could exist in large numbers in any country; and let us
further suppose that the community lived through the winter, and
consequently required a store of honey: there can in this case be no doubt
that it would be an advantage to our imaginary humble-bee if a slight
modification of her instincts led her to make her waxen cells near
together, so as to intersect a little; for a wall in common even to two
adjoining cells would save some little labour and wax. Hence, it would
continually be more and more advantageous to our humble-bees, if they were
to make their cells more and more regular, nearer together, and aggregated
into a mass, like the cells of the Melipona; for in this case a large part
of the bounding surface of each cell would serve to bound the adjoining
cells, and much labour and wax would be saved. Again, from the same cause,
it would be advantageous to the Melipona, if she were to make her cells
closer together, and more regular in every way than at present; for then,
as we have seen, the spherical surfaces would wholly disappear and be
replaced by plane surfaces; and the Melipona would make a comb as perfect
as that of the hive-bee. Beyond this stage of perfection in architecture,
natural selection could not lead; for the comb of the hive-bee, as far as
we can see, is absolutely perfect in economising labour and wax.

Thus, as I believe, the most wonderful of all known instincts, that of the
hive-bee, can be explained by natural selection having taken advantage of
numerous, successive, slight modifications of simpler instincts; natural
selection having, by slow degrees, more and more perfectly led the bees to
sweep equal spheres at a given distance from each other in a double layer,
and to build up and excavate the wax along the planes of intersection. The
bees, of course, no more knowing that they swept their spheres at one
particular distance from each other, than they know what are the several
angles of the hexagonal prisms and of the basal rhombic plates; the motive
power of the process of natural selection having been the construction of
cells of due strength and of the proper size and shape for the larvae,
this being effected with the greatest possible economy of labour and wax;
that individual swarm which thus made the best cells with least labour,
and least waste of honey in the secretion of wax, having succeeded best,
and having transmitted their newly-acquired economical instincts to new
swarms, which in their turn will have had the best chance of succeeding in
the struggle for existence.

OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED TO INSTINCTS:
NEUTER AND STERILE INSECTS.

It has been objected to the foregoing view of the origin of instincts that
"the variations of structure and of instinct must have been simultaneous
and accurately adjusted to each other, as a modification in the one
without an immediate corresponding change in the other would have been
fatal." The force of this objection rests entirely on the assumption that
the changes in the instincts and structure are abrupt. To take as an
illustration the case of the larger titmouse, (Parus major) alluded to in
a previous chapter; this bird often holds the seeds of the yew between its
feet on a branch, and hammers with its beak till it gets at the kernel.
Now what special difficulty would there be in natural selection preserving
all the slight individual variations in the shape of the beak, which were
better and better adapted to break open the seeds, until a beak was
formed, as well constructed for this purpose as that of the nuthatch, at
the same time that habit, or compulsion, or spontaneous variations of
taste, led the bird to become more and more of a seed-eater? In this case
the beak is supposed to be slowly modified by natural selection,
subsequently to, but in accordance with, slowly changing habits or taste;
but let the feet of the titmouse vary and grow larger from correlation
with the beak, or from any other unknown cause, and it is not improbable
that such larger feet would lead the bird to climb more and more until it
acquired the remarkable climbing instinct and power of the nuthatch. In
this case a gradual change of structure is supposed to lead to changed
instinctive habits. To take one more case: few instincts are more
remarkable than that which leads the swift of the Eastern Islands to make
its nest wholly of inspissated saliva. Some birds build their nests of
mud, believed to be moistened with saliva; and one of the swifts of North
America makes its nest (as I have seen) of sticks agglutinated with
saliva, and even with flakes of this substance. Is it then very improbable
that the natural selection of individual swifts, which secreted more and
more saliva, should at last produce a species with instincts leading it to
neglect other materials and to make its nest exclusively of inspissated
saliva? And so in other cases. It must, however, be admitted that in many
instances we cannot conjecture whether it was instinct or structure which
first varied.

No doubt many instincts of very difficult explanation could be opposed to
the theory of natural selection—cases, in which we cannot see how an
instinct could have originated; cases, in which no intermediate gradations
are known to exist; cases of instincts of such trifling importance, that
they could hardly have been acted on by natural selection; cases of
instincts almost identically the same in animals so remote in the scale of
nature that we cannot account for their similarity by inheritance from a
common progenitor, and consequently must believe that they were
independently acquired through natural selection. I will not here enter on
these several cases, but will confine myself to one special difficulty,
which at first appeared to me insuperable, and actually fatal to the whole
theory. I allude to the neuters or sterile females in insect communities:
for these neuters often differ widely in instinct and in structure from
both the males and fertile females, and yet, from being sterile, they
cannot propagate their kind.

The subject well deserves to be discussed at great length, but I will here
take only a single case, that of working or sterile ants. How the workers
have been rendered sterile is a difficulty; but not much greater than that
of any other striking modification of structure; for it can be shown that
some insects and other articulate animals in a state of nature
occasionally become sterile; and if such insects had been social, and it
had been profitable to the community that a number should have been
annually born capable of work, but incapable of procreation, I can see no
especial difficulty in this having been effected through natural
selection. But I must pass over this preliminary difficulty. The great
difficulty lies in the working ants differing widely from both the males
and the fertile females in structure, as in the shape of the thorax, and
in being destitute of wings and sometimes of eyes, and in instinct. As far
as instinct alone is concerned, the wonderful difference in this respect
between the workers and the perfect females would have been better
exemplified by the hive-bee. If a working ant or other neuter insect had
been an ordinary animal, I should have unhesitatingly assumed that all its
characters had been slowly acquired through natural selection; namely, by
individuals having been born with slight profitable modifications, which
were inherited by the offspring, and that these again varied and again
were selected, and so onwards. But with the working ant we have an insect
differing greatly from its parents, yet absolutely sterile; so that it
could never have transmitted successively acquired modifications of
structure or instinct to its progeny. It may well be asked how it is
possible to reconcile this case with the theory of natural selection?

First, let it be remembered that we have innumerable instances, both in
our domestic productions and in those in a state of nature, of all sorts
of differences of inherited structure which are correlated with certain
ages and with either sex. We have differences correlated not only with one
sex, but with that short period when the reproductive system is active, as
in the nuptial plumage of many birds, and in the hooked jaws of the male
salmon. We have even slight differences in the horns of different breeds
of cattle in relation to an artificially imperfect state of the male sex;
for oxen of certain breeds have longer horns than the oxen of other
breeds, relatively to the length of the horns in both the bulls and cows
of these same breeds. Hence, I can see no great difficulty in any
character becoming correlated with the sterile condition of certain
members of insect communities; the difficulty lies in understanding how
such correlated modifications of structure could have been slowly
accumulated by natural selection.

This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be applied
to the family, as well as to the individual, and may thus gain the desired
end. Breeders of cattle wish the flesh and fat to be well marbled
together. An animal thus characterized has been slaughtered, but the
breeder has gone with confidence to the same stock and has succeeded. Such
faith may be placed in the power of selection that a breed of cattle,
always yielding oxen with extraordinarily long horns, could, it is
probable, be formed by carefully watching which individual bulls and cows,
when matched, produced oxen with the longest horns; and yet no one ox
would ever have propagated its kind. Here is a better and real
illustration: According to M. Verlot, some varieties of the double annual
stock, from having been long and carefully selected to the right degree,
always produce a large proportion of seedlings bearing double and quite
sterile flowers, but they likewise yield some single and fertile plants.
These latter, by which alone the variety can be propagated, may be
compared with the fertile male and female ants, and the double sterile
plants with the neuters of the same community. As with the varieties of
the stock, so with social insects, selection has been applied to the
family, and not to the individual, for the sake of gaining a serviceable
end. Hence, we may conclude that slight modifications of structure or of
instinct, correlated with the sterile condition of certain members of the
community, have proved advantageous; consequently the fertile males and
females have flourished, and transmitted to their fertile offspring a
tendency to produce sterile members with the same modifications. This
process must have been repeated many times, until that prodigious amount
of difference between the fertile and sterile females of the same species
has been produced which we see in many social insects.

But we have not as yet touched on the acme of the difficulty; namely, the
fact that the neuters of several ants differ, not only from the fertile
females and males, but from each other, sometimes to an almost incredible
degree, and are thus divided into two or even three castes. The castes,
moreover, do not generally graduate into each other, but are perfectly
well defined; being as distinct from each other as are any two species of
the same genus, or rather as any two genera of the same family. Thus, in
Eciton, there are working and soldier neuters, with jaws and instincts
extraordinarily different: in Cryptocerus, the workers of one caste alone
carry a wonderful sort of shield on their heads, the use of which is quite
unknown: in the Mexican Myrmecocystus, the workers of one caste never
leave the nest; they are fed by the workers of another caste, and they
have an enormously developed abdomen which secretes a sort of honey,
supplying the place of that excreted by the aphides, or the domestic
cattle as they may be called, which our European ants guard and imprison.

It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful
and well-established facts at once annihilate the theory. In the simpler
case of neuter insects all of one caste, which, as I believe, have been
rendered different from the fertile males and females through natural
selection, we may conclude from the analogy of ordinary variations, that
the successive, slight, profitable modifications did not first arise in
all the neuters in the same nest, but in some few alone; and that by the
survival of the communities with females which produced most neuters
having the advantageous modification, all the neuters ultimately came to
be thus characterized. According to this view we ought occasionally to
find in the same nest neuter-insects, presenting gradations of structure;
and this we do find, even not rarely, considering how few neuter-insects
out of Europe have been carefully examined. Mr. F. Smith has shown that
the neuters of several British ants differ surprisingly from each other in
size and sometimes in colour; and that the extreme forms can be linked
together by individuals taken out of the same nest: I have myself compared
perfect gradations of this kind. It sometimes happens that the larger or
the smaller sized workers are the most numerous; or that both large and
small are numerous, while those of an intermediate size are scanty in
numbers. Formica flava has larger and smaller workers, with some few of
intermediate size; and, in this species, as Mr. F. Smith has observed, the
larger workers have simple eyes (ocelli), which, though small, can be
plainly distinguished, whereas the smaller workers have their ocelli
rudimentary. Having carefully dissected several specimens of these
workers, I can affirm that the eyes are far more rudimentary in the
smaller workers than can be accounted for merely by their proportionately
lesser size; and I fully believe, though I dare not assert so positively,
that the workers of intermediate size have their ocelli in an exactly
intermediate condition. So that here we have two bodies of sterile workers
in the same nest, differing not only in size, but in their organs of
vision, yet connected by some few members in an intermediate condition. I
may digress by adding, that if the smaller workers had been the most
useful to the community, and those males and females had been continually
selected, which produced more and more of the smaller workers, until all
the workers were in this condition; we should then have had a species of
ant with neuters in nearly the same condition as those of Myrmica. For the
workers of Myrmica have not even rudiments of ocelli, though the male and
female ants of this genus have well-developed ocelli.

I may give one other case: so confidently did I expect occasionally to
find gradations of important structures between the different castes of
neuters in the same species, that I gladly availed myself of Mr. F.
Smith's offer of numerous specimens from the same nest of the driver ant
(Anomma) of West Africa. The reader will perhaps best appreciate the
amount of difference in these workers by my giving, not the actual
measurements, but a strictly accurate illustration: the difference was the
same as if we were to see a set of workmen building a house, of whom many
were five feet four inches high, and many sixteen feet high; but we must
in addition suppose that the larger workmen had heads four instead of
three times as big as those of the smaller men, and jaws nearly five times
as big. The jaws, moreover, of the working ants of the several sizes
differed wonderfully in shape, and in the form and number of the teeth.
But the important fact for us is that, though the workers can be grouped
into castes of different sizes, yet they graduate insensibly into each
other, as does the widely-different structure of their jaws. I speak
confidently on this latter point, as Sir J. Lubbock made drawings for me,
with the camera lucida, of the jaws which I dissected from the workers of
the several sizes. Mr. Bates, in his interesting "Naturalist on the
Amazons," has described analogous cases.

With these facts before me, I believe that natural selection, by acting on
the fertile ants or parents, could form a species which should regularly
produce neuters, all of large size with one form of jaw, or all of small
size with widely different jaws; or lastly, and this is the greatest
difficulty, one set of workers of one size and structure, and
simultaneously another set of workers of a different size and structure; a
graduated series having first been formed, as in the case of the driver
ant, and then the extreme forms having been produced in greater and
greater numbers, through the survival of the parents which generated them,
until none with an intermediate structure were produced.

An analogous explanation has been given by Mr. Wallace, of the equally
complex case, of certain Malayan butterflies regularly appearing under two
or even three distinct female forms; and by Fritz Muller, of certain
Brazilian crustaceans likewise appearing under two widely distinct male
forms. But this subject need not here be discussed.

I have now explained how, I believe, the wonderful fact of two distinctly
defined castes of sterile workers existing in the same nest, both widely
different from each other and from their parents, has originated. We can
see how useful their production may have been to a social community of
ants, on the same principle that the division of labour is useful to
civilised man. Ants, however, work by inherited instincts and by inherited
organs or tools, while man works by acquired knowledge and manufactured
instruments. But I must confess, that, with all my faith in natural
selection, I should never have anticipated that this principle could have
been efficient in so high a degree, had not the case of these neuter
insects led me to this conclusion. I have, therefore, discussed this case,
at some little but wholly insufficient length, in order to show the power
of natural selection, and likewise because this is by far the most serious
special difficulty which my theory has encountered. The case, also, is
very interesting, as it proves that with animals, as with plants, any
amount of modification may be effected by the accumulation of numerous,
slight, spontaneous variations, which are in any way profitable, without
exercise or habit having been brought into play. For peculiar habits,
confined to the workers of sterile females, however long they might be
followed, could not possibly affect the males and fertile females, which
alone leave descendants. I am surprised that no one has advanced this
demonstrative case of neuter insects, against the well-known doctrine of
inherited habit, as advanced by Lamarck.

SUMMARY.

I have endeavoured in this chapter briefly to show that the mental
qualities of our domestic animals vary, and that the variations are
inherited. Still more briefly I have attempted to show that instincts vary
slightly in a state of nature. No one will dispute that instincts are of
the highest importance to each animal. Therefore, there is no real
difficulty, under changing conditions of life, in natural selection
accumulating to any extent slight modifications of instinct which are in
any way useful. In many cases habit or use and disuse have probably come
into play. I do not pretend that the facts given in this chapter
strengthen in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it. On the other hand,
the fact that instincts are not always absolutely perfect and are liable
to mistakes; that no instinct can be shown to have been produced for the
good of other animals, though animals take advantage of the instincts of
others; that the canon in natural history, of "Natura non facit saltum,"
is applicable to instincts as well as to corporeal structure, and is
plainly explicable on the foregoing views, but is otherwise inexplicable—all
tend to corroborate the theory of natural selection.

This theory is also strengthened by some few other facts in regard to
instincts; as by that common case of closely allied, but distinct,
species, when inhabiting distant parts of the world and living under
considerably different conditions of life, yet often retaining nearly the
same instincts. For instance, we can understand, on the principle of
inheritance, how it is that the thrush of tropical South America lines its
nest with mud, in the same peculiar manner as does our British thrush; how
it is that the Hornbills of Africa and India have the same extraordinary
instinct of plastering up and imprisoning the females in a hole in a tree,
with only a small hole left in the plaster through which the males feed
them and their young when hatched; how it is that the male wrens
(Troglodytes) of North America, build "cock-nests," to roost in, like the
males of our Kitty-wrens,—a habit wholly unlike that of any other
known bird. Finally, it may not be a logical deduction, but to my
imagination it is far more satisfactory to look at such instincts as the
young cuckoo ejecting its foster-brothers, ants making slaves, the larvae
of ichneumonidae feeding within the live bodies of caterpillars, not as
specially endowed or created instincts, but as small consequences of one
general law leading to the advancement of all organic beings—namely,
multiply, vary, let the strongest live and the weakest die.



CHAPTER IX. HYBRIDISM.


 Distinction between the sterility of first crosses and of
 hybrids—Sterility various in degree, not universal, affected by close
 interbreeding, removed by domestication—Laws governing the sterility
 of hybrids—Sterility not a special endowment, but incidental on
 other differences, not accumulated by natural selection—Causes of
 the sterility of first crosses and of hybrids—Parallelism between the
 effects of changed conditions of life and of crossing—Dimorphism and
 trimorphism—Fertility of varieties when crossed and of their mongrel
 offspring not universal—Hybrids and mongrels compared independently of
 their fertility—Summary.

The view commonly entertained by naturalists is that species, when
intercrossed, have been specially endowed with sterility, in order to
prevent their confusion. This view certainly seems at first highly
probable, for species living together could hardly have been kept distinct
had they been capable of freely crossing. The subject is in many ways
important for us, more especially as the sterility of species when first
crossed, and that of their hybrid offspring, cannot have been acquired, as
I shall show, by the preservation of successive profitable degrees of
sterility. It is an incidental result of differences in the reproductive
systems of the parent-species.

In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded; namely, the
sterility of species when first crossed, and the sterility of the hybrids
produced from them.

Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no offspring.
Hybrids, on the other hand, have their reproductive organs functionally
impotent, as may be clearly seen in the state of the male element in both
plants and animals; though the formative organs themselves are perfect in
structure, as far as the microscope reveals. In the first case the two
sexual elements which go to form the embryo are perfect; in the second
case they are either not at all developed, or are imperfectly developed.
This distinction is important, when the cause of the sterility, which is
common to the two cases, has to be considered. The distinction probably
has been slurred over, owing to the sterility in both cases being looked
on as a special endowment, beyond the province of our reasoning powers.

The fertility of varieties, that is of the forms known or believed to be
descended from common parents, when crossed, and likewise the fertility of
their mongrel offspring, is, with reference to my theory, of equal
importance with the sterility of species; for it seems to make a broad and
clear distinction between varieties and species.

DEGREES OF STERILITY.

First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of
those two conscientious and admirable observers, Kolreuter and Gartner,
who almost devoted their lives to this subject, without being deeply
impressed with the high generality of some degree of sterility. Kolreuter
makes the rule universal; but then he cuts the knot, for in ten cases in
which he found two forms, considered by most authors as distinct species,
quite fertile together, he unhesitatingly ranks them as varieties.
Gartner, also, makes the rule equally universal; and he disputes the
entire fertility of Kolreuter's ten cases. But in these and in many other
cases, Gartner is obliged carefully to count the seeds, in order to show
that there is any degree of sterility. He always compares the maximum
number of seeds produced by two species when first crossed, and the
maximum produced by their hybrid offspring, with the average number
produced by both pure parent-species in a state of nature. But causes of
serious error here intervene: a plant, to be hybridised, must be
castrated, and, what is often more important, must be secluded in order to
prevent pollen being brought to it by insects from other plants. Nearly
all the plants experimented on by Gartner were potted, and were kept in a
chamber in his house. That these processes are often injurious to the
fertility of a plant cannot be doubted; for Gartner gives in his table
about a score of cases of plants which he castrated, and artificially
fertilised with their own pollen, and (excluding all cases such as the
Leguminosae, in which there is an acknowledged difficulty in the
manipulation) half of these twenty plants had their fertility in some
degree impaired. Moreover, as Gartner repeatedly crossed some forms, such
as the common red and blue pimpernels (Anagallis arvensis and coerulea),
which the best botanists rank as varieties, and found them absolutely
sterile, we may doubt whether many species are really so sterile, when
intercrossed, as he believed.

It is certain, on the one hand, that the sterility of various species when
crossed is so different in degree and graduates away so insensibly, and,
on the other hand, that the fertility of pure species is so easily
affected by various circumstances, that for all practical purposes it is
most difficult to say where perfect fertility ends and sterility begins. I
think no better evidence of this can be required than that the two most
experienced observers who have ever lived, namely Kolreuter and Gartner,
arrived at diametrically opposite conclusions in regard to some of the
very same forms. It is also most instructive to compare—but I have
not space here to enter on details—the evidence advanced by our best
botanists on the question whether certain doubtful forms should be ranked
as species or varieties, with the evidence from fertility adduced by
different hybridisers, or by the same observer from experiments made
during different years. It can thus be shown that neither sterility nor
fertility affords any certain distinction between species and varieties.
The evidence from this source graduates away, and is doubtful in the same
degree as is the evidence derived from other constitutional and structural
differences.

In regard to the sterility of hybrids in successive generations; though
Gartner was enabled to rear some hybrids, carefully guarding them from a
cross with either pure parent, for six or seven, and in one case for ten
generations, yet he asserts positively that their fertility never
increases, but generally decreases greatly and suddenly. With respect to
this decrease, it may first be noticed that when any deviation in
structure or constitution is common to both parents, this is often
transmitted in an augmented degree to the offspring; and both sexual
elements in hybrid plants are already affected in some degree. But I
believe that their fertility has been diminished in nearly all these cases
by an independent cause, namely, by too close interbreeding. I have made
so many experiments and collected so many facts, showing on the one hand
that an occasional cross with a distinct individual or variety increases
the vigour and fertility of the offspring, and on the other hand that very
close interbreeding lessens their vigour and fertility, that I cannot
doubt the correctness of this conclusion. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits of insects
must be carefully prevented during the flowering season: hence hybrids, if
left to themselves, will generally be fertilised during each generation by
pollen from the same flower; and this would probably be injurious to their
fertility, already lessened by their hybrid origin. I am strengthened in
this conviction by a remarkable statement repeatedly made by Gartner,
namely, that if even the less fertile hybrids be artificially fertilised
with hybrid pollen of the same kind, their fertility, notwithstanding the
frequent ill effects from manipulation, sometimes decidedly increases, and
goes on increasing. Now, in the process of artificial fertilisation,
pollen is as often taken by chance (as I know from my own experience) from
the anthers of another flower, as from the anthers of the flower itself
which is to be fertilised; so that a cross between two flowers, though
probably often on the same plant, would be thus effected. Moreover,
whenever complicated experiments are in progress, so careful an observer
as Gartner would have castrated his hybrids, and this would have insured
in each generation a cross with pollen from a distinct flower, either from
the same plant or from another plant of the same hybrid nature. And thus,
the strange fact of an increase of fertility in the successive generations
of ARTIFICIALLY FERTILISED hybrids, in contrast with those spontaneously
self-fertilised, may, as I believe, be accounted for by too close
interbreeding having been avoided.

Now let us turn to the results arrived at by a third most experienced
hybridiser, namely, the Hon. and Rev. W. Herbert. He is as emphatic in his
conclusion that some hybrids are perfectly fertile—as fertile as the
pure parent-species—as are Kolreuter and Gartner that some degree of
sterility between distinct species is a universal law of nature. He
experimented on some of the very same species as did Gartner. The
difference in their results may, I think, be in part accounted for by
Herbert's great horticultural skill, and by his having hot-houses at his
command. Of his many important statements I will here give only a single
one as an example, namely, that "every ovule in a pod of Crinum capense
fertilised by C. revolutum produced a plant, which I never saw to occur in
a case of its natural fecundation." So that here we have perfect, or even
more than commonly perfect fertility, in a first cross between two
distinct species.

This case of the Crinum leads me to refer to a singular fact, namely, that
individual plants of certain species of Lobelia, Verbascum and Passiflora,
can easily be fertilised by the pollen from a distinct species, but not by
pollen from the same plant, though this pollen can be proved to be
perfectly sound by fertilising other plants or species. In the genus
Hippeastrum, in Corydalis as shown by Professor Hildebrand, in various
orchids as shown by Mr. Scott and Fritz Muller, all the individuals are in
this peculiar condition. So that with some species, certain abnormal
individuals, and in other species all the individuals, can actually be
hybridised much more readily than they can be fertilised by pollen from
the same individual plant! To give one instance, a bulb of Hippeastrum
aulicum produced four flowers; three were fertilised by Herbert with their
own pollen, and the fourth was subsequently fertilised by the pollen of a
compound hybrid descended from three distinct species: the result was that
"the ovaries of the three first flowers soon ceased to grow, and after a
few days perished entirely, whereas the pod impregnated by the pollen of
the hybrid made vigorous growth and rapid progress to maturity, and bore
good seed, which vegetated freely." Mr. Herbert tried similar experiments
during many years, and always with the same result. These cases serve to
show on what slight and mysterious causes the lesser or greater fertility
of a species sometimes depends.

The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, etc., have been crossed, yet many of these hybrids
seed freely. For instance, Herbert asserts that a hybrid from Calceolaria
integrifolia and plantaginea, species most widely dissimilar in general
habit, "reproduces itself as perfectly as if it had been a natural species
from the mountains of Chile." I have taken some pains to ascertain the
degree of fertility of some of the complex crosses of Rhododendrons, and I
am assured that many of them are perfectly fertile. Mr. C. Noble, for
instance, informs me that he raises stocks for grafting from a hybrid
between Rhod. ponticum and catawbiense, and that this hybrid "seeds as
freely as it is possible to imagine." Had hybrids, when fairly treated,
always gone on decreasing in fertility in each successive generation, as
Gartner believed to be the case, the fact would have been notorious to
nurserymen. Horticulturists raise large beds of the same hybrid, and such
alone are fairly treated, for by insect agency the several individuals are
allowed to cross freely with each other, and the injurious influence of
close interbreeding is thus prevented. Any one may readily convince
himself of the efficiency of insect agency by examining the flowers of the
more sterile kinds of hybrid Rhododendrons, which produce no pollen, for
he will find on their stigmas plenty of pollen brought from other flowers.

In regard to animals, much fewer experiments have been carefully tried
than with plants. If our systematic arrangements can be trusted, that is,
if the genera of animals are as distinct from each other as are the genera
of plants, then we may infer that animals more widely distinct in the
scale of nature can be crossed more easily than in the case of plants; but
the hybrids themselves are, I think, more sterile. It should, however, be
borne in mind that, owing to few animals breeding freely under
confinement, few experiments have been fairly tried: for instance, the
canary-bird has been crossed with nine distinct species of finches, but,
as not one of these breeds freely in confinement, we have no right to
expect that the first crosses between them and the canary, or that their
hybrids, should be perfectly fertile. Again, with respect to the fertility
in successive generations of the more fertile hybrid animals, I hardly
know of an instance in which two families of the same hybrid have been
raised at the same time from different parents, so as to avoid the ill
effects of close interbreeding. On the contrary, brothers and sisters have
usually been crossed in each successive generation, in opposition to the
constantly repeated admonition of every breeder. And in this case, it is
not at all surprising that the inherent sterility in the hybrids should
have gone on increasing.

Although I know of hardly any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have reason to believe that the
hybrids from Cervulus vaginalis and Reevesii, and from Phasianus colchicus
with P. torquatus, are perfectly fertile. M. Quatrefages states that the
hybrids from two moths (Bombyx cynthia and arrindia) were proved in Paris
to be fertile inter se for eight generations. It has lately been asserted
that two such distinct species as the hare and rabbit, when they can be
got to breed together, produce offspring, which are highly fertile when
crossed with one of the parent-species. The hybrids from the common and
Chinese geese (A. cygnoides), species which are so different that they are
generally ranked in distinct genera, have often bred in this country with
either pure parent, and in one single instance they have bred inter se.
This was effected by Mr. Eyton, who raised two hybrids from the same
parents, but from different hatches; and from these two birds he raised no
less than eight hybrids (grandchildren of the pure geese) from one nest.
In India, however, these cross-bred geese must be far more fertile; for I
am assured by two eminently capable judges, namely Mr. Blyth and Captain
Hutton, that whole flocks of these crossed geese are kept in various parts
of the country; and as they are kept for profit, where neither pure
parent-species exists, they must certainly be highly or perfectly fertile.

With our domesticated animals, the various races when crossed together are
quite fertile; yet in many cases they are descended from two or more wild
species. From this fact we must conclude either that the aboriginal
parent-species at first produced perfectly fertile hybrids, or that the
hybrids subsequently reared under domestication became quite fertile. This
latter alternative, which was first propounded by Pallas, seems by far the
most probable, and can, indeed, hardly be doubted. It is, for instance,
almost certain that our dogs are descended from several wild stocks; yet,
with perhaps the exception of certain indigenous domestic dogs of South
America, all are quite fertile together; but analogy makes me greatly
doubt, whether the several aboriginal species would at first have freely
bred together and have produced quite fertile hybrids. So again I have
lately acquired decisive evidence that the crossed offspring from the
Indian humped and common cattle are inter se perfectly fertile; and from
the observations by Rutimeyer on their important osteological differences,
as well as from those by Mr. Blyth on their differences in habits, voice,
constitution, etc., these two forms must be regarded as good and distinct
species. The same remarks may be extended to the two chief races of the
pig. We must, therefore, either give up the belief of the universal
sterility of species when crossed; or we must look at this sterility in
animals, not as an indelible characteristic, but as one capable of being
removed by domestication.

Finally, considering all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility,
both in first crosses and in hybrids, is an extremely general result; but
that it cannot, under our present state of knowledge, be considered as
absolutely universal.

LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

We will now consider a little more in detail the laws governing the
sterility of first crosses and of hybrids. Our chief object will be to see
whether or not these laws indicate that species have been specially
endowed with this quality, in order to prevent their crossing and blending
together in utter confusion. The following conclusions are drawn up
chiefly from Gartner's admirable work on the hybridisation of plants. I
have taken much pains to ascertain how far they apply to animals, and,
considering how scanty our knowledge is in regard to hybrid animals, I
have been surprised to find how generally the same rules apply to both
kingdoms.

It has been already remarked, that the degree of fertility, both of first
crosses and of hybrids, graduates from zero to perfect fertility. It is
surprising in how many curious ways this gradation can be shown; but only
the barest outline of the facts can here be given. When pollen from a
plant of one family is placed on the stigma of a plant of a distinct
family, it exerts no more influence than so much inorganic dust. From this
absolute zero of fertility, the pollen of different species applied to the
stigma of some one species of the same genus, yields a perfect gradation
in the number of seeds produced, up to nearly complete or even quite
complete fertility; and, as we have seen, in certain abnormal cases, even
to an excess of fertility, beyond that which the plant's own pollen
produces. So in hybrids themselves, there are some which never have
produced, and probably never would produce, even with the pollen of the
pure parents, a single fertile seed: but in some of these cases a first
trace of fertility may be detected, by the pollen of one of the pure
parent-species causing the flower of the hybrid to wither earlier than it
otherwise would have done; and the early withering of the flower is well
known to be a sign of incipient fertilisation. From this extreme degree of
sterility we have self-fertilised hybrids producing a greater and greater
number of seeds up to perfect fertility.

The hybrids raised from two species which are very difficult to cross, and
which rarely produce any offspring, are generally very sterile; but the
parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced—two classes of facts which
are generally confounded together—is by no means strict. There are
many cases, in which two pure species, as in the genus Verbascum, can be
united with unusual facility, and produce numerous hybrid offspring, yet
these hybrids are remarkably sterile. On the other hand, there are species
which can be crossed very rarely, or with extreme difficulty, but the
hybrids, when at last produced, are very fertile. Even within the limits
of the same genus, for instance in Dianthus, these two opposite cases
occur.

The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is that of pure species. But the
fertility of first crosses is likewise innately variable; for it is not
always the same in degree when the same two species are crossed under the
same circumstances; it depends in part upon the constitution of the
individuals which happen to have been chosen for the experiment. So it is
with hybrids, for their degree of fertility is often found to differ
greatly in the several individuals raised from seed out of the same
capsule and exposed to the same conditions.

By the term systematic affinity is meant, the general resemblance between
species in structure and constitution. Now the fertility of first crosses,
and of the hybrids produced from them, is largely governed by their
systematic affinity. This is clearly shown by hybrids never having been
raised between species ranked by systematists in distinct families; and on
the other hand, by very closely allied species generally uniting with
facility. But the correspondence between systematic affinity and the
facility of crossing is by no means strict. A multitude of cases could be
given of very closely allied species which will not unite, or only with
extreme difficulty; and on the other hand of very distinct species which
unite with the utmost facility. In the same family there may be a genus,
as Dianthus, in which very many species can most readily be crossed; and
another genus, as Silene, in which the most persevering efforts have
failed to produce between extremely close species a single hybrid. Even
within the limits of the same genus, we meet with this same difference;
for instance, the many species of Nicotiana have been more largely crossed
than the species of almost any other genus; but Gartner found that N.
acuminata, which is not a particularly distinct species, obstinately
failed to fertilise, or to be fertilised, by no less than eight other
species of Nicotiana. Many analogous facts could be given.

No one has been able to point out what kind or what amount of difference,
in any recognisable character, is sufficient to prevent two species
crossing. It can be shown that plants most widely different in habit and
general appearance, and having strongly marked differences in every part
of the flower, even in the pollen, in the fruit, and in the cotyledons,
can be crossed. Annual and perennial plants, deciduous and evergreen
trees, plants inhabiting different stations and fitted for extremely
different climates, can often be crossed with ease.

By a reciprocal cross between two species, I mean the case, for instance,
of a female-ass being first crossed by a stallion, and then a mare by a
male-ass: these two species may then be said to have been reciprocally
crossed. There is often the widest possible difference in the facility of
making reciprocal crosses. Such cases are highly important, for they prove
that the capacity in any two species to cross is often completely
independent of their systematic affinity, that is of any difference in
their structure or constitution, excepting in their reproductive systems.
The diversity of the result in reciprocal crosses between the same two
species was long ago observed by Kolreuter. To give an instance: Mirabilis
jalapa can easily be fertilised by the pollen of M. longiflora, and the
hybrids thus produced are sufficiently fertile; but Kolreuter tried more
than two hundred times, during eight following years, to fertilise
reciprocally M. longiflora with the pollen of M. jalapa, and utterly
failed. Several other equally striking cases could be given. Thuret has
observed the same fact with certain sea-weeds or Fuci. Gartner, moreover,
found that this difference of facility in making reciprocal crosses is
extremely common in a lesser degree. He has observed it even between
closely related forms (as Matthiola annua and glabra) which many botanists
rank only as varieties. It is also a remarkable fact that hybrids raised
from reciprocal crosses, though of course compounded of the very same two
species, the one species having first been used as the father and then as
the mother, though they rarely differ in external characters, yet
generally differ in fertility in a small, and occasionally in a high
degree.

Several other singular rules could be given from Gartner: for instance,
some species have a remarkable power of crossing with other species; other
species of the same genus have a remarkable power of impressing their
likeness on their hybrid offspring; but these two powers do not at all
necessarily go together. There are certain hybrids which, instead of
having, as is usual, an intermediate character between their two parents,
always closely resemble one of them; and such hybrids, though externally
so like one of their pure parent-species, are with rare exceptions
extremely sterile. So again among hybrids which are usually intermediate
in structure between their parents, exceptional and abnormal individuals
sometimes are born, which closely resemble one of their pure parents; and
these hybrids are almost always utterly sterile, even when the other
hybrids raised from seed from the same capsule have a considerable degree
of fertility. These facts show how completely the fertility of a hybrid
may be independent of its external resemblance to either pure parent.

Considering the several rules now given, which govern the fertility of
first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess; that their fertility, besides being
eminently susceptible to favourable and unfavourable conditions, is
innately variable; that it is by no means always the same in degree in the
first cross and in the hybrids produced from this cross; that the
fertility of hybrids is not related to the degree in which they resemble
in external appearance either parent; and lastly, that the facility of
making a first cross between any two species is not always governed by
their systematic affinity or degree of resemblance to each other. This
latter statement is clearly proved by the difference in the result of
reciprocal crosses between the same two species, for, according as the one
species or the other is used as the father or the mother, there is
generally some difference, and occasionally the widest possible
difference, in the facility of effecting an union. The hybrids, moreover,
produced from reciprocal crosses often differ in fertility.

Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded in
nature? I think not. For why should the sterility be so extremely
different in degree, when various species are crossed, all of which we
must suppose it would be equally important to keep from blending together?
Why should the degree of sterility be innately variable in the individuals
of the same species? Why should some species cross with facility and yet
produce very sterile hybrids; and other species cross with extreme
difficulty, and yet produce fairly fertile hybrids? Why should there often
be so great a difference in the result of a reciprocal cross between the
same two species? Why, it may even be asked, has the production of hybrids
been permitted? To grant to species the special power of producing
hybrids, and then to stop their further propagation by different degrees
of sterility, not strictly related to the facility of the first union
between their parents, seems a strange arrangement.

The foregoing rules and facts, on the other hand, appear to me clearly to
indicate that the sterility, both of first crosses and of hybrids, is
simply incidental or dependent on unknown differences in their
reproductive systems; the differences being of so peculiar and limited a
nature, that, in reciprocal crosses between the same two species, the male
sexual element of the one will often freely act on the female sexual
element of the other, but not in a reversed direction. It will be
advisable to explain a little more fully, by an example, what I mean by
sterility being incidental on other differences, and not a specially
endowed quality. As the capacity of one plant to be grafted or budded on
another is unimportant for their welfare in a state of nature, I presume
that no one will suppose that this capacity is a SPECIALLY endowed
quality, but will admit that it is incidental on differences in the laws
of growth of the two plants. We can sometimes see the reason why one tree
will not take on another from differences in their rate of growth, in the
hardness of their wood, in the period of the flow or nature of their sap,
etc.; but in a multitude of cases we can assign no reason whatever. Great
diversity in the size of two plants, one being woody and the other
herbaceous, one being evergreen and the other deciduous, and adaptation to
widely different climates, does not always prevent the two grafting
together. As in hybridisation, so with grafting, the capacity is limited
by systematic affinity, for no one has been able to graft together trees
belonging to quite distinct families; and, on the other hand, closely
allied species and varieties of the same species, can usually, but not
invariably, be grafted with ease. But this capacity, as in hybridisation,
is by no means absolutely governed by systematic affinity. Although many
distinct genera within the same family have been grafted together, in
other cases species of the same genus will not take on each other. The
pear can be grafted far more readily on the quince, which is ranked as a
distinct genus, than on the apple, which is a member of the same genus.
Even different varieties of the pear take with different degrees of
facility on the quince; so do different varieties of the apricot and peach
on certain varieties of the plum.

As Gartner found that there was sometimes an innate difference in
different INDIVIDUALS of the same two species in crossing; so Sagaret
believes this to be the case with different individuals of the same two
species in being grafted together. As in reciprocal crosses, the facility
of effecting an union is often very far from equal, so it sometimes is in
grafting. The common gooseberry, for instance, cannot be grafted on the
currant, whereas the currant will take, though with difficulty, on the
gooseberry.

We have seen that the sterility of hybrids which have their reproductive
organs in an imperfect condition, is a different case from the difficulty
of uniting two pure species, which have their reproductive organs perfect;
yet these two distinct classes of cases run to a large extent parallel.
Something analogous occurs in grafting; for Thouin found that three
species of Robinia, which seeded freely on their own roots, and which
could be grafted with no great difficulty on a fourth species, when thus
grafted were rendered barren. On the other hand, certain species of
Sorbus, when grafted on other species, yielded twice as much fruit as when
on their own roots. We are reminded by this latter fact of the
extraordinary cases of Hippeastrum, Passiflora, etc., which seed much more
freely when fertilised with the pollen of a distinct species than when
fertilised with pollen from the same plant.

We thus see that, although there is a clear and great difference between
the mere adhesion of grafted stocks and the union of the male and female
elements in the act of reproduction, yet that there is a rude degree of
parallelism in the results of grafting and of crossing distinct species.
And as we must look at the curious and complex laws governing the facility
with which trees can be grafted on each other as incidental on unknown
differences in their vegetative systems, so I believe that the still more
complex laws governing the facility of first crosses are incidental on
unknown differences in their reproductive systems. These differences in
both cases follow, to a certain extent, as might have been expected,
systematic affinity, by which term every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The
facts by no means seem to indicate that the greater or lesser difficulty
of either grafting or crossing various species has been a special
endowment; although in the case of crossing, the difficulty is as
important for the endurance and stability of specific forms as in the case
of grafting it is unimportant for their welfare.

ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

At one time it appeared to me probable, as it has to others, that the
sterility of first crosses and of hybrids might have been slowly acquired
through the natural selection of slightly lessened degrees of fertility,
which, like any other variation, spontaneously appeared in certain
individuals of one variety when crossed with those of another variety. For
it would clearly be advantageous to two varieties or incipient species if
they could be kept from blending, on the same principle that, when man is
selecting at the same time two varieties, it is necessary that he should
keep them separate. In the first place, it may be remarked that species
inhabiting distinct regions are often sterile when crossed; now it could
clearly have been of no advantage to such separated species to have been
rendered mutually sterile, and consequently this could not have been
effected through natural selection; but it may perhaps be argued, that, if
a species was rendered sterile with some one compatriot, sterility with
other species would follow as a necessary contingency. In the second
place, it is almost as much opposed to the theory of natural selection as
to that of special creation, that in reciprocal crosses the male element
of one form should have been rendered utterly impotent on a second form,
while at the same time the male element of this second form is enabled
freely to fertilise the first form; for this peculiar state of the
reproductive system could hardly have been advantageous to either species.

In considering the probability of natural selection having come into
action, in rendering species mutually sterile, the greatest difficulty
will be found to lie in the existence of many graduated steps, from
slightly lessened fertility to absolute sterility. It may be admitted that
it would profit an incipient species, if it were rendered in some slight
degree sterile when crossed with its parent form or with some other
variety; for thus fewer bastardised and deteriorated offspring would be
produced to commingle their blood with the new species in process of
formation. But he who will take the trouble to reflect on the steps by
which this first degree of sterility could be increased through natural
selection to that high degree which is common with so many species, and
which is universal with species which have been differentiated to a
generic or family rank, will find the subject extraordinarily complex.
After mature reflection, it seems to me that this could not have been
effected through natural selection. Take the case of any two species
which, when crossed, produced few and sterile offspring; now, what is
there which could favour the survival of those individuals which happened
to be endowed in a slightly higher degree with mutual infertility, and
which thus approached by one small step towards absolute sterility? Yet an
advance of this kind, if the theory of natural selection be brought to
bear, must have incessantly occurred with many species, for a multitude
are mutually quite barren. With sterile neuter insects we have reason to
believe that modifications in their structure and fertility have been
slowly accumulated by natural selection, from an advantage having been
thus indirectly given to the community to which they belonged over other
communities of the same species; but an individual animal not belonging to
a social community, if rendered slightly sterile when crossed with some
other variety, would not thus itself gain any advantage or indirectly give
any advantage to the other individuals of the same variety, thus leading
to their preservation.

But it would be superfluous to discuss this question in detail: for with
plants we have conclusive evidence that the sterility of crossed species
must be due to some principle, quite independent of natural selection.
Both Gartner and Kolreuter have proved that in genera including numerous
species, a series can be formed from species which when crossed yield
fewer and fewer seeds, to species which never produce a single seed, but
yet are affected by the pollen of certain other species, for the germen
swells. It is here manifestly impossible to select the more sterile
individuals, which have already ceased to yield seeds; so that this acme
of sterility, when the germen alone is effected, cannot have been gained
through selection; and from the laws governing the various grades of
sterility being so uniform throughout the animal and vegetable kingdoms,
we may infer that the cause, whatever it may be, is the same or nearly the
same in all cases.

We will now look a little closer at the probable nature of the differences
between species which induce sterility in first crosses and in hybrids. In
the case of first crosses, the greater or less difficulty in effecting a
union and in obtaining offspring apparently depends on several distinct
causes. There must sometimes be a physical impossibility in the male
element reaching the ovule, as would be the case with a plant having a
pistil too long for the pollen-tubes to reach the ovarium. It has also
been observed that when the pollen of one species is placed on the stigma
of a distantly allied species, though the pollen-tubes protrude, they do
not penetrate the stigmatic surface. Again, the male element may reach the
female element, but be incapable of causing an embryo to be developed, as
seems to have been the case with some of Thuret's experiments on Fuci. No
explanation can be given of these facts, any more than why certain trees
cannot be grafted on others. Lastly, an embryo may be developed, and then
perish at an early period. This latter alternative has not been
sufficiently attended to; but I believe, from observations communicated to
me by Mr. Hewitt, who has had great experience in hybridising pheasants
and fowls, that the early death of the embryo is a very frequent cause of
sterility in first crosses. Mr. Salter has recently given the results of
an examination of about 500 eggs produced from various crosses between
three species of Gallus and their hybrids; the majority of these eggs had
been fertilised; and in the majority of the fertilised eggs, the embryos
had either been partially developed and had then perished, or had become
nearly mature, but the young chickens had been unable to break through the
shell. Of the chickens which were born, more than four-fifths died within
the first few days, or at latest weeks, "without any obvious cause,
apparently from mere inability to live;" so that from the 500 eggs only
twelve chickens were reared. With plants, hybridized embryos probably
often perish in a like manner; at least it is known that hybrids raised
from very distinct species are sometimes weak and dwarfed, and perish at
an early age; of which fact Max Wichura has recently given some striking
cases with hybrid willows. It may be here worth noticing that in some
cases of parthenogenesis, the embryos within the eggs of silk moths which
had not been fertilised, pass through their early stages of development
and then perish like the embryos produced by a cross between distinct
species. Until becoming acquainted with these facts, I was unwilling to
believe in the frequent early death of hybrid embryos; for hybrids, when
once born, are generally healthy and long-lived, as we see in the case of
the common mule. Hybrids, however, are differently circumstanced before
and after birth: when born and living in a country where their two parents
live, they are generally placed under suitable conditions of life. But a
hybrid partakes of only half of the nature and constitution of its mother;
it may therefore, before birth, as long as it is nourished within its
mother's womb, or within the egg or seed produced by the mother, be
exposed to conditions in some degree unsuitable, and consequently be
liable to perish at an early period; more especially as all very young
beings are eminently sensitive to injurious or unnatural conditions of
life. But after all, the cause more probably lies in some imperfection in
the original act of impregnation, causing the embryo to be imperfectly
developed, rather than in the conditions to which it is subsequently
exposed.

In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is somewhat different. I have more than
once alluded to a large body of facts showing that, when animals and
plants are removed from their natural conditions, they are extremely
liable to have their reproductive systems seriously affected. This, in
fact, is the great bar to the domestication of animals. Between the
sterility thus superinduced and that of hybrids, there are many points of
similarity. In both cases the sterility is independent of general health,
and is often accompanied by excess of size or great luxuriance. In both
cases the sterility occurs in various degrees; in both, the male element
is the most liable to be affected; but sometimes the female more than the
male. In both, the tendency goes to a certain extent with systematic
affinity, for whole groups of animals and plants are rendered impotent by
the same unnatural conditions; and whole groups of species tend to produce
sterile hybrids. On the other hand, one species in a group will sometimes
resist great changes of conditions with unimpaired fertility; and certain
species in a group will produce unusually fertile hybrids. No one can tell
till he tries, whether any particular animal will breed under confinement,
or any exotic plant seed freely under culture; nor can he tell till he
tries, whether any two species of a genus will produce more or less
sterile hybrids. Lastly, when organic beings are placed during several
generations under conditions not natural to them, they are extremely
liable to vary, which seems to be partly due to their reproductive systems
having been specially affected, though in a lesser degree than when
sterility ensues. So it is with hybrids, for their offspring in successive
generations are eminently liable to vary, as every experimentalist has
observed.

Thus we see that when organic beings are placed under new and unnatural
conditions, and when hybrids are produced by the unnatural crossing of two
species, the reproductive system, independently of the general state of
health, is affected in a very similar manner. In the one case, the
conditions of life have been disturbed, though often in so slight a degree
as to be inappreciable by us; in the other case, or that of hybrids, the
external conditions have remained the same, but the organisation has been
disturbed by two distinct structures and constitutions, including of
course the reproductive systems, having been blended into one. For it is
scarcely possible that two organisations should be compounded into one,
without some disturbance occurring in the development, or periodical
action, or mutual relations of the different parts and organs one to
another or to the conditions of life. When hybrids are able to breed inter
se, they transmit to their offspring from generation to generation the
same compounded organisation, and hence we need not be surprised that
their sterility, though in some degree variable, does not diminish; it is
even apt to increase, this being generally the result, as before
explained, of too close interbreeding. The above view of the sterility of
hybrids being caused by two constitutions being compounded into one has
been strongly maintained by Max Wichura.

It must, however, be owned that we cannot understand, on the above or any
other view, several facts with respect to the sterility of hybrids; for
instance, the unequal fertility of hybrids produced from reciprocal
crosses; or the increased sterility in those hybrids which occasionally
and exceptionally resemble closely either pure parent. Nor do I pretend
that the foregoing remarks go to the root of the matter: no explanation is
offered why an organism, when placed under unnatural conditions, is
rendered sterile. All that I have attempted to show is, that in two cases,
in some respects allied, sterility is the common result—in the one
case from the conditions of life having been disturbed, in the other case
from the organisation having been disturbed by two organisations being
compounded into one.

A similar parallelism holds good with an allied yet very different class
of facts. It is an old and almost universal belief, founded on a
considerable body of evidence, which I have elsewhere given, that slight
changes in the conditions of life are beneficial to all living things. We
see this acted on by farmers and gardeners in their frequent exchanges of
seed, tubers, etc., from one soil or climate to another, and back again.
During the convalescence of animals, great benefit is derived from almost
any change in their habits of life. Again, both with plants and animals,
there is the clearest evidence that a cross between individuals of the
same species, which differ to a certain extent, gives vigour and fertility
to the offspring; and that close interbreeding continued during several
generations between the nearest relations, if these be kept under the same
conditions of life, almost always leads to decreased size, weakness, or
sterility.

Hence it seems that, on the one hand, slight changes in the conditions of
life benefit all organic beings, and on the other hand, that slight
crosses, that is, crosses between the males and females of the same
species, which have been subjected to slightly different conditions, or
which have slightly varied, give vigour and fertility to the offspring.
But, as we have seen, organic beings long habituated to certain uniform
conditions under a state of nature, when subjected, as under confinement,
to a considerable change in their conditions, very frequently are rendered
more or less sterile; and we know that a cross between two forms that have
become widely or specifically different, produce hybrids which are almost
always in some degree sterile. I am fully persuaded that this double
parallelism is by no means an accident or an illusion. He who is able to
explain why the elephant, and a multitude of other animals, are incapable
of breeding when kept under only partial confinement in their native
country, will be able to explain the primary cause of hybrids being so
generally sterile. He will at the same time be able to explain how it is
that the races of some of our domesticated animals, which have often been
subjected to new and not uniform conditions, are quite fertile together,
although they are descended from distinct species, which would probably
have been sterile if aboriginally crossed. The above two parallel series
of facts seem to be connected together by some common but unknown bond,
which is essentially related to the principle of life; this principle,
according to Mr. Herbert Spencer, being that life depends on, or consists
in, the incessant action and reaction of various forces, which, as
throughout nature, are always tending towards an equilibrium; and when
this tendency is slightly disturbed by any change, the vital forces gain
in power.

RECIPROCAL DIMORPHISM AND TRIMORPHISM.

This subject may be here briefly discussed, and will be found to throw
some light on hybridism. Several plants belonging to distinct orders
present two forms, which exist in about equal numbers and which differ in
no respect except in their reproductive organs; one form having a long
pistil with short stamens, the other a short pistil with long stamens; the
two having differently sized pollen-grains. With trimorphic plants there
are three forms likewise differing in the lengths of their pistils and
stamens, in the size and colour of the pollen-grains, and in some other
respects; and as in each of the three forms there are two sets of stamens,
the three forms possess altogether six sets of stamens and three kinds of
pistils. These organs are so proportioned in length to each other that
half the stamens in two of the forms stand on a level with the stigma of
the third form. Now I have shown, and the result has been confirmed by
other observers, that in order to obtain full fertility with these plants,
it is necessary that the stigma of the one form should be fertilised by
pollen taken from the stamens of corresponding height in another form. So
that with dimorphic species two unions, which may be called legitimate,
are fully fertile; and two, which may be called illegitimate, are more or
less infertile. With trimorphic species six unions are legitimate, or
fully fertile, and twelve are illegitimate, or more or less infertile.

The infertility which may be observed in various dimorphic and trimorphic
plants, when they are illegitimately fertilised, that is by pollen taken
from stamens not corresponding in height with the pistil, differs much in
degree, up to absolute and utter sterility; just in the same manner as
occurs in crossing distinct species. As the degree of sterility in the
latter case depends in an eminent degree on the conditions of life being
more or less favourable, so I have found it with illegitimate unions. It
is well known that if pollen of a distinct species be placed on the stigma
of a flower, and its own pollen be afterwards, even after a considerable
interval of time, placed on the same stigma, its action is so strongly
prepotent that it generally annihilates the effect of the foreign pollen;
so it is with the pollen of the several forms of the same species, for
legitimate pollen is strongly prepotent over illegitimate pollen, when
both are placed on the same stigma. I ascertained this by fertilising
several flowers, first illegitimately, and twenty-four hours afterwards
legitimately, with pollen taken from a peculiarly coloured variety, and
all the seedlings were similarly coloured; this shows that the legitimate
pollen, though applied twenty-four hours subsequently, had wholly
destroyed or prevented the action of the previously applied illegitimate
pollen. Again, as in making reciprocal crosses between the same two
species, there is occasionally a great difference in the result, so the
same thing occurs with trimorphic plants; for instance, the mid-styled
form of Lythrum salicaria was illegitimately fertilised with the greatest
ease by pollen from the longer stamens of the short-styled form, and
yielded many seeds; but the latter form did not yield a single seed when
fertilised by the longer stamens of the mid-styled form.

In all these respects, and in others which might be added, the forms of
the same undoubted species, when illegitimately united, behave in exactly
the same manner as do two distinct species when crossed. This led me
carefully to observe during four years many seedlings, raised from several
illegitimate unions. The chief result is that these illegitimate plants,
as they may be called, are not fully fertile. It is possible to raise from
dimorphic species, both long-styled and short-styled illegitimate plants,
and from trimorphic plants all three illegitimate forms. These can then be
properly united in a legitimate manner. When this is done, there is no
apparent reason why they should not yield as many seeds as did their
parents when legitimately fertilised. But such is not the case. They are
all infertile, in various degrees; some being so utterly and incurably
sterile that they did not yield during four seasons a single seed or even
seed-capsule. The sterility of these illegitimate plants, when united with
each other in a legitimate manner, may be strictly compared with that of
hybrids when crossed inter se. If, on the other hand, a hybrid is crossed
with either pure parent-species, the sterility is usually much lessened:
and so it is when an illegitimate plant is fertilised by a legitimate
plant. In the same manner as the sterility of hybrids does not always run
parallel with the difficulty of making the first cross between the two
parent-species, so that sterility of certain illegitimate plants was
unusually great, while the sterility of the union from which they were
derived was by no means great. With hybrids raised from the same
seed-capsule the degree of sterility is innately variable, so it is in a
marked manner with illegitimate plants. Lastly, many hybrids are profuse
and persistent flowerers, while other and more sterile hybrids produce few
flowers, and are weak, miserable dwarfs; exactly similar cases occur with
the illegitimate offspring of various dimorphic and trimorphic plants.

Altogether there is the closest identity in character and behaviour
between illegitimate plants and hybrids. It is hardly an exaggeration to
maintain that illegitimate plants are hybrids, produced within the limits
of the same species by the improper union of certain forms, while ordinary
hybrids are produced from an improper union between so-called distinct
species. We have also already seen that there is the closest similarity in
all respects between first illegitimate unions and first crosses between
distinct species. This will perhaps be made more fully apparent by an
illustration; we may suppose that a botanist found two well-marked
varieties (and such occur) of the long-styled form of the trimorphic
Lythrum salicaria, and that he determined to try by crossing whether they
were specifically distinct. He would find that they yielded only about
one-fifth of the proper number of seed, and that they behaved in all the
other above specified respects as if they had been two distinct species.
But to make the case sure, he would raise plants from his supposed
hybridised seed, and he would find that the seedlings were miserably
dwarfed and utterly sterile, and that they behaved in all other respects
like ordinary hybrids. He might then maintain that he had actually proved,
in accordance with the common view, that his two varieties were as good
and as distinct species as any in the world; but he would be completely
mistaken.

The facts now given on dimorphic and trimorphic plants are important,
because they show us, first, that the physiological test of lessened
fertility, both in first crosses and in hybrids, is no safe criterion of
specific distinction; secondly, because we may conclude that there is some
unknown bond which connects the infertility of illegitimate unions with
that of their illegitimate offspring, and we are led to extend the same
view to first crosses and hybrids; thirdly, because we find, and this
seems to me of especial importance, that two or three forms of the same
species may exist and may differ in no respect whatever, either in
structure or in constitution, relatively to external conditions, and yet
be sterile when united in certain ways. For we must remember that it is
the union of the sexual elements of individuals of the same form, for
instance, of two long-styled forms, which results in sterility; while it
is the union of the sexual elements proper to two distinct forms which is
fertile. Hence the case appears at first sight exactly the reverse of what
occurs, in the ordinary unions of the individuals of the same species and
with crosses between distinct species. It is, however, doubtful whether
this is really so; but I will not enlarge on this obscure subject.

We may, however, infer as probable from the consideration of dimorphic and
trimorphic plants, that the sterility of distinct species when crossed and
of their hybrid progeny, depends exclusively on the nature of their sexual
elements, and not on any difference in their structure or general
constitution. We are also led to this same conclusion by considering
reciprocal crosses, in which the male of one species cannot be united, or
can be united with great difficulty, with the female of a second species,
while the converse cross can be effected with perfect facility. That
excellent observer, Gartner, likewise concluded that species when crossed
are sterile owing to differences confined to their reproductive systems.

FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING, NOT
UNIVERSAL.

It may be urged as an overwhelming argument that there must be some
essential distinction between species and varieties inasmuch as the
latter, however much they may differ from each other in external
appearance, cross with perfect facility, and yield perfectly fertile
offspring. With some exceptions, presently to be given, I fully admit that
this is the rule. But the subject is surrounded by difficulties, for,
looking to varieties produced under nature, if two forms hitherto reputed
to be varieties be found in any degree sterile together, they are at once
ranked by most naturalists as species. For instance, the blue and red
pimpernel, which are considered by most botanists as varieties, are said
by Gartner to be quite sterile when crossed, and he consequently ranks
them as undoubted species. If we thus argue in a circle, the fertility of
all varieties produced under nature will assuredly have to be granted.

If we turn to varieties, produced, or supposed to have been produced,
under domestication, we are still involved in some doubt. For when it is
stated, for instance, that certain South American indigenous domestic dogs
do not readily unite with European dogs, the explanation which will occur
to everyone, and probably the true one, is that they are descended from
aboriginally distinct species. Nevertheless the perfect fertility of so
many domestic races, differing widely from each other in appearance, for
instance, those of the pigeon, or of the cabbage, is a remarkable fact;
more especially when we reflect how many species there are, which, though
resembling each other most closely, are utterly sterile when intercrossed.
Several considerations, however, render the fertility of domestic
varieties less remarkable. In the first place, it may be observed that the
amount of external difference between two species is no sure guide to
their degree of mutual sterility, so that similar differences in the case
of varieties would be no sure guide. It is certain that with species the
cause lies exclusively in differences in their sexual constitution. Now
the varying conditions to which domesticated animals and cultivated plants
have been subjected, have had so little tendency towards modifying the
reproductive system in a manner leading to mutual sterility, that we have
good grounds for admitting the directly opposite doctrine of Pallas,
namely, that such conditions generally eliminate this tendency; so that
the domesticated descendants of species, which in their natural state
probably would have been in some degree sterile when crossed, become
perfectly fertile together. With plants, so far is cultivation from giving
a tendency towards sterility between distinct species, that in several
well-authenticated cases already alluded to, certain plants have been
affected in an opposite manner, for they have become self-impotent, while
still retaining the capacity of fertilising, and being fertilised by,
other species. If the Pallasian doctrine of the elimination of sterility
through long-continued domestication be admitted, and it can hardly be
rejected, it becomes in the highest degree improbable that similar
conditions long-continued should likewise induce this tendency; though in
certain cases, with species having a peculiar constitution, sterility
might occasionally be thus caused. Thus, as I believe, we can understand
why, with domesticated animals, varieties have not been produced which are
mutually sterile; and why with plants only a few such cases, immediately
to be given, have been observed.

The real difficulty in our present subject is not, as it appears to me,
why domestic varieties have not become mutually infertile when crossed,
but why this has so generally occurred with natural varieties, as soon as
they have been permanently modified in a sufficient degree to take rank as
species. We are far from precisely knowing the cause; nor is this
surprising, seeing how profoundly ignorant we are in regard to the normal
and abnormal action of the reproductive system. But we can see that
species, owing to their struggle for existence with numerous competitors,
will have been exposed during long periods of time to more uniform
conditions, than have domestic varieties; and this may well make a wide
difference in the result. For we know how commonly wild animals and
plants, when taken from their natural conditions and subjected to
captivity, are rendered sterile; and the reproductive functions of organic
beings which have always lived under natural conditions would probably in
like manner be eminently sensitive to the influence of an unnatural cross.
Domesticated productions, on the other hand, which, as shown by the mere
fact of their domestication, were not originally highly sensitive to
changes in their conditions of life, and which can now generally resist
with undiminished fertility repeated changes of conditions, might be
expected to produce varieties, which would be little liable to have their
reproductive powers injuriously affected by the act of crossing with other
varieties which had originated in a like manner.

I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it is impossible to resist the
evidence of the existence of a certain amount of sterility in the few
following cases, which I will briefly abstract. The evidence is at least
as good as that from which we believe in the sterility of a multitude of
species. The evidence is also derived from hostile witnesses, who in all
other cases consider fertility and sterility as safe criterions of
specific distinction. Gartner kept, during several years, a dwarf kind of
maize with yellow seeds, and a tall variety with red seeds growing near
each other in his garden; and although these plants have separated sexes,
they never naturally crossed. He then fertilised thirteen flowers of the
one kind with pollen of the other; but only a single head produced any
seed, and this one head produced only five grains. Manipulation in this
case could not have been injurious, as the plants have separated sexes. No
one, I believe, has suspected that these varieties of maize are distinct
species; and it is important to notice that the hybrid plants thus raised
were themselves PERFECTLY fertile; so that even Gartner did not venture to
consider the two varieties as specifically distinct.

Girou de Buzareingues crossed three varieties of gourd, which like the
maize has separated sexes, and he asserts that their mutual fertilisation
is by so much the less easy as their differences are greater. How far
these experiments may be trusted, I know not; but the forms experimented
on are ranked by Sagaret, who mainly founds his classification by the test
of infertility, as varieties, and Naudin has come to the same conclusion.

The following case is far more remarkable, and seems at first incredible;
but it is the result of an astonishing number of experiments made during
many years on nine species of Verbascum, by so good an observer and so
hostile a witness as Gartner: namely, that the yellow and white varieties
when crossed produce less seed than the similarly coloured varieties of
the same species. Moreover, he asserts that, when yellow and white
varieties of one species are crossed with yellow and white varieties of a
DISTINCT species, more seed is produced by the crosses between the
similarly coloured flowers, than between those which are differently
coloured. Mr. Scott also has experimented on the species and varieties of
Verbascum; and although unable to confirm Gartner's results on the
crossing of the distinct species, he finds that the dissimilarly coloured
varieties of the same species yield fewer seeds, in the proportion of
eighty-six to 100, than the similarly coloured varieties. Yet these
varieties differ in no respect, except in the colour of their flowers; and
one variety can sometimes be raised from the seed of another.

Kolreuter, whose accuracy has been confirmed by every subsequent observer,
has proved the remarkable fact that one particular variety of the common
tobacco was more fertile than the other varieties, when crossed with a
widely distinct species. He experimented on five forms which are commonly
reputed to be varieties, and which he tested by the severest trial,
namely, by reciprocal crosses, and he found their mongrel offspring
perfectly fertile. But one of these five varieties, when used either as
the father or mother, and crossed with the Nicotiana glutinosa, always
yielded hybrids not so sterile as those which were produced from the four
other varieties when crossed with N. glutinosa. Hence the reproductive
system of this one variety must have been in some manner and in some
degree modified.

From these facts it can no longer be maintained that varieties when
crossed are invariably quite fertile. From the great difficulty of
ascertaining the infertility of varieties in a state of nature, for a
supposed variety, if proved to be infertile in any degree, would almost
universally be ranked as a species; from man attending only to external
characters in his domestic varieties, and from such varieties not having
been exposed for very long periods to uniform conditions of life; from
these several considerations we may conclude that fertility does not
constitute a fundamental distinction between varieties and species when
crossed. The general sterility of crossed species may safely be looked at,
not as a special acquirement or endowment, but as incidental on changes of
an unknown nature in their sexual elements.

HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY.

Independently of the question of fertility, the offspring of species and
of varieties when crossed may be compared in several other respects.
Gartner, whose strong wish it was to draw a distinct line between species
and varieties, could find very few, and, as it seems to me, quite
unimportant differences between the so-called hybrid offspring of species,
and the so-called mongrel offspring of varieties. And, on the other hand,
they agree most closely in many important respects.

I shall here discuss this subject with extreme brevity. The most important
distinction is, that in the first generation mongrels are more variable
than hybrids; but Gartner admits that hybrids from species which have long
been cultivated are often variable in the first generation; and I have
myself seen striking instances of this fact. Gartner further admits that
hybrids between very closely allied species are more variable than those
from very distinct species; and this shows that the difference in the
degree of variability graduates away. When mongrels and the more fertile
hybrids are propagated for several generations, an extreme amount of
variability in the offspring in both cases is notorious; but some few
instances of both hybrids and mongrels long retaining a uniform character
could be given. The variability, however, in the successive generations of
mongrels is, perhaps, greater than in hybrids.

This greater variability in mongrels than in hybrids does not seem at all
surprising. For the parents of mongrels are varieties, and mostly domestic
varieties (very few experiments having been tried on natural varieties),
and this implies that there has been recent variability; which would often
continue and would augment that arising from the act of crossing. The
slight variability of hybrids in the first generation, in contrast with
that in the succeeding generations, is a curious fact and deserves
attention. For it bears on the view which I have taken of one of the
causes of ordinary variability; namely, that the reproductive system, from
being eminently sensitive to changed conditions of life, fails under these
circumstances to perform its proper function of producing offspring
closely similar in all respects to the parent-form. Now, hybrids in the
first generation are descended from species (excluding those long
cultivated) which have not had their reproductive systems in any way
affected, and they are not variable; but hybrids themselves have their
reproductive systems seriously affected, and their descendants are highly
variable.

But to return to our comparison of mongrels and hybrids: Gartner states
that mongrels are more liable than hybrids to revert to either parent
form; but this, if it be true, is certainly only a difference in degree.
Moreover, Gartner expressly states that the hybrids from long cultivated
plants are more subject to reversion than hybrids from species in their
natural state; and this probably explains the singular difference in the
results arrived at by different observers. Thus Max Wichura doubts whether
hybrids ever revert to their parent forms, and he experimented on
uncultivated species of willows, while Naudin, on the other hand, insists
in the strongest terms on the almost universal tendency to reversion in
hybrids, and he experimented chiefly on cultivated plants. Gartner further
states that when any two species, although most closely allied to each
other, are crossed with a third species, the hybrids are widely different
from each other; whereas if two very distinct varieties of one species are
crossed with another species, the hybrids do not differ much. But this
conclusion, as far as I can make out, is founded on a single experiment;
and seems directly opposed to the results of several experiments made by
Kolreuter.

Such alone are the unimportant differences which Gartner is able to point
out between hybrid and mongrel plants. On the other hand, the degrees and
kinds of resemblance in mongrels and in hybrids to their respective
parents, more especially in hybrids produced from nearly related species,
follow, according to Gartner the same laws. When two species are crossed,
one has sometimes a prepotent power of impressing its likeness on the
hybrid. So I believe it to be with varieties of plants; and with animals,
one variety certainly often has this prepotent power over another variety.
Hybrid plants produced from a reciprocal cross generally resemble each
other closely, and so it is with mongrel plants from a reciprocal cross.
Both hybrids and mongrels can be reduced to either pure parent form, by
repeated crosses in successive generations with either parent.

These several remarks are apparently applicable to animals; but the
subject is here much complicated, partly owing to the existence of
secondary sexual characters; but more especially owing to prepotency in
transmitting likeness running more strongly in one sex than in the other,
both when one species is crossed with another and when one variety is
crossed with another variety. For instance, I think those authors are
right who maintain that the ass has a prepotent power over the horse, so
that both the mule and the hinny resemble more closely the ass than the
horse; but that the prepotency runs more strongly in the male than in the
female ass, so that the mule, which is an offspring of the male ass and
mare, is more like an ass than is the hinny, which is the offspring of the
female-ass and stallion.

Much stress has been laid by some authors on the supposed fact, that it is
only with mongrels that the offspring are not intermediate in character,
but closely resemble one of their parents; but this does sometimes occur
with hybrids, yet I grant much less frequently than with mongrels. Looking
to the cases which I have collected of cross-bred animals closely
resembling one parent, the resemblances seem chiefly confined to
characters almost monstrous in their nature, and which have suddenly
appeared—such as albinism, melanism, deficiency of tail or horns, or
additional fingers and toes; and do not relate to characters which have
been slowly acquired through selection. A tendency to sudden reversions to
the perfect character of either parent would, also, be much more likely to
occur with mongrels, which are descended from varieties often suddenly
produced and semi-monstrous in character, than with hybrids, which are
descended from species slowly and naturally produced. On the whole, I
entirely agree with Dr. Prosper Lucas, who, after arranging an enormous
body of facts with respect to animals, comes to the conclusion that the
laws of resemblance of the child to its parents are the same, whether the
two parents differ little or much from each other, namely, in the union of
individuals of the same variety, or of different varieties, or of distinct
species.

Independently of the question of fertility and sterility, in all other
respects there seems to be a general and close similarity in the offspring
of crossed species, and of crossed varieties. If we look at species as
having been specially created, and at varieties as having been produced by
secondary laws, this similarity would be an astonishing fact. But it
harmonises perfectly with the view that there is no essential distinction
between species and varieties.

SUMMARY OF CHAPTER.

First crosses between forms, sufficiently distinct to be ranked as
species, and their hybrids, are very generally, but not universally,
sterile. The sterility is of all degrees, and is often so slight that the
most careful experimentalists have arrived at diametrically opposite
conclusions in ranking forms by this test. The sterility is innately
variable in individuals of the same species, and is eminently susceptible
to action of favourable and unfavourable conditions. The degree of
sterility does not strictly follow systematic affinity, but is governed by
several curious and complex laws. It is generally different, and sometimes
widely different in reciprocal crosses between the same two species. It is
not always equal in degree in a first cross and in the hybrids produced
from this cross.

In the same manner as in grafting trees, the capacity in one species or
variety to take on another, is incidental on differences, generally of an
unknown nature, in their vegetative systems, so in crossing, the greater
or less facility of one species to unite with another is incidental on
unknown differences in their reproductive systems. There is no more reason
to think that species have been specially endowed with various degrees of
sterility to prevent their crossing and blending in nature, than to think
that trees have been specially endowed with various and somewhat analogous
degrees of difficulty in being grafted together in order to prevent their
inarching in our forests.

The sterility of first crosses and of their hybrid progeny has not been
acquired through natural selection. In the case of first crosses it seems
to depend on several circumstances; in some instances in chief part on the
early death of the embryo. In the case of hybrids, it apparently depends
on their whole organisation having been disturbed by being compounded from
two distinct forms; the sterility being closely allied to that which so
frequently affects pure species, when exposed to new and unnatural
conditions of life. He who will explain these latter cases will be able to
explain the sterility of hybrids. This view is strongly supported by a
parallelism of another kind: namely, that, firstly, slight changes in the
conditions of life add to the vigour and fertility of all organic beings;
and secondly, that the crossing of forms, which have been exposed to
slightly different conditions of life, or which have varied, favours the
size, vigour and fertility of their offspring. The facts given on the
sterility of the illegitimate unions of dimorphic and trimorphic plants
and of their illegitimate progeny, perhaps render it probable that some
unknown bond in all cases connects the degree of fertility of first unions
with that of their offspring. The consideration of these facts on
dimorphism, as well as of the results of reciprocal crosses, clearly leads
to the conclusion that the primary cause of the sterility of crossed
species is confined to differences in their sexual elements. But why, in
the case of distinct species, the sexual elements should so generally have
become more or less modified, leading to their mutual infertility, we do
not know; but it seems to stand in some close relation to species having
been exposed for long periods of time to nearly uniform conditions of
life.

It is not surprising that the difficulty in crossing any two species, and
the sterility of their hybrid offspring, should in most cases correspond,
even if due to distinct causes: for both depend on the amount of
difference between the species which are crossed. Nor is it surprising
that the facility of effecting a first cross, and the fertility of the
hybrids thus produced, and the capacity of being grafted together—though
this latter capacity evidently depends on widely different circumstances—should
all run, to a certain extent, parallel with the systematic affinity of the
forms subjected to experiment; for systematic affinity includes
resemblances of all kinds.

First crosses between forms known to be varieties, or sufficiently alike
to be considered as varieties, and their mongrel offspring, are very
generally, but not, as is so often stated, invariably fertile. Nor is this
almost universal and perfect fertility surprising, when it is remembered
how liable we are to argue in a circle with respect to varieties in a
state of nature; and when we remember that the greater number of varieties
have been produced under domestication by the selection of mere external
differences, and that they have not been long exposed to uniform
conditions of life. It should also be especially kept in mind, that
long-continued domestication tends to eliminate sterility, and is
therefore little likely to induce this same quality. Independently of the
question of fertility, in all other respects there is the closest general
resemblance between hybrids and mongrels, in their variability, in their
power of absorbing each other by repeated crosses, and in their
inheritance of characters from both parent-forms. Finally, then, although
we are as ignorant of the precise cause of the sterility of first crosses
and of hybrids as we are why animals and plants removed from their natural
conditions become sterile, yet the facts given in this chapter do not seem
to me opposed to the belief that species aboriginally existed as
varieties.



CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.


 On the absence of intermediate varieties at the present day—On the
 nature of extinct intermediate varieties; on their number—On the lapse
 of time, as inferred from the rate of denudation and of deposition
 number—On the lapse of time as estimated by years—On the poorness of
 our palaeontological collections—On the intermittence of geological
 formations—On the denudation of granitic areas—On the absence of
 intermediate varieties in any one formation—On the sudden appearance
 of groups of species—On their sudden appearance in the lowest known
 fossiliferous strata—Antiquity of the habitable earth.

In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely, the distinctness of specific forms
and their not being blended together by innumerable transitional links, is
a very obvious difficulty. I assigned reasons why such links do not
commonly occur at the present day under the circumstances apparently most
favourable for their presence, namely, on an extensive and continuous area
with graduated physical conditions. I endeavoured to show, that the life
of each species depends in a more important manner on the presence of
other already defined organic forms, than on climate, and, therefore, that
the really governing conditions of life do not graduate away quite
insensibly like heat or moisture. I endeavoured, also, to show that
intermediate varieties, from existing in lesser numbers than the forms
which they connect, will generally be beaten out and exterminated during
the course of further modification and improvement. The main cause,
however, of innumerable intermediate links not now occurring everywhere
throughout nature depends, on the very process of natural selection,
through which new varieties continually take the places of and supplant
their parent-forms. But just in proportion as this process of
extermination has acted on an enormous scale, so must the number of
intermediate varieties, which have formerly existed, be truly enormous.
Why then is not every geological formation and every stratum full of such
intermediate links? Geology assuredly does not reveal any such finely
graduated organic chain; and this, perhaps, is the most obvious and
serious objection which can be urged against my theory. The explanation
lies, as I believe, in the extreme imperfection of the geological record.

In the first place, it should always be borne in mind what sort of
intermediate forms must, on the theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid picturing to
myself forms DIRECTLY intermediate between them. But this is a wholly
false view; we should always look for forms intermediate between each
species and a common but unknown progenitor; and the progenitor will
generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and pouter pigeons
are both descended from the rock-pigeon; if we possessed all the
intermediate varieties which have ever existed, we should have an
extremely close series between both and the rock-pigeon; but we should
have no varieties directly intermediate between the fantail and pouter;
none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds. These
two breeds, moreover, have become so much modified, that, if we had no
historical or indirect evidence regarding their origin, it would not have
been possible to have determined from a mere comparison of their structure
with that of the rock-pigeon, C. livia, whether they had descended from
this species or from some other allied species, such as C. oenas.

So with natural species, if we look to forms very distinct, for instance
to the horse and tapir, we have no reason to suppose that links directly
intermediate between them ever existed, but between each and an unknown
common parent. The common parent will have had in its whole organisation
much general resemblance to the tapir and to the horse; but in some points
of structure may have differed considerably from both, even perhaps more
than they differ from each other. Hence, in all such cases, we should be
unable to recognise the parent-form of any two or more species, even if we
closely compared the structure of the parent with that of its modified
descendants, unless at the same time we had a nearly perfect chain of the
intermediate links.

It is just possible, by the theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and in
this case DIRECT intermediate links will have existed between them. But
such a case would imply that one form had remained for a very long period
unaltered, whilst its descendants had undergone a vast amount of change;
and the principle of competition between organism and organism, between
child and parent, will render this a very rare event; for in all cases the
new and improved forms of life tend to supplant the old and unimproved
forms.

By the theory of natural selection all living species have been connected
with the parent-species of each genus, by differences not greater than we
see between the natural and domestic varieties of the same species at the
present day; and these parent-species, now generally extinct, have in
their turn been similarly connected with more ancient forms; and so on
backwards, always converging to the common ancestor of each great class.
So that the number of intermediate and transitional links, between all
living and extinct species, must have been inconceivably great. But
assuredly, if this theory be true, such have lived upon the earth.

ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF DEPOSITION AND EXTENT
OF DENUDATION.

Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected that time cannot have
sufficed for so great an amount of organic change, all changes having been
effected slowly. It is hardly possible for me to recall to the reader who
is not a practical geologist, the facts leading the mind feebly to
comprehend the lapse of time. He who can read Sir Charles Lyell's grand
work on the Principles of Geology, which the future historian will
recognise as having produced a revolution in natural science, and yet does
not admit how vast have been the past periods of time, may at once close
this volume. Not that it suffices to study the Principles of Geology, or
to read special treatises by different observers on separate formations,
and to mark how each author attempts to give an inadequate idea of the
duration of each formation, or even of each stratum. We can best gain some
idea of past time by knowing the agencies at work; and learning how deeply
the surface of the land has been denuded, and how much sediment has been
deposited. As Lyell has well remarked, the extent and thickness of our
sedimentary formations are the result and the measure of the denudation
which the earth's crust has elsewhere undergone. Therefore a man should
examine for himself the great piles of superimposed strata, and watch the
rivulets bringing down mud, and the waves wearing away the sea-cliffs, in
order to comprehend something about the duration of past time, the
monuments of which we see all around us.

It is good to wander along the coast, when formed of moderately hard
rocks, and mark the process of degradation. The tides in most cases reach
the cliffs only for a short time twice a day, and the waves eat into them
only when they are charged with sand or pebbles; for there is good
evidence that pure water effects nothing in wearing away rock. At last the
base of the cliff is undermined, huge fragments fall down, and these
remaining fixed, have to be worn away atom by atom, until after being
reduced in size they can be rolled about by the waves, and then they are
more quickly ground into pebbles, sand, or mud. But how often do we see
along the bases of retreating cliffs rounded boulders, all thickly clothed
by marine productions, showing how little they are abraded and how seldom
they are rolled about! Moreover, if we follow for a few miles any line of
rocky cliff, which is undergoing degradation, we find that it is only here
and there, along a short length or round a promontory, that the cliffs are
at the present time suffering. The appearance of the surface and the
vegetation show that elsewhere years have elapsed since the waters washed
their base.

We have, however, recently learned from the observations of Ramsay, in the
van of many excellent observers—of Jukes, Geikie, Croll and others,
that subaerial degradation is a much more important agency than
coast-action, or the power of the waves. The whole surface of the land is
exposed to the chemical action of the air and of the rainwater, with its
dissolved carbonic acid, and in colder countries to frost; the
disintegrated matter is carried down even gentle slopes during heavy rain,
and to a greater extent than might be supposed, especially in arid
districts, by the wind; it is then transported by the streams and rivers,
which, when rapid deepen their channels, and triturate the fragments. On a
rainy day, even in a gently undulating country, we see the effects of
subaerial degradation in the muddy rills which flow down every slope.
Messrs. Ramsay and Whitaker have shown, and the observation is a most
striking one, that the great lines of escarpment in the Wealden district
and those ranging across England, which formerly were looked at as ancient
sea-coasts, cannot have been thus formed, for each line is composed of one
and the same formation, while our sea-cliffs are everywhere formed by the
intersection of various formations. This being the case, we are compelled
to admit that the escarpments owe their origin in chief part to the rocks
of which they are composed, having resisted subaerial denudation better
than the surrounding surface; this surface consequently has been gradually
lowered, with the lines of harder rock left projecting. Nothing impresses
the mind with the vast duration of time, according to our ideas of time,
more forcibly than the conviction thus gained that subaerial agencies,
which apparently have so little power, and which seem to work so slowly,
have produced great results.

When thus impressed with the slow rate at which the land is worn away
through subaerial and littoral action, it is good, in order to appreciate
the past duration of time, to consider, on the one hand, the masses of
rock which have been removed over many extensive areas, and on the other
hand the thickness of our sedimentary formations. I remember having been
much struck when viewing volcanic islands, which have been worn by the
waves and pared all round into perpendicular cliffs of one or two thousand
feet in height; for the gentle slope of the lava-streams, due to their
formerly liquid state, showed at a glance how far the hard, rocky beds had
once extended into the open ocean. The same story is told still more
plainly by faults—those great cracks along which the strata have
been upheaved on one side, or thrown down on the other, to the height or
depth of thousands of feet; for since the crust cracked, and it makes no
great difference whether the upheaval was sudden, or, as most geologists
now believe, was slow and effected by many starts, the surface of the land
has been so completely planed down that no trace of these vast
dislocations is externally visible. The Craven fault, for instance,
extends for upward of thirty miles, and along this line the vertical
displacement of the strata varies from 600 to 3,000 feet. Professor Ramsay
has published an account of a downthrow in Anglesea of 2,300 feet; and he
informs me that he fully believes that there is one in Merionethshire of
12,000 feet; yet in these cases there is nothing on the surface of the
land to show such prodigious movements; the pile of rocks on either side
of the crack having been smoothly swept away.

On the other hand, in all parts of the world the piles of sedimentary
strata are of wonderful thickness. In the Cordillera, I estimated one mass
of conglomerate at ten thousand feet; and although conglomerates have
probably been accumulated at a quicker rate than finer sediments, yet from
being formed of worn and rounded pebbles, each of which bears the stamp of
time, they are good to show how slowly the mass must have been heaped
together. Professor Ramsay has given me the maximum thickness, from actual
measurement in most cases, of the successive formations in DIFFERENT parts
of Great Britain; and this is the result:—

                                            Feet
 Palaeozoic strata (not including igneous beds)..57,154
 Secondary strata................................13,190
 Tertiary strata..................................2,240
 —making altogether 72,584 feet;

that is, very nearly thirteen and three-quarters British miles. Some of
these formations, which are represented in England by thin beds, are
thousands of feet in thickness on the Continent. Moreover, between each
successive formation we have, in the opinion of most geologists, blank
periods of enormous length. So that the lofty pile of sedimentary rocks in
Britain gives but an inadequate idea of the time which has elapsed during
their accumulation. The consideration of these various facts impresses the
mind almost in the same manner as does the vain endeavour to grapple with
the idea of eternity.

Nevertheless this impression is partly false. Mr. Croll, in an interesting
paper, remarks that we do not err "in forming too great a conception of
the length of geological periods," but in estimating them by years. When
geologists look at large and complicated phenomena, and then at the
figures representing several million years, the two produce a totally
different effect on the mind, and the figures are at once pronounced too
small. In regard to subaerial denudation, Mr. Croll shows, by calculating
the known amount of sediment annually brought down by certain rivers,
relatively to their areas of drainage, that 1,000 feet of solid rock, as
it became gradually disintegrated, would thus be removed from the mean
level of the whole area in the course of six million years. This seems an
astonishing result, and some considerations lead to the suspicion that it
may be too large, but if halved or quartered it is still very surprising.
Few of us, however, know what a million really means: Mr. Croll gives the
following illustration: Take a narrow strip of paper, eighty-three feet
four inches in length, and stretch it along the wall of a large hall; then
mark off at one end the tenth of an inch. This tenth of an inch will
represent one hundred years, and the entire strip a million years. But let
it be borne in mind, in relation to the subject of this work, what a
hundred years implies, represented as it is by a measure utterly
insignificant in a hall of the above dimensions. Several eminent breeders,
during a single lifetime, have so largely modified some of the higher
animals, which propagate their kind much more slowly than most of the
lower animals, that they have formed what well deserves to be called a new
sub-breed. Few men have attended with due care to any one strain for more
than half a century, so that a hundred years represents the work of two
breeders in succession. It is not to be supposed that species in a state
of nature ever change so quickly as domestic animals under the guidance of
methodical selection. The comparison would be in every way fairer with the
effects which follow from unconscious selection, that is, the preservation
of the most useful or beautiful animals, with no intention of modifying
the breed; but by this process of unconscious selection, various breeds
have been sensibly changed in the course of two or three centuries.

Species, however, probably change much more slowly, and within the same
country only a few change at the same time. This slowness follows from all
the inhabitants of the same country being already so well adapted to each
other, that new places in the polity of nature do not occur until after
long intervals, due to the occurrence of physical changes of some kind, or
through the immigration of new forms. Moreover, variations or individual
differences of the right nature, by which some of the inhabitants might be
better fitted to their new places under the altered circumstance, would
not always occur at once. Unfortunately we have no means of determining,
according to the standard of years, how long a period it takes to modify a
species; but to the subject of time we must return.

ON THE POORNESS OF PALAEONTOLOGICAL COLLECTIONS.

Now let us turn to our richest museums, and what a paltry display we
behold! That our collections are imperfect is admitted by every one. The
remark of that admirable palaeontologist, Edward Forbes, should never be
forgotten, namely, that very many fossil species are known and named from
single and often broken specimens, or from a few specimens collected on
some one spot. Only a small portion of the surface of the earth has been
geologically explored, and no part with sufficient care, as the important
discoveries made every year in Europe prove. No organism wholly soft can
be preserved. Shells and bones decay and disappear when left on the bottom
of the sea, where sediment is not accumulating. We probably take a quite
erroneous view, when we assume that sediment is being deposited over
nearly the whole bed of the sea, at a rate sufficiently quick to embed and
preserve fossil remains. Throughout an enormously large proportion of the
ocean, the bright blue tint of the water bespeaks its purity. The many
cases on record of a formation conformably covered, after an immense
interval of time, by another and later formation, without the underlying
bed having suffered in the interval any wear and tear, seem explicable
only on the view of the bottom of the sea not rarely lying for ages in an
unaltered condition. The remains which do become embedded, if in sand or
gravel, will, when the beds are upraised, generally be dissolved by the
percolation of rain water charged with carbonic acid. Some of the many
kinds of animals which live on the beach between high and low water mark
seem to be rarely preserved. For instance, the several species of the
Chthamalinae (a sub-family of sessile cirripedes) coat the rocks all over
the world in infinite numbers: they are all strictly littoral, with the
exception of a single Mediterranean species, which inhabits deep water and
this has been found fossil in Sicily, whereas not one other species has
hitherto been found in any tertiary formation: yet it is known that the
genus Chthamalus existed during the Chalk period. Lastly, many great
deposits, requiring a vast length of time for their accumulation, are
entirely destitute of organic remains, without our being able to assign
any reason: one of the most striking instances is that of the Flysch
formation, which consists of shale and sandstone, several thousand,
occasionally even six thousand feet in thickness, and extending for at
least 300 miles from Vienna to Switzerland; and although this great mass
has been most carefully searched, no fossils, except a few vegetable
remains, have been found.

With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that our
evidence is fragmentary in an extreme degree. For instance, until recently
not a land-shell was known belonging to either of these vast periods, with
the exception of one species discovered by Sir C. Lyell and Dr. Dawson in
the carboniferous strata of North America; but now land-shells have been
found in the lias. In regard to mammiferous remains, a glance at the
historical table published in Lyell's Manual, will bring home the truth,
how accidental and rare is their preservation, far better than pages of
detail. Nor is their rarity surprising, when we remember how large a
proportion of the bones of tertiary mammals have been discovered either in
caves or in lacustrine deposits; and that not a cave or true lacustrine
bed is known belonging to the age of our secondary or palaeozoic
formations.

But the imperfection in the geological record largely results from another
and more important cause than any of the foregoing; namely, from the
several formations being separated from each other by wide intervals of
time. This doctrine has been emphatically admitted by many geologists and
palaeontologists, who, like E. Forbes, entirely disbelieve in the change
of species. When we see the formations tabulated in written works, or when
we follow them in nature, it is difficult to avoid believing that they are
closely consecutive. But we know, for instance, from Sir R. Murchison's
great work on Russia, what wide gaps there are in that country between the
superimposed formations; so it is in North America, and in many other
parts of the world. The most skilful geologist, if his attention had been
confined exclusively to these large territories, would never have
suspected that during the periods which were blank and barren in his own
country, great piles of sediment, charged with new and peculiar forms of
life, had elsewhere been accumulated. And if, in every separate territory,
hardly any idea can be formed of the length of time which has elapsed
between the consecutive formations, we may infer that this could nowhere
be ascertained. The frequent and great changes in the mineralogical
composition of consecutive formations, generally implying great changes in
the geography of the surrounding lands, whence the sediment was derived,
accord with the belief of vast intervals of time having elapsed between
each formation.

We can, I think, see why the geological formations of each region are
almost invariably intermittent; that is, have not followed each other in
close sequence. Scarcely any fact struck me more when examining many
hundred miles of the South American coasts, which have been upraised
several hundred feet within the recent period, than the absence of any
recent deposits sufficiently extensive to last for even a short geological
period. Along the whole west coast, which is inhabited by a peculiar
marine fauna, tertiary beds are so poorly developed that no record of
several successive and peculiar marine faunas will probably be preserved
to a distant age. A little reflection will explain why, along the rising
coast of the western side of South America, no extensive formations with
recent or tertiary remains can anywhere be found, though the supply of
sediment must for ages have been great, from the enormous degradation of
the coast rocks and from the muddy streams entering the sea. The
explanation, no doubt, is that the littoral and sub-littoral deposits are
continually worn away, as soon as they are brought up by the slow and
gradual rising of the land within the grinding action of the coast-waves.

We may, I think, conclude that sediment must be accumulated in extremely
thick, solid, or extensive masses, in order to withstand the incessant
action of the waves, when first upraised and during subsequent
oscillations of level, as well as the subsequent subaerial degradation.
Such thick and extensive accumulations of sediment may be formed in two
ways; either in profound depths of the sea, in which case the bottom will
not be inhabited by so many and such varied forms of life as the more
shallow seas; and the mass when upraised will give an imperfect record of
the organisms which existed in the neighbourhood during the period of its
accumulation. Or sediment may be deposited to any thickness and extent
over a shallow bottom, if it continue slowly to subside. In this latter
case, as long as the rate of subsidence and supply of sediment nearly
balance each other, the sea will remain shallow and favourable for many
and varied forms, and thus a rich fossiliferous formation, thick enough,
when upraised, to resist a large amount of denudation, may be formed.

I am convinced that nearly all our ancient formations, which are
throughout the greater part of their thickness RICH IN FOSSILS, have thus
been formed during subsidence. Since publishing my views on this subject
in 1845, I have watched the progress of geology, and have been surprised
to note how author after author, in treating of this or that great
formation, has come to the conclusion that it was accumulated during
subsidence. I may add, that the only ancient tertiary formation on the
west coast of South America, which has been bulky enough to resist such
degradation as it has as yet suffered, but which will hardly last to a
distant geological age, was deposited during a downward oscillation of
level, and thus gained considerable thickness.

All geological facts tell us plainly that each area has undergone numerous
slow oscillations of level, and apparently these oscillations have
affected wide spaces. Consequently, formations rich in fossils and
sufficiently thick and extensive to resist subsequent degradation, will
have been formed over wide spaces during periods of subsidence, but only
where the supply of sediment was sufficient to keep the sea shallow and to
embed and preserve the remains before they had time to decay. On the other
hand, as long as the bed of the sea remained stationary, THICK deposits
cannot have been accumulated in the shallow parts, which are the most
favourable to life. Still less can this have happened during the alternate
periods of elevation; or, to speak more accurately, the beds which were
then accumulated will generally have been destroyed by being upraised and
brought within the limits of the coast-action.

These remarks apply chiefly to littoral and sublittoral deposits. In the
case of an extensive and shallow sea, such as that within a large part of
the Malay Archipelago, where the depth varies from thirty or forty to
sixty fathoms, a widely extended formation might be formed during a period
of elevation, and yet not suffer excessively from denudation during its
slow upheaval; but the thickness of the formation could not be great, for
owing to the elevatory movement it would be less than the depth in which
it was formed; nor would the deposit be much consolidated, nor be capped
by overlying formations, so that it would run a good chance of being worn
away by atmospheric degradation and by the action of the sea during
subsequent oscillations of level. It has, however, been suggested by Mr.
Hopkins, that if one part of the area, after rising and before being
denuded, subsided, the deposit formed during the rising movement, though
not thick, might afterwards become protected by fresh accumulations, and
thus be preserved for a long period.

Mr. Hopkins also expresses his belief that sedimentary beds of
considerable horizontal extent have rarely been completely destroyed. But
all geologists, excepting the few who believe that our present metamorphic
schists and plutonic rocks once formed the primordial nucleus of the
globe, will admit that these latter rocks have been stripped of their
covering to an enormous extent. For it is scarcely possible that such
rocks could have been solidified and crystallised while uncovered; but if
the metamorphic action occurred at profound depths of the ocean, the
former protecting mantle of rock may not have been very thick. Admitting
then that gneiss, mica-schist, granite, diorite, etc., were once
necessarily covered up, how can we account for the naked and extensive
areas of such rocks in many parts of the world, except on the belief that
they have subsequently been completely denuded of all overlying strata?
That such extensive areas do exist cannot be doubted: the granitic region
of Parime is described by Humboldt as being at least nineteen times as
large as Switzerland. South of the Amazon, Boue colours an area composed
of rocks of this nature as equal to that of Spain, France, Italy, part of
Germany, and the British Islands, all conjoined. This region has not been
carefully explored, but from the concurrent testimony of travellers, the
granitic area is very large: thus Von Eschwege gives a detailed section of
these rocks, stretching from Rio de Janeiro for 260 geographical miles
inland in a straight line; and I travelled for 150 miles in another
direction, and saw nothing but granitic rocks. Numerous specimens,
collected along the whole coast, from near Rio de Janeiro to the mouth of
the Plata, a distance of 1,100 geographical miles, were examined by me,
and they all belonged to this class. Inland, along the whole northern bank
of the Plata, I saw, besides modern tertiary beds, only one small patch of
slightly metamorphosed rock, which alone could have formed a part of the
original capping of the granitic series. Turning to a well-known region,
namely, to the United States and Canada, as shown in Professor H.D.
Rogers' beautiful map, I have estimated the areas by cutting out and
weighing the paper, and I find that the metamorphic (excluding the
"semi-metamorphic") and granite rocks exceed, in the proportion of 19 to
12.5, the whole of the newer Palaeozoic formations. In many regions the
metamorphic and granite rocks would be found much more widely extended
than they appear to be, if all the sedimentary beds were removed which
rest unconformably on them, and which could not have formed part of the
original mantle under which they were crystallised. Hence, it is probable
that in some parts of the world whole formations have been completely
denuded, with not a wreck left behind.

One remark is here worth a passing notice. During periods of elevation the
area of the land and of the adjoining shoal parts of the sea will be
increased and new stations will often be formed—all circumstances
favourable, as previously explained, for the formation of new varieties
and species; but during such periods there will generally be a blank in
the geological record. On the other hand, during subsidence, the inhabited
area and number of inhabitants will decrease (excepting on the shores of a
continent when first broken up into an archipelago), and consequently
during subsidence, though there will be much extinction, few new varieties
or species will be formed; and it is during these very periods of
subsidence that the deposits which are richest in fossils have been
accumulated.

ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES IN ANY SINGLE FORMATION.

From these several considerations it cannot be doubted that the geological
record, viewed as a whole, is extremely imperfect; but if we confine our
attention to any one formation, it becomes much more difficult to
understand why we do not therein find closely graduated varieties between
the allied species which lived at its commencement and at its close.
Several cases are on record of the same species presenting varieties in
the upper and lower parts of the same formation. Thus Trautschold gives a
number of instances with Ammonites, and Hilgendorf has described a most
curious case of ten graduated forms of Planorbis multiformis in the
successive beds of a fresh-water formation in Switzerland. Although each
formation has indisputably required a vast number of years for its
deposition, several reasons can be given why each should not commonly
include a graduated series of links between the species which lived at its
commencement and close, but I cannot assign due proportional weight to the
following considerations.

Although each formation may mark a very long lapse of years, each probably
is short compared with the period requisite to change one species into
another. I am aware that two palaeontologists, whose opinions are worthy
of much deference, namely Bronn and Woodward, have concluded that the
average duration of each formation is twice or thrice as long as the
average duration of specific forms. But insuperable difficulties, as it
seems to me, prevent us from coming to any just conclusion on this head.
When we see a species first appearing in the middle of any formation, it
would be rash in the extreme to infer that it had not elsewhere previously
existed. So again, when we find a species disappearing before the last
layers have been deposited, it would be equally rash to suppose that it
then became extinct. We forget how small the area of Europe is compared
with the rest of the world; nor have the several stages of the same
formation throughout Europe been correlated with perfect accuracy.

We may safely infer that with marine animals of all kinds there has been a
large amount of migration due to climatal and other changes; and when we
see a species first appearing in any formation, the probability is that it
only then first immigrated into that area. It is well known, for instance,
that several species appear somewhat earlier in the palaeozoic beds of
North America than in those of Europe; time having apparently been
required for their migration from the American to the European seas. In
examining the latest deposits, in various quarters of the world, it has
everywhere been noted, that some few still existing species are common in
the deposit, but have become extinct in the immediately surrounding sea;
or, conversely, that some are now abundant in the neighbouring sea, but
are rare or absent in this particular deposit. It is an excellent lesson
to reflect on the ascertained amount of migration of the inhabitants of
Europe during the glacial epoch, which forms only a part of one whole
geological period; and likewise to reflect on the changes of level, on the
extreme change of climate, and on the great lapse of time, all included
within this same glacial period. Yet it may be doubted whether, in any
quarter of the world, sedimentary deposits, INCLUDING FOSSIL REMAINS, have
gone on accumulating within the same area during the whole of this period.
It is not, for instance, probable that sediment was deposited during the
whole of the glacial period near the mouth of the Mississippi, within that
limit of depth at which marine animals can best flourish: for we know that
great geographical changes occurred in other parts of America during this
space of time. When such beds as were deposited in shallow water near the
mouth of the Mississippi during some part of the glacial period shall have
been upraised, organic remains will probably first appear and disappear at
different levels, owing to the migrations of species and to geographical
changes. And in the distant future, a geologist, examining these beds,
would be tempted to conclude that the average duration of life of the
embedded fossils had been less than that of the glacial period, instead of
having been really far greater, that is, extending from before the glacial
epoch to the present day.

In order to get a perfect gradation between two forms in the upper and
lower parts of the same formation, the deposit must have gone on
continuously accumulating during a long period, sufficient for the slow
process of modification; hence, the deposit must be a very thick one; and
the species undergoing change must have lived in the same district
throughout the whole time. But we have seen that a thick formation,
fossiliferous throughout its entire thickness, can accumulate only during
a period of subsidence; and to keep the depth approximately the same,
which is necessary that the same marine species may live on the same
space, the supply of sediment must nearly counterbalance the amount of
subsidence. But this same movement of subsidence will tend to submerge the
area whence the sediment is derived, and thus diminish the supply, whilst
the downward movement continues. In fact, this nearly exact balancing
between the supply of sediment and the amount of subsidence is probably a
rare contingency; for it has been observed by more than one
palaeontologist that very thick deposits are usually barren of organic
remains, except near their upper or lower limits.

It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation composed
of beds of widely different mineralogical composition, we may reasonably
suspect that the process of deposition has been more or less interrupted.
Nor will the closest inspection of a formation give us any idea of the
length of time which its deposition may have consumed. Many instances
could be given of beds, only a few feet in thickness, representing
formations which are elsewhere thousands of feet in thickness, and which
must have required an enormous period for their accumulation; yet no one
ignorant of this fact would have even suspected the vast lapse of time
represented by the thinner formation. Many cases could be given of the
lower beds of a formation having been upraised, denuded, submerged, and
then re-covered by the upper beds of the same formation—facts,
showing what wide, yet easily overlooked, intervals have occurred in its
accumulation. In other cases we have the plainest evidence in great
fossilised trees, still standing upright as they grew, of many long
intervals of time and changes of level during the process of deposition,
which would not have been suspected, had not the trees been preserved:
thus Sir C. Lyell and Dr. Dawson found carboniferous beds 1,400 feet thick
in Nova Scotia, with ancient root-bearing strata, one above the other, at
no less than sixty-eight different levels. Hence, when the same species
occurs at the bottom, middle, and top of a formation, the probability is
that it has not lived on the same spot during the whole period of
deposition, but has disappeared and reappeared, perhaps many times, during
the same geological period. Consequently if it were to undergo a
considerable amount of modification during the deposition of any one
geological formation, a section would not include all the fine
intermediate gradations which must on our theory have existed, but abrupt,
though perhaps slight, changes of form.

It is all-important to remember that naturalists have no golden rule by
which to distinguish species and varieties; they grant some little
variability to each species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank both as species,
unless they are enabled to connect them together by the closest
intermediate gradations; and this, from the reasons just assigned, we can
seldom hope to effect in any one geological section. Supposing B and C to
be two species, and a third, A, to be found in an older and underlying
bed; even if A were strictly intermediate between B and C, it would simply
be ranked as a third and distinct species, unless at the same time it
could be closely connected by intermediate varieties with either one or
both forms. Nor should it be forgotten, as before explained, that A might
be the actual progenitor of B and C, and yet would not necessarily be
strictly intermediate between them in all respects. So that we might
obtain the parent-species and its several modified descendants from the
lower and upper beds of the same formation, and unless we obtained
numerous transitional gradations, we should not recognise their
blood-relationship, and should consequently rank them as distinct species.

It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the very
fine species of D'Orbigny and others into the rank of varieties; and on
this view we do find the kind of evidence of change which on the theory we
ought to find. Look again at the later tertiary deposits, which include
many shells believed by the majority of naturalists to be identical with
existing species; but some excellent naturalists, as Agassiz and Pictet,
maintain that all these tertiary species are specifically distinct, though
the distinction is admitted to be very slight; so that here, unless we
believe that these eminent naturalists have been misled by their
imaginations, and that these late tertiary species really present no
difference whatever from their living representatives, or unless we admit,
in opposition to the judgment of most naturalists, that these tertiary
species are all truly distinct from the recent, we have evidence of the
frequent occurrence of slight modifications of the kind required. If we
look to rather wider intervals of time, namely, to distinct but
consecutive stages of the same great formation, we find that the embedded
fossils, though universally ranked as specifically different, yet are far
more closely related to each other than are the species found in more
widely separated formations; so that here again we have undoubted evidence
of change in the direction required by the theory; but to this latter
subject I shall return in the following chapter.

With animals and plants that propagate rapidly and do not wander much,
there is reason to suspect, as we have formerly seen, that their varieties
are generally at first local; and that such local varieties do not spread
widely and supplant their parent-form until they have been modified and
perfected in some considerable degree. According to this view, the chance
of discovering in a formation in any one country all the early stages of
transition between any two forms, is small, for the successive changes are
supposed to have been local or confined to some one spot. Most marine
animals have a wide range; and we have seen that with plants it is those
which have the widest range, that oftenest present varieties, so that,
with shells and other marine animals, it is probable that those which had
the widest range, far exceeding the limits of the known geological
formations in Europe, have oftenest given rise, first to local varieties
and ultimately to new species; and this again would greatly lessen the
chance of our being able to trace the stages of transition in any one
geological formation.

It is a more important consideration, leading to the same result, as
lately insisted on by Dr. Falconer, namely, that the period during which
each species underwent modification, though long as measured by years, was
probably short in comparison with that during which it remained without
undergoing any change.

It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties, and thus proved to be the same species, until many
specimens are collected from many places; and with fossil species this can
rarely be done. We shall, perhaps, best perceive the improbability of our
being enabled to connect species by numerous, fine, intermediate, fossil
links, by asking ourselves whether, for instance, geologists at some
future period will be able to prove that our different breeds of cattle,
sheep, horses, and dogs are descended from a single stock or from several
aboriginal stocks; or, again, whether certain sea-shells inhabiting the
shores of North America, which are ranked by some conchologists as
distinct species from their European representatives, and by other
conchologists as only varieties, are really varieties, or are, as it is
called, specifically distinct. This could be effected by the future
geologist only by his discovering in a fossil state numerous intermediate
gradations; and such success is improbable in the highest degree.

It has been asserted over and over again, by writers who believe in the
immutability of species, that geology yields no linking forms. This
assertion, as we shall see in the next chapter, is certainly erroneous. As
Sir J. Lubbock has remarked, "Every species is a link between other allied
forms." If we take a genus having a score of species, recent and extinct,
and destroy four-fifths of them, no one doubts that the remainder will
stand much more distinct from each other. If the extreme forms in the
genus happen to have been thus destroyed, the genus itself will stand more
distinct from other allied genera. What geological research has not
revealed, is the former existence of infinitely numerous gradations, as
fine as existing varieties, connecting together nearly all existing and
extinct species. But this ought not to be expected; yet this has been
repeatedly advanced as a most serious objection against my views.

It may be worth while to sum up the foregoing remarks on the causes of the
imperfection of the geological record under an imaginary illustration. The
Malay Archipelago is about the size of Europe from the North Cape to the
Mediterranean, and from Britain to Russia; and therefore equals all the
geological formations which have been examined with any accuracy,
excepting those of the United States of America. I fully agree with Mr.
Godwin-Austen, that the present condition of the Malay Archipelago, with
its numerous large islands separated by wide and shallow seas, probably
represents the former state of Europe, while most of our formations were
accumulating. The Malay Archipelago is one of the richest regions in
organic beings; yet if all the species were to be collected which have
ever lived there, how imperfectly would they represent the natural history
of the world!

But we have every reason to believe that the terrestrial productions of
the archipelago would be preserved in an extremely imperfect manner in the
formations which we suppose to be there accumulating. Not many of the
strictly littoral animals, or of those which lived on naked submarine
rocks, would be embedded; and those embedded in gravel or sand would not
endure to a distant epoch. Wherever sediment did not accumulate on the bed
of the sea, or where it did not accumulate at a sufficient rate to protect
organic bodies from decay, no remains could be preserved.

Formations rich in fossils of many kinds, and of thickness sufficient to
last to an age as distant in futurity as the secondary formations lie in
the past, would generally be formed in the archipelago only during periods
of subsidence. These periods of subsidence would be separated from each
other by immense intervals of time, during which the area would be either
stationary or rising; whilst rising, the fossiliferous formations on the
steeper shores would be destroyed, almost as soon as accumulated, by the
incessant coast-action, as we now see on the shores of South America. Even
throughout the extensive and shallow seas within the archipelago,
sedimentary beds could hardly be accumulated of great thickness during the
periods of elevation, or become capped and protected by subsequent
deposits, so as to have a good chance of enduring to a very distant
future. During the periods of subsidence, there would probably be much
extinction of life; during the periods of elevation, there would be much
variation, but the geological record would then be less perfect.

It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with a
contemporaneous accumulation of sediment, would EXCEED the average
duration of the same specific forms; and these contingencies are
indispensable for the preservation of all the transitional gradations
between any two or more species. If such gradations were not all fully
preserved, transitional varieties would merely appear as so many new,
though closely allied species. It is also probable that each great period
of subsidence would be interrupted by oscillations of level, and that
slight climatical changes would intervene during such lengthy periods; and
in these cases the inhabitants of the archipelago would migrate, and no
closely consecutive record of their modifications could be preserved in
any one formation.

Very many of the marine inhabitants of the archipelago now range thousands
of miles beyond its confines; and analogy plainly leads to the belief that
it would be chiefly these far-ranging species, though only some of them,
which would oftenest produce new varieties; and the varieties would at
first be local or confined to one place, but if possessed of any decided
advantage, or when further modified and improved, they would slowly spread
and supplant their parent-forms. When such varieties returned to their
ancient homes, as they would differ from their former state in a nearly
uniform, though perhaps extremely slight degree, and as they would be
found embedded in slightly different sub-stages of the same formation,
they would, according to the principles followed by many palaeontologists,
be ranked as new and distinct species.

If then there be some degree of truth in these remarks, we have no right
to expect to find, in our geological formations, an infinite number of
those fine transitional forms, which, on our theory, have connected all
the past and present species of the same group into one long and branching
chain of life. We ought only to look for a few links, and such assuredly
we do find—some more distantly, some more closely, related to each
other; and these links, let them be ever so close, if found in different
stages of the same formation, would, by many palaeontologists, be ranked
as distinct species. But I do not pretend that I should ever have
suspected how poor was the record in the best preserved geological
sections, had not the absence of innumerable transitional links between
the species which lived at the commencement and close of each formation,
pressed so hardly on my theory.

ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED SPECIES.

The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palaeontologists—for
instance, by Agassiz, Pictet, and Sedgwick, as a fatal objection to the
belief in the transmutation of species. If numerous species, belonging to
the same genera or families, have really started into life at once, the
fact would be fatal to the theory of evolution through natural selection.
For the development by this means of a group of forms, all of which are
descended from some one progenitor, must have been an extremely slow
process; and the progenitors must have lived long before their modified
descendants. But we continually overrate the perfection of the geological
record, and falsely infer, because certain genera or families have not
been found beneath a certain stage, that they did not exist before that
stage. In all cases positive palaeontological evidence may be implicitly
trusted; negative evidence is worthless, as experience has so often shown.
We continually forget how large the world is, compared with the area over
which our geological formations have been carefully examined; we forget
that groups of species may elsewhere have long existed, and have slowly
multiplied, before they invaded the ancient archipelagoes of Europe and
the United States. We do not make due allowance for the enormous intervals
of time which have elapsed between our consecutive formations, longer
perhaps in many cases than the time required for the accumulation of each
formation. These intervals will have given time for the multiplication of
species from some one parent-form: and in the succeeding formation, such
groups or species will appear as if suddenly created.

I may here recall a remark formerly made, namely, that it might require a
long succession of ages to adapt an organism to some new and peculiar line
of life, for instance, to fly through the air; and consequently that the
transitional forms would often long remain confined to some one region;
but that, when this adaptation had once been effected, and a few species
had thus acquired a great advantage over other organisms, a comparatively
short time would be necessary to produce many divergent forms, which would
spread rapidly and widely throughout the world. Professor Pictet, in his
excellent Review of this work, in commenting on early transitional forms,
and taking birds as an illustration, cannot see how the successive
modifications of the anterior limbs of a supposed prototype could possibly
have been of any advantage. But look at the penguins of the Southern
Ocean; have not these birds their front limbs in this precise intermediate
state of "neither true arms nor true wings?" Yet these birds hold their
place victoriously in the battle for life; for they exist in infinite
numbers and of many kinds. I do not suppose that we here see the real
transitional grades through which the wings of birds have passed; but what
special difficulty is there in believing that it might profit the modified
descendants of the penguin, first to become enabled to flap along the
surface of the sea like the logger-headed duck, and ultimately to rise
from its surface and glide through the air?

I will now give a few examples to illustrate the foregoing remarks, and to
show how liable we are to error in supposing that whole groups of species
have suddenly been produced. Even in so short an interval as that between
the first and second editions of Pictet's great work on Palaeontology,
published in 1844-46 and in 1853-57, the conclusions on the first
appearance and disappearance of several groups of animals have been
considerably modified; and a third edition would require still further
changes. I may recall the well-known fact that in geological treatises,
published not many years ago, mammals were always spoken of as having
abruptly come in at the commencement of the tertiary series. And now one
of the richest known accumulations of fossil mammals belongs to the middle
of the secondary series; and true mammals have been discovered in the new
red sandstone at nearly the commencement of this great series. Cuvier used
to urge that no monkey occurred in any tertiary stratum; but now extinct
species have been discovered in India, South America and in Europe, as far
back as the miocene stage. Had it not been for the rare accident of the
preservation of footsteps in the new red sandstone of the United States,
who would have ventured to suppose that no less than at least thirty
different bird-like animals, some of gigantic size, existed during that
period? Not a fragment of bone has been discovered in these beds. Not long
ago, palaeontologists maintained that the whole class of birds came
suddenly into existence during the eocene period; but now we know, on the
authority of Professor Owen, that a bird certainly lived during the
deposition of the upper greensand; and still more recently, that strange
bird, the Archeopteryx, with a long lizard-like tail, bearing a pair of
feathers on each joint, and with its wings furnished with two free claws,
has been discovered in the oolitic slates of Solenhofen. Hardly any recent
discovery shows more forcibly than this how little we as yet know of the
former inhabitants of the world.

I may give another instance, which, from having passed under my own eyes
has much struck me. In a memoir on Fossil Sessile Cirripedes, I stated
that, from the large number of existing and extinct tertiary species; from
the extraordinary abundance of the individuals of many species all over
the world, from the Arctic regions to the equator, inhabiting various
zones of depths, from the upper tidal limits to fifty fathoms; from the
perfect manner in which specimens are preserved in the oldest tertiary
beds; from the ease with which even a fragment of a valve can be
recognised; from all these circumstances, I inferred that, had sessile
cirripedes existed during the secondary periods, they would certainly have
been preserved and discovered; and as not one species had then been
discovered in beds of this age, I concluded that this great group had been
suddenly developed at the commencement of the tertiary series. This was a
sore trouble to me, adding, as I then thought, one more instance of the
abrupt appearance of a great group of species. But my work had hardly been
published, when a skilful palaeontologist, M. Bosquet, sent me a drawing
of a perfect specimen of an unmistakable sessile cirripede, which he had
himself extracted from the chalk of Belgium. And, as if to make the case
as striking as possible, this cirripede was a Chthamalus, a very common,
large, and ubiquitous genus, of which not one species has as yet been
found even in any tertiary stratum. Still more recently, a Pyrgoma, a
member of a distinct subfamily of sessile cirripedes, has been discovered
by Mr. Woodward in the upper chalk; so that we now have abundant evidence
of the existence of this group of animals during the secondary period.

The case most frequently insisted on by palaeontologists of the apparently
sudden appearance of a whole group of species, is that of the teleostean
fishes, low down, according to Agassiz, in the Chalk period. This group
includes the large majority of existing species. But certain Jurassic and
Triassic forms are now commonly admitted to be teleostean; and even some
palaeozoic forms have thus been classed by one high authority. If the
teleosteans had really appeared suddenly in the northern hemisphere at the
commencement of the chalk formation, the fact would have been highly
remarkable; but it would not have formed an insuperable difficulty, unless
it could likewise have been shown that at the same period the species were
suddenly and simultaneously developed in other quarters of the world. It
is almost superfluous to remark that hardly any fossil-fish are known from
south of the equator; and by running through Pictet's Palaeontology it
will be seen that very few species are known from several formations in
Europe. Some few families of fish now have a confined range; the
teleostean fishes might formerly have had a similarly confined range, and
after having been largely developed in some one sea, have spread widely.
Nor have we any right to suppose that the seas of the world have always
been so freely open from south to north as they are at present. Even at
this day, if the Malay Archipelago were converted into land, the tropical
parts of the Indian Ocean would form a large and perfectly enclosed basin,
in which any great group of marine animals might be multiplied; and here
they would remain confined, until some of the species became adapted to a
cooler climate, and were enabled to double the southern capes of Africa or
Australia, and thus reach other and distant seas.

From these considerations, from our ignorance of the geology of other
countries beyond the confines of Europe and the United States, and from
the revolution in our palaeontological knowledge effected by the
discoveries of the last dozen years, it seems to me to be about as rash to
dogmatize on the succession of organic forms throughout the world, as it
would be for a naturalist to land for five minutes on a barren point in
Australia, and then to discuss the number and range of its productions.

ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES IN THE LOWEST KNOWN
FOSSILIFEROUS STRATA.

There is another and allied difficulty, which is much more serious. I
allude to the manner in which species belonging to several of the main
divisions of the animal kingdom suddenly appear in the lowest known
fossiliferous rocks. Most of the arguments which have convinced me that
all the existing species of the same group are descended from a single
progenitor, apply with equal force to the earliest known species. For
instance, it cannot be doubted that all the Cambrian and Silurian
trilobites are descended from some one crustacean, which must have lived
long before the Cambrian age, and which probably differed greatly from any
known animal. Some of the most ancient animals, as the Nautilus, Lingula,
etc., do not differ much from living species; and it cannot on our theory
be supposed, that these old species were the progenitors of all the
species belonging to the same groups which have subsequently appeared, for
they are not in any degree intermediate in character.

Consequently, if the theory be true, it is indisputable that before the
lowest Cambrian stratum was deposited long periods elapsed, as long as, or
probably far longer than, the whole interval from the Cambrian age to the
present day; and that during these vast periods the world swarmed with
living creatures. Here we encounter a formidable objection; for it seems
doubtful whether the earth, in a fit state for the habitation of living
creatures, has lasted long enough. Sir W. Thompson concludes that the
consolidation of the crust can hardly have occurred less than twenty or
more than four hundred million years ago, but probably not less than
ninety-eight or more than two hundred million years. These very wide
limits show how doubtful the data are; and other elements may have
hereafter to be introduced into the problem. Mr. Croll estimates that
about sixty million years have elapsed since the Cambrian period, but
this, judging from the small amount of organic change since the
commencement of the Glacial epoch, appears a very short time for the many
and great mutations of life, which have certainly occurred since the
Cambrian formation; and the previous one hundred and forty million years
can hardly be considered as sufficient for the development of the varied
forms of life which already existed during the Cambrian period. It is,
however, probable, as Sir William Thompson insists, that the world at a
very early period was subjected to more rapid and violent changes in its
physical conditions than those now occurring; and such changes would have
tended to induce changes at a corresponding rate in the organisms which
then existed.

To the question why we do not find rich fossiliferous deposits belonging
to these assumed earliest periods prior to the Cambrian system, I can give
no satisfactory answer. Several eminent geologists, with Sir R. Murchison
at their head, were until recently convinced that we beheld in the organic
remains of the lowest Silurian stratum the first dawn of life. Other
highly competent judges, as Lyell and E. Forbes, have disputed this
conclusion. We should not forget that only a small portion of the world is
known with accuracy. Not very long ago M. Barrande added another and lower
stage, abounding with new and peculiar species, beneath the then known
Silurian system; and now, still lower down in the Lower Cambrian
formation, Mr Hicks has found South Wales beds rich in trilobites, and
containing various molluscs and annelids. The presence of phosphatic
nodules and bituminous matter, even in some of the lowest azotic rocks,
probably indicates life at these periods; and the existence of the Eozoon
in the Laurentian formation of Canada is generally admitted. There are
three great series of strata beneath the Silurian system in Canada, in the
lowest of which the Eozoon is found. Sir W. Logan states that their
"united thickness may possibly far surpass that of all the succeeding
rocks, from the base of the palaeozoic series to the present time. We are
thus carried back to a period so remote, that the appearance of the
so-called primordial fauna (of Barrande) may by some be considered as a
comparatively modern event." The Eozoon belongs to the most lowly
organised of all classes of animals, but is highly organised for its
class; it existed in countless numbers, and, as Dr. Dawson has remarked,
certainly preyed on other minute organic beings, which must have lived in
great numbers. Thus the words, which I wrote in 1859, about the existence
of living beings long before the Cambrian period, and which are almost the
same with those since used by Sir W. Logan, have proved true.
Nevertheless, the difficulty of assigning any good reason for the absence
of vast piles of strata rich in fossils beneath the Cambrian system is
very great. It does not seem probable that the most ancient beds have been
quite worn away by denudation, or that their fossils have been wholly
obliterated by metamorphic action, for if this had been the case we should
have found only small remnants of the formations next succeeding them in
age, and these would always have existed in a partially metamorphosed
condition. But the descriptions which we possess of the Silurian deposits
over immense territories in Russia and in North America, do not support
the view that the older a formation is the more invariably it has suffered
extreme denudation and metamorphism.

The case at present must remain inexplicable; and may be truly urged as a
valid argument against the views here entertained. To show that it may
hereafter receive some explanation, I will give the following hypothesis.
From the nature of the organic remains which do not appear to have
inhabited profound depths, in the several formations of Europe and of the
United States; and from the amount of sediment, miles in thickness, of
which the formations are composed, we may infer that from first to last
large islands or tracts of land, whence the sediment was derived, occurred
in the neighbourhood of the now existing continents of Europe and North
America. This same view has since been maintained by Agassiz and others.
But we do not know what was the state of things in the intervals between
the several successive formations; whether Europe and the United States
during these intervals existed as dry land, or as a submarine surface near
land, on which sediment was not deposited, or as the bed of an open and
unfathomable sea.

Looking to the existing oceans, which are thrice as extensive as the land,
we see them studded with many islands; but hardly one truly oceanic island
(with the exception of New Zealand, if this can be called a truly oceanic
island) is as yet known to afford even a remnant of any palaeozoic or
secondary formation. Hence, we may perhaps infer, that during the
palaeozoic and secondary periods, neither continents nor continental
islands existed where our oceans now extend; for had they existed,
palaeozoic and secondary formations would in all probability have been
accumulated from sediment derived from their wear and tear; and would have
been at least partially upheaved by the oscillations of level, which must
have intervened during these enormously long periods. If, then, we may
infer anything from these facts, we may infer that, where our oceans now
extend, oceans have extended from the remotest period of which we have any
record; and on the other hand, that where continents now exist, large
tracts of land have existed, subjected, no doubt, to great oscillations of
level, since the Cambrian period. The coloured map appended to my volume
on Coral Reefs, led me to conclude that the great oceans are still mainly
areas of subsidence, the great archipelagoes still areas of oscillations
of level, and the continents areas of elevation. But we have no reason to
assume that things have thus remained from the beginning of the world. Our
continents seem to have been formed by a preponderance, during many
oscillations of level, of the force of elevation. But may not the areas of
preponderant movement have changed in the lapse of ages? At a period long
antecedent to the Cambrian epoch, continents may have existed where oceans
are now spread out, and clear and open oceans may have existed where our
continents now stand. Nor should we be justified in assuming that if, for
instance, the bed of the Pacific Ocean were now converted into a continent
we should there find sedimentary formations, in recognisable condition,
older than the Cambrian strata, supposing such to have been formerly
deposited; for it might well happen that strata which had subsided some
miles nearer to the centre of the earth, and which had been pressed on by
an enormous weight of superincumbent water, might have undergone far more
metamorphic action than strata which have always remained nearer to the
surface. The immense areas in some parts of the world, for instance in
South America, of naked metamorphic rocks, which must have been heated
under great pressure, have always seemed to me to require some special
explanation; and we may perhaps believe that we see in these large areas
the many formations long anterior to the Cambrian epoch in a completely
metamorphosed and denuded condition.

The several difficulties here discussed, namely, that, though we find in
our geological formations many links between the species which now exist
and which formerly existed, we do not find infinitely numerous fine
transitional forms closely joining them all together. The sudden manner in
which several groups of species first appear in our European formations,
the almost entire absence, as at present known, of formations rich in
fossils beneath the Cambrian strata, are all undoubtedly of the most
serious nature. We see this in the fact that the most eminent
palaeontologists, namely, Cuvier, Agassiz, Barrande, Pictet, Falconer, E.
Forbes, etc., and all our greatest geologists, as Lyell, Murchison,
Sedgwick, etc., have unanimously, often vehemently, maintained the
immutability of species. But Sir Charles Lyell now gives the support of
his high authority to the opposite side, and most geologists and
palaeontologists are much shaken in their former belief. Those who believe
that the geological record is in any degree perfect, will undoubtedly at
once reject my theory. For my part, following out Lyell's metaphor, I look
at the geological record as a history of the world imperfectly kept and
written in a changing dialect. Of this history we possess the last volume
alone, relating only to two or three countries. Of this volume, only here
and there a short chapter has been preserved, and of each page, only here
and there a few lines. Each word of the slowly-changing language, more or
less different in the successive chapters, may represent the forms of
life, which are entombed in our consecutive formations, and which falsely
appear to have been abruptly introduced. On this view the difficulties
above discussed are greatly diminished or even disappear.



CHAPTER XI. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.


 On the slow and successive appearance of new species—On their different
 rates of change—Species once lost do not reappear—Groups of species
 follow the same general rules in their appearance and disappearance as
 do single species—On extinction—On simultaneous changes in the forms
 of life throughout the world—On the affinities of extinct species to
 each other and to living species—On the state of development of
 ancient forms—On the succession of the same types within the same
 areas—Summary of preceding and present chapters.

Let us now see whether the several facts and laws relating to the
geological succession of organic beings accord best with the common view
of the immutability of species, or with that of their slow and gradual
modification, through variation and natural selection.

New species have appeared very slowly, one after another, both on the land
and in the waters. Lyell has shown that it is hardly possible to resist
the evidence on this head in the case of the several tertiary stages; and
every year tends to fill up the blanks between the stages, and to make the
proportion between the lost and existing forms more gradual. In some of
the most recent beds, though undoubtedly of high antiquity if measured by
years, only one or two species are extinct, and only one or two are new,
having appeared there for the first time, either locally, or, as far as we
know, on the face of the earth. The secondary formations are more broken;
but, as Bronn has remarked, neither the appearance nor disappearance of
the many species embedded in each formation has been simultaneous.

Species belonging to different genera and classes have not changed at the
same rate, or in the same degree. In the older tertiary beds a few living
shells may still be found in the midst of a multitude of extinct forms.
Falconer has given a striking instance of a similar fact, for an existing
crocodile is associated with many lost mammals and reptiles in the
sub-Himalayan deposits. The Silurian Lingula differs but little from the
living species of this genus; whereas most of the other Silurian Molluscs
and all the Crustaceans have changed greatly. The productions of the land
seem to have changed at a quicker rate than those of the sea, of which a
striking instance has been observed in Switzerland. There is some reason
to believe that organisms high in the scale, change more quickly than
those that are low: though there are exceptions to this rule. The amount
of organic change, as Pictet has remarked, is not the same in each
successive so-called formation. Yet if we compare any but the most closely
related formations, all the species will be found to have undergone some
change. When a species has once disappeared from the face of the earth, we
have no reason to believe that the same identical form ever reappears. The
strongest apparent exception to this latter rule is that of the so-called
"colonies" of M. Barrande, which intrude for a period in the midst of an
older formation, and then allow the pre-existing fauna to reappear; but
Lyell's explanation, namely, that it is a case of temporary migration from
a distinct geographical province, seems satisfactory.

These several facts accord well with our theory, which includes no fixed
law of development, causing all the inhabitants of an area to change
abruptly, or simultaneously, or to an equal degree. The process of
modification must be slow, and will generally affect only a few species at
the same time; for the variability of each species is independent of that
of all others. Whether such variations or individual differences as may
arise will be accumulated through natural selection in a greater or less
degree, thus causing a greater or less amount of permanent modification,
will depend on many complex contingencies—on the variations being of
a beneficial nature, on the freedom of intercrossing, on the slowly
changing physical conditions of the country, on the immigration of new
colonists, and on the nature of the other inhabitants with which the
varying species come into competition. Hence it is by no means surprising
that one species should retain the same identical form much longer than
others; or, if changing, should change in a less degree. We find similar
relations between the existing inhabitants of distinct countries; for
instance, the land-shells and coleopterous insects of Madeira have come to
differ considerably from their nearest allies on the continent of Europe,
whereas the marine shells and birds have remained unaltered. We can
perhaps understand the apparently quicker rate of change in terrestrial
and in more highly organised productions compared with marine and lower
productions, by the more complex relations of the higher beings to their
organic and inorganic conditions of life, as explained in a former
chapter. When many of the inhabitants of any area have become modified and
improved, we can understand, on the principle of competition, and from the
all-important relations of organism to organism in the struggle for life,
that any form which did not become in some degree modified and improved,
would be liable to extermination. Hence, we see why all the species in the
same region do at last, if we look to long enough intervals of time,
become modified; for otherwise they would become extinct.

In members of the same class the average amount of change, during long and
equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of enduring formations, rich in fossils, depends on great
masses of sediment being deposited on subsiding areas, our formations have
been almost necessarily accumulated at wide and irregularly intermittent
intervals of time; consequently the amount of organic change exhibited by
the fossils embedded in consecutive formations is not equal. Each
formation, on this view, does not mark a new and complete act of creation,
but only an occasional scene, taken almost at hazard, in an ever slowly
changing drama.

We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and inorganic,
should recur. For though the offspring of one species might be adapted
(and no doubt this has occurred in innumerable instances) to fill the
place of another species in the economy of nature, and thus supplant it;
yet the two forms—the old and the new—would not be identically
the same; for both would almost certainly inherit different characters
from their distinct progenitors; and organisms already differing would
vary in a different manner. For instance, it is possible, if all our
fantail-pigeons were destroyed, that fanciers might make a new breed
hardly distinguishable from the present breed; but if the parent
rock-pigeon were likewise destroyed, and under nature we have every reason
to believe that parent forms are generally supplanted and exterminated by
their improved offspring, it is incredible that a fantail, identical with
the existing breed, could be raised from any other species of pigeon, or
even from any other well established race of the domestic pigeon, for the
successive variations would almost certainly be in some degree different,
and the newly-formed variety would probably inherit from its progenitor
some characteristic differences.

Groups of species, that is, genera and families, follow the same general
rules in their appearance and disappearance as do single species, changing
more or less quickly, and in a greater or lesser degree. A group, when it
has once disappeared, never reappears; that is, its existence, as long as
it lasts, is continuous. I am aware that there are some apparent
exceptions to this rule, but the exceptions are surprisingly few, so few
that E. Forbes, Pictet, and Woodward (though all strongly opposed to such
views as I maintain) admit its truth; and the rule strictly accords with
the theory. For all the species of the same group, however long it may
have lasted, are the modified descendants one from the other, and all from
a common progenitor. In the genus Lingula, for instance, the species which
have successively appeared at all ages must have been connected by an
unbroken series of generations, from the lowest Silurian stratum to the
present day.

We have seen in the last chapter that whole groups of species sometimes
falsely appear to have been abruptly developed; and I have attempted to
give an explanation of this fact, which if true would be fatal to my
views. But such cases are certainly exceptional; the general rule being a
gradual increase in number, until the group reaches its maximum, and then,
sooner or later, a gradual decrease. If the number of the species included
within a genus, or the number of the genera within a family, be
represented by a vertical line of varying thickness, ascending through the
successive geological formations, in which the species are found, the line
will sometimes falsely appear to begin at its lower end, not in a sharp
point, but abruptly; it then gradually thickens upwards, often keeping of
equal thickness for a space, and ultimately thins out in the upper beds,
marking the decrease and final extinction of the species. This gradual
increase in number of the species of a group is strictly conformable with
the theory; for the species of the same genus, and the genera of the same
family, can increase only slowly and progressively; the process of
modification and the production of a number of allied forms necessarily
being a slow and gradual process, one species first giving rise to two or
three varieties, these being slowly converted into species, which in their
turn produce by equally slow steps other varieties and species, and so on,
like the branching of a great tree from a single stem, till the group
becomes large.

ON EXTINCTION.

We have as yet only spoken incidentally of the disappearance of species
and of groups of species. On the theory of natural selection, the
extinction of old forms and the production of new and improved forms are
intimately connected together. The old notion of all the inhabitants of
the earth having been swept away by catastrophes at successive periods is
very generally given up, even by those geologists, as Elie de Beaumont,
Murchison, Barrande, etc., whose general views would naturally lead them
to this conclusion. On the contrary, we have every reason to believe, from
the study of the tertiary formations, that species and groups of species
gradually disappear, one after another, first from one spot, then from
another, and finally from the world. In some few cases, however, as by the
breaking of an isthmus and the consequent irruption of a multitude of new
inhabitants into an adjoining sea, or by the final subsidence of an
island, the process of extinction may have been rapid. Both single species
and whole groups of species last for very unequal periods; some groups, as
we have seen, have endured from the earliest known dawn of life to the
present day; some have disappeared before the close of the palaeozoic
period. No fixed law seems to determine the length of time during which
any single species or any single genus endures. There is reason to believe
that the extinction of a whole group of species is generally a slower
process than their production: if their appearance and disappearance be
represented, as before, by a vertical line of varying thickness the line
is found to taper more gradually at its upper end, which marks the
progress of extermination, than at its lower end, which marks the first
appearance and the early increase in number of the species. In some cases,
however, the extermination of whole groups, as of ammonites, towards the
close of the secondary period, has been wonderfully sudden.

The extinction of species has been involved in the most gratuitous
mystery. Some authors have even supposed that, as the individual has a
definite length of life, so have species a definite duration. No one can
have marvelled more than I have done at the extinction of species. When I
found in La Plata the tooth of a horse embedded with the remains of
Mastodon, Megatherium, Toxodon and other extinct monsters, which all
co-existed with still living shells at a very late geological period, I
was filled with astonishment; for, seeing that the horse, since its
introduction by the Spaniards into South America, has run wild over the
whole country and has increased in numbers at an unparalleled rate, I
asked myself what could so recently have exterminated the former horse
under conditions of life apparently so favourable. But my astonishment was
groundless. Professor Owen soon perceived that the tooth, though so like
that of the existing horse, belonged to an extinct species. Had this horse
been still living, but in some degree rare, no naturalist would have felt
the least surprise at its rarity; for rarity is the attribute of a vast
number of species of all classes, in all countries. If we ask ourselves
why this or that species is rare, we answer that something is unfavourable
in its conditions of life; but what that something is, we can hardly ever
tell. On the supposition of the fossil horse still existing as a rare
species, we might have felt certain, from the analogy of all other
mammals, even of the slow-breeding elephant, and from the history of the
naturalisation of the domestic horse in South America, that under more
favourable conditions it would in a very few years have stocked the whole
continent. But we could not have told what the unfavourable conditions
were which checked its increase, whether some one or several
contingencies, and at what period of the horse's life, and in what degree
they severally acted. If the conditions had gone on, however slowly,
becoming less and less favourable, we assuredly should not have perceived
the fact, yet the fossil horse would certainly have become rarer and
rarer, and finally extinct—its place being seized on by some more
successful competitor.

It is most difficult always to remember that the increase of every living
creature is constantly being checked by unperceived hostile agencies; and
that these same unperceived agencies are amply sufficient to cause rarity,
and finally extinction. So little is this subject understood, that I have
heard surprise repeatedly expressed at such great monsters as the Mastodon
and the more ancient Dinosaurians having become extinct; as if mere bodily
strength gave victory in the battle of life. Mere size, on the contrary,
would in some cases determine, as has been remarked by Owen, quicker
extermination, from the greater amount of requisite food. Before man
inhabited India or Africa, some cause must have checked the continued
increase of the existing elephant. A highly capable judge, Dr. Falconer,
believes that it is chiefly insects which, from incessantly harassing and
weakening the elephant in India, check its increase; and this was Bruce's
conclusion with respect to the African elephant in Abyssinia. It is
certain that insects and blood-sucking bats determine the existence of the
larger naturalised quadrupeds in several parts of South America.

We see in many cases in the more recent tertiary formations that rarity
precedes extinction; and we know that this has been the progress of events
with those animals which have been exterminated, either locally or wholly,
through man's agency. I may repeat what I published in 1845, namely, that
to admit that species generally become rare before they become extinct—to
feel no surprise at the rarity of a species, and yet to marvel greatly
when the species ceases to exist, is much the same as to admit that
sickness in the individual is the forerunner of death—to feel no
surprise at sickness, but, when the sick man dies, to wonder and to
suspect that he died by some deed of violence.

The theory of natural selection is grounded on the belief that each new
variety and ultimately each new species, is produced and maintained by
having some advantage over those with which it comes into competition; and
the consequent extinction of less-favoured forms almost inevitably
follows. It is the same with our domestic productions: when a new and
slightly improved variety has been raised, it at first supplants the less
improved varieties in the same neighbourhood; when much improved it is
transported far and near, like our short-horn cattle, and takes the place
of other breeds in other countries. Thus the appearance of new forms and
the disappearance of old forms, both those naturally and artificially
produced, are bound together. In flourishing groups, the number of new
specific forms which have been produced within a given time has at some
periods probably been greater than the number of the old specific forms
which have been exterminated; but we know that species have not gone on
indefinitely increasing, at least during the later geological epochs, so
that, looking to later times, we may believe that the production of new
forms has caused the extinction of about the same number of old forms.

The competition will generally be most severe, as formerly explained and
illustrated by examples, between the forms which are most like each other
in all respects. Hence the improved and modified descendants of a species
will generally cause the extermination of the parent-species; and if many
new forms have been developed from any one species, the nearest allies of
that species, i.e. the species of the same genus, will be the most liable
to extermination. Thus, as I believe, a number of new species descended
from one species, that is a new genus, comes to supplant an old genus,
belonging to the same family. But it must often have happened that a new
species belonging to some one group has seized on the place occupied by a
species belonging to a distinct group, and thus have caused its
extermination. If many allied forms be developed from the successful
intruder, many will have to yield their places; and it will generally be
the allied forms, which will suffer from some inherited inferiority in
common. But whether it be species belonging to the same or to a distinct
class, which have yielded their places to other modified and improved
species, a few of the sufferers may often be preserved for a long time,
from being fitted to some peculiar line of life, or from inhabiting some
distant and isolated station, where they will have escaped severe
competition. For instance, some species of Trigonia, a great genus of
shells in the secondary formations, survive in the Australian seas; and a
few members of the great and almost extinct group of Ganoid fishes still
inhabit our fresh waters. Therefore, the utter extinction of a group is
generally, as we have seen, a slower process than its production.

With respect to the apparently sudden extermination of whole families or
orders, as of Trilobites at the close of the palaeozoic period, and of
Ammonites at the close of the secondary period, we must remember what has
been already said on the probable wide intervals of time between our
consecutive formations; and in these intervals there may have been much
slow extermination. Moreover, when, by sudden immigration or by unusually
rapid development, many species of a new group have taken possession of an
area, many of the older species will have been exterminated in a
correspondingly rapid manner; and the forms which thus yield their places
will commonly be allied, for they will partake of the same inferiority in
common.

Thus, as it seems to me, the manner in which single species and whole
groups of species become extinct accords well with the theory of natural
selection. We need not marvel at extinction; if we must marvel, let it be
at our presumption in imagining for a moment that we understand the many
complex contingencies on which the existence of each species depends. If
we forget for an instant that each species tends to increase inordinately,
and that some check is always in action, yet seldom perceived by us, the
whole economy of nature will be utterly obscured. Whenever we can
precisely say why this species is more abundant in individuals than that;
why this species and not another can be naturalised in a given country;
then, and not until then, we may justly feel surprise why we cannot
account for the extinction of any particular species or group of species.

ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THE WORLD.

Scarcely any palaeontological discovery is more striking than the fact
that the forms of life change almost simultaneously throughout the world.
Thus our European Chalk formation can be recognised in many distant
regions, under the most different climates, where not a fragment of the
mineral chalk itself can be found; namely, in North America, in equatorial
South America, in Tierra del Fuego, at the Cape of Good Hope, and in the
peninsula of India. For at these distant points, the organic remains in
certain beds present an unmistakable resemblance to those of the Chalk. It
is not that the same species are met with; for in some cases not one
species is identically the same, but they belong to the same families,
genera, and sections of genera, and sometimes are similarly characterised
in such trifling points as mere superficial sculpture. Moreover, other
forms, which are not found in the Chalk of Europe, but which occur in the
formations either above or below, occur in the same order at these distant
points of the world. In the several successive palaeozoic formations of
Russia, Western Europe and North America, a similar parallelism in the
forms of life has been observed by several authors; so it is, according to
Lyell, with the European and North American tertiary deposits. Even if the
few fossil species which are common to the Old and New Worlds were kept
wholly out of view, the general parallelism in the successive forms of
life, in the palaeozoic and tertiary stages, would still be manifest, and
the several formations could be easily correlated.

These observations, however, relate to the marine inhabitants of the
world: we have not sufficient data to judge whether the productions of the
land and of fresh water at distant points change in the same parallel
manner. We may doubt whether they have thus changed: if the Megatherium,
Mylodon, Macrauchenia, and Toxodon had been brought to Europe from La
Plata, without any information in regard to their geological position, no
one would have suspected that they had co-existed with sea-shells all
still living; but as these anomalous monsters co-existed with the Mastodon
and Horse, it might at least have been inferred that they had lived during
one of the later tertiary stages.

When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that this
expression relates to the same year, or even to the same century, or even
that it has a very strict geological sense; for if all the marine animals
now living in Europe, and all those that lived in Europe during the
pleistocene period (a very remote period as measured by years, including
the whole glacial epoch) were compared with those now existing in South
America or in Australia, the most skilful naturalist would hardly be able
to say whether the present or the pleistocene inhabitants of Europe
resembled most closely those of the southern hemisphere. So, again,
several highly competent observers maintain that the existing productions
of the United States are more closely related to those which lived in
Europe during certain late tertiary stages, than to the present
inhabitants of Europe; and if this be so, it is evident that fossiliferous
beds now deposited on the shores of North America would hereafter be
liable to be classed with somewhat older European beds. Nevertheless,
looking to a remotely future epoch, there can be little doubt that all the
more modern MARINE formations, namely, the upper pliocene, the pleistocene
and strictly modern beds of Europe, North and South America, and
Australia, from containing fossil remains in some degree allied, and from
not including those forms which are found only in the older underlying
deposits, would be correctly ranked as simultaneous in a geological sense.

The fact of the forms of life changing simultaneously in the above large
sense, at distant parts of the world, has greatly struck those admirable
observers, MM. de Verneuil and d'Archiac. After referring to the
parallelism of the palaeozoic forms of life in various parts of Europe,
they add, "If struck by this strange sequence, we turn our attention to
North America, and there discover a series of analogous phenomena, it will
appear certain that all these modifications of species, their extinction,
and the introduction of new ones, cannot be owing to mere changes in
marine currents or other causes more or less local and temporary, but
depend on general laws which govern the whole animal kingdom." M. Barrande
has made forcible remarks to precisely the same effect. It is, indeed,
quite futile to look to changes of currents, climate, or other physical
conditions, as the cause of these great mutations in the forms of life
throughout the world, under the most different climates. We must, as
Barrande has remarked, look to some special law. We shall see this more
clearly when we treat of the present distribution of organic beings, and
find how slight is the relation between the physical conditions of various
countries and the nature of their inhabitants.

This great fact of the parallel succession of the forms of life throughout
the world, is explicable on the theory of natural selection. New species
are formed by having some advantage over older forms; and the forms, which
are already dominant, or have some advantage over the other forms in their
own country, give birth to the greatest number of new varieties or
incipient species. We have distinct evidence on this head, in the plants
which are dominant, that is, which are commonest and most widely diffused,
producing the greatest number of new varieties. It is also natural that
the dominant, varying and far-spreading species, which have already
invaded, to a certain extent, the territories of other species, should be
those which would have the best chance of spreading still further, and of
giving rise in new countries to other new varieties and species. The
process of diffusion would often be very slow, depending on climatal and
geographical changes, on strange accidents, and on the gradual
acclimatization of new species to the various climates through which they
might have to pass, but in the course of time the dominant forms would
generally succeed in spreading and would ultimately prevail. The diffusion
would, it is probable, be slower with the terrestrial inhabitants of
distinct continents than with the marine inhabitants of the continuous
sea. We might therefore expect to find, as we do find, a less strict
degree of parallelism in the succession of the productions of the land
than with those of the sea.

Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the world,
accords well with the principle of new species having been formed by
dominant species spreading widely and varying; the new species thus
produced being themselves dominant, owing to their having had some
advantage over their already dominant parents, as well as over other
species; and again spreading, varying, and producing new forms. The old
forms which are beaten and which yield their places to the new and
victorious forms, will generally be allied in groups, from inheriting some
inferiority in common; and, therefore, as new and improved groups spread
throughout the world, old groups disappear from the world; and the
succession of forms everywhere tends to correspond both in their first
appearance and final disappearance.

There is one other remark connected with this subject worth making. I have
given my reasons for believing that most of our great formations, rich in
fossils, were deposited during periods of subsidence; and that blank
intervals of vast duration, as far as fossils are concerned, occurred
during the periods when the bed of the sea was either stationary or
rising, and likewise when sediment was not thrown down quickly enough to
embed and preserve organic remains. During these long and blank intervals
I suppose that the inhabitants of each region underwent a considerable
amount of modification and extinction, and that there was much migration
from other parts of the world. As we have reason to believe that large
areas are affected by the same movement, it is probable that strictly
contemporaneous formations have often been accumulated over very wide
spaces in the same quarter of the world; but we are very far from having
any right to conclude that this has invariably been the case, and that
large areas have invariably been affected by the same movements. When two
formations have been deposited in two regions during nearly, but not
exactly, the same period, we should find in both, from the causes
explained in the foregoing paragraphs, the same general succession in the
forms of life; but the species would not exactly correspond; for there
will have been a little more time in the one region than in the other for
modification, extinction, and immigration.

I suspect that cases of this nature occur in Europe. Mr. Prestwich, in his
admirable Memoirs on the eocene deposits of England and France, is able to
draw a close general parallelism between the successive stages in the two
countries; but when he compares certain stages in England with those in
France, although he finds in both a curious accordance in the numbers of
the species belonging to the same genera, yet the species themselves
differ in a manner very difficult to account for considering the proximity
of the two areas, unless, indeed, it be assumed that an isthmus separated
two seas inhabited by distinct, but contemporaneous faunas. Lyell has made
similar observations on some of the later tertiary formations. Barrande,
also, shows that there is a striking general parallelism in the successive
Silurian deposits of Bohemia and Scandinavia; nevertheless he finds a
surprising amount of difference in the species. If the several formations
in these regions have not been deposited during the same exact periods—a
formation in one region often corresponding with a blank interval in the
other—and if in both regions the species have gone on slowly
changing during the accumulation of the several formations and during the
long intervals of time between them; in this case the several formations
in the two regions could be arranged in the same order, in accordance with
the general succession of the forms of life, and the order would falsely
appear to be strictly parallel; nevertheless the species would not all be
the same in the apparently corresponding stages in the two regions.

ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, AND TO LIVING FORMS.

Let us now look to the mutual affinities of extinct and living species.
All fall into a few grand classes; and this fact is at once explained on
the principle of descent. The more ancient any form is, the more, as a
general rule, it differs from living forms. But, as Buckland long ago
remarked, extinct species can all be classed either in still existing
groups, or between them. That the extinct forms of life help to fill up
the intervals between existing genera, families, and orders, is certainly
true; but as this statement has often been ignored or even denied, it may
be well to make some remarks on this subject, and to give some instances.
If we confine our attention either to the living or to the extinct species
of the same class, the series is far less perfect than if we combine both
into one general system. In the writings of Professor Owen we continually
meet with the expression of generalised forms, as applied to extinct
animals; and in the writings of Agassiz, of prophetic or synthetic types;
and these terms imply that such forms are, in fact, intermediate or
connecting links. Another distinguished palaeontologist, M. Gaudry, has
shown in the most striking manner that many of the fossil mammals
discovered by him in Attica serve to break down the intervals between
existing genera. Cuvier ranked the Ruminants and Pachyderms as two of the
most distinct orders of mammals; but so many fossil links have been
disentombed that Owen has had to alter the whole classification, and has
placed certain Pachyderms in the same sub-order with ruminants; for
example, he dissolves by gradations the apparently wide interval between
the pig and the camel. The Ungulata or hoofed quadrupeds are now divided
into the even-toed or odd-toed divisions; but the Macrauchenia of South
America connects to a certain extent these two grand divisions. No one
will deny that the Hipparion is intermediate between the existing horse
and certain other ungulate forms. What a wonderful connecting link in the
chain of mammals is the Typotherium from South America, as the name given
to it by Professor Gervais expresses, and which cannot be placed in any
existing order. The Sirenia form a very distinct group of the mammals, and
one of the most remarkable peculiarities in existing dugong and lamentin
is the entire absence of hind limbs, without even a rudiment being left;
but the extinct Halitherium had, according to Professor Flower, an
ossified thigh-bone "articulated to a well-defined acetabulum in the
pelvis," and it thus makes some approach to ordinary hoofed quadrupeds, to
which the Sirenia are in other respects allied. The cetaceans or whales
are widely different from all other mammals, but the tertiary Zeuglodon
and Squalodon, which have been placed by some naturalists in an order by
themselves, are considered by Professor Huxley to be undoubtedly
cetaceans, "and to constitute connecting links with the aquatic
carnivora."

Even the wide interval between birds and reptiles has been shown by the
naturalist just quoted to be partially bridged over in the most unexpected
manner, on the one hand, by the ostrich and extinct Archeopteryx, and on
the other hand by the Compsognathus, one of the Dinosaurians—that
group which includes the most gigantic of all terrestrial reptiles.
Turning to the Invertebrata, Barrande asserts, a higher authority could
not be named, that he is every day taught that, although palaeozoic
animals can certainly be classed under existing groups, yet that at this
ancient period the groups were not so distinctly separated from each other
as they now are.

Some writers have objected to any extinct species, or group of species,
being considered as intermediate between any two living species, or groups
of species. If by this term it is meant that an extinct form is directly
intermediate in all its characters between two living forms or groups, the
objection is probably valid. But in a natural classification many fossil
species certainly stand between living species, and some extinct genera
between living genera, even between genera belonging to distinct families.
The most common case, especially with respect to very distinct groups,
such as fish and reptiles, seems to be that, supposing them to be
distinguished at the present day by a score of characters, the ancient
members are separated by a somewhat lesser number of characters, so that
the two groups formerly made a somewhat nearer approach to each other than
they now do.

It is a common belief that the more ancient a form is, by so much the more
it tends to connect by some of its characters groups now widely separated
from each other. This remark no doubt must be restricted to those groups
which have undergone much change in the course of geological ages; and it
would be difficult to prove the truth of the proposition, for every now
and then even a living animal, as the Lepidosiren, is discovered having
affinities directed towards very distinct groups. Yet if we compare the
older Reptiles and Batrachians, the older Fish, the older Cephalopods, and
the eocene Mammals, with the recent members of the same classes, we must
admit that there is truth in the remark.

Let us see how far these several facts and inferences accord with the
theory of descent with modification. As the subject is somewhat complex, I
must request the reader to turn to the diagram in the fourth chapter. We
may suppose that the numbered letters in italics represent genera, and the
dotted lines diverging from them the species in each genus. The diagram is
much too simple, too few genera and too few species being given, but this
is unimportant for us. The horizontal lines may represent successive
geological formations, and all the forms beneath the uppermost line may be
considered as extinct. The three existing genera, a14, q14, p14, will form
a small family; b14 and f14, a closely allied family or subfamily; and
o14, i14, m14, a third family. These three families, together with the
many extinct genera on the several lines of descent diverging from the
parent form (A) will form an order; for all will have inherited something
in common from their ancient progenitor. On the principle of the continued
tendency to divergence of character, which was formerly illustrated by
this diagram, the more recent any form is the more it will generally
differ from its ancient progenitor. Hence, we can understand the rule that
the most ancient fossils differ most from existing forms. We must not,
however, assume that divergence of character is a necessary contingency;
it depends solely on the descendants from a species being thus enabled to
seize on many and different places in the economy of nature. Therefore it
is quite possible, as we have seen in the case of some Silurian forms,
that a species might go on being slightly modified in relation to its
slightly altered conditions of life, and yet retain throughout a vast
period the same general characteristics. This is represented in the
diagram by the letter F14.

All the many forms, extinct and recent, descended from (A), make, as
before remarked, one order; and this order, from the continued effects of
extinction and divergence of character, has become divided into several
sub-families and families, some of which are supposed to have perished at
different periods, and some to have endured to the present day.

By looking at the diagram we can see that if many of the extinct forms
supposed to be embedded in the successive formations, were discovered at
several points low down in the series, the three existing families on the
uppermost line would be rendered less distinct from each other. If, for
instance, the genera a1, a5, a10, f8, m3, m6, m9, were disinterred, these
three families would be so closely linked together that they probably
would have to be united into one great family, in nearly the same manner
as has occurred with ruminants and certain pachyderms. Yet he who objected
to consider as intermediate the extinct genera, which thus link together
the living genera of three families, would be partly justified, for they
are intermediate, not directly, but only by a long and circuitous course
through many widely different forms. If many extinct forms were to be
discovered above one of the middle horizontal lines or geological
formations—for instance, above No. VI.—but none from beneath
this line, then only two of the families (those on the left hand a14,
etc., and b14, etc.) would have to be united into one; and there would
remain two families which would be less distinct from each other than they
were before the discovery of the fossils. So again, if the three families
formed of eight genera (a14 to m14), on the uppermost line, be supposed to
differ from each other by half-a-dozen important characters, then the
families which existed at a period marked VI would certainly have differed
from each other by a less number of characters; for they would at this
early stage of descent have diverged in a less degree from their common
progenitor. Thus it comes that ancient and extinct genera are often in a
greater or less degree intermediate in character between their modified
descendants, or between their collateral relations.

Under nature the process will be far more complicated than is represented
in the diagram; for the groups will have been more numerous; they will
have endured for extremely unequal lengths of time, and will have been
modified in various degrees. As we possess only the last volume of the
geological record, and that in a very broken condition, we have no right
to expect, except in rare cases, to fill up the wide intervals in the
natural system, and thus to unite distinct families or orders. All that we
have a right to expect is, that those groups which have, within known
geological periods, undergone much modification, should in the older
formations make some slight approach to each other; so that the older
members should differ less from each other in some of their characters
than do the existing members of the same groups; and this by the
concurrent evidence of our best palaeontologists is frequently the case.

Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each
other and to living forms, are explained in a satisfactory manner. And
they are wholly inexplicable on any other view.

On this same theory, it is evident that the fauna during any one great
period in the earth's history will be intermediate in general character
between that which preceded and that which succeeded it. Thus the species
which lived at the sixth great stage of descent in the diagram are the
modified offspring of those which lived at the fifth stage, and are the
parents of those which became still more modified at the seventh stage;
hence they could hardly fail to be nearly intermediate in character
between the forms of life above and below. We must, however, allow for the
entire extinction of some preceding forms, and in any one region for the
immigration of new forms from other regions, and for a large amount of
modification during the long and blank intervals between the successive
formations. Subject to these allowances, the fauna of each geological
period undoubtedly is intermediate in character, between the preceding and
succeeding faunas. I need give only one instance, namely, the manner in
which the fossils of the Devonian system, when this system was first
discovered, were at once recognised by palaeontologists as intermediate in
character between those of the overlying carboniferous and underlying
Silurian systems. But each fauna is not necessarily exactly intermediate,
as unequal intervals of time have elapsed between consecutive formations.

It is no real objection to the truth of the statement that the fauna of
each period as a whole is nearly intermediate in character between the
preceding and succeeding faunas, that certain genera offer exceptions to
the rule. For instance, the species of mastodons and elephants, when
arranged by Dr. Falconer in two series—in the first place according
to their mutual affinities, and in the second place according to their
periods of existence—do not accord in arrangement. The species
extreme in character are not the oldest or the most recent; nor are those
which are intermediate in character, intermediate in age. But supposing
for an instant, in this and other such cases, that the record of the first
appearance and disappearance of the species was complete, which is far
from the case, we have no reason to believe that forms successively
produced necessarily endure for corresponding lengths of time. A very
ancient form may occasionally have lasted much longer than a form
elsewhere subsequently produced, especially in the case of terrestrial
productions inhabiting separated districts. To compare small things with
great; if the principal living and extinct races of the domestic pigeon
were arranged in serial affinity, this arrangement would not closely
accord with the order in time of their production, and even less with the
order of their disappearance; for the parent rock-pigeon still lives; and
many varieties between the rock-pigeon and the carrier have become
extinct; and carriers which are extreme in the important character of
length of beak originated earlier than short-beaked tumblers, which are at
the opposite end of the series in this respect.

Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is
the fact, insisted on by all palaeontologists, that fossils from two
consecutive formations are far more closely related to each other, than
are the fossils from two remote formations. Pictet gives as a well-known
instance, the general resemblance of the organic remains from the several
stages of the Chalk formation, though the species are distinct in each
stage. This fact alone, from its generality, seems to have shaken
Professor Pictet in his belief in the immutability of species. He who is
acquainted with the distribution of existing species over the globe, will
not attempt to account for the close resemblance of distinct species in
closely consecutive formations, by the physical conditions of the ancient
areas having remained nearly the same. Let it be remembered that the forms
of life, at least those inhabiting the sea, have changed almost
simultaneously throughout the world, and therefore under the most
different climates and conditions. Consider the prodigious vicissitudes of
climate during the pleistocene period, which includes the whole glacial
epoch, and note how little the specific forms of the inhabitants of the
sea have been affected.

On the theory of descent, the full meaning of the fossil remains from
closely consecutive formations, being closely related, though ranked as
distinct species, is obvious. As the accumulation of each formation has
often been interrupted, and as long blank intervals have intervened
between successive formations, we ought not to expect to find, as I
attempted to show in the last chapter, in any one or in any two
formations, all the intermediate varieties between the species which
appeared at the commencement and close of these periods: but we ought to
find after intervals, very long as measured by years, but only moderately
long as measured geologically, closely allied forms, or, as they have been
called by some authors, representative species; and these assuredly we do
find. We find, in short, such evidence of the slow and scarcely sensible
mutations of specific forms, as we have the right to expect.

ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED WITH LIVING FORMS.

We have seen in the fourth chapter that the degree of differentiation and
specialisation of the parts in organic beings, when arrived at maturity,
is the best standard, as yet suggested, of their degree of perfection or
highness. We have also seen that, as the specialisation of parts is an
advantage to each being, so natural selection will tend to render the
organisation of each being more specialised and perfect, and in this sense
higher; not but that it may leave many creatures with simple and
unimproved structures fitted for simple conditions of life, and in some
cases will even degrade or simplify the organisation, yet leaving such
degraded beings better fitted for their new walks of life. In another and
more general manner, new species become superior to their predecessors;
for they have to beat in the struggle for life all the older forms, with
which they come into close competition. We may therefore conclude that if
under a nearly similar climate the eocene inhabitants of the world could
be put into competition with the existing inhabitants, the former would be
beaten and exterminated by the latter, as would the secondary by the
eocene, and the palaeozoic by the secondary forms. So that by this
fundamental test of victory in the battle for life, as well as by the
standard of the specialisation of organs, modern forms ought, on the
theory of natural selection, to stand higher than ancient forms. Is this
the case? A large majority of palaeontologists would answer in the
affirmative; and it seems that this answer must be admitted as true,
though difficult of proof.

It is no valid objection to this conclusion, that certain Brachiopods have
been but slightly modified from an extremely remote geological epoch; and
that certain land and fresh-water shells have remained nearly the same,
from the time when, as far as is known, they first appeared. It is not an
insuperable difficulty that Foraminifera have not, as insisted on by Dr.
Carpenter, progressed in organisation since even the Laurentian epoch; for
some organisms would have to remain fitted for simple conditions of life,
and what could be better fitted for this end than these lowly organised
Protozoa? Such objections as the above would be fatal to my view, if it
included advance in organisation as a necessary contingent. They would
likewise be fatal, if the above Foraminifera, for instance, could be
proved to have first come into existence during the Laurentian epoch, or
the above Brachiopods during the Cambrian formation; for in this case,
there would not have been time sufficient for the development of these
organisms up to the standard which they had then reached. When advanced up
to any given point, there is no necessity, on the theory of natural
selection, for their further continued process; though they will, during
each successive age, have to be slightly modified, so as to hold their
places in relation to slight changes in their conditions. The foregoing
objections hinge on the question whether we really know how old the world
is, and at what period the various forms of life first appeared; and this
may well be disputed.

The problem whether organisation on the whole has advanced is in many ways
excessively intricate. The geological record, at all times imperfect, does
not extend far enough back to show with unmistakable clearness that within
the known history of the world organisation has largely advanced. Even at
the present day, looking to members of the same class, naturalists are not
unanimous which forms ought to be ranked as highest: thus, some look at
the selaceans or sharks, from their approach in some important points of
structure to reptiles, as the highest fish; others look at the teleosteans
as the highest. The ganoids stand intermediate between the selaceans and
teleosteans; the latter at the present day are largely preponderant in
number; but formerly selaceans and ganoids alone existed; and in this
case, according to the standard of highness chosen, so will it be said
that fishes have advanced or retrograded in organisation. To attempt to
compare members of distinct types in the scale of highness seems hopeless;
who will decide whether a cuttle-fish be higher than a bee—that
insect which the great Von Baer believed to be "in fact more highly
organised than a fish, although upon another type?" In the complex
struggle for life it is quite credible that crustaceans, not very high in
their own class, might beat cephalopods, the highest molluscs; and such
crustaceans, though not highly developed, would stand very high in the
scale of invertebrate animals, if judged by the most decisive of all
trials—the law of battle. Beside these inherent difficulties in
deciding which forms are the most advanced in organisation, we ought not
solely to compare the highest members of a class at any two periods—though
undoubtedly this is one and perhaps the most important element in striking
a balance—but we ought to compare all the members, high and low, at
two periods. At an ancient epoch the highest and lowest molluscoidal
animals, namely, cephalopods and brachiopods, swarmed in numbers; at the
present time both groups are greatly reduced, while others, intermediate
in organisation, have largely increased; consequently some naturalists
maintain that molluscs were formerly more highly developed than at
present; but a stronger case can be made out on the opposite side, by
considering the vast reduction of brachiopods, and the fact that our
existing cephalopods, though few in number, are more highly organised than
their ancient representatives. We ought also to compare the relative
proportional numbers, at any two periods, of the high and low classes
throughout the world: if, for instance, at the present day fifty thousand
kinds of vertebrate animals exist, and if we knew that at some former
period only ten thousand kinds existed, we ought to look at this increase
in number in the highest class, which implies a great displacement of
lower forms, as a decided advance in the organisation of the world. We
thus see how hopelessly difficult it is to compare with perfect fairness,
under such extremely complex relations, the standard of organisation of
the imperfectly-known faunas of successive periods.

We shall appreciate this difficulty more clearly by looking to certain
existing faunas and floras. From the extraordinary manner in which
European productions have recently spread over New Zealand, and have
seized on places which must have been previously occupied by the
indigenes, we must believe, that if all the animals and plants of Great
Britain were set free in New Zealand, a multitude of British forms would
in the course of time become thoroughly naturalized there, and would
exterminate many of the natives. On the other hand, from the fact that
hardly a single inhabitant of the southern hemisphere has become wild in
any part of Europe, we may well doubt whether, if all the productions of
New Zealand were set free in Great Britain, any considerable number would
be enabled to seize on places now occupied by our native plants and
animals. Under this point of view, the productions of Great Britain stand
much higher in the scale than those of New Zealand. Yet the most skilful
naturalist, from an examination of the species of the two countries, could
not have foreseen this result.

Agassiz and several other highly competent judges insist that ancient
animals resemble to a certain extent the embryos of recent animals
belonging to the same classes; and that the geological succession of
extinct forms is nearly parallel with the embryological development of
existing forms. This view accords admirably well with our theory. In a
future chapter I shall attempt to show that the adult differs from its
embryo, owing to variations having supervened at a not early age, and
having been inherited at a corresponding age. This process, whilst it
leaves the embryo almost unaltered, continually adds, in the course of
successive generations, more and more difference to the adult. Thus the
embryo comes to be left as a sort of picture, preserved by nature, of the
former and less modified condition of the species. This view may be true,
and yet may never be capable of proof. Seeing, for instance, that the
oldest known mammals, reptiles, and fishes strictly belong to their proper
classes, though some of these old forms are in a slight degree less
distinct from each other than are the typical members of the same groups
at the present day, it would be vain to look for animals having the common
embryological character of the Vertebrata, until beds rich in fossils are
discovered far beneath the lowest Cambrian strata—a discovery of
which the chance is small.

ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE
LATER TERTIARY PERIODS.

Mr. Clift many years ago showed that the fossil mammals from the
Australian caves were closely allied to the living marsupials of that
continent. In South America, a similar relationship is manifest, even to
an uneducated eye, in the gigantic pieces of armour, like those of the
armadillo, found in several parts of La Plata; and Professor Owen has
shown in the most striking manner that most of the fossil mammals, buried
there in such numbers, are related to South American types. This
relationship is even more clearly seen in the wonderful collection of
fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was so
much impressed with these facts that I strongly insisted, in 1839 and
1845, on this "law of the succession of types,"—on "this wonderful
relationship in the same continent between the dead and the living."
Professor Owen has subsequently extended the same generalisation to the
mammals of the Old World. We see the same law in this author's
restorations of the extinct and gigantic birds of New Zealand. We see it
also in the birds of the caves of Brazil. Mr. Woodward has shown that the
same law holds good with sea-shells, but, from the wide distribution of
most molluscs, it is not well displayed by them. Other cases could be
added, as the relation between the extinct and living land-shells of
Madeira; and between the extinct and living brackish water-shells of the
Aralo-Caspian Sea.

Now, what does this remarkable law of the succession of the same types
within the same areas mean? He would be a bold man who, after comparing
the present climate of Australia and of parts of South America, under the
same latitude, would attempt to account, on the one hand through
dissimilar physical conditions, for the dissimilarity of the inhabitants
of these two continents; and, on the other hand through similarity of
conditions, for the uniformity of the same types in each continent during
the later tertiary periods. Nor can it be pretended that it is an
immutable law that marsupials should have been chiefly or solely produced
in Australia; or that Edentata and other American types should have been
solely produced in South America. For we know that Europe in ancient times
was peopled by numerous marsupials; and I have shown in the publications
above alluded to, that in America the law of distribution of terrestrial
mammals was formerly different from what it now is. North America formerly
partook strongly of the present character of the southern half of the
continent; and the southern half was formerly more closely allied, than it
is at present, to the northern half. In a similar manner we know, from
Falconer and Cautley's discoveries, that Northern India was formerly more
closely related in its mammals to Africa than it is at the present time.
Analogous facts could be given in relation to the distribution of marine
animals.

On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the same
areas, is at once explained; for the inhabitants of each quarter of the
world will obviously tend to leave in that quarter, during the next
succeeding period of time, closely allied though in some degree modified
descendants. If the inhabitants of one continent formerly differed greatly
from those of another continent, so will their modified descendants still
differ in nearly the same manner and degree. But after very long intervals
of time, and after great geographical changes, permitting much
intermigration, the feebler will yield to the more dominant forms, and
there will be nothing immutable in the distribution of organic beings.

It may be asked in ridicule whether I suppose that the megatherium and
other allied huge monsters, which formerly lived in South America, have
left behind them the sloth, armadillo, and anteater, as their degenerate
descendants. This cannot for an instant be admitted. These huge animals
have become wholly extinct, and have left no progeny. But in the caves of
Brazil there are many extinct species which are closely allied in size and
in all other characters to the species still living in South America; and
some of these fossils may have been the actual progenitors of the living
species. It must not be forgotten that, on our theory, all the species of
the same genus are the descendants of some one species; so that, if six
genera, each having eight species, be found in one geological formation,
and in a succeeding formation there be six other allied or representative
genera, each with the same number of species, then we may conclude that
generally only one species of each of the older genera has left modified
descendants, which constitute the new genera containing the several
species; the other seven species of each old genus having died out and
left no progeny. Or, and this will be a far commoner case, two or three
species in two or three alone of the six older genera will be the parents
of the new genera: the other species and the other old genera having
become utterly extinct. In failing orders, with the genera and species
decreasing in numbers as is the case with the Edentata of South America,
still fewer genera and species will leave modified blood-descendants.

SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.

I have attempted to show that the geological record is extremely
imperfect; that only a small portion of the globe has been geologically
explored with care; that only certain classes of organic beings have been
largely preserved in a fossil state; that the number both of specimens and
of species, preserved in our museums, is absolutely as nothing compared
with the number of generations which must have passed away even during a
single formation; that, owing to subsidence being almost necessary for the
accumulation of deposits rich in fossil species of many kinds, and thick
enough to outlast future degradation, great intervals of time must have
elapsed between most of our successive formations; that there has probably
been more extinction during the periods of subsidence, and more variation
during the periods of elevation, and during the latter the record will
have been least perfectly kept; that each single formation has not been
continuously deposited; that the duration of each formation is probably
short compared with the average duration of specific forms; that migration
has played an important part in the first appearance of new forms in any
one area and formation; that widely ranging species are those which have
varied most frequently, and have oftenest given rise to new species; that
varieties have at first been local; and lastly, although each species must
have passed through numerous transitional stages, it is probable that the
periods, during which each underwent modification, though many and long as
measured by years, have been short in comparison with the periods during
which each remained in an unchanged condition. These causes, taken
conjointly, will to a large extent explain why—though we do find
many links—we do not find interminable varieties, connecting
together all extinct and existing forms by the finest graduated steps. It
should also be constantly borne in mind that any linking variety between
two forms, which might be found, would be ranked, unless the whole chain
could be perfectly restored, as a new and distinct species; for it is not
pretended that we have any sure criterion by which species and varieties
can be discriminated.

He who rejects this view of the imperfection of the geological record,
will rightly reject the whole theory. For he may ask in vain where are the
numberless transitional links which must formerly have connected the
closely allied or representative species, found in the successive stages
of the same great formation? He may disbelieve in the immense intervals of
time which must have elapsed between our consecutive formations; he may
overlook how important a part migration has played, when the formations of
any one great region, as those of Europe, are considered; he may urge the
apparent, but often falsely apparent, sudden coming in of whole groups of
species. He may ask where are the remains of those infinitely numerous
organisms which must have existed long before the Cambrian system was
deposited? We now know that at least one animal did then exist; but I can
answer this last question only by supposing that where our oceans now
extend they have extended for an enormous period, and where our
oscillating continents now stand they have stood since the commencement of
the Cambrian system; but that, long before that epoch, the world presented
a widely different aspect; and that the older continents, formed of
formations older than any known to us, exist now only as remnants in a
metamorphosed condition, or lie still buried under the ocean.

Passing from these difficulties, the other great leading facts in
palaeontology agree admirably with the theory of descent with modification
through variation and natural selection. We can thus understand how it is
that new species come in slowly and successively; how species of different
classes do not necessarily change together, or at the same rate, or in the
same degree; yet in the long run that all undergo modification to some
extent. The extinction of old forms is the almost inevitable consequence
of the production of new forms. We can understand why, when a species has
once disappeared, it never reappears. Groups of species increase in
numbers slowly, and endure for unequal periods of time; for the process of
modification is necessarily slow, and depends on many complex
contingencies. The dominant species belonging to large and dominant groups
tend to leave many modified descendants, which form new sub-groups and
groups. As these are formed, the species of the less vigorous groups, from
their inferiority inherited from a common progenitor, tend to become
extinct together, and to leave no modified offspring on the face of the
earth. But the utter extinction of a whole group of species has sometimes
been a slow process, from the survival of a few descendants, lingering in
protected and isolated situations. When a group has once wholly
disappeared, it does not reappear; for the link of generation has been
broken.

We can understand how it is that dominant forms which spread widely and
yield the greatest number of varieties tend to people the world with
allied, but modified, descendants; and these will generally succeed in
displacing the groups which are their inferiors in the struggle for
existence. Hence, after long intervals of time, the productions of the
world appear to have changed simultaneously.

We can understand how it is that all the forms of life, ancient and
recent, make together a few grand classes. We can understand, from the
continued tendency to divergence of character, why the more ancient a form
is, the more it generally differs from those now living. Why ancient and
extinct forms often tend to fill up gaps between existing forms, sometimes
blending two groups, previously classed as distinct into one; but more
commonly bringing them only a little closer together. The more ancient a
form is, the more often it stands in some degree intermediate between
groups now distinct; for the more ancient a form is, the more nearly it
will be related to, and consequently resemble, the common progenitor of
groups, since become widely divergent. Extinct forms are seldom directly
intermediate between existing forms; but are intermediate only by a long
and circuitous course through other extinct and different forms. We can
clearly see why the organic remains of closely consecutive formations are
closely allied; for they are closely linked together by generation. We can
clearly see why the remains of an intermediate formation are intermediate
in character.

The inhabitants of the world at each successive period in its history have
beaten their predecessors in the race for life, and are, in so far, higher
in the scale, and their structure has generally become more specialised;
and this may account for the common belief held by so many
palaeontologists, that organisation on the whole has progressed. Extinct
and ancient animals resemble to a certain extent the embryos of the more
recent animals belonging to the same classes, and this wonderful fact
receives a simple explanation according to our views. The succession of
the same types of structure within the same areas during the later
geological periods ceases to be mysterious, and is intelligible on the
principle of inheritance.

If, then, the geological record be as imperfect as many believe, and it
may at least be asserted that the record cannot be proved to be much more
perfect, the main objections to the theory of natural selection are
greatly diminished or disappear. On the other hand, all the chief laws of
palaeontology plainly proclaim, as it seems to me, that species have been
produced by ordinary generation: old forms having been supplanted by new
and improved forms of life, the products of variation and the survival of
the fittest.



CHAPTER XII. GEOGRAPHICAL DISTRIBUTION.


 Present distribution cannot be accounted for by differences in physical
 conditions—Importance of barriers—Affinity of the productions of the
 same continent—Centres of creation—Means of dispersal by changes of
 climate and of the level of the land, and by occasional means—Dispersal
 during the Glacial period—Alternate Glacial periods in the North and
 South.

In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the
similarity nor the dissimilarity of the inhabitants of various regions can
be wholly accounted for by climatal and other physical conditions. Of
late, almost every author who has studied the subject has come to this
conclusion. The case of America alone would almost suffice to prove its
truth; for if we exclude the arctic and northern temperate parts, all
authors agree that one of the most fundamental divisions in geographical
distribution is that between the New and Old Worlds; yet if we travel over
the vast American continent, from the central parts of the United States
to its extreme southern point, we meet with the most diversified
conditions; humid districts, arid deserts, lofty mountains, grassy plains,
forests, marshes, lakes and great rivers, under almost every temperature.
There is hardly a climate or condition in the Old World which cannot be
paralleled in the New—at least so closely as the same species
generally require. No doubt small areas can be pointed out in the Old
World hotter than any in the New World; but these are not inhabited by a
fauna different from that of the surrounding districts; for it is rare to
find a group of organisms confined to a small area, of which the
conditions are peculiar in only a slight degree. Notwithstanding this
general parallelism in the conditions of Old and New Worlds, how widely
different are their living productions!

In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latitudes 25
and 35 degrees, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas and
floras more utterly dissimilar. Or, again, we may compare the productions
of South America south of latitude 35 degrees with those north of 25
degrees, which consequently are separated by a space of ten degrees of
latitude, and are exposed to considerably different conditions; yet they
are incomparably more closely related to each other than they are to the
productions of Australia or Africa under nearly the same climate.
Analogous facts could be given with respect to the inhabitants of the sea.

A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the productions of
various regions. We see this in the great difference in nearly all the
terrestrial productions of the New and Old Worlds, excepting in the
northern parts, where the land almost joins, and where, under a slightly
different climate, there might have been free migration for the northern
temperate forms, as there now is for the strictly arctic productions. We
see the same fact in the great difference between the inhabitants of
Australia, Africa, and South America under the same latitude; for these
countries are almost as much isolated from each other as is possible. On
each continent, also, we see the same fact; for on the opposite sides of
lofty and continuous mountain-ranges, and of great deserts and even of
large rivers, we find different productions; though as mountain chains,
deserts, etc., are not as impassable, or likely to have endured so long,
as the oceans separating continents, the differences are very inferior in
degree to those characteristic of distinct continents.

Turning to the sea, we find the same law. The marine inhabitants of the
eastern and western shores of South America are very distinct, with
extremely few shells, crustacea, or echinodermata in common; but Dr.
Gunther has recently shown that about thirty per cent of the fishes are
the same on the opposite sides of the isthmus of Panama; and this fact has
led naturalists to believe that the isthmus was formerly open. Westward of
the shores of America, a wide space of open ocean extends, with not an
island as a halting-place for emigrants; here we have a barrier of another
kind, and as soon as this is passed we meet in the eastern islands of the
Pacific with another and totally distinct fauna. So that three marine
faunas range northward and southward in parallel lines not far from each
other, under corresponding climate; but from being separated from each
other by impassable barriers, either of land or open sea, they are almost
wholly distinct. On the other hand, proceeding still further westward from
the eastern islands of the tropical parts of the Pacific, we encounter no
impassable barriers, and we have innumerable islands as halting-places, or
continuous coasts, until, after travelling over a hemisphere, we come to
the shores of Africa; and over this vast space we meet with no
well-defined and distinct marine faunas. Although so few marine animals
are common to the above-named three approximate faunas of Eastern and
Western America and the eastern Pacific islands, yet many fishes range
from the Pacific into the Indian Ocean, and many shells are common to the
eastern islands of the Pacific and the eastern shores of Africa on almost
exactly opposite meridians of longitude.

A third great fact, partly included in the foregoing statement, is the
affinity of the productions of the same continent or of the same sea,
though the species themselves are distinct at different points and
stations. It is a law of the widest generality, and every continent offers
innumerable instances. Nevertheless, the naturalist, in travelling, for
instance, from north to south, never fails to be struck by the manner in
which successive groups of beings, specifically distinct, though nearly
related, replace each other. He hears from closely allied, yet distinct
kinds of birds, notes nearly similar, and sees their nests similarly
constructed, but not quite alike, with eggs coloured in nearly the same
manner. The plains near the Straits of Magellan are inhabited by one
species of Rhea (American ostrich), and northward the plains of La Plata
by another species of the same genus; and not by a true ostrich or emu,
like those inhabiting Africa and Australia under the same latitude. On
these same plains of La Plata we see the agouti and bizcacha, animals
having nearly the same habits as our hares and rabbits, and belonging to
the same order of Rodents, but they plainly display an American type of
structure. We ascend the lofty peaks of the Cordillera, and we find an
alpine species of bizcacha; we look to the waters, and we do not find the
beaver or muskrat, but the coypu and capybara, rodents of the South
American type. Innumerable other instances could be given. If we look to
the islands off the American shore, however much they may differ in
geological structure, the inhabitants are essentially American, though
they may be all peculiar species. We may look back to past ages, as shown
in the last chapter, and we find American types then prevailing on the
American continent and in the American seas. We see in these facts some
deep organic bond, throughout space and time, over the same areas of land
and water, independently of physical conditions. The naturalist must be
dull who is not led to inquire what this bond is.

The bond is simply inheritance, that cause which alone, as far as we
positively know, produces organisms quite like each other, or, as we see
in the case of varieties, nearly alike. The dissimilarity of the
inhabitants of different regions may be attributed to modification through
variation and natural selection, and probably in a subordinate degree to
the definite influence of different physical conditions. The degrees of
dissimilarity will depend on the migration of the more dominant forms of
life from one region into another having been more or less effectually
prevented, at periods more or less remote—on the nature and number
of the former immigrants—and on the action of the inhabitants on
each other in leading to the preservation of different modifications; the
relation of organism to organism in the struggle for life being, as I have
already often remarked, the most important of all relations. Thus the high
importance of barriers comes into play by checking migration; as does time
for the slow process of modification through natural selection.
Widely-ranging species, abounding in individuals, which have already
triumphed over many competitors in their own widely-extended homes, will
have the best chance of seizing on new places, when they spread out into
new countries. In their new homes they will be exposed to new conditions,
and will frequently undergo further modification and improvement; and thus
they will become still further victorious, and will produce groups of
modified descendants. On this principle of inheritance with modification
we can understand how it is that sections of genera, whole genera, and
even families, are confined to the same areas, as is so commonly and
notoriously the case.

There is no evidence, as was remarked in the last chapter, of the
existence of any law of necessary development. As the variability of each
species is an independent property, and will be taken advantage of by
natural selection, only so far as it profits each individual in its
complex struggle for life, so the amount of modification in different
species will be no uniform quantity. If a number of species, after having
long competed with each other in their old home, were to migrate in a body
into a new and afterwards isolated country, they would be little liable to
modification; for neither migration nor isolation in themselves effect
anything. These principles come into play only by bringing organisms into
new relations with each other and in a lesser degree with the surrounding
physical conditions. As we have seen in the last chapter that some forms
have retained nearly the same character from an enormously remote
geological period, so certain species have migrated over vast spaces, and
have not become greatly or at all modified.

According to these views, it is obvious that the several species of the
same genus, though inhabiting the most distant quarters of the world, must
originally have proceeded from the same source, as they are descended from
the same progenitor. In the case of those species which have undergone,
during whole geological periods, little modification, there is not much
difficulty in believing that they have migrated from the same region; for
during the vast geographical and climatical changes which have supervened
since ancient times, almost any amount of migration is possible. But in
many other cases, in which we have reason to believe that the species of a
genus have been produced within comparatively recent times, there is great
difficulty on this head. It is also obvious that the individuals of the
same species, though now inhabiting distant and isolated regions, must
have proceeded from one spot, where their parents were first produced:
for, as has been explained, it is incredible that individuals identically
the same should have been produced from parents specifically distinct.

SINGLE CENTRES OF SUPPOSED CREATION.

We are thus brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at one or more
points of the earth's surface. Undoubtedly there are many cases of extreme
difficulty in understanding how the same species could possibly have
migrated from some one point to the several distant and isolated points,
where now found. Nevertheless the simplicity of the view that each species
was first produced within a single region captivates the mind. He who
rejects it, rejects the vera causa of ordinary generation with subsequent
migration, and calls in the agency of a miracle. It is universally
admitted, that in most cases the area inhabited by a species is
continuous; and that when a plant or animal inhabits two points so distant
from each other, or with an interval of such a nature, that the space
could not have been easily passed over by migration, the fact is given as
something remarkable and exceptional. The incapacity of migrating across a
wide sea is more clear in the case of terrestrial mammals than perhaps
with any other organic beings; and, accordingly, we find no inexplicable
instances of the same mammals inhabiting distant points of the world. No
geologist feels any difficulty in Great Britain possessing the same
quadrupeds with the rest of Europe, for they were no doubt once united.
But if the same species can be produced at two separate points, why do we
not find a single mammal common to Europe and Australia or South America?
The conditions of life are nearly the same, so that a multitude of
European animals and plants have become naturalised in America and
Australia; and some of the aboriginal plants are identically the same at
these distant points of the northern and southern hemispheres? The answer,
as I believe, is, that mammals have not been able to migrate, whereas some
plants, from their varied means of dispersal, have migrated across the
wide and broken interspaces. The great and striking influence of barriers
of all kinds, is intelligible only on the view that the great majority of
species have been produced on one side, and have not been able to migrate
to the opposite side. Some few families, many subfamilies, very many
genera, a still greater number of sections of genera, are confined to a
single region; and it has been observed by several naturalists that the
most natural genera, or those genera in which the species are most closely
related to each other, are generally confined to the same country, or if
they have a wide range that their range is continuous. What a strange
anomaly it would be if a directly opposite rule were to prevail when we go
down one step lower in the series, namely to the individuals of the same
species, and these had not been, at least at first, confined to some one
region!

Hence, it seems to me, as it has to many other naturalists, that the view
of each species having been produced in one area alone, and having
subsequently migrated from that area as far as its powers of migration and
subsistence under past and present conditions permitted, is the most
probable. Undoubtedly many cases occur in which we cannot explain how the
same species could have passed from one point to the other. But the
geographical and climatical changes which have certainly occurred within
recent geological times, must have rendered discontinuous the formerly
continuous range of many species. So that we are reduced to consider
whether the exceptions to continuity of range are so numerous, and of so
grave a nature, that we ought to give up the belief, rendered probable by
general considerations, that each species has been produced within one
area, and has migrated thence as far as it could. It would be hopelessly
tedious to discuss all the exceptional cases of the same species, now
living at distant and separated points; nor do I for a moment pretend that
any explanation could be offered of many instances. But, after some
preliminary remarks, I will discuss a few of the most striking classes of
facts, namely, the existence of the same species on the summits of distant
mountain ranges, and at distant points in the Arctic and Antarctic
regions; and secondly (in the following chapter), the wide distribution of
fresh water productions; and thirdly, the occurrence of the same
terrestrial species on islands and on the nearest mainland, though
separated by hundreds of miles of open sea. If the existence of the same
species at distant and isolated points of the earth's surface can in many
instances be explained on the view of each species having migrated from a
single birthplace; then, considering our ignorance with respect to former
climatical and geographical changes, and to the various occasional means
of transport, the belief that a single birthplace is the law seems to me
incomparably the safest.

In discussing this subject we shall be enabled at the same time to
consider a point equally important for us, namely, whether the several
species of a genus which must on our theory all be descended from a common
progenitor, can have migrated, undergoing modification during their
migration from some one area. If, when most of the species inhabiting one
region are different from those of another region, though closely allied
to them, it can be shown that migration from the one region to the other
has probably occurred at some former period, our general view will be much
strengthened; for the explanation is obvious on the principle of descent
with modification. A volcanic island, for instance, upheaved and formed at
the distance of a few hundreds of miles from a continent, would probably
receive from it in the course of time a few colonists, and their
descendants, though modified, would still be related by inheritance to the
inhabitants of that continent. Cases of this nature are common, and are,
as we shall hereafter see, inexplicable on the theory of independent
creation. This view of the relation of the species of one region to those
of another, does not differ much from that advanced by Mr. Wallace, who
concludes that "every species has come into existence coincident both in
space and time with a pre-existing closely allied species." And it is now
well known that he attributes this coincidence to descent with
modification.

The question of single or multiple centres of creation differs from
another though allied question, namely, whether all the individuals of the
same species are descended from a single pair, or single hermaphrodite, or
whether, as some authors suppose, from many individuals simultaneously
created. With organic beings which never intercross, if such exist, each
species, must be descended from a succession of modified varieties, that
have supplanted each other, but have never blended with other individuals
or varieties of the same species, so that, at each successive stage of
modification, all the individuals of the same form will be descended from
a single parent. But in the great majority of cases, namely, with all
organisms which habitually unite for each birth, or which occasionally
intercross, the individuals of the same species inhabiting the same area
will be kept nearly uniform by intercrossing; so that many individuals
will go on simultaneously changing, and the whole amount of modification
at each stage will not be due to descent from a single parent. To
illustrate what I mean: our English race-horses differ from the horses of
every other breed; but they do not owe their difference and superiority to
descent from any single pair, but to continued care in the selecting and
training of many individuals during each generation.

Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of "single
centres of creation," I must say a few words on the means of dispersal.

MEANS OF DISPERSAL.

Sir C. Lyell and other authors have ably treated this subject. I can give
here only the briefest abstract of the more important facts. Change of
climate must have had a powerful influence on migration. A region now
impassable to certain organisms from the nature of its climate, might have
been a high road for migration, when the climate was different. I shall,
however, presently have to discuss this branch of the subject in some
detail. Changes of level in the land must also have been highly
influential: a narrow isthmus now separates two marine faunas; submerge
it, or let it formerly have been submerged, and the two faunas will now
blend together, or may formerly have blended. Where the sea now extends,
land may at a former period have connected islands or possibly even
continents together, and thus have allowed terrestrial productions to pass
from one to the other. No geologist disputes that great mutations of level
have occurred within the period of existing organisms. Edward Forbes
insisted that all the islands in the Atlantic must have been recently
connected with Europe or Africa, and Europe likewise with America. Other
authors have thus hypothetically bridged over every ocean, and united
almost every island with some mainland. If, indeed, the arguments used by
Forbes are to be trusted, it must be admitted that scarcely a single
island exists which has not recently been united to some continent. This
view cuts the Gordian knot of the dispersal of the same species to the
most distant points, and removes many a difficulty; but to the best of my
judgment we are not authorized in admitting such enormous geographical
changes within the period of existing species. It seems to me that we have
abundant evidence of great oscillations in the level of the land or sea;
but not of such vast changes in the position and extension of our
continents, as to have united them within the recent period to each other
and to the several intervening oceanic islands. I freely admit the former
existence of many islands, now buried beneath the sea, which may have
served as halting places for plants and for many animals during their
migration. In the coral-producing oceans such sunken islands are now
marked by rings of coral or atolls standing over them. Whenever it is
fully admitted, as it will some day be, that each species has proceeded
from a single birthplace, and when in the course of time we know something
definite about the means of distribution, we shall be enabled to speculate
with security on the former extension of the land. But I do not believe
that it will ever be proved that within the recent period most of our
continents which now stand quite separate, have been continuously, or
almost continuously united with each other, and with the many existing
oceanic islands. Several facts in distribution—such as the great
difference in the marine faunas on the opposite sides of almost every
continent—the close relation of the tertiary inhabitants of several
lands and even seas to their present inhabitants—the degree of
affinity between the mammals inhabiting islands with those of the nearest
continent, being in part determined (as we shall hereafter see) by the
depth of the intervening ocean—these and other such facts are
opposed to the admission of such prodigious geographical revolutions
within the recent period, as are necessary on the view advanced by Forbes
and admitted by his followers. The nature and relative proportions of the
inhabitants of oceanic islands are likewise opposed to the belief of their
former continuity of continents. Nor does the almost universally volcanic
composition of such islands favour the admission that they are the wrecks
of sunken continents; if they had originally existed as continental
mountain ranges, some at least of the islands would have been formed, like
other mountain summits, of granite, metamorphic schists, old fossiliferous
and other rocks, instead of consisting of mere piles of volcanic matter.

I must now say a few words on what are called accidental means, but which
more properly should be called occasional means of distribution. I shall
here confine myself to plants. In botanical works, this or that plant is
often stated to be ill adapted for wide dissemination; but the greater or
less facilities for transport across the sea may be said to be almost
wholly unknown. Until I tried, with Mr. Berkeley's aid, a few experiments,
it was not even known how far seeds could resist the injurious action of
sea-water. To my surprise I found that out of eighty-seven kinds,
sixty-four germinated after an immersion of twenty-eight days, and a few
survived an immersion of 137 days. It deserves notice that certain orders
were far more injured than others: nine Leguminosae were tried, and, with
one exception, they resisted the salt-water badly; seven species of the
allied orders, Hydrophyllaceae and Polemoniaceae, were all killed by a
month's immersion. For convenience sake I chiefly tried small seeds
without the capsules or fruit; and as all of these sank in a few days,
they could not have been floated across wide spaces of the sea, whether or
not they were injured by salt water. Afterwards I tried some larger
fruits, capsules, etc., and some of these floated for a long time. It is
well known what a difference there is in the buoyancy of green and
seasoned timber; and it occurred to me that floods would often wash into
the sea dried plants or branches with seed-capsules or fruit attached to
them. Hence I was led to dry the stems and branches of ninety-four plants
with ripe fruit, and to place them on sea-water. The majority sank
quickly, but some which, whilst green, floated for a very short time, when
dried floated much longer; for instance, ripe hazel-nuts sank immediately,
but when dried they floated for ninety days, and afterwards when planted
germinated; an asparagus plant with ripe berries floated for twenty-three
days, when dried it floated for eighty-five days, and the seeds afterwards
germinated: the ripe seeds of Helosciadium sank in two days, when dried
they floated for above ninety days, and afterwards germinated. Altogether,
out of the ninety-four dried plants, eighteen floated for above
twenty-eight days; and some of the eighteen floated for a very much longer
period. So that as 64/87 kinds of seeds germinated after an immersion of
twenty-eight days; and as 18/94 distinct species with ripe fruit (but not
all the same species as in the foregoing experiment) floated, after being
dried, for above twenty-eight days, we may conclude, as far as anything
can be inferred from these scanty facts, that the seeds of 14/100 kinds of
plants of any country might be floated by sea-currents during twenty-eight
days, and would retain their power of germination. In Johnston's Physical
Atlas, the average rate of the several Atlantic currents is thirty-three
miles per diem (some currents running at the rate of sixty miles per
diem); on this average, the seeds of 14/100 plants belonging to one
country might be floated across 924 miles of sea to another country; and
when stranded, if blown by an inland gale to a favourable spot, would
germinate.

Subsequently to my experiments, M. Martens tried similar ones, but in a
much better manner, for he placed the seeds in a box in the actual sea, so
that they were alternately wet and exposed to the air like really floating
plants. He tried ninety-eight seeds, mostly different from mine, but he
chose many large fruits, and likewise seeds, from plants which live near
the sea; and this would have favoured both the average length of their
flotation and their resistance to the injurious action of the salt-water.
On the other hand, he did not previously dry the plants or branches with
the fruit; and this, as we have seen, would have caused some of them to
have floated much longer. The result was that 18/98 of his seeds of
different kinds floated for forty-two days, and were then capable of
germination. But I do not doubt that plants exposed to the waves would
float for a less time than those protected from violent movement as in our
experiments. Therefore, it would perhaps be safer to assume that the seeds
of about 10/100 plants of a flora, after having been dried, could be
floated across a space of sea 900 miles in width, and would then
germinate. The fact of the larger fruits often floating longer than the
small, is interesting; as plants with large seeds or fruit which, as Alph.
de Candolle has shown, generally have restricted ranges, could hardly be
transported by any other means.

Seeds may be occasionally transported in another manner. Drift timber is
thrown up on most islands, even on those in the midst of the widest
oceans; and the natives of the coral islands in the Pacific procure stones
for their tools, solely from the roots of drifted trees, these stones
being a valuable royal tax. I find that when irregularly shaped stones are
embedded in the roots of trees, small parcels of earth are very frequently
enclosed in their interstices and behind them, so perfectly that not a
particle could be washed away during the longest transport: out of one
small portion of earth thus COMPLETELY enclosed by the roots of an oak
about fifty years old, three dicotyledonous plants germinated: I am
certain of the accuracy of this observation. Again, I can show that the
carcasses of birds, when floating on the sea, sometimes escape being
immediately devoured; and many kinds of seeds in the crops of floating
birds long retain their vitality: peas and vetches, for instance, are
killed by even a few days' immersion in sea-water; but some taken out of
the crop of a pigeon, which had floated on artificial sea-water for thirty
days, to my surprise nearly all germinated.

Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the ocean.
We may safely assume that under such circumstances their rate of flight
would often be thirty-five miles an hour; and some authors have given a
far higher estimate. I have never seen an instance of nutritious seeds
passing through the intestines of a bird; but hard seeds of fruit pass
uninjured through even the digestive organs of a turkey. In the course of
two months, I picked up in my garden twelve kinds of seeds, out of the
excrement of small birds, and these seemed perfect, and some of them,
which were tried, germinated. But the following fact is more important:
the crops of birds do not secrete gastric juice, and do not, as I know by
trial, injure in the least the germination of seeds; now, after a bird has
found and devoured a large supply of food, it is positively asserted that
all the grains do not pass into the gizzard for twelve or even eighteen
hours. A bird in this interval might easily be blown to the distance of
five hundred miles, and hawks are known to look out for tired birds, and
the contents of their torn crops might thus readily get scattered. Some
hawks and owls bolt their prey whole, and after an interval of from twelve
to twenty hours, disgorge pellets, which, as I know from experiments made
in the Zoological Gardens, include seeds capable of germination. Some
seeds of the oat, wheat, millet, canary, hemp, clover, and beet germinated
after having been from twelve to twenty-one hours in the stomachs of
different birds of prey; and two seeds of beet grew after having been thus
retained for two days and fourteen hours. Fresh-water fish, I find, eat
seeds of many land and water plants; fish are frequently devoured by
birds, and thus the seeds might be transported from place to place. I
forced many kinds of seeds into the stomachs of dead fish, and then gave
their bodies to fishing-eagles, storks, and pelicans; these birds, after
an interval of many hours, either rejected the seeds in pellets or passed
them in their excrement; and several of these seeds retained the power of
germination. Certain seeds, however, were always killed by this process.

Locusts are sometimes blown to great distances from the land. I myself
caught one 370 miles from the coast of Africa, and have heard of others
caught at greater distances. The Rev. R.T. Lowe informed Sir C. Lyell that
in November, 1844, swarms of locusts visited the island of Madeira. They
were in countless numbers, as thick as the flakes of snow in the heaviest
snowstorm, and extended upward as far as could be seen with a telescope.
During two or three days they slowly careered round and round in an
immense ellipse, at least five or six miles in diameter, and at night
alighted on the taller trees, which were completely coated with them. They
then disappeared over the sea, as suddenly as they had appeared, and have
not since visited the island. Now, in parts of Natal it is believed by
some farmers, though on insufficient evidence, that injurious seeds are
introduced into their grass-land in the dung left by the great flights of
locusts which often visit that country. In consequence of this belief Mr.
Weale sent me in a letter a small packet of the dried pellets, out of
which I extracted under the microscope several seeds, and raised from them
seven grass plants, belonging to two species, of two genera. Hence a swarm
of locusts, such as that which visited Madeira, might readily be the means
of introducing several kinds of plants into an island lying far from the
mainland.

Although the beaks and feet of birds are generally clean, earth sometimes
adheres to them: in one case I removed sixty-one grains, and in another
case twenty-two grains of dry argillaceous earth from the foot of a
partridge, and in the earth there was a pebble as large as the seed of a
vetch. Here is a better case: the leg of a woodcock was sent to me by a
friend, with a little cake of dry earth attached to the shank, weighing
only nine grains; and this contained a seed of the toad-rush (Juncus
bufonius) which germinated and flowered. Mr. Swaysland, of Brighton, who
during the last forty years has paid close attention to our migratory
birds, informs me that he has often shot wagtails (Motacillae), wheatears,
and whinchats (Saxicolae), on their first arrival on our shores, before
they had alighted; and he has several times noticed little cakes of earth
attached to their feet. Many facts could be given showing how generally
soil is charged with seeds. For instance, Professor Newton sent me the leg
of a red-legged partridge (Caccabis rufa) which had been wounded and could
not fly, with a ball of hard earth adhering to it, and weighing six and a
half ounces. The earth had been kept for three years, but when broken,
watered and placed under a bell glass, no less than eighty-two plants
sprung from it: these consisted of twelve monocotyledons, including the
common oat, and at least one kind of grass, and of seventy dicotyledons,
which consisted, judging from the young leaves, of at least three distinct
species. With such facts before us, can we doubt that the many birds which
are annually blown by gales across great spaces of ocean, and which
annually migrate—for instance, the millions of quails across the
Mediterranean—must occasionally transport a few seeds embedded in
dirt adhering to their feet or beaks? But I shall have to recur to this
subject.

As icebergs are known to be sometimes loaded with earth and stones, and
have even carried brushwood, bones, and the nest of a land-bird, it can
hardly be doubted that they must occasionally, as suggested by Lyell, have
transported seeds from one part to another of the arctic and antarctic
regions; and during the Glacial period from one part of the now temperate
regions to another. In the Azores, from the large number of plants common
to Europe, in comparison with the species on the other islands of the
Atlantic, which stand nearer to the mainland, and (as remarked by Mr. H.C.
Watson) from their somewhat northern character, in comparison with the
latitude, I suspected that these islands had been partly stocked by
ice-borne seeds during the Glacial epoch. At my request Sir C. Lyell wrote
to M. Hartung to inquire whether he had observed erratic boulders on these
islands, and he answered that he had found large fragments of granite and
other rocks, which do not occur in the archipelago. Hence we may safely
infer that icebergs formerly landed their rocky burdens on the shores of
these mid-ocean islands, and it is at least possible that they may have
brought thither the seeds of northern plants.

Considering that these several means of transport, and that other means,
which without doubt remain to be discovered, have been in action year
after year for tens of thousands of years, it would, I think, be a
marvellous fact if many plants had not thus become widely transported.
These means of transport are sometimes called accidental, but this is not
strictly correct: the currents of the sea are not accidental, nor is the
direction of prevalent gales of wind. It should be observed that scarcely
any means of transport would carry seeds for very great distances; for
seeds do not retain their vitality when exposed for a great length of time
to the action of sea water; nor could they be long carried in the crops or
intestines of birds. These means, however, would suffice for occasional
transport across tracts of sea some hundred miles in breadth, or from
island to island, or from a continent to a neighbouring island, but not
from one distant continent to another. The floras of distant continents
would not by such means become mingled; but would remain as distinct as
they now are. The currents, from their course, would never bring seeds
from North America to Britain, though they might and do bring seeds from
the West Indies to our western shores, where, if not killed by their very
long immersion in salt water, they could not endure our climate. Almost
every year, one or two land-birds are blown across the whole Atlantic
Ocean, from North America to the western shores of Ireland and England;
but seeds could be transported by these rare wanderers only by one means,
namely, by dirt adhering to their feet or beaks, which is in itself a rare
accident. Even in this case, how small would be the chance of a seed
falling on favourable soil, and coming to maturity! But it would be a
great error to argue that because a well-stocked island, like Great
Britain, has not, as far as is known (and it would be very difficult to
prove this), received within the last few centuries, through occasional
means of transport, immigrants from Europe or any other continent, that a
poorly-stocked island, though standing more remote from the mainland,
would not receive colonists by similar means. Out of a hundred kinds of
seeds or animals transported to an island, even if far less well-stocked
than Britain, perhaps not more than one would be so well fitted to its new
home, as to become naturalised. But this is no valid argument against what
would be effected by occasional means of transport, during the long lapse
of geological time, whilst the island was being upheaved, and before it
had become fully stocked with inhabitants. On almost bare land, with few
or no destructive insects or birds living there, nearly every seed which
chanced to arrive, if fitted for the climate, would germinate and survive.

DISPERSAL DURING THE GLACIAL PERIOD.

The identity of many plants and animals, on mountain-summits, separated
from each other by hundreds of miles of lowlands, where Alpine species
could not possibly exist, is one of the most striking cases known of the
same species living at distant points, without the apparent possibility of
their having migrated from one point to the other. It is indeed a
remarkable fact to see so many plants of the same species living on the
snowy regions of the Alps or Pyrenees, and in the extreme northern parts
of Europe; but it is far more remarkable, that the plants on the White
Mountains, in the United States of America, are all the same with those of
Labrador, and nearly all the same, as we hear from Asa Gray, with those on
the loftiest mountains of Europe. Even as long ago as 1747, such facts led
Gmelin to conclude that the same species must have been independently
created at many distinct points; and we might have remained in this same
belief, had not Agassiz and others called vivid attention to the Glacial
period, which, as we shall immediately see, affords a simple explanation
of these facts. We have evidence of almost every conceivable kind, organic
and inorganic, that, within a very recent geological period, central
Europe and North America suffered under an Arctic climate. The ruins of a
house burnt by fire do not tell their tale more plainly than do the
mountains of Scotland and Wales, with their scored flanks, polished
surfaces, and perched boulders, of the icy streams with which their
valleys were lately filled. So greatly has the climate of Europe changed,
that in Northern Italy, gigantic moraines, left by old glaciers, are now
clothed by the vine and maize. Throughout a large part of the United
States, erratic boulders and scored rocks plainly reveal a former cold
period.

The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained by Edward Forbes, is substantially as
follows. But we shall follow the changes more readily, by supposing a new
glacial period slowly to come on, and then pass away, as formerly
occurred. As the cold came on, and as each more southern zone became
fitted for the inhabitants of the north, these would take the places of
the former inhabitants of the temperate regions. The latter, at the same
time would travel further and further southward, unless they were stopped
by barriers, in which case they would perish. The mountains would become
covered with snow and ice, and their former Alpine inhabitants would
descend to the plains. By the time that the cold had reached its maximum,
we should have an arctic fauna and flora, covering the central parts of
Europe, as far south as the Alps and Pyrenees, and even stretching into
Spain. The now temperate regions of the United States would likewise be
covered by arctic plants and animals and these would be nearly the same
with those of Europe; for the present circumpolar inhabitants, which we
suppose to have everywhere travelled southward, are remarkably uniform
round the world.

As the warmth returned, the arctic forms would retreat northward, closely
followed up in their retreat by the productions of the more temperate
regions. And as the snow melted from the bases of the mountains, the
arctic forms would seize on the cleared and thawed ground, always
ascending, as the warmth increased and the snow still further disappeared,
higher and higher, whilst their brethren were pursuing their northern
journey. Hence, when the warmth had fully returned, the same species,
which had lately lived together on the European and North American
lowlands, would again be found in the arctic regions of the Old and New
Worlds, and on many isolated mountain-summits far distant from each other.

Thus we can understand the identity of many plants at points so immensely
remote as the mountains of the United States and those of Europe. We can
thus also understand the fact that the Alpine plants of each
mountain-range are more especially related to the arctic forms living due
north or nearly due north of them: for the first migration when the cold
came on, and the re-migration on the returning warmth, would generally
have been due south and north. The Alpine plants, for example, of
Scotland, as remarked by Mr. H.C. Watson, and those of the Pyrenees, as
remarked by Ramond, are more especially allied to the plants of northern
Scandinavia; those of the United States to Labrador; those of the
mountains of Siberia to the arctic regions of that country. These views,
grounded as they are on the perfectly well-ascertained occurrence of a
former Glacial period, seem to me to explain in so satisfactory a manner
the present distribution of the Alpine and Arctic productions of Europe
and America, that when in other regions we find the same species on
distant mountain-summits, we may almost conclude, without other evidence,
that a colder climate formerly permitted their migration across the
intervening lowlands, now become too warm for their existence.

As the arctic forms moved first southward and afterwards backward to the
north, in unison with the changing climate, they will not have been
exposed during their long migrations to any great diversity of
temperature; and as they all migrated in a body together, their mutual
relations will not have been much disturbed. Hence, in accordance with the
principles inculcated in this volume, these forms will not have been
liable to much modification. But with the Alpine productions, left
isolated from the moment of the returning warmth, first at the bases and
ultimately on the summits of the mountains, the case will have been
somewhat different; for it is not likely that all the same arctic species
will have been left on mountain ranges far distant from each other, and
have survived there ever since; they will also, in all probability, have
become mingled with ancient Alpine species, which must have existed on the
mountains before the commencement of the Glacial epoch, and which during
the coldest period will have been temporarily driven down to the plains;
they will, also, have been subsequently exposed to somewhat different
climatical influences. Their mutual relations will thus have been in some
degree disturbed; consequently they will have been liable to modification;
and they have been modified; for if we compare the present Alpine plants
and animals of the several great European mountain ranges, one with
another, though many of the species remain identically the same, some
exist as varieties, some as doubtful forms or sub-species and some as
distinct yet closely allied species representing each other on the several
ranges.

In the foregoing illustration, I have assumed that at the commencement of
our imaginary Glacial period, the arctic productions were as uniform round
the polar regions as they are at the present day. But it is also necessary
to assume that many sub-arctic and some few temperate forms were the same
round the world, for some of the species which now exist on the lower
mountain slopes and on the plains of North America and Europe are the
same; and it may be asked how I account for this degree of uniformity of
the sub-arctic and temperate forms round the world, at the commencement of
the real Glacial period. At the present day, the sub-arctic and northern
temperate productions of the Old and New Worlds are separated from each
other by the whole Atlantic Ocean and by the northern part of the Pacific.
During the Glacial period, when the inhabitants of the Old and New Worlds
lived further southwards than they do at present, they must have been
still more completely separated from each other by wider spaces of ocean;
so that it may well be asked how the same species could then or previously
have entered the two continents. The explanation, I believe, lies in the
nature of the climate before the commencement of the Glacial period. At
this, the newer Pliocene period, the majority of the inhabitants of the
world were specifically the same as now, and we have good reason to
believe that the climate was warmer than at the present day. Hence, we may
suppose that the organisms which now live under latitude 60 degrees, lived
during the Pliocene period further north, under the Polar Circle, in
latitude 66-67 degrees; and that the present arctic productions then lived
on the broken land still nearer to the pole. Now, if we look at a
terrestrial globe, we see under the Polar Circle that there is almost
continuous land from western Europe through Siberia, to eastern America.
And this continuity of the circumpolar land, with the consequent freedom
under a more favourable climate for intermigration, will account for the
supposed uniformity of the sub-arctic and temperate productions of the Old
and New Worlds, at a period anterior to the Glacial epoch.

Believing, from reasons before alluded to, that our continents have long
remained in nearly the same relative position, though subjected to great
oscillations of level, I am strongly inclined to extend the above view,
and to infer that during some earlier and still warmer period, such as the
older Pliocene period, a large number of the same plants and animals
inhabited the almost continuous circumpolar land; and that these plants
and animals, both in the Old and New Worlds, began slowly to migrate
southwards as the climate became less warm, long before the commencement
of the Glacial period. We now see, as I believe, their descendants, mostly
in a modified condition, in the central parts of Europe and the United
States. On this view we can understand the relationship with very little
identity, between the productions of North America and Europe—a
relationship which is highly remarkable, considering the distance of the
two areas, and their separation by the whole Atlantic Ocean. We can
further understand the singular fact remarked on by several observers that
the productions of Europe and America during the later tertiary stages
were more closely related to each other than they are at the present time;
for during these warmer periods the northern parts of the Old and New
Worlds will have been almost continuously united by land, serving as a
bridge, since rendered impassable by cold, for the intermigration of their
inhabitants.

During the slowly decreasing warmth of the Pliocene period, as soon as the
species in common, which inhabited the New and Old Worlds, migrated south
of the Polar Circle, they will have been completely cut off from each
other. This separation, as far as the more temperate productions are
concerned, must have taken place long ages ago. As the plants and animals
migrated southward, they will have become mingled in the one great region
with the native American productions, and would have had to compete with
them; and in the other great region, with those of the Old World.
Consequently we have here everything favourable for much modification—for
far more modification than with the Alpine productions, left isolated,
within a much more recent period, on the several mountain ranges and on
the arctic lands of Europe and North America. Hence, it has come, that
when we compare the now living productions of the temperate regions of the
New and Old Worlds, we find very few identical species (though Asa Gray
has lately shown that more plants are identical than was formerly
supposed), but we find in every great class many forms, which some
naturalists rank as geographical races, and others as distinct species;
and a host of closely allied or representative forms which are ranked by
all naturalists as specifically distinct.

As on the land, so in the waters of the sea, a slow southern migration of
a marine fauna, which, during the Pliocene or even a somewhat earlier
period, was nearly uniform along the continuous shores of the Polar
Circle, will account, on the theory of modification, for many closely
allied forms now living in marine areas completely sundered. Thus, I
think, we can understand the presence of some closely allied, still
existing and extinct tertiary forms, on the eastern and western shores of
temperate North America; and the still more striking fact of many closely
allied crustaceans (as described in Dana's admirable work), some fish and
other marine animals, inhabiting the Mediterranean and the seas of Japan—these
two areas being now completely separated by the breadth of a whole
continent and by wide spaces of ocean.

These cases of close relationship in species either now or formerly
inhabiting the seas on the eastern and western shores of North America,
the Mediterranean and Japan, and the temperate lands of North America and
Europe, are inexplicable on the theory of creation. We cannot maintain
that such species have been created alike, in correspondence with the
nearly similar physical conditions of the areas; for if we compare, for
instance, certain parts of South America with parts of South Africa or
Australia, we see countries closely similar in all their physical
conditions, with their inhabitants utterly dissimilar.

ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH.

But we must return to our more immediate subject. I am convinced that
Forbes's view may be largely extended. In Europe we meet with the plainest
evidence of the Glacial period, from the western shores of Britain to the
Ural range, and southward to the Pyrenees. We may infer from the frozen
mammals and nature of the mountain vegetation, that Siberia was similarly
affected. In the Lebanon, according to Dr. Hooker, perpetual snow formerly
covered the central axis, and fed glaciers which rolled 4,000 feet down
the valleys. The same observer has recently found great moraines at a low
level on the Atlas range in North Africa. Along the Himalaya, at points
900 miles apart, glaciers have left the marks of their former low descent;
and in Sikkim, Dr. Hooker saw maize growing on ancient and gigantic
moraines. Southward of the Asiatic continent, on the opposite side of the
equator, we know, from the excellent researches of Dr. J. Haast and Dr.
Hector, that in New Zealand immense glaciers formerly descended to a low
level; and the same plants, found by Dr. Hooker on widely separated
mountains in this island tell the same story of a former cold period. From
facts communicated to me by the Rev. W.B. Clarke, it appears also that
there are traces of former glacial action on the mountains of the
south-eastern corner of Australia.

Looking to America: in the northern half, ice-borne fragments of rock have
been observed on the eastern side of the continent, as far south as
latitude 36 and 37 degrees, and on the shores of the Pacific, where the
climate is now so different, as far south as latitude 46 degrees. Erratic
boulders have, also, been noticed on the Rocky Mountains. In the
Cordillera of South America, nearly under the equator, glaciers once
extended far below their present level. In central Chile I examined a vast
mound of detritus with great boulders, crossing the Portillo valley,
which, there can hardly be a doubt, once formed a huge moraine; and Mr. D.
Forbes informs me that he found in various parts of the Cordillera, from
latitude 13 to 30 degrees south, at about the height of 12,000 feet,
deeply-furrowed rocks, resembling those with which he was familiar in
Norway, and likewise great masses of detritus, including grooved pebbles.
Along this whole space of the Cordillera true glaciers do not now exist
even at much more considerable heights. Further south, on both sides of
the continent, from latitude 41 degrees to the southernmost extremity, we
have the clearest evidence of former glacial action, in numerous immense
boulders transported far from their parent source.

From these several facts, namely, from the glacial action having extended
all round the northern and southern hemispheres—from the period
having been in a geological sense recent in both hemispheres—from
its having lasted in both during a great length of time, as may be
inferred from the amount of work effected—and lastly, from glaciers
having recently descended to a low level along the whole line of the
Cordillera, it at one time appeared to me that we could not avoid the
conclusion that the temperature of the whole world had been simultaneously
lowered during the Glacial period. But now, Mr. Croll, in a series of
admirable memoirs, has attempted to show that a glacial condition of
climate is the result of various physical causes, brought into operation
by an increase in the eccentricity of the earth's orbit. All these causes
tend towards the same end; but the most powerful appears to be the
indirect influence of the eccentricity of the orbit upon oceanic currents.
According to Mr. Croll, cold periods regularly recur every ten or fifteen
thousand years; and these at long intervals are extremely severe, owing to
certain contingencies, of which the most important, as Sir C. Lyell has
shown, is the relative position of the land and water. Mr. Croll believes
that the last great glacial period occurred about 240,000 years ago, and
endured, with slight alterations of climate, for about 160,000 years. With
respect to more ancient glacial periods, several geologists are convinced,
from direct evidence, that such occurred during the miocene and eocene
formations, not to mention still more ancient formations. But the most
important result for us, arrived at by Mr. Croll, is that whenever the
northern hemisphere passes through a cold period the temperature of the
southern hemisphere is actually raised, with the winters rendered much
milder, chiefly through changes in the direction of the ocean currents. So
conversely it will be with the northern hemisphere, while the southern
passes through a glacial period. This conclusion throws so much light on
geographical distribution that I am strongly inclined to trust in it; but
I will first give the facts which demand an explanation.

In South America, Dr. Hooker has shown that besides many closely allied
species, between forty and fifty of the flowering plants of Tierra del
Fuego, forming no inconsiderable part of its scanty flora, are common to
North America and Europe, enormously remote as these areas in opposite
hemispheres are from each other. On the lofty mountains of equatorial
America a host of peculiar species belonging to European genera occur. On
the Organ Mountains of Brazil some few temperate European, some Antarctic
and some Andean genera were found by Gardner which do not exist in the low
intervening hot countries. On the Silla of Caraccas the illustrious
Humboldt long ago found species belonging to genera characteristic of the
Cordillera.

In Africa, several forms characteristic of Europe, and some few
representatives of the flora of the Cape of Good Hope, occur on the
mountains of Abyssinia. At the Cape of Good Hope a very few European
species, believed not to have been introduced by man, and on the mountains
several representative European forms are found which have not been
discovered in the intertropical parts of Africa. Dr. Hooker has also
lately shown that several of the plants living on the upper parts of the
lofty island of Fernando Po, and on the neighbouring Cameroon Mountains,
in the Gulf of Guinea, are closely related to those on the mountains of
Abyssinia, and likewise to those of temperate Europe. It now also appears,
as I hear from Dr. Hooker, that some of these same temperate plants have
been discovered by the Rev. R.T. Lowe on the mountains of the Cape Verde
Islands. This extension of the same temperate forms, almost under the
equator, across the whole continent of Africa and to the mountains of the
Cape Verde archipelago, is one of the most astonishing facts ever recorded
in the distribution of plants.

On the Himalaya, and on the isolated mountain ranges of the peninsula of
India, on the heights of Ceylon, and on the volcanic cones of Java, many
plants occur either identically the same or representing each other, and
at the same time representing plants of Europe not found in the
intervening hot lowlands. A list of the genera of plants collected on the
loftier peaks of Java, raises a picture of a collection made on a hillock
in Europe. Still more striking is the fact that peculiar Australian forms
are represented by certain plants growing on the summits of the mountains
of Borneo. Some of these Australian forms, as I hear from Dr. Hooker,
extend along the heights of the peninsula of Malacca, and are thinly
scattered on the one hand over India, and on the other hand as far north
as Japan.

On the southern mountains of Australia, Dr. F. Muller has discovered
several European species; other species, not introduced by man, occur on
the lowlands; and a long list can be given, as I am informed by Dr.
Hooker, of European genera, found in Australia, but not in the
intermediate torrid regions. In the admirable "Introduction to the Flora
of New Zealand," by Dr. Hooker, analogous and striking facts are given in
regard to the plants of that large island. Hence, we see that certain
plants growing on the more lofty mountains of the tropics in all parts of
the world, and on the temperate plains of the north and south, are either
the same species or varieties of the same species. It should, however, be
observed that these plants are not strictly arctic forms; for, as Mr. H.C.
Watson has remarked, "in receding from polar toward equatorial latitudes,
the Alpine or mountain flora really become less and less Arctic." Besides
these identical and closely allied forms, many species inhabiting the same
widely sundered areas, belong to genera not now found in the intermediate
tropical lowlands.

These brief remarks apply to plants alone; but some few analogous facts
could be given in regard to terrestrial animals. In marine productions,
similar cases likewise occur; as an example, I may quote a statement by
the highest authority, Prof. Dana, that "it is certainly a wonderful fact
that New Zealand should have a closer resemblance in its crustacea to
Great Britain, its antipode, than to any other part of the world." Sir J.
Richardson, also, speaks of the reappearance on the shores of New Zealand,
Tasmania, etc., of northern forms of fish. Dr. Hooker informs me that
twenty-five species of Algae are common to New Zealand and to Europe, but
have not been found in the intermediate tropical seas.

From the foregoing facts, namely, the presence of temperate forms on the
highlands across the whole of equatorial Africa, and along the Peninsula
of India, to Ceylon and the Malay Archipelago, and in a less well-marked
manner across the wide expanse of tropical South America, it appears
almost certain that at some former period, no doubt during the most severe
part of a Glacial period, the lowlands of these great continents were
everywhere tenanted under the equator by a considerable number of
temperate forms. At this period the equatorial climate at the level of the
sea was probably about the same with that now experienced at the height of
from five to six thousand feet under the same latitude, or perhaps even
rather cooler. During this, the coldest period, the lowlands under the
equator must have been clothed with a mingled tropical and temperate
vegetation, like that described by Hooker as growing luxuriantly at the
height of from four to five thousand feet on the lower slopes of the
Himalaya, but with perhaps a still greater preponderance of temperate
forms. So again in the mountainous island of Fernando Po, in the Gulf of
Guinea, Mr. Mann found temperate European forms beginning to appear at the
height of about five thousand feet. On the mountains of Panama, at the
height of only two thousand feet, Dr. Seemann found the vegetation like
that of Mexico, "with forms of the torrid zone harmoniously blended with
those of the temperate."

Now let us see whether Mr. Croll's conclusion that when the northern
hemisphere suffered from the extreme cold of the great Glacial period, the
southern hemisphere was actually warmer, throws any clear light on the
present apparently inexplicable distribution of various organisms in the
temperate parts of both hemispheres, and on the mountains of the tropics.
The Glacial period, as measured by years, must have been very long; and
when we remember over what vast spaces some naturalised plants and animals
have spread within a few centuries, this period will have been ample for
any amount of migration. As the cold became more and more intense, we know
that Arctic forms invaded the temperate regions; and from the facts just
given, there can hardly be a doubt that some of the more vigorous,
dominant and widest-spreading temperate forms invaded the equatorial
lowlands. The inhabitants of these hot lowlands would at the same time
have migrated to the tropical and subtropical regions of the south, for
the southern hemisphere was at this period warmer. On the decline of the
Glacial period, as both hemispheres gradually recovered their former
temperature, the northern temperate forms living on the lowlands under the
equator, would have been driven to their former homes or have been
destroyed, being replaced by the equatorial forms returning from the
south. Some, however, of the northern temperate forms would almost
certainly have ascended any adjoining high land, where, if sufficiently
lofty, they would have long survived like the Arctic forms on the
mountains of Europe. They might have survived, even if the climate was not
perfectly fitted for them, for the change of temperature must have been
very slow, and plants undoubtedly possess a certain capacity for
acclimatisation, as shown by their transmitting to their offspring
different constitutional powers of resisting heat and cold.

In the regular course of events the southern hemisphere would in its turn
be subjected to a severe Glacial period, with the northern hemisphere
rendered warmer; and then the southern temperate forms would invade the
equatorial lowlands. The northern forms which had before been left on the
mountains would now descend and mingle with the southern forms. These
latter, when the warmth returned, would return to their former homes,
leaving some few species on the mountains, and carrying southward with
them some of the northern temperate forms which had descended from their
mountain fastnesses. Thus, we should have some few species identically the
same in the northern and southern temperate zones and on the mountains of
the intermediate tropical regions. But the species left during a long time
on these mountains, or in opposite hemispheres, would have to compete with
many new forms and would be exposed to somewhat different physical
conditions; hence, they would be eminently liable to modification, and
would generally now exist as varieties or as representative species; and
this is the case. We must, also, bear in mind the occurrence in both
hemispheres of former Glacial periods; for these will account, in
accordance with the same principles, for the many quite distinct species
inhabiting the same widely separated areas, and belonging to genera not
now found in the intermediate torrid zones.

It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many more
identical or slightly modified species have migrated from the north to the
south, than in a reversed direction. We see, however, a few southern forms
on the mountains of Borneo and Abyssinia. I suspect that this preponderant
migration from the north to the south is due to the greater extent of land
in the north, and to the northern forms having existed in their own homes
in greater numbers, and having consequently been advanced through natural
selection and competition to a higher stage of perfection, or dominating
power, than the southern forms. And thus, when the two sets became
commingled in the equatorial regions, during the alternations of the
Glacial periods, the northern forms were the more powerful and were able
to hold their places on the mountains, and afterwards migrate southward
with the southern forms; but not so the southern in regard to the northern
forms. In the same manner, at the present day, we see that very many
European productions cover the ground in La Plata, New Zealand, and to a
lesser degree in Australia, and have beaten the natives; whereas extremely
few southern forms have become naturalised in any part of the northern
hemisphere, though hides, wool, and other objects likely to carry seeds
have been largely imported into Europe during the last two or three
centuries from La Plata and during the last forty or fifty years from
Australia. The Neilgherrie Mountains in India, however, offer a partial
exception; for here, as I hear from Dr. Hooker, Australian forms are
rapidly sowing themselves and becoming naturalised. Before the last great
Glacial period, no doubt the intertropical mountains were stocked with
endemic Alpine forms; but these have almost everywhere yielded to the more
dominant forms generated in the larger areas and more efficient workshops
of the north. In many islands the native productions are nearly equalled,
or even outnumbered, by those which have become naturalised; and this is
the first stage towards their extinction. Mountains are islands on the
land; and their inhabitants have yielded to those produced within the
larger areas of the north, just in the same way as the inhabitants of real
islands have everywhere yielded and are still yielding to continental
forms naturalised through man's agency.

The same principles apply to the distribution of terrestrial animals and
of marine productions, in the northern and southern temperate zones, and
on the intertropical mountains. When, during the height of the Glacial
period, the ocean-currents were widely different to what they now are,
some of the inhabitants of the temperate seas might have reached the
equator; of these a few would perhaps at once be able to migrate
southwards, by keeping to the cooler currents, while others might remain
and survive in the colder depths until the southern hemisphere was in its
turn subjected to a glacial climate and permitted their further progress;
in nearly the same manner as, according to Forbes, isolated spaces
inhabited by Arctic productions exist to the present day in the deeper
parts of the northern temperate seas.

I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which now
live so widely separated in the north and south, and sometimes on the
intermediate mountain ranges, are removed on the views above given. The
exact lines of migration cannot be indicated. We cannot say why certain
species and not others have migrated; why certain species have been
modified and have given rise to new forms, while others have remained
unaltered. We cannot hope to explain such facts, until we can say why one
species and not another becomes naturalised by man's agency in a foreign
land; why one species ranges twice or thrice as far, and is twice or
thrice as common, as another species within their own homes.

Various special difficulties also remain to be solved; for instance, the
occurrence, as shown by Dr. Hooker, of the same plants at points so
enormously remote as Kerguelen Land, New Zealand, and Fuegia; but
icebergs, as suggested by Lyell, may have been concerned in their
dispersal. The existence at these and other distant points of the southern
hemisphere, of species, which, though distinct, belong to genera
exclusively confined to the south, is a more remarkable case. Some of
these species are so distinct, that we cannot suppose that there has been
time since the commencement of the last Glacial period for their migration
and subsequent modification to the necessary degree. The facts seem to
indicate that distinct species belonging to the same genera have migrated
in radiating lines from a common centre; and I am inclined to look in the
southern, as in the northern hemisphere, to a former and warmer period,
before the commencement of the last Glacial period, when the Antarctic
lands, now covered with ice, supported a highly peculiar and isolated
flora. It may be suspected that before this flora was exterminated during
the last Glacial epoch, a few forms had been already widely dispersed to
various points of the southern hemisphere by occasional means of
transport, and by the aid, as halting-places, of now sunken islands. Thus
the southern shores of America, Australia, and New Zealand may have become
slightly tinted by the same peculiar forms of life.

Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alternations of climate
throughout the world on geographical distribution. And we have now seen
that Mr. Croll's conclusion that successive Glacial periods in the one
hemisphere coincide with warmer periods in the opposite hemisphere,
together with the admission of the slow modification of species, explains
a multitude of facts in the distribution of the same and of the allied
forms of life in all parts of the globe. The living waters have flowed
during one period from the north and during another from the south, and in
both cases have reached the equator; but the stream of life has flowed
with greater force from the north than in the opposite direction, and has
consequently more freely inundated the south. As the tide leaves its drift
in horizontal lines, rising higher on the shores where the tide rises
highest, so have the living waters left their living drift on our mountain
summits, in a line gently rising from the Arctic lowlands to a great
latitude under the equator. The various beings thus left stranded may be
compared with savage races of man, driven up and surviving in the mountain
fastnesses of almost every land, which serves as a record, full of
interest to us, of the former inhabitants of the surrounding lowlands.



CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION—continued.


 Distribution of fresh-water productions—On the inhabitants of oceanic
 islands—Absence of Batrachians and of terrestrial Mammals—On
 the relation of the inhabitants of islands to those of the nearest
 mainland—On colonisation from the nearest source with subsequent
 modification—Summary of the last and present chapters.

FRESH-WATER PRODUCTIONS.

As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently a
still more formidable barrier, that they would never have extended to
distant countries. But the case is exactly the reverse. Not only have many
fresh-water species, belonging to different classes, an enormous range,
but allied species prevail in a remarkable manner throughout the world.
When first collecting in the fresh waters of Brazil, I well remember
feeling much surprise at the similarity of the fresh-water insects,
shells, etc., and at the dissimilarity of the surrounding terrestrial
beings, compared with those of Britain.

But the wide ranging power of fresh-water productions can, I think, in
most cases be explained by their having become fitted, in a manner highly
useful to them, for short and frequent migrations from pond to pond, or
from stream to stream, within their own countries; and liability to wide
dispersal would follow from this capacity as an almost necessary
consequence. We can here consider only a few cases; of these, some of the
most difficult to explain are presented by fish. It was formerly believed
that the same fresh-water species never existed on two continents distant
from each other. But Dr. Gunther has lately shown that the Galaxias
attenuatus inhabits Tasmania, New Zealand, the Falkland Islands and the
mainland of South America. This is a wonderful case, and probably
indicates dispersal from an Antarctic centre during a former warm period.
This case, however, is rendered in some degree less surprising by the
species of this genus having the power of crossing by some unknown means
considerable spaces of open ocean: thus there is one species common to New
Zealand and to the Auckland Islands, though separated by a distance of
about 230 miles. On the same continent fresh-water fish often range
widely, and as if capriciously; for in two adjoining river systems some of
the species may be the same and some wholly different.

It is probable that they are occasionally transported by what may be
called accidental means. Thus fishes still alive are not very rarely
dropped at distant points by whirlwinds; and it is known that the ova
retain their vitality for a considerable time after removal from the
water. Their dispersal may, however, be mainly attributed to changes in
the level of the land within the recent period, causing rivers to flow
into each other. Instances, also, could be given of this having occurred
during floods, without any change of level. The wide differences of the
fish on the opposite sides of most mountain-ranges, which are continuous
and consequently must, from an early period, have completely prevented the
inosculation of the river systems on the two sides, leads to the same
conclusion. Some fresh-water fish belong to very ancient forms, and in
such cases there will have been ample time for great geographical changes,
and consequently time and means for much migration. Moreover, Dr. Gunther
has recently been led by several considerations to infer that with fishes
the same forms have a long endurance. Salt-water fish can with care be
slowly accustomed to live in fresh water; and, according to Valenciennes,
there is hardly a single group of which all the members are confined to
fresh water, so that a marine species belonging to a fresh-water group
might travel far along the shores of the sea, and could, it is probable,
become adapted without much difficulty to the fresh waters of a distant
land.

Some species of fresh-water shells have very wide ranges, and allied
species which, on our theory, are descended from a common parent, and must
have proceeded from a single source, prevail throughout the world. Their
distribution at first perplexed me much, as their ova are not likely to be
transported by birds; and the ova, as well as the adults, are immediately
killed by sea-water. I could not even understand how some naturalised
species have spread rapidly throughout the same country. But two facts,
which I have observed—and many others no doubt will be discovered—throw
some light on this subject. When ducks suddenly emerge from a pond covered
with duck-weed, I have twice seen these little plants adhering to their
backs; and it has happened to me, in removing a little duck-weed from one
aquarium to another, that I have unintentionally stocked the one with
fresh-water shells from the other. But another agency is perhaps more
effectual: I suspended the feet of a duck in an aquarium, where many ova
of fresh-water shells were hatching; and I found that numbers of the
extremely minute and just-hatched shells crawled on the feet, and clung to
them so firmly that when taken out of the water they could not be jarred
off, though at a somewhat more advanced age they would voluntarily drop
off. These just-hatched molluscs, though aquatic in their nature, survived
on the duck's feet, in damp air, from twelve to twenty hours; and in this
length of time a duck or heron might fly at least six or seven hundred
miles, and if blown across the sea to an oceanic island, or to any other
distant point, would be sure to alight on a pool or rivulet. Sir Charles
Lyell informs me that a Dyticus has been caught with an Ancylus (a
fresh-water shell like a limpet) firmly adhering to it; and a water-beetle
of the same family, a Colymbetes, once flew on board the "Beagle," when
forty-five miles distant from the nearest land: how much farther it might
have been blown by a favouring gale no one can tell.

With respect to plants, it has long been known what enormous ranges many
fresh-water, and even marsh-species, have, both over continents and to the
most remote oceanic islands. This is strikingly illustrated, according to
Alph. de Candolle, in those large groups of terrestrial plants, which have
very few aquatic members; for the latter seem immediately to acquire, as
if in consequence, a wide range. I think favourable means of dispersal
explain this fact. I have before mentioned that earth occasionally adheres
in some quantity to the feet and beaks of birds. Wading birds, which
frequent the muddy edges of ponds, if suddenly flushed, would be the most
likely to have muddy feet. Birds of this order wander more than those of
any other; and are occasionally found on the most remote and barren
islands of the open ocean; they would not be likely to alight on the
surface of the sea, so that any dirt on their feet would not be washed
off; and when gaining the land, they would be sure to fly to their natural
fresh-water haunts. I do not believe that botanists are aware how charged
the mud of ponds is with seeds: I have tried several little experiments,
but will here give only the most striking case: I took in February three
tablespoonfuls of mud from three different points, beneath water, on the
edge of a little pond; this mud when dry weighed only 6 and 3/4 ounces; I
kept it covered up in my study for six months, pulling up and counting
each plant as it grew; the plants were of many kinds, and were altogether
537 in number; and yet the viscid mud was all contained in a breakfast
cup! Considering these facts, I think it would be an inexplicable
circumstance if water-birds did not transport the seeds of fresh-water
plants to unstocked ponds and streams, situated at very distant points.
The same agency may have come into play with the eggs of some of the
smaller fresh-water animals.

Other and unknown agencies probably have also played a part. I have stated
that fresh-water fish eat some kinds of seeds, though they reject many
other kinds after having swallowed them; even small fish swallow seeds of
moderate size, as of the yellow water-lily and Potamogeton. Herons and
other birds, century after century, have gone on daily devouring fish;
they then take flight and go to other waters, or are blown across the sea;
and we have seen that seeds retain their power of germination, when
rejected many hours afterwards in pellets or in the excrement. When I saw
the great size of the seeds of that fine water-lily, the Nelumbium, and
remembered Alph. de Candolle's remarks on the distribution of this plant,
I thought that the means of its dispersal must remain inexplicable; but
Audubon states that he found the seeds of the great southern water-lily
(probably according to Dr. Hooker, the Nelumbium luteum) in a heron's
stomach. Now this bird must often have flown with its stomach thus well
stocked to distant ponds, and, then getting a hearty meal of fish, analogy
makes me believe that it would have rejected the seeds in the pellet in a
fit state for germination.

In considering these several means of distribution, it should be
remembered that when a pond or stream is first formed, for instance on a
rising islet, it will be unoccupied; and a single seed or egg will have a
good chance of succeeding. Although there will always be a struggle for
life between the inhabitants of the same pond, however few in kind, yet as
the number even in a well-stocked pond is small in comparison with the
number of species inhabiting an equal area of land, the competition
between them will probably be less severe than between terrestrial
species; consequently an intruder from the waters of a foreign country
would have a better chance of seizing on a new place, than in the case of
terrestrial colonists. We should also remember that many fresh-water
productions are low in the scale of nature, and we have reason to believe
that such beings become modified more slowly than the high; and this will
give time for the migration of aquatic species. We should not forget the
probability of many fresh-water forms having formerly ranged continuously
over immense areas, and then having become extinct at intermediate points.
But the wide distribution of fresh-water plants, and of the lower animals,
whether retaining the same identical form, or in some degree modified,
apparently depends in main part on the wide dispersal of their seeds and
eggs by animals, more especially by fresh-water birds, which have great
powers of flight, and naturally travel from one piece of water to another.

ON THE INHABITANTS OF OCEANIC ISLANDS.

We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty with respect to
distribution, on the view that not only all the individuals of the same
species have migrated from some one area, but that allied species,
although now inhabiting the most distant points, have proceeded from a
single area, the birthplace of their early progenitors. I have already
given my reasons for disbelieving in continental extensions within the
period of existing species on so enormous a scale that all the many
islands of the several oceans were thus stocked with their present
terrestrial inhabitants. This view removes many difficulties, but it does
not accord with all the facts in regard to the productions of islands. In
the following remarks I shall not confine myself to the mere question of
dispersal, but shall consider some other cases bearing on the truth of the
two theories of independent creation and of descent with modification.

The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. New Zealand, for instance,
with its lofty mountains and diversified stations, extending over 780
miles of latitude, together with the outlying islands of Auckland,
Campbell and Chatham, contain altogether only 960 kinds of flowering
plants; if we compare this moderate number with the species which swarm
over equal areas in Southwestern Australia or at the Cape of Good Hope, we
must admit that some cause, independently of different physical
conditions, has given rise to so great a difference in number. Even the
uniform county of Cambridge has 847 plants, and the little island of
Anglesea 764, but a few ferns and a few introduced plants are included in
these numbers, and the comparison in some other respects is not quite
fair. We have evidence that the barren island of Ascension aboriginally
possessed less than half-a-dozen flowering plants; yet many species have
now become naturalised on it, as they have in New Zealand and on every
other oceanic island which can be named. In St. Helena there is reason to
believe that the naturalised plants and animals have nearly or quite
exterminated many native productions. He who admits the doctrine of the
creation of each separate species, will have to admit that a sufficient
number of the best adapted plants and animals were not created for oceanic
islands; for man has unintentionally stocked them far more fully and
perfectly than did nature.

Although in oceanic islands the species are few in number, the proportion
of endemic kinds (i.e. those found nowhere else in the world) is often
extremely large. If we compare, for instance, the number of endemic
land-shells in Madeira, or of endemic birds in the Galapagos Archipelago,
with the number found on any continent, and then compare the area of the
island with that of the continent, we shall see that this is true. This
fact might have been theoretically expected, for, as already explained,
species occasionally arriving, after long intervals of time in the new and
isolated district, and having to compete with new associates, would be
eminently liable to modification, and would often produce groups of
modified descendants. But it by no means follows that, because in an
island nearly all the species of one class are peculiar, those of another
class, or of another section of the same class, are peculiar; and this
difference seems to depend partly on the species which are not modified
having immigrated in a body, so that their mutual relations have not been
much disturbed; and partly on the frequent arrival of unmodified
immigrants from the mother-country, with which the insular forms have
intercrossed. It should be borne in mind that the offspring of such
crosses would certainly gain in vigour; so that even an occasional cross
would produce more effect than might have been anticipated. I will give a
few illustrations of the foregoing remarks: in the Galapagos Islands there
are twenty-six land birds; of these twenty-one (or perhaps twenty-three)
are peculiar; whereas of the eleven marine birds only two are peculiar;
and it is obvious that marine birds could arrive at these islands much
more easily and frequently than land-birds. Bermuda, on the other hand,
which lies at about the same distance from North America as the Galapagos
Islands do from South America, and which has a very peculiar soil, does
not possess a single endemic land bird; and we know from Mr. J.M. Jones's
admirable account of Bermuda, that very many North American birds
occasionally or even frequently visit this island. Almost every year, as I
am informed by Mr. E.V. Harcourt, many European and African birds are
blown to Madeira; this island is inhabited by ninety-nine kinds, of which
one alone is peculiar, though very closely related to a European form; and
three or four other species are confined to this island and to the
Canaries. So that the islands of Bermuda and Madeira have been stocked
from the neighbouring continents with birds, which for long ages have
there struggled together, and have become mutually co-adapted. Hence, when
settled in their new homes, each kind will have been kept by the others to
its proper place and habits, and will consequently have been but little
liable to modification. Any tendency to modification will also have been
checked by intercrossing with the unmodified immigrants, often arriving
from the mother-country. Madeira again is inhabited by a wonderful number
of peculiar land-shells, whereas not one species of sea-shell is peculiar
to its shores: now, though we do not know how sea-shells are dispersed,
yet we can see that their eggs or larvae, perhaps attached to seaweed or
floating timber, or to the feet of wading birds, might be transported
across three or four hundred miles of open sea far more easily than
land-shells. The different orders of insects inhabiting Madeira present
nearly parallel cases.

Oceanic islands are sometimes deficient in animals of certain whole
classes, and their places are occupied by other classes; thus in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless birds,
take, or recently took, the place of mammals. Although New Zealand is here
spoken of as an oceanic island, it is in some degree doubtful whether it
should be so ranked; it is of large size, and is not separated from
Australia by a profoundly deep sea; from its geological character and the
direction of its mountain ranges, the Rev. W.B. Clarke has lately
maintained that this island, as well as New Caledonia, should be
considered as appurtenances of Australia. Turning to plants, Dr. Hooker
has shown that in the Galapagos Islands the proportional numbers of the
different orders are very different from what they are elsewhere. All such
differences in number, and the absence of certain whole groups of animals
and plants, are generally accounted for by supposed differences in the
physical conditions of the islands; but this explanation is not a little
doubtful. Facility of immigration seems to have been fully as important as
the nature of the conditions.

Many remarkable little facts could be given with respect to the
inhabitants of oceanic islands. For instance, in certain islands not
tenanted by a single mammal, some of the endemic plants have beautifully
hooked seeds; yet few relations are more manifest than that hooks serve
for the transportal of seeds in the wool or fur of quadrupeds. But a
hooked seed might be carried to an island by other means; and the plant
then becoming modified would form an endemic species, still retaining its
hooks, which would form a useless appendage, like the shrivelled wings
under the soldered wing-covers of many insular beetles. Again, islands
often possess trees or bushes belonging to orders which elsewhere include
only herbaceous species; now trees, as Alph. de Candolle has shown,
generally have, whatever the cause may be, confined ranges. Hence trees
would be little likely to reach distant oceanic islands; and an herbaceous
plant, which had no chance of successfully competing with the many fully
developed trees growing on a continent, might, when established on an
island, gain an advantage over other herbaceous plants by growing taller
and taller and overtopping them. In this case, natural selection would
tend to add to the stature of the plant, to whatever order it belonged,
and thus first convert it into a bush and then into a tree.

ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON OCEANIC ISLANDS.

With respect to the absence of whole orders of animals on oceanic islands,
Bory St. Vincent long ago remarked that Batrachians (frogs, toads, newts)
are never found on any of the many islands with which the great oceans are
studded. I have taken pains to verify this assertion, and have found it
true, with the exception of New Zealand, New Caledonia, the Andaman
Islands, and perhaps the Solomon Islands and the Seychelles. But I have
already remarked that it is doubtful whether New Zealand and New Caledonia
ought to be classed as oceanic islands; and this is still more doubtful
with respect to the Andaman and Solomon groups and the Seychelles. This
general absence of frogs, toads and newts on so many true oceanic islands
cannot be accounted for by their physical conditions; indeed it seems that
islands are peculiarly fitted for these animals; for frogs have been
introduced into Madeira, the Azores, and Mauritius, and have multiplied so
as to become a nuisance. But as these animals and their spawn are
immediately killed (with the exception, as far as known, of one Indian
species) by sea-water, there would be great difficulty in their
transportal across the sea, and therefore we can see why they do not exist
on strictly oceanic islands. But why, on the theory of creation, they
should not have been created there, it would be very difficult to explain.

Mammals offer another and similar case. I have carefully searched the
oldest voyages, and have not found a single instance, free from doubt, of
a terrestrial mammal (excluding domesticated animals kept by the natives)
inhabiting an island situated above 300 miles from a continent or great
continental island; and many islands situated at a much less distance are
equally barren. The Falkland Islands, which are inhabited by a wolf-like
fox, come nearest to an exception; but this group cannot be considered as
oceanic, as it lies on a bank in connection with the mainland at a
distance of about 280 miles; moreover, icebergs formerly brought boulders
to its western shores, and they may have formerly transported foxes, as
now frequently happens in the arctic regions. Yet it cannot be said that
small islands will not support at least small mammals, for they occur in
many parts of the world on very small islands, when lying close to a
continent; and hardly an island can be named on which our smaller
quadrupeds have not become naturalised and greatly multiplied. It cannot
be said, on the ordinary view of creation, that there has not been time
for the creation of mammals; many volcanic islands are sufficiently
ancient, as shown by the stupendous degradation which they have suffered,
and by their tertiary strata: there has also been time for the production
of endemic species belonging to other classes; and on continents it is
known that new species of mammals appear and disappear at a quicker rate
than other and lower animals. Although terrestrial mammals do not occur on
oceanic islands, aerial mammals do occur on almost every island. New
Zealand possesses two bats found nowhere else in the world: Norfolk
Island, the Viti Archipelago, the Bonin Islands, the Caroline and Marianne
Archipelagoes, and Mauritius, all possess their peculiar bats. Why, it may
be asked, has the supposed creative force produced bats and no other
mammals on remote islands? On my view this question can easily be
answered; for no terrestrial mammal can be transported across a wide space
of sea, but bats can fly across. Bats have been seen wandering by day far
over the Atlantic Ocean; and two North American species, either regularly
or occasionally, visit Bermuda, at the distance of 600 miles from the
mainland. I hear from Mr. Tomes, who has specially studied this family,
that many species have enormous ranges, and are found on continents and on
far distant islands. Hence, we have only to suppose that such wandering
species have been modified in their new homes in relation to their new
position, and we can understand the presence of endemic bats on oceanic
islands, with the absence of all other terrestrial mammals.

Another interesting relation exists, namely, between the depth of the sea
separating islands from each other, or from the nearest continent, and the
degree of affinity of their mammalian inhabitants. Mr. Windsor Earl has
made some striking observations on this head, since greatly extended by
Mr. Wallace's admirable researches, in regard to the great Malay
Archipelago, which is traversed near Celebes by a space of deep ocean, and
this separates two widely distinct mammalian faunas. On either side, the
islands stand on a moderately shallow submarine bank, and these islands
are inhabited by the same or by closely allied quadrupeds. I have not as
yet had time to follow up this subject in all quarters of the world; but
as far as I have gone, the relation holds good. For instance, Britain is
separated by a shallow channel from Europe, and the mammals are the same
on both sides; and so it is with all the islands near the shores of
Australia. The West Indian Islands, on the other hand, stand on a deeply
submerged bank, nearly one thousand fathoms in depth, and here we find
American forms, but the species and even the genera are quite distinct. As
the amount of modification which animals of all kinds undergo partly
depends on the lapse of time, and as the islands which are separated from
each other, or from the mainland, by shallow channels, are more likely to
have been continuously united within a recent period than the islands
separated by deeper channels, we can understand how it is that a relation
exists between the depth of the sea separating two mammalian faunas, and
the degree of their affinity, a relation which is quite inexplicable on
the theory of independent acts of creation.

The foregoing statements in regard to the inhabitants of oceanic islands,
namely, the fewness of the species, with a large proportion consisting of
endemic forms—the members of certain groups, but not those of other
groups in the same class, having been modified—the absence of
certain whole orders, as of batrachians and of terrestrial mammals,
notwithstanding the presence of aerial bats, the singular proportions of
certain orders of plants, herbaceous forms having been developed into
trees, etc., seem to me to accord better with the belief in the efficiency
of occasional means of transport, carried on during a long course of time,
than with the belief in the former connection of all oceanic islands with
the nearest continent; for on this latter view it is probable that the
various classes would have immigrated more uniformly, and from the species
having entered in a body, their mutual relations would not have been much
disturbed, and consequently, they would either have not been modified, or
all the species in a more equable manner.

I do not deny that there are many and serious difficulties in
understanding how many of the inhabitants of the more remote islands,
whether still retaining the same specific form or subsequently modified,
have reached their present homes. But the probability of other islands
having once existed as halting-places, of which not a wreck now remains,
must not be overlooked. I will specify one difficult case. Almost all
oceanic islands, even the most isolated and smallest, are inhabited by
land-shells, generally by endemic species, but sometimes by species found
elsewhere striking instances of which have been given by Dr. A.A. Gould in
relation to the Pacific. Now it is notorious that land-shells are easily
killed by sea-water; their eggs, at least such as I have tried, sink in it
and are killed. Yet there must be some unknown, but occasionally efficient
means for their transportal. Would the just-hatched young sometimes adhere
to the feet of birds roosting on the ground and thus get transported? It
occurred to me that land-shells, when hybernating and having a membranous
diaphragm over the mouth of the shell, might be floated in chinks of
drifted timber across moderately wide arms of the sea. And I find that
several species in this state withstand uninjured an immersion in
sea-water during seven days. One shell, the Helix pomatia, after having
been thus treated, and again hybernating, was put into sea-water for
twenty days and perfectly recovered. During this length of time the shell
might have been carried by a marine country of average swiftness to a
distance of 660 geographical miles. As this Helix has a thick calcareous
operculum I removed it, and when it had formed a new membranous one, I
again immersed it for fourteen days in sea-water, and again it recovered
and crawled away. Baron Aucapitaine has since tried similar experiments.
He placed 100 land-shells, belonging to ten species, in a box pierced with
holes, and immersed it for a fortnight in the sea. Out of the hundred
shells twenty-seven recovered. The presence of an operculum seems to have
been of importance, as out of twelve specimens of Cyclostoma elegans,
which is thus furnished, eleven revived. It is remarkable, seeing how well
the Helix pomatia resisted with me the salt-water, that not one of
fifty-four specimens belonging to four other species of Helix tried by
Aucapitaine recovered. It is, however, not at all probable that
land-shells have often been thus transported; the feet of birds offer a
more probable method.

ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO THOSE OF THE NEAREST
MAINLAND.

The most striking and important fact for us is the affinity of the species
which inhabit islands to those of the nearest mainland, without being
actually the same. Numerous instances could be given. The Galapagos
Archipelago, situated under the equator, lies at a distance of between 500
and 600 miles from the shores of South America. Here almost every product
of the land and of the water bears the unmistakable stamp of the American
continent. There are twenty-six land birds. Of these twenty-one, or
perhaps twenty-three, are ranked as distinct species, and would commonly
be assumed to have been here created; yet the close affinity of most of
these birds to American species is manifest in every character in their
habits, gestures, and tones of voice. So it is with the other animals, and
with a large proportion of the plants, as shown by Dr. Hooker in his
admirable Flora of this archipelago. The naturalist, looking at the
inhabitants of these volcanic islands in the Pacific, distant several
hundred miles from the continent, feels that he is standing on American
land. Why should this be so? Why should the species which are supposed to
have been created in the Galapagos Archipelago, and nowhere else, bear so
plainly the stamp of affinity to those created in America? There is
nothing in the conditions of life, in the geological nature of the
islands, in their height or climate, or in the proportions in which the
several classes are associated together, which closely resembles the
conditions of the South American coast. In fact, there is a considerable
dissimilarity in all these respects. On the other hand, there is a
considerable degree of resemblance in the volcanic nature of the soil, in
the climate, height, and size of the islands, between the Galapagos and
Cape Verde Archipelagos: but what an entire and absolute difference in
their inhabitants! The inhabitants of the Cape Verde Islands are related
to those of Africa, like those of the Galapagos to America. Facts, such as
these, admit of no sort of explanation on the ordinary view of independent
creation; whereas, on the view here maintained, it is obvious that the
Galapagos Islands would be likely to receive colonists from America,
whether by occasional means of transport or (though I do not believe in
this doctrine) by formerly continuous land, and the Cape Verde Islands
from Africa; such colonists would be liable to modification—the
principle of inheritance still betraying their original birthplace.

Many analogous facts could be given: indeed it is an almost universal rule
that the endemic productions of islands are related to those of the
nearest continent, or of the nearest large island. The exceptions are few,
and most of them can be explained. Thus, although Kerguelen Land stands
nearer to Africa than to America, the plants are related, and that very
closely, as we know from Dr. Hooker's account, to those of America: but on
the view that this island has been mainly stocked by seeds brought with
earth and stones on icebergs, drifted by the prevailing currents, this
anomaly disappears. New Zealand in its endemic plants is much more closely
related to Australia, the nearest mainland, than to any other region: and
this is what might have been expected; but it is also plainly related to
South America, which, although the next nearest continent, is so
enormously remote, that the fact becomes an anomaly. But this difficulty
partially disappears on the view that New Zealand, South America, and the
other southern lands, have been stocked in part from a nearly intermediate
though distant point, namely, from the antarctic islands, when they were
clothed with vegetation, during a warmer tertiary period, before the
commencement of the last Glacial period. The affinity, which, though
feeble, I am assured by Dr. Hooker is real, between the flora of the
south-western corner of Australia and of the Cape of Good Hope, is a far
more remarkable case; but this affinity is confined to the plants, and
will, no doubt, some day be explained.

The same law which has determined the relationship between the inhabitants
of islands and the nearest mainland, is sometimes displayed on a small
scale, but in a most interesting manner, within the limits of the same
archipelago. Thus each separate island of the Galapagos Archipelago is
tenanted, and the fact is a marvellous one, by many distinct species; but
these species are related to each other in a very much closer manner than
to the inhabitants of the American continent, or of any other quarter of
the world. This is what might have been expected, for islands situated so
near to each other would almost necessarily receive immigrants from the
same original source, and from each other. But how is it that many of the
immigrants have been differently modified, though only in a small degree,
in islands situated within sight of each other, having the same geological
nature, the same height, climate, etc? This long appeared to me a great
difficulty: but it arises in chief part from the deeply-seated error of
considering the physical conditions of a country as the most important;
whereas it cannot be disputed that the nature of the other species with
which each has to compete, is at least as important, and generally a far
more important element of success. Now if we look to the species which
inhabit the Galapagos Archipelago, and are likewise found in other parts
of the world, we find that they differ considerably in the several
islands. This difference might indeed have been expected if the islands
have been stocked by occasional means of transport—a seed, for
instance, of one plant having been brought to one island, and that of
another plant to another island, though all proceeding from the same
general source. Hence, when in former times an immigrant first settled on
one of the islands, or when it subsequently spread from one to another, it
would undoubtedly be exposed to different conditions in the different
islands, for it would have to compete with a different set of organisms; a
plant, for instance, would find the ground best-fitted for it occupied by
somewhat different species in the different islands, and would be exposed
to the attacks of somewhat different enemies. If, then, it varied, natural
selection would probably favour different varieties in the different
islands. Some species, however, might spread and yet retain the same
character throughout the group, just as we see some species spreading
widely throughout a continent and remaining the same.

The really surprising fact in this case of the Galapagos Archipelago, and
in a lesser degree in some analogous cases, is that each new species after
being formed in any one island, did not spread quickly to the other
islands. But the islands, though in sight of each other, are separated by
deep arms of the sea, in most cases wider than the British Channel, and
there is no reason to suppose that they have at any former period been
continuously united. The currents of the sea are rapid and deep between
the islands, and gales of wind are extraordinarily rare; so that the
islands are far more effectually separated from each other than they
appear on a map. Nevertheless, some of the species, both of those found in
other parts of the world and of those confined to the archipelago, are
common to the several islands; and we may infer from the present manner of
distribution that they have spread from one island to the others. But we
often take, I think, an erroneous view of the probability of closely
allied species invading each other's territory, when put into free
intercommunication. Undoubtedly, if one species has any advantage over
another, it will in a very brief time wholly or in part supplant it; but
if both are equally well fitted for their own places, both will probably
hold their separate places for almost any length of time. Being familiar
with the fact that many species, naturalised through man's agency, have
spread with astonishing rapidity over wide areas, we are apt to infer that
most species would thus spread; but we should remember that the species
which become naturalised in new countries are not generally closely allied
to the aboriginal inhabitants, but are very distinct forms, belonging in a
large proportion of cases, as shown by Alph. de Candolle, to distinct
genera. In the Galapagos Archipelago, many even of the birds, though so
well adapted for flying from island to island, differ on the different
islands; thus there are three closely allied species of mocking-thrush,
each confined to its own island. Now let us suppose the mocking-thrush of
Chatham Island to be blown to Charles Island, which has its own
mocking-thrush; why should it succeed in establishing itself there? We may
safely infer that Charles Island is well stocked with its own species, for
annually more eggs are laid and young birds hatched than can possibly be
reared; and we may infer that the mocking-thrush peculiar to Charles
Island is at least as well fitted for its home as is the species peculiar
to Chatham Island. Sir C. Lyell and Mr. Wollaston have communicated to me
a remarkable fact bearing on this subject; namely, that Madeira and the
adjoining islet of Porto Santo possess many distinct but representative
species of land-shells, some of which live in crevices of stone; and
although large quantities of stone are annually transported from Porto
Santo to Madeira, yet this latter island has not become colonised by the
Porto Santo species: nevertheless, both islands have been colonised by
some European land-shells, which no doubt had some advantage over the
indigenous species. From these considerations I think we need not greatly
marvel at the endemic species which inhabit the several islands of the
Galapagos Archipelago not having all spread from island to island. On the
same continent, also, pre-occupation has probably played an important part
in checking the commingling of the species which inhabit different
districts with nearly the same physical conditions. Thus, the south-east
and south-west corners of Australia have nearly the same physical
conditions, and are united by continuous land, yet they are inhabited by a
vast number of distinct mammals, birds, and plants; so it is, according to
Mr. Bates, with the butterflies and other animals inhabiting the great,
open, and continuous valley of the Amazons.

The same principle which governs the general character of the inhabitants
of oceanic islands, namely, the relation to the source whence colonists
could have been most easily derived, together with their subsequent
modification, is of the widest application throughout nature. We see this
on every mountain-summit, in every lake and marsh. For Alpine species,
excepting in as far as the same species have become widely spread during
the Glacial epoch, are related to those of the surrounding lowlands; thus
we have in South America, Alpine humming-birds, Alpine rodents, Alpine
plants, etc., all strictly belonging to American forms; and it is obvious
that a mountain, as it became slowly upheaved, would be colonised from the
surrounding lowlands. So it is with the inhabitants of lakes and marshes,
excepting in so far as great facility of transport has allowed the same
forms to prevail throughout large portions of the world. We see the same
principle in the character of most of the blind animals inhabiting the
caves of America and of Europe. Other analogous facts could be given. It
will, I believe, be found universally true, that wherever in two regions,
let them be ever so distant, many closely allied or representative species
occur, there will likewise be found some identical species; and wherever
many closely-allied species occur, there will be found many forms which
some naturalists rank as distinct species, and others as mere varieties;
these doubtful forms showing us the steps in the process of modification.

The relation between the power and extent of migration in certain species,
either at the present or at some former period, and the existence at
remote points of the world of closely allied species, is shown in another
and more general way. Mr. Gould remarked to me long ago, that in those
genera of birds which range over the world, many of the species have very
wide ranges. I can hardly doubt that this rule is generally true, though
difficult of proof. Among mammals, we see it strikingly displayed in Bats,
and in a lesser degree in the Felidae and Canidae. We see the same rule in
the distribution of butterflies and beetles. So it is with most of the
inhabitants of fresh water, for many of the genera in the most distinct
classes range over the world, and many of the species have enormous
ranges. It is not meant that all, but that some of the species have very
wide ranges in the genera which range very widely. Nor is it meant that
the species in such genera have, on an average, a very wide range; for
this will largely depend on how far the process of modification has gone;
for instance, two varieties of the same species inhabit America and
Europe, and thus the species has an immense range; but, if variation were
to be carried a little further, the two varieties would be ranked as
distinct species, and their range would be greatly reduced. Still less is
it meant, that species which have the capacity of crossing barriers and
ranging widely, as in the case of certain powerfully-winged birds, will
necessarily range widely; for we should never forget that to range widely
implies not only the power of crossing barriers, but the more important
power of being victorious in distant lands in the struggle for life with
foreign associates. But according to the view that all the species of a
genus, though distributed to the most remote points of the world, are
descended from a single progenitor, we ought to find, and I believe as a
general rule we do find, that some at least of the species range very
widely.

We should bear in mind that many genera in all classes are of ancient
origin, and the species in this case will have had ample time for
dispersal and subsequent modification. There is also reason to believe,
from geological evidence, that within each great class the lower organisms
change at a slower rate than the higher; consequently they will have had a
better chance of ranging widely and of still retaining the same specific
character. This fact, together with that of the seeds and eggs of most
lowly organised forms being very minute and better fitted for distant
transportal, probably accounts for a law which has long been observed, and
which has lately been discussed by Alph. de Candolle in regard to plants,
namely, that the lower any group of organisms stands the more widely it
ranges.

The relations just discussed—namely, lower organisms ranging more
widely than the higher—some of the species of widely-ranging genera
themselves ranging widely—such facts, as alpine, lacustrine, and
marsh productions being generally related to those which live on the
surrounding low lands and dry lands—the striking relationship
between the inhabitants of islands and those of the nearest mainland—the
still closer relationship of the distinct inhabitants of the islands of
the same archipelago—are inexplicable on the ordinary view of the
independent creation of each species, but are explicable if we admit
colonisation from the nearest or readiest source, together with the
subsequent adaptation of the colonists to their new homes.

SUMMARY OF THE LAST AND PRESENT CHAPTERS.

In these chapters I have endeavoured to show that if we make due allowance
for our ignorance of the full effects of changes of climate and of the
level of the land, which have certainly occurred within the recent period,
and of other changes which have probably occurred—if we remember how
ignorant we are with respect to the many curious means of occasional
transport—if we bear in mind, and this is a very important
consideration, how often a species may have ranged continuously over a
wide area, and then have become extinct in the intermediate tracts—the
difficulty is not insuperable in believing that all the individuals of the
same species, wherever found, are descended from common parents. And we
are led to this conclusion, which has been arrived at by many naturalists
under the designation of single centres of creation, by various general
considerations, more especially from the importance of barriers of all
kinds, and from the analogical distribution of subgenera, genera, and
families.

With respect to distinct species belonging to the same genus, which on our
theory have spread from one parent-source; if we make the same allowances
as before for our ignorance, and remember that some forms of life have
changed very slowly, enormous periods of time having been thus granted for
their migration, the difficulties are far from insuperable; though in this
case, as in that of the individuals of the same species, they are often
great.

As exemplifying the effects of climatical changes on distribution, I have
attempted to show how important a part the last Glacial period has played,
which affected even the equatorial regions, and which, during the
alternations of the cold in the north and the south, allowed the
productions of opposite hemispheres to mingle, and left some of them
stranded on the mountain-summits in all parts of the world. As showing how
diversified are the means of occasional transport, I have discussed at
some little length the means of dispersal of fresh-water productions.

If the difficulties be not insuperable in admitting that in the long
course of time all the individuals of the same species, and likewise of
the several species belonging to the same genus, have proceeded from some
one source; then all the grand leading facts of geographical distribution
are explicable on the theory of migration, together with subsequent
modification and the multiplication of new forms. We can thus understand
the high importance of barriers, whether of land or water, in not only
separating but in apparently forming the several zoological and botanical
provinces. We can thus understand the concentration of related species
within the same areas; and how it is that under different latitudes, for
instance, in South America, the inhabitants of the plains and mountains,
of the forests, marshes, and deserts, are linked together in so mysterious
a manner, and are likewise linked to the extinct beings which formerly
inhabited the same continent. Bearing in mind that the mutual relation of
organism to organism is of the highest importance, we can see why two
areas, having nearly the same physical conditions, should often be
inhabited by very different forms of life; for according to the length of
time which has elapsed since the colonists entered one of the regions, or
both; according to the nature of the communication which allowed certain
forms and not others to enter, either in greater or lesser numbers;
according or not as those which entered happened to come into more or less
direct competition with each other and with the aborigines; and according
as the immigrants were capable of varying more or less rapidly, there
would ensue in the to or more regions, independently of their physical
conditions, infinitely diversified conditions of life; there would be an
almost endless amount of organic action and reaction, and we should find
some groups of beings greatly, and some only slightly modified; some
developed in great force, some existing in scanty numbers—and this
we do find in the several great geographical provinces of the world.

On these same principles we can understand, as I have endeavoured to show,
why oceanic islands should have few inhabitants, but that of these, a
large proportion should be endemic or peculiar; and why, in relation to
the means of migration, one group of beings should have all its species
peculiar, and another group, even within the same class, should have all
its species the same with those in an adjoining quarter of the world. We
can see why whole groups of organisms, as batrachians and terrestrial
mammals, should be absent from oceanic islands, whilst the most isolated
islands should possess their own peculiar species of aerial mammals or
bats. We can see why, in islands, there should be some relation between
the presence of mammals, in a more or less modified condition, and the
depth of the sea between such islands and the mainland. We can clearly see
why all the inhabitants of an archipelago, though specifically distinct on
the several islets, should be closely related to each other, and should
likewise be related, but less closely, to those of the nearest continent,
or other source whence immigrants might have been derived. We can see why,
if there exist very closely allied or representative species in two areas,
however distant from each other, some identical species will almost always
there be found.

As the late Edward Forbes often insisted, there is a striking parallelism
in the laws of life throughout time and space; the laws governing the
succession of forms in past times being nearly the same with those
governing at the present time the differences in different areas. We see
this in many facts. The endurance of each species and group of species is
continuous in time; for the apparent exceptions to the rule are so few
that they may fairly be attributed to our not having as yet discovered in
an intermediate deposit certain forms which are absent in it, but which
occur above and below: so in space, it certainly is the general rule that
the area inhabited by a single species, or by a group of species, is
continuous, and the exceptions, which are not rare, may, as I have
attempted to show, be accounted for by former migrations under different
circumstances, or through occasional means of transport, or by the species
having become extinct in the intermediate tracts. Both in time and space
species and groups of species have their points of maximum development.
Groups of species, living during the same period of time, or living within
the same area, are often characterised by trifling features in common, as
of sculpture or colour. In looking to the long succession of past ages, as
in looking to distant provinces throughout the world, we find that species
in certain classes differ little from each other, whilst those in another
class, or only in a different section of the same order, differ greatly
from each other. In both time and space the lowly organised members of
each class generally change less than the highly organised; but there are
in both cases marked exceptions to the rule. According to our theory,
these several relations throughout time and space are intelligible; for
whether we look to the allied forms of life which have changed during
successive ages, or to those which have changed after having migrated into
distant quarters, in both cases they are connected by the same bond of
ordinary generation; in both cases the laws of variation have been the
same, and modifications have been accumulated by the same means of natural
selection.



CHAPTER XIV. MUTUAL AFFINITIES OF ORGANIC BEINGS:

MORPHOLOGY—EMBRYOLOGY—RUDIMENTARY ORGANS.


 Classification, groups subordinate to groups—Natural system—Rules and
 difficulties in classification, explained on the theory of descent
 with modification—Classification of varieties—Descent always used in
 classification—Analogical or adaptive characters—Affinities,
 general, complex and radiating—Extinction separates and defines
 groups—Morphology, between members of the same class, between parts of
 the same individual—Embryology, laws of, explained by variations not
 supervening at an early age, and being inherited at a corresponding
 age—Rudimentary organs; their origin explained—Summary.

CLASSIFICATION.

From the most remote period in the history of the world organic beings
have been found to resemble each other in descending degrees, so that they
can be classed in groups under groups. This classification is not
arbitrary like the grouping of the stars in constellations. The existence
of groups would have been of simple significance, if one group had been
exclusively fitted to inhabit the land, and another the water; one to feed
on flesh, another on vegetable matter, and so on; but the case is widely
different, for it is notorious how commonly members of even the same
subgroup have different habits. In the second and fourth chapters, on
Variation and on Natural Selection, I have attempted to show that within
each country it is the widely ranging, the much diffused and common, that
is the dominant species, belonging to the larger genera in each class,
which vary most. The varieties, or incipient species, thus produced,
ultimately become converted into new and distinct species; and these, on
the principle of inheritance, tend to produce other new and dominant
species. Consequently the groups which are now large, and which generally
include many dominant species, tend to go on increasing in size. I further
attempted to show that from the varying descendants of each species trying
to occupy as many and as different places as possible in the economy of
nature, they constantly tend to diverge in character. This latter
conclusion is supported by observing the great diversity of forms, which,
in any small area, come into the closest competition, and by certain facts
in naturalisation.

I attempted also to show that there is a steady tendency in the forms
which are increasing in number and diverging in character, to supplant and
exterminate the preceding, less divergent and less improved forms. I
request the reader to turn to the diagram illustrating the action, as
formerly explained, of these several principles; and he will see that the
inevitable result is, that the modified descendants proceeding from one
progenitor become broken up into groups subordinate to groups. In the
diagram each letter on the uppermost line may represent a genus including
several species; and the whole of the genera along this upper line form
together one class, for all are descended from one ancient parent, and,
consequently, have inherited something in common. But the three genera on
the left hand have, on this same principle, much in common, and form a
subfamily, distinct from that containing the next two genera on the right
hand, which diverged from a common parent at the fifth stage of descent.
These five genera have also much in common, though less than when grouped
in subfamilies; and they form a family distinct from that containing the
three genera still further to the right hand, which diverged at an earlier
period. And all these genera, descended from (A), form an order distinct
from the genera descended from (I). So that we here have many species
descended from a single progenitor grouped into genera; and the genera
into subfamilies, families and orders, all under one great class. The
grand fact of the natural subordination of organic beings in groups under
groups, which, from its familiarity, does not always sufficiently strike
us, is in my judgment thus explained. No doubt organic beings, like all
other objects, can be classed in many ways, either artificially by single
characters, or more naturally by a number of characters. We know, for
instance, that minerals and the elemental substances can be thus arranged.
In this case there is of course no relation to genealogical succession,
and no cause can at present be assigned for their falling into groups. But
with organic beings the case is different, and the view above given
accords with their natural arrangement in group under group; and no other
explanation has ever been attempted.

Naturalists, as we have seen, try to arrange the species, genera and
families in each class, on what is called the Natural System. But what is
meant by this system? Some authors look at it merely as a scheme for
arranging together those living objects which are most alike, and for
separating those which are most unlike; or as an artificial method of
enunciating, as briefly as possible, general propositions—that is,
by one sentence to give the characters common, for instance, to all
mammals, by another those common to all carnivora, by another those common
to the dog-genus, and then, by adding a single sentence, a full
description is given of each kind of dog. The ingenuity and utility of
this system are indisputable. But many naturalists think that something
more is meant by the Natural System; they believe that it reveals the plan
of the Creator; but unless it be specified whether order in time or space,
or both, or what else is meant by the plan of the Creator, it seems to me
that nothing is thus added to our knowledge. Expressions such as that
famous one by Linnæus, which we often meet with in a more or less
concealed form, namely, that the characters do not make the genus, but
that the genus gives the characters, seem to imply that some deeper bond
is included in our classifications than mere resemblance. I believe that
this is the case, and that community of descent—the one known cause
of close similarity in organic beings—is the bond, which, though
observed by various degrees of modification, is partially revealed to us
by our classifications.

Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification either
gives some unknown plan of creation, or is simply a scheme for enunciating
general propositions and of placing together the forms most like each
other. It might have been thought (and was in ancient times thought) that
those parts of the structure which determined the habits of life, and the
general place of each being in the economy of nature, would be of very
high importance in classification. Nothing can be more false. No one
regards the external similarity of a mouse to a shrew, of a dugong to a
whale, of a whale to a fish, as of any importance. These resemblances,
though so intimately connected with the whole life of the being, are
ranked as merely "adaptive or analogical characters;" but to the
consideration of these resemblances we shall recur. It may even be given
as a general rule, that the less any part of the organisation is concerned
with special habits, the more important it becomes for classification. As
an instance: Owen, in speaking of the dugong, says, "The generative
organs, being those which are most remotely related to the habits and food
of an animal, I have always regarded as affording very clear indications
of its true affinities. We are least likely in the modifications of these
organs to mistake a merely adaptive for an essential character." With
plants how remarkable it is that the organs of vegetation, on which their
nutrition and life depend, are of little signification; whereas the organs
of reproduction, with their product the seed and embryo, are of paramount
importance! So again, in formerly discussing certain morphological
characters which are not functionally important, we have seen that they
are often of the highest service in classification. This depends on their
constancy throughout many allied groups; and their constancy chiefly
depends on any slight deviations not having been preserved and accumulated
by natural selection, which acts only on serviceable characters.

That the mere physiological importance of an organ does not determine its
classificatory value, is almost proved by the fact, that in allied groups,
in which the same organ, as we have every reason to suppose, has nearly
the same physiological value, its classificatory value is widely
different. No naturalist can have worked at any group without being struck
with this fact; and it has been fully acknowledged in the writings of
almost every author. It will suffice to quote the highest authority,
Robert Brown, who, in speaking of certain organs in the Proteaceae, says
their generic importance, "like that of all their parts, not only in this,
but, as I apprehend in every natural family, is very unequal, and in some
cases seems to be entirely lost." Again, in another work he says, the
genera of the Connaraceae "differ in having one or more ovaria, in the
existence or absence of albumen, in the imbricate or valvular aestivation.
Any one of these characters singly is frequently of more than generic
importance, though here even, when all taken together, they appear
insufficient to separate Cnestis from Connarus." To give an example among
insects: in one great division of the Hymenoptera, the antennae, as
Westwood has remarked, are most constant in structure; in another division
they differ much, and the differences are of quite subordinate value in
classification; yet no one will say that the antennae in these two
divisions of the same order are of unequal physiological importance. Any
number of instances could be given of the varying importance for
classification of the same important organ within the same group of
beings.

Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this
condition are often of much value in classification. No one will dispute
that the rudimentary teeth in the upper jaws of young ruminants, and
certain rudimentary bones of the leg, are highly serviceable in exhibiting
the close affinity between Ruminants and Pachyderms. Robert Brown has
strongly insisted on the fact that the position of the rudimentary florets
is of the highest importance in the classification of the Grasses.

Numerous instances could be given of characters derived from parts which
must be considered of very trifling physiological importance, but which
are universally admitted as highly serviceable in the definition of whole
groups. For instance, whether or not there is an open passage from the
nostrils to the mouth, the only character, according to Owen, which
absolutely distinguishes fishes and reptiles—the inflection of the
angle of the lower jaw in Marsupials—the manner in which the wings
of insects are folded—mere colour in certain Algae—mere
pubescence on parts of the flower in grasses—the nature of the
dermal covering, as hair or feathers, in the Vertebrata. If the
Ornithorhynchus had been covered with feathers instead of hair, this
external and trifling character would have been considered by naturalists
as an important aid in determining the degree of affinity of this strange
creature to birds.

The importance, for classification, of trifling characters, mainly depends
on their being correlated with many other characters of more or less
importance. The value indeed of an aggregate of characters is very evident
in natural history. Hence, as has often been remarked, a species may
depart from its allies in several characters, both of high physiological
importance, and of almost universal prevalence, and yet leave us in no
doubt where it should be ranked. Hence, also, it has been found that a
classification founded on any single character, however important that may
be, has always failed; for no part of the organisation is invariably
constant. The importance of an aggregate of characters, even when none are
important, alone explains the aphorism enunciated by Linnæus, namely,
that the characters do not give the genus, but the genus gives the
character; for this seems founded on the appreciation of many trifling
points of resemblance, too slight to be defined. Certain plants, belonging
to the Malpighiaceae, bear perfect and degraded flowers; in the latter, as
A. de Jussieu has remarked, "The greater number of the characters proper
to the species, to the genus, to the family, to the class, disappear, and
thus laugh at our classification." When Aspicarpa produced in France,
during several years, only these degraded flowers, departing so
wonderfully in a number of the most important points of structure from the
proper type of the order, yet M. Richard sagaciously saw, as Jussieu
observes, that this genus should still be retained among the
Malpighiaceae. This case well illustrates the spirit of our
classifications.

Practically, when naturalists are at work, they do not trouble themselves
about the physiological value of the characters which they use in defining
a group or in allocating any particular species. If they find a character
nearly uniform, and common to a great number of forms, and not common to
others, they use it as one of high value; if common to some lesser number,
they use it as of subordinate value. This principle has been broadly
confessed by some naturalists to be the true one; and by none more clearly
than by that excellent botanist, Aug. St. Hilaire. If several trifling
characters are always found in combination, though no apparent bond of
connexion can be discovered between them, especial value is set on them.
As in most groups of animals, important organs, such as those for
propelling the blood, or for aerating it, or those for propagating the
race, are found nearly uniform, they are considered as highly serviceable
in classification; but in some groups all these, the most important vital
organs, are found to offer characters of quite subordinate value. Thus, as
Fritz Muller has lately remarked, in the same group of crustaceans,
Cypridina is furnished with a heart, while in two closely allied genera,
namely Cypris and Cytherea, there is no such organ; one species of
Cypridina has well-developed branchiae, while another species is destitute
of them.

We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for a natural classification
of course includes all ages. But it is by no means obvious, on the
ordinary view, why the structure of the embryo should be more important
for this purpose than that of the adult, which alone plays its full part
in the economy of nature. Yet it has been strongly urged by those great
naturalists, Milne Edwards and Agassiz, that embryological characters are
the most important of all; and this doctrine has very generally been
admitted as true. Nevertheless, their importance has sometimes been
exaggerated, owing to the adaptive characters of larvae not having been
excluded; in order to show this, Fritz Muller arranged, by the aid of such
characters alone, the great class of crustaceans, and the arrangement did
not prove a natural one. But there can be no doubt that embryonic,
excluding larval characters, are of the highest value for classification,
not only with animals but with plants. Thus the main divisions of
flowering plants are founded on differences in the embryo—on the
number and position of the cotyledons, and on the mode of development of
the plumule and radicle. We shall immediately see why these characters
possess so high a value in classification, namely, from the natural system
being genealogical in its arrangement.

Our classifications are often plainly influenced by chains of affinities.
Nothing can be easier than to define a number of characters common to all
birds; but with crustaceans, any such definition has hitherto been found
impossible. There are crustaceans at the opposite ends of the series,
which have hardly a character in common; yet the species at both ends,
from being plainly allied to others, and these to others, and so onwards,
can be recognised as unequivocally belonging to this, and to no other
class of the Articulata.

Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of
closely allied forms. Temminck insists on the utility or even necessity of
this practice in certain groups of birds; and it has been followed by
several entomologists and botanists.

Finally, with respect to the comparative value of the various groups of
species, such as orders, suborders, families, subfamilies, and genera,
they seem to be, at least at present, almost arbitrary. Several of the
best botanists, such as Mr. Bentham and others, have strongly insisted on
their arbitrary value. Instances could be given among plants and insects,
of a group first ranked by practised naturalists as only a genus, and then
raised to the rank of a subfamily or family; and this has been done, not
because further research has detected important structural differences, at
first overlooked, but because numerous allied species, with slightly
different grades of difference, have been subsequently discovered.

All the foregoing rules and aids and difficulties in classification may be
explained, if I do not greatly deceive myself, on the view that the
natural system is founded on descent with modification—that the
characters which naturalists consider as showing true affinity between any
two or more species, are those which have been inherited from a common
parent, all true classification being genealogical—that community of
descent is the hidden bond which naturalists have been unconsciously
seeking, and not some unknown plan of creation, or the enunciation of
general propositions, and the mere putting together and separating objects
more or less alike.

But I must explain my meaning more fully. I believe that the ARRANGEMENT
of the groups within each class, in due subordination and relation to each
other, must be strictly genealogical in order to be natural; but that the
AMOUNT of difference in the several branches or groups, though allied in
the same degree in blood to their common progenitor, may differ greatly,
being due to the different degrees of modification which they have
undergone; and this is expressed by the forms being ranked under different
genera, families, sections or orders. The reader will best understand what
is meant, if he will take the trouble to refer to the diagram in the
fourth chapter. We will suppose the letters A to L to represent allied
genera existing during the Silurian epoch, and descended from some still
earlier form. In three of these genera (A, F, and I) a species has
transmitted modified descendants to the present day, represented by the
fifteen genera (a14 to z14) on the uppermost horizontal line. Now, all
these modified descendants from a single species are related in blood or
descent in the same degree. They may metaphorically be called cousins to
the same millionth degree, yet they differ widely and in different degrees
from each other. The forms descended from A, now broken up into two or
three families, constitute a distinct order from those descended from I,
also broken up into two families. Nor can the existing species descended
from A be ranked in the same genus with the parent A, or those from I with
parent I. But the existing genus F14 may be supposed to have been but
slightly modified, and it will then rank with the parent genus F; just as
some few still living organisms belong to Silurian genera. So that the
comparative value of the differences between these organic beings, which
are all related to each other in the same degree in blood, has come to be
widely different. Nevertheless, their genealogical ARRANGEMENT remains
strictly true, not only at the present time, but at each successive period
of descent. All the modified descendants from A will have inherited
something in common from their common parent, as will all the descendants
from I; so will it be with each subordinate branch of descendants at each
successive stage. If, however, we suppose any descendant of A or of I to
have become so much modified as to have lost all traces of its parentage
in this case, its place in the natural system will be lost, as seems to
have occurred with some few existing organisms. All the descendants of the
genus F, along its whole line of descent, are supposed to have been but
little modified, and they form a single genus. But this genus, though much
isolated, will still occupy its proper intermediate position. The
representation of the groups as here given in the diagram on a flat
surface, is much too simple. The branches ought to have diverged in all
directions. If the names of the groups had been simply written down in a
linear series the representation would have been still less natural; and
it is notoriously not possible to represent in a series, on a flat
surface, the affinities which we discover in nature among the beings of
the same group. Thus, the natural system is genealogical in its
arrangement, like a pedigree. But the amount of modification which the
different groups have undergone has to be expressed by ranking them under
different so-called genera, subfamilies, families, sections, orders, and
classes.

It may be worth while to illustrate this view of classification, by taking
the case of languages. If we possessed a perfect pedigree of mankind, a
genealogical arrangement of the races of man would afford the best
classification of the various languages now spoken throughout the world;
and if all extinct languages, and all intermediate and slowly changing
dialects, were to be included, such an arrangement would be the only
possible one. Yet it might be that some ancient languages had altered very
little and had given rise to few new languages, whilst others had altered
much owing to the spreading, isolation and state of civilisation of the
several co-descended races, and had thus given rise to many new dialects
and languages. The various degrees of difference between the languages of
the same stock would have to be expressed by groups subordinate to groups;
but the proper or even the only possible arrangement would still be
genealogical; and this would be strictly natural, as it would connect
together all languages, extinct and recent, by the closest affinities, and
would give the filiation and origin of each tongue.

In confirmation of this view, let us glance at the classification of
varieties, which are known or believed to be descended from a single
species. These are grouped under the species, with the subvarieties under
the varieties; and in some cases, as with the domestic pigeon, with
several other grades of difference. Nearly the same rules are followed as
in classifying species. Authors have insisted on the necessity of
arranging varieties on a natural instead of an artificial system; we are
cautioned, for instance, not to class two varieties of the pine-apple
together, merely because their fruit, though the most important part,
happens to be nearly identical; no one puts the Swedish and common turnip
together, though the esculent and thickened stems are so similar. Whatever
part is found to be most constant, is used in classing varieties: thus the
great agriculturist Marshall says the horns are very useful for this
purpose with cattle, because they are less variable than the shape or
colour of the body, etc.; whereas with sheep the horns are much less
serviceable, because less constant. In classing varieties, I apprehend
that if we had a real pedigree, a genealogical classification would be
universally preferred; and it has been attempted in some cases. For we
might feel sure, whether there had been more or less modification, that
the principle of inheritance would keep the forms together which were
allied in the greatest number of points. In tumbler pigeons, though some
of the subvarieties differ in the important character of the length of the
beak, yet all are kept together from having the common habit of tumbling;
but the short-faced breed has nearly or quite lost this habit;
nevertheless, without any thought on the subject, these tumblers are kept
in the same group, because allied in blood and alike in some other
respects.

With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade, that
of species, the two sexes; and how enormously these sometimes differ in
the most important characters is known to every naturalist: scarcely a
single fact can be predicated in common of the adult males and
hermaphrodites of certain cirripedes, and yet no one dreams of separating
them. As soon as the three Orchidean forms, Monachanthus, Myanthus, and
Catasetum, which had previously been ranked as three distinct genera, were
known to be sometimes produced on the same plant, they were immediately
considered as varieties; and now I have been able to show that they are
the male, female, and hermaphrodite forms of the same species. The
naturalist includes as one species the various larval stages of the same
individual, however much they may differ from each other and from the
adult; as well as the so-called alternate generations of Steenstrup, which
can only in a technical sense be considered as the same individual. He
includes monsters and varieties, not from their partial resemblance to the
parent-form, but because they are descended from it.

As descent has universally been used in classing together the individuals
of the same species, though the males and females and larvae are sometimes
extremely different; and as it has been used in classing varieties which
have undergone a certain, and sometimes a considerable amount of
modification, may not this same element of descent have been unconsciously
used in grouping species under genera, and genera under higher groups, all
under the so-called natural system? I believe it has been unconsciously
used; and thus only can I understand the several rules and guides which
have been followed by our best systematists. As we have no written
pedigrees, we are forced to trace community of descent by resemblances of
any kind. Therefore, we choose those characters which are the least likely
to have been modified, in relation to the conditions of life to which each
species has been recently exposed. Rudimentary structures on this view are
as good as, or even sometimes better than other parts of the organisation.
We care not how trifling a character may be—let it be the mere
inflection of the angle of the jaw, the manner in which an insect's wing
is folded, whether the skin be covered by hair or feathers—if it
prevail throughout many and different species, especially those having
very different habits of life, it assumes high value; for we can account
for its presence in so many forms with such different habits, only by
inheritance from a common parent. We may err in this respect in regard to
single points of structure, but when several characters, let them be ever
so trifling, concur throughout a large group of beings having different
habits, we may feel almost sure, on the theory of descent, that these
characters have been inherited from a common ancestor; and we know that
such aggregated characters have especial value in classification.

We can understand why a species or a group of species may depart from its
allies, in several of its most important characteristics, and yet be
safely classed with them. This may be safely done, and is often done, as
long as a sufficient number of characters, let them be ever so
unimportant, betrays the hidden bond of community of descent. Let two
forms have not a single character in common, yet, if these extreme forms
are connected together by a chain of intermediate groups, we may at once
infer their community of descent, and we put them all into the same class.
As we find organs of high physiological importance—those which serve
to preserve life under the most diverse conditions of existence—are
generally the most constant, we attach especial value to them; but if
these same organs, in another group or section of a group, are found to
differ much, we at once value them less in our classification. We shall
presently see why embryological characters are of such high classificatory
importance. Geographical distribution may sometimes be brought usefully
into play in classing large genera, because all the species of the same
genus, inhabiting any distinct and isolated region, are in all probability
descended from the same parents.

ANALOGICAL RESEMBLANCES.

We can understand, on the above views, the very important distinction
between real affinities and analogical or adaptive resemblances. Lamarck
first called attention to this subject, and he has been ably followed by
Macleay and others. The resemblance in the shape of the body and in the
fin-like anterior limbs between dugongs and whales, and between these two
orders of mammals and fishes, are analogical. So is the resemblance
between a mouse and a shrew-mouse (Sorex), which belong to different
orders; and the still closer resemblance, insisted on by Mr. Mivart,
between the mouse and a small marsupial animal (Antechinus) of Australia.
These latter resemblances may be accounted for, as it seems to me, by
adaptation for similarly active movements through thickets and herbage,
together with concealment from enemies.

Among insects there are innumerable instances; thus Linnæus, misled by
external appearances, actually classed an homopterous insect as a moth. We
see something of the same kind even with our domestic varieties, as in the
strikingly similar shape of the body in the improved breeds of the Chinese
and common pig, which are descended from distinct species; and in the
similarly thickened stems of the common and specifically distinct Swedish
turnip. The resemblance between the greyhound and race-horse is hardly
more fanciful than the analogies which have been drawn by some authors
between widely different animals.

On the view of characters being of real importance for classification,
only in so far as they reveal descent, we can clearly understand why
analogical or adaptive characters, although of the utmost importance to
the welfare of the being, are almost valueless to the systematist. For
animals, belonging to two most distinct lines of descent, may have become
adapted to similar conditions, and thus have assumed a close external
resemblance; but such resemblances will not reveal—will rather tend
to conceal their blood-relationship. We can thus also understand the
apparent paradox, that the very same characters are analogical when one
group is compared with another, but give true affinities when the members
of the same group are compared together: thus the shape of the body and
fin-like limbs are only analogical when whales are compared with fishes,
being adaptations in both classes for swimming through the water; but
between the the several members of the whale family, the shape of the body
and the fin-like limbs offer characters exhibiting true affinity; for as
these parts are so nearly similar throughout the whole family, we cannot
doubt that they have been inherited from a common ancestor. So it is with
fishes.

Numerous cases could be given of striking resemblances in quite distinct
beings between single parts or organs, which have been adapted for the
same functions. A good instance is afforded by the close resemblance of
the jaws of the dog and Tasmanian wolf or Thylacinus—animals which
are widely sundered in the natural system. But this resemblance is
confined to general appearance, as in the prominence of the canines, and
in the cutting shape of the molar teeth. For the teeth really differ much:
thus the dog has on each side of the upper jaw four pre-molars and only
two molars; while the Thylacinus has three pre-molars and four molars. The
molars also differ much in the two animals in relative size and structure.
The adult dentition is preceded by a widely different milk dentition. Any
one may, of course, deny that the teeth in either case have been adapted
for tearing flesh, through the natural selection of successive variations;
but if this be admitted in the one case, it is unintelligible to me that
it should be denied in the other. I am glad to find that so high an
authority as Professor Flower has come to this same conclusion.

The extraordinary cases given in a former chapter, of widely different
fishes possessing electric organs—of widely different insects
possessing luminous organs—and of orchids and asclepiads having
pollen-masses with viscid discs, come under this same head of analogical
resemblances. But these cases are so wonderful that they were introduced
as difficulties or objections to our theory. In all such cases some
fundamental difference in the growth or development of the parts, and
generally in their matured structure, can be detected. The end gained is
the same, but the means, though appearing superficially to be the same,
are essentially different. The principle formerly alluded to under the
term of ANALOGICAL VARIATION has probably in these cases often come into
play; that is, the members of the same class, although only distantly
allied, have inherited so much in common in their constitution, that they
are apt to vary under similar exciting causes in a similar manner; and
this would obviously aid in the acquirement through natural selection of
parts or organs, strikingly like each other, independently of their direct
inheritance from a common progenitor.

As species belonging to distinct classes have often been adapted by
successive slight modifications to live under nearly similar circumstances—to
inhabit, for instance, the three elements of land, air and water—we
can perhaps understand how it is that a numerical parallelism has
sometimes been observed between the subgroups of distinct classes. A
naturalist, struck with a parallelism of this nature, by arbitrarily
raising or sinking the value of the groups in several classes (and all our
experience shows that their valuation is as yet arbitrary), could easily
extend the parallelism over a wide range; and thus the septenary, quinary,
quaternary and ternary classifications have probably arisen.

There is another and curious class of cases in which close external
resemblance does not depend on adaptation to similar habits of life, but
has been gained for the sake of protection. I allude to the wonderful
manner in which certain butterflies imitate, as first described by Mr.
Bates, other and quite distinct species. This excellent observer has shown
that in some districts of South America, where, for instance, an Ithomia
abounds in gaudy swarms, another butterfly, namely, a Leptalis, is often
found mingled in the same flock; and the latter so closely resembles the
Ithomia in every shade and stripe of colour, and even in the shape of its
wings, that Mr. Bates, with his eyes sharpened by collecting during eleven
years, was, though always on his guard, continually deceived. When the
mockers and the mocked are caught and compared, they are found to be very
different in essential structure, and to belong not only to distinct
genera, but often to distinct families. Had this mimicry occurred in only
one or two instances, it might have been passed over as a strange
coincidence. But, if we proceed from a district where one Leptalis
imitates an Ithomia, another mocking and mocked species, belonging to the
same two genera, equally close in their resemblance, may be found.
Altogether no less than ten genera are enumerated, which include species
that imitate other butterflies. The mockers and mocked always inhabit the
same region; we never find an imitator living remote from the form which
it imitates. The mockers are almost invariably rare insects; the mocked in
almost every case abounds in swarms. In the same district in which a
species of Leptalis closely imitates an Ithomia, there are sometimes other
Lepidoptera mimicking the same Ithomia: so that in the same place, species
of three genera of butterflies and even a moth are found all closely
resembling a butterfly belonging to a fourth genus. It deserves especial
notice that many of the mimicking forms of the Leptalis, as well as of the
mimicked forms, can be shown by a graduated series to be merely varieties
of the same species; while others are undoubtedly distinct species. But
why, it may be asked, are certain forms treated as the mimicked and others
as the mimickers? Mr. Bates satisfactorily answers this question by
showing that the form which is imitated keeps the usual dress of the group
to which it belongs, while the counterfeiters have changed their dress and
do not resemble their nearest allies.

We are next led to enquire what reason can be assigned for certain
butterflies and moths so often assuming the dress of another and quite
distinct form; why, to the perplexity of naturalists, has nature
condescended to the tricks of the stage? Mr. Bates has, no doubt, hit on
the true explanation. The mocked forms, which always abound in numbers,
must habitually escape destruction to a large extent, otherwise they could
not exist in such swarms; and a large amount of evidence has now been
collected, showing that they are distasteful to birds and other
insect-devouring animals. The mocking forms, on the other hand, that
inhabit the same district, are comparatively rare, and belong to rare
groups; hence, they must suffer habitually from some danger, for
otherwise, from the number of eggs laid by all butterflies, they would in
three or four generations swarm over the whole country. Now if a member of
one of these persecuted and rare groups were to assume a dress so like
that of a well-protected species that it continually deceived the
practised eyes of an entomologist, it would often deceive predaceous birds
and insects, and thus often escape destruction. Mr. Bates may almost be
said to have actually witnessed the process by which the mimickers have
come so closely to resemble the mimicked; for he found that some of the
forms of Leptalis which mimic so many other butterflies, varied in an
extreme degree. In one district several varieties occurred, and of these
one alone resembled, to a certain extent, the common Ithomia of the same
district. In another district there were two or three varieties, one of
which was much commoner than the others, and this closely mocked another
form of Ithomia. From facts of this nature, Mr. Bates concludes that the
Leptalis first varies; and when a variety happens to resemble in some
degree any common butterfly inhabiting the same district, this variety,
from its resemblance to a flourishing and little persecuted kind, has a
better chance of escaping destruction from predaceous birds and insects,
and is consequently oftener preserved; "the less perfect degrees of
resemblance being generation after generation eliminated, and only the
others left to propagate their kind." So that here we have an excellent
illustration of natural selection.

Messrs. Wallace and Trimen have likewise described several equally
striking cases of imitation in the Lepidoptera of the Malay Archipelago
and Africa, and with some other insects. Mr. Wallace has also detected one
such case with birds, but we have none with the larger quadrupeds. The
much greater frequency of imitation with insects than with other animals,
is probably the consequence of their small size; insects cannot defend
themselves, excepting indeed the kinds furnished with a sting, and I have
never heard of an instance of such kinds mocking other insects, though
they are mocked; insects cannot easily escape by flight from the larger
animals which prey on them; therefore, speaking metaphorically, they are
reduced, like most weak creatures, to trickery and dissimulation.

It should be observed that the process of imitation probably never
commenced between forms widely dissimilar in colour. But, starting with
species already somewhat like each other, the closest resemblance, if
beneficial, could readily be gained by the above means, and if the
imitated form was subsequently and gradually modified through any agency,
the imitating form would be led along the same track, and thus be altered
to almost any extent, so that it might ultimately assume an appearance or
colouring wholly unlike that of the other members of the family to which
it belonged. There is, however, some difficulty on this head, for it is
necessary to suppose in some cases that ancient members belonging to
several distinct groups, before they had diverged to their present extent,
accidentally resembled a member of another and protected group in a
sufficient degree to afford some slight protection, this having given the
basis for the subsequent acquisition of the most perfect resemblance.

ON THE NATURE OF THE AFFINITIES CONNECTING ORGANIC BEINGS.

As the modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages which made the groups to which they
belong large and their parents dominant, they are almost sure to spread
widely, and to seize on more and more places in the economy of nature. The
larger and more dominant groups within each class thus tend to go on
increasing in size, and they consequently supplant many smaller and
feebler groups. Thus, we can account for the fact that all organisms,
recent and extinct, are included under a few great orders and under still
fewer classes. As showing how few the higher groups are in number, and how
widely they are spread throughout the world, the fact is striking that the
discovery of Australia has not added an insect belonging to a new class,
and that in the vegetable kingdom, as I learn from Dr. Hooker, it has
added only two or three families of small size.

In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character during
the long-continued process of modification, how it is that the more
ancient forms of life often present characters in some degree intermediate
between existing groups. As some few of the old and intermediate forms
having transmitted to the present day descendants but little modified,
these constitute our so-called osculant or aberrant groups. The more
aberrant any form is, the greater must be the number of connecting forms
which have been exterminated and utterly lost. And we have evidence of
aberrant groups having suffered severely from extinction, for they are
almost always represented by extremely few species; and such species as do
occur are generally very distinct from each other, which again implies
extinction. The genera Ornithorhynchus and Lepidosiren, for example, would
not have been less aberrant had each been represented by a dozen species,
instead of as at present by a single one, or by two or three. We can, I
think, account for this fact only by looking at aberrant groups as forms
which have been conquered by more successful competitors, with a few
members still preserved under unusually favourable conditions.

Mr. Waterhouse has remarked that when a member belonging to one group of
animals exhibits an affinity to a quite distinct group, this affinity in
most cases is general and not special: thus, according to Mr. Waterhouse,
of all Rodents, the bizcacha is most nearly related to Marsupials; but in
the points in which it approaches this order, its relations are general,
that is, not to any one Marsupial species more than to another. As these
points of affinity are believed to be real and not merely adaptive, they
must be due in accordance with our view to inheritance from a common
progenitor. Therefore, we must suppose either that all Rodents, including
the bizcacha, branched off from some ancient Marsupial, which will
naturally have been more or less intermediate in character with respect to
all existing Marsupials; or that both Rodents and Marsupials branched off
from a common progenitor, and that both groups have since undergone much
modification in divergent directions. On either view we must suppose that
the bizcacha has retained, by inheritance, more of the character of its
ancient progenitor than have other Rodents; and therefore it will not be
specially related to any one existing Marsupial, but indirectly to all or
nearly all Marsupials, from having partially retained the character of
their common progenitor, or of some early member of the group. On the
other hand, of all Marsupials, as Mr. Waterhouse has remarked, the
Phascolomys resembles most nearly, not any one species, but the general
order of Rodents. In this case, however, it may be strongly suspected that
the resemblance is only analogical, owing to the Phascolomys having become
adapted to habits like those of a Rodent. The elder De Candolle has made
nearly similar observations on the general nature of the affinities of
distinct families of plants.

On the principle of the multiplication and gradual divergence in character
of the species descended from a common progenitor, together with their
retention by inheritance of some characters in common, we can understand
the excessively complex and radiating affinities by which all the members
of the same family or higher group are connected together. For the common
progenitor of a whole family, now broken up by extinction into distinct
groups and subgroups, will have transmitted some of its characters,
modified in various ways and degrees, to all the species; and they will
consequently be related to each other by circuitous lines of affinity of
various lengths (as may be seen in the diagram so often referred to),
mounting up through many predecessors. As it is difficult to show the
blood-relationship between the numerous kindred of any ancient and noble
family, even by the aid of a genealogical tree, and almost impossible to
do so without this aid, we can understand the extraordinary difficulty
which naturalists have experienced in describing, without the aid of a
diagram, the various affinities which they perceive between the many
living and extinct members of the same great natural class.

Extinction, as we have seen in the fourth chapter, has played an important
part in defining and widening the intervals between the several groups in
each class. We may thus account for the distinctness of whole classes from
each other—for instance, of birds from all other vertebrate animals—by
the belief that many ancient forms of life have been utterly lost, through
which the early progenitors of birds were formerly connected with the
early progenitors of the other and at that time less differentiated
vertebrate classes. There has been much less extinction of the forms of
life which once connected fishes with Batrachians. There has been still
less within some whole classes, for instance the Crustacea, for here the
most wonderfully diverse forms are still linked together by a long and
only partially broken chain of affinities. Extinction has only defined the
groups: it has by no means made them; for if every form which has ever
lived on this earth were suddenly to reappear, though it would be quite
impossible to give definitions by which each group could be distinguished,
still a natural classification, or at least a natural arrangement, would
be possible. We shall see this by turning to the diagram: the letters, A
to L, may represent eleven Silurian genera, some of which have produced
large groups of modified descendants, with every link in each branch and
sub-branch still alive; and the links not greater than those between
existing varieties. In this case it would be quite impossible to give
definitions by which the several members of the several groups could be
distinguished from their more immediate parents and descendants. Yet the
arrangement in the diagram would still hold good and would be natural;
for, on the principle of inheritance, all the forms descended, for
instance from A, would have something in common. In a tree we can
distinguish this or that branch, though at the actual fork the two unite
and blend together. We could not, as I have said, define the several
groups; but we could pick out types, or forms, representing most of the
characters of each group, whether large or small, and thus give a general
idea of the value of the differences between them. This is what we should
be driven to, if we were ever to succeed in collecting all the forms in
any one class which have lived throughout all time and space. Assuredly we
shall never succeed in making so perfect a collection: nevertheless, in
certain classes, we are tending toward this end; and Milne Edwards has
lately insisted, in an able paper, on the high importance of looking to
types, whether or not we can separate and define the groups to which such
types belong.

Finally, we have seen that natural selection, which follows from the
struggle for existence, and which almost inevitably leads to extinction
and divergence of character in the descendants from any one
parent-species, explains that great and universal feature in the
affinities of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the individuals of
both sexes and of all ages under one species, although they may have but
few characters in common; we use descent in classing acknowledged
varieties, however different they may be from their parents; and I believe
that this element of descent is the hidden bond of connexion which
naturalists have sought under the term of the Natural System. On this idea
of the natural system being, in so far as it has been perfected,
genealogical in its arrangement, with the grades of difference expressed
by the terms genera, families, orders, etc., we can understand the rules
which we are compelled to follow in our classification. We can understand
why we value certain resemblances far more than others; why we use
rudimentary and useless organs, or others of trifling physiological
importance; why, in finding the relations between one group and another,
we summarily reject analogical or adaptive characters, and yet use these
same characters within the limits of the same group. We can clearly see
how it is that all living and extinct forms can be grouped together within
a few great classes; and how the several members of each class are
connected together by the most complex and radiating lines of affinities.
We shall never, probably, disentangle the inextricable web of the
affinities between the members of any one class; but when we have a
distinct object in view, and do not look to some unknown plan of creation,
we may hope to make sure but slow progress.

Professor Haeckel in his "Generelle Morphologie" and in another works, has
recently brought his great knowledge and abilities to bear on what he
calls phylogeny, or the lines of descent of all organic beings. In drawing
up the several series he trusts chiefly to embryological characters, but
receives aid from homologous and rudimentary organs, as well as from the
successive periods at which the various forms of life are believed to have
first appeared in our geological formations. He has thus boldly made a
great beginning, and shows us how classification will in the future be
treated.

MORPHOLOGY.

We have seen that the members of the same class, independently of their
habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term "unity of
type;" or by saying that the several parts and organs in the different
species of the class are homologous. The whole subject is included under
the general term of Morphology. This is one of the most interesting
departments of natural history, and may almost be said to be its very
soul. What can be more curious than that the hand of a man, formed for
grasping, that of a mole for digging, the leg of the horse, the paddle of
the porpoise, and the wing of the bat, should all be constructed on the
same pattern, and should include similar bones, in the same relative
positions? How curious it is, to give a subordinate though striking
instance, that the hind feet of the kangaroo, which are so well fitted for
bounding over the open plains—those of the climbing, leaf-eating
koala, equally well fitted for grasping the branches of trees—those
of the ground-dwelling, insect or root-eating, bandicoots—and those
of some other Australian marsupials—should all be constructed on the
same extraordinary type, namely with the bones of the second and third
digits extremely slender and enveloped within the same skin, so that they
appear like a single toe furnished with two claws. Notwithstanding this
similarity of pattern, it is obvious that the hind feet of these several
animals are used for as widely different purposes as it is possible to
conceive. The case is rendered all the more striking by the American
opossums, which follow nearly the same habits of life as some of their
Australian relatives, having feet constructed on the ordinary plan.
Professor Flower, from whom these statements are taken, remarks in
conclusion: "We may call this conformity to type, without getting much
nearer to an explanation of the phenomenon;" and he then adds "but is it
not powerfully suggestive of true relationship, of inheritance from a
common ancestor?"

Geoffroy St. Hilaire has strongly insisted on the high importance of
relative position or connexion in homologous parts; they may differ to
almost any extent in form and size, and yet remain connected together in
the same invariable order. We never find, for instance, the bones of the
arm and forearm, or of the thigh and leg, transposed. Hence the same names
can be given to the homologous bones in widely different animals. We see
the same great law in the construction of the mouths of insects: what can
be more different than the immensely long spiral proboscis of a
sphinx-moth, the curious folded one of a bee or bug, and the great jaws of
a beetle? Yet all these organs, serving for such widely different
purposes, are formed by infinitely numerous modifications of an upper lip,
mandibles, and two pairs of maxillae. The same law governs the
construction of the mouths and limbs of crustaceans. So it is with the
flowers of plants.

Nothing can be more hopeless than to attempt to explain this similarity of
pattern in members of the same class, by utility or by the doctrine of
final causes. The hopelessness of the attempt has been expressly admitted
by Owen in his most interesting work on the "Nature of Limbs." On the
ordinary view of the independent creation of each being, we can only say
that so it is; that it has pleased the Creator to construct all the
animals and plants in each great class on a uniform plan; but this is not
a scientific explanation.

The explanation is to a large extent simple, on the theory of the
selection of successive slight modifications, each being profitable in
some way to the modified form, but often affecting by correlation other
parts of the organisation. In changes of this nature, there will be little
or no tendency to alter the original pattern, or to transpose the parts.
The bones of a limb might be shortened and flattened to any extent,
becoming at the same time enveloped in thick membrane, so as to serve as a
fin; or a webbed hand might have all its bones, or certain bones,
lengthened to any extent, with the membrane connecting them increased, so
as to serve as a wing; yet all these modifications would not tend to alter
the framework of the bones or the relative connexion of the parts. If we
suppose that an early progenitor—the archetype, as it may be called—of
all mammals, birds and reptiles, had its limbs constructed on the existing
general pattern, for whatever purpose they served, we can at once perceive
the plain signification of the homologous construction of the limbs
throughout the class. So with the mouths of insects, we have only to
suppose that their common progenitor had an upper lip, mandibles, and two
pairs of maxillae, these parts being perhaps very simple in form; and then
natural selection will account for the infinite diversity in structure and
function of the mouths of insects. Nevertheless, it is conceivable that
the general pattern of an organ might become so much obscured as to be
finally lost, by the reduction and ultimately by the complete abortion of
certain parts, by the fusion of other parts, and by the doubling or
multiplication of others, variations which we know to be within the limits
of possibility. In the paddles of the gigantic extinct sea-lizards, and in
the mouths of certain suctorial crustaceans, the general pattern seems
thus to have become partially obscured.

There is another and equally curious branch of our subject; namely, serial
homologies, or the comparison of the different parts or organs in the same
individual, and not of the same parts or organs in different members of
the same class. Most physiologists believe that the bones of the skull are
homologous—that is, correspond in number and in relative connexion—with
the elemental parts of a certain number of vertebrae. The anterior and
posterior limbs in all the higher vertebrate classes are plainly
homologous. So it is with the wonderfully complex jaws and legs of
crustaceans. It is familiar to almost every one, that in a flower the
relative position of the sepals, petals, stamens, and pistils, as well as
their intimate structure, are intelligible on the view that they consist
of metamorphosed leaves, arranged in a spire. In monstrous plants, we
often get direct evidence of the possibility of one organ being
transformed into another; and we can actually see, during the early or
embryonic stages of development in flowers, as well as in crustaceans and
many other animals, that organs, which when mature become extremely
different are at first exactly alike.

How inexplicable are the cases of serial homologies on the ordinary view
of creation! Why should the brain be enclosed in a box composed of such
numerous and such extraordinarily shaped pieces of bone apparently
representing vertebrae? As Owen has remarked, the benefit derived from the
yielding of the separate pieces in the act of parturition by mammals, will
by no means explain the same construction in the skulls of birds and
reptiles. Why should similar bones have been created to form the wing and
the leg of a bat, used as they are for such totally different purposes,
namely flying and walking? Why should one crustacean, which has an
extremely complex mouth formed of many parts, consequently always have
fewer legs; or conversely, those with many legs have simpler mouths? Why
should the sepals, petals, stamens, and pistils, in each flower, though
fitted for such distinct purposes, be all constructed on the same pattern?

On the theory of natural selection, we can, to a certain extent, answer
these questions. We need not here consider how the bodies of some animals
first became divided into a series of segments, or how they became divided
into right and left sides, with corresponding organs, for such questions
are almost beyond investigation. It is, however, probable that some serial
structures are the result of cells multiplying by division, entailing the
multiplication of the parts developed from such cells. It must suffice for
our purpose to bear in mind that an indefinite repetition of the same part
or organ is the common characteristic, as Owen has remarked, of all low or
little specialised forms; therefore the unknown progenitor of the
Vertebrata probably possessed many vertebrae; the unknown progenitor of
the Articulata, many segments; and the unknown progenitor of flowering
plants, many leaves arranged in one or more spires. We have also formerly
seen that parts many times repeated are eminently liable to vary, not only
in number, but in form. Consequently such parts, being already present in
considerable numbers, and being highly variable, would naturally afford
the materials for adaptation to the most different purposes; yet they
would generally retain, through the force of inheritance, plain traces of
their original or fundamental resemblance. They would retain this
resemblance all the more, as the variations, which afforded the basis for
their subsequent modification through natural selection, would tend from
the first to be similar; the parts being at an early stage of growth
alike, and being subjected to nearly the same conditions. Such parts,
whether more or less modified, unless their common origin became wholly
obscured, would be serially homologous.

In the great class of molluscs, though the parts in distinct species can
be shown to be homologous, only a few serial homologies; such as the
valves of Chitons, can be indicated; that is, we are seldom enabled to say
that one part is homologous with another part in the same individual. And
we can understand this fact; for in molluscs, even in the lowest members
of the class, we do not find nearly so much indefinite repetition of any
one part as we find in the other great classes of the animal and vegetable
kingdoms.

But morphology is a much more complex subject than it at first appears, as
has lately been well shown in a remarkable paper by Mr. E. Ray Lankester,
who has drawn an important distinction between certain classes of cases
which have all been equally ranked by naturalists as homologous. He
proposes to call the structures which resemble each other in distinct
animals, owing to their descent from a common progenitor with subsequent
modification, "homogenous"; and the resemblances which cannot thus be
accounted for, he proposes to call "homoplastic". For instance, he
believes that the hearts of birds and mammals are as a whole homogenous—that
is, have been derived from a common progenitor; but that the four cavities
of the heart in the two classes are homoplastic—that is, have been
independently developed. Mr. Lankester also adduces the close resemblance
of the parts on the right and left sides of the body, and in the
successive segments of the same individual animal; and here we have parts
commonly called homologous which bear no relation to the descent of
distinct species from a common progenitor. Homoplastic structures are the
same with those which I have classed, though in a very imperfect manner,
as analogous modifications or resemblances. Their formation may be
attributed in part to distinct organisms, or to distinct parts of the same
organism, having varied in an analogous manner; and in part to similar
modifications, having been preserved for the same general purpose or
function, of which many instances have been given.

Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae; the jaws of crabs as metamorphosed legs; the stamens and
pistils in flowers as metamorphosed leaves; but it would in most cases be
more correct, as Professor Huxley has remarked, to speak of both skull and
vertebrae, jaws and legs, etc., as having been metamorphosed, not one from
the other, as they now exist, but from some common and simpler element.
Most naturalists, however, use such language only in a metaphorical sense:
they are far from meaning that during a long course of descent, primordial
organs of any kind—vertebrae in the one case and legs in the other—have
actually been converted into skulls or jaws. Yet so strong is the
appearance of this having occurred that naturalists can hardly avoid
employing language having this plain signification. According to the views
here maintained, such language may be used literally; and the wonderful
fact of the jaws, for instance, of a crab retaining numerous characters,
which they probably would have retained through inheritance, if they had
really been metamorphosed from true though extremely simple legs, is in
part explained.

DEVELOPMENT AND EMBRYOLOGY.

This is one of the most important subjects in the whole round of natural
history. The metamorphoses of insects, with which every one is familiar,
are generally effected abruptly by a few stages; but the transformations
are in reality numerous and gradual, though concealed. A certain
ephemerous insect (Chloeon) during its development, moults, as shown by
Sir J. Lubbock, above twenty times, and each time undergoes a certain
amount of change; and in this case we see the act of metamorphosis
performed in a primary and gradual manner. Many insects, and especially
certain crustaceans, show us what wonderful changes of structure can be
effected during development. Such changes, however, reach their acme in
the so-called alternate generations of some of the lower animals. It is,
for instance, an astonishing fact that a delicate branching coralline,
studded with polypi, and attached to a submarine rock, should produce,
first by budding and then by transverse division, a host of huge floating
jelly-fishes; and that these should produce eggs, from which are hatched
swimming animalcules, which attach themselves to rocks and become
developed into branching corallines; and so on in an endless cycle. The
belief in the essential identity of the process of alternate generation
and of ordinary metamorphosis has been greatly strengthened by Wagner's
discovery of the larva or maggot of a fly, namely the Cecidomyia,
producing asexually other larvae, and these others, which finally are
developed into mature males and females, propagating their kind in the
ordinary manner by eggs.

It may be worth notice that when Wagner's remarkable discovery was first
announced, I was asked how was it possible to account for the larvae of
this fly having acquired the power of a sexual reproduction. As long as
the case remained unique no answer could be given. But already Grimm has
shown that another fly, a Chironomus, reproduces itself in nearly the same
manner, and he believes that this occurs frequently in the order. It is
the pupa, and not the larva, of the Chironomus which has this power; and
Grimm further shows that this case, to a certain extent, "unites that of
the Cecidomyia with the parthenogenesis of the Coccidae;" the term
parthenogenesis implying that the mature females of the Coccidae are
capable of producing fertile eggs without the concourse of the male.
Certain animals belonging to several classes are now known to have the
power of ordinary reproduction at an unusually early age; and we have only
to accelerate parthenogenetic reproduction by gradual steps to an earlier
and earlier age—Chironomus showing us an almost exactly intermediate
stage, viz., that of the pupa—and we can perhaps account for the
marvellous case of the Cecidomyia.

It has already been stated that various parts in the same individual,
which are exactly alike during an early embryonic period, become widely
different and serve for widely different purposes in the adult state. So
again it has been shown that generally the embryos of the most distinct
species belonging to the same class are closely similar, but become, when
fully developed, widely dissimilar. A better proof of this latter fact
cannot be given than the statement by Von Baer that "the embryos of
mammalia, of birds, lizards and snakes, probably also of chelonia, are in
the earliest states exceedingly like one another, both as a whole and in
the mode of development of their parts; so much so, in fact, that we can
often distinguish the embryos only by their size. In my possession are two
little embryos in spirit, whose names I have omitted to attach, and at
present I am quite unable to say to what class they belong. They may be
lizards or small birds, or very young mammalia, so complete is the
similarity in the mode of formation of the head and trunk in these
animals. The extremities, however, are still absent in these embryos. But
even if they had existed in the earliest stage of their development we
should learn nothing, for the feet of lizards and mammals, the wings and
feet of birds, no less than the hands and feet of man, all arise from the
same fundamental form." The larvae of most crustaceans, at corresponding
stages of development, closely resemble each other, however different the
adults may become; and so it is with very many other animals. A trace of
the law of embryonic resemblance occasionally lasts till a rather late
age: thus birds of the same genus, and of allied genera, often resemble
each other in their immature plumage; as we see in the spotted feathers in
the young of the thrush group. In the cat tribe, most of the species when
adult are striped or spotted in lines; and stripes or spots can be plainly
distinguished in the whelp of the lion and the puma. We occasionally,
though rarely, see something of the same kind in plants; thus the first
leaves of the ulex or furze, and the first leaves of the phyllodineous
acacias, are pinnate or divided like the ordinary leaves of the
leguminosae.

The points of structure, in which the embryos of widely different animals
within the same class resemble each other, often have no direct relation
to their conditions of existence. We cannot, for instance, suppose that in
the embryos of the vertebrata the peculiar loop-like courses of the
arteries near the branchial slits are related to similar conditions—in
the young mammal which is nourished in the womb of its mother, in the egg
of the bird which is hatched in a nest, and in the spawn of a frog under
water. We have no more reason to believe in such a relation than we have
to believe that the similar bones in the hand of a man, wing of a bat, and
fin of a porpoise, are related to similar conditions of life. No one
supposes that the stripes on the whelp of a lion, or the spots on the
young blackbird, are of any use to these animals.

The case, however, is different when an animal, during any part of its
embryonic career, is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on,
the adaptation of the larva to its conditions of life is just as perfect
and as beautiful as in the adult animal. In how important a manner this
has acted, has recently been well shown by Sir J. Lubbock in his remarks
on the close similarity of the larvae of some insects belonging to very
different orders, and on the dissimilarity of the larvae of other insects
within the same order, according to their habits of life. Owing to such
adaptations the similarity of the larvae of allied animals is sometimes
greatly obscured; especially when there is a division of labour during the
different stages of development, as when the same larva has during one
stage to search for food, and during another stage has to search for a
place of attachment. Cases can even be given of the larvae of allied
species, or groups of species, differing more from each other than do the
adults. In most cases, however, the larvae, though active, still obey,
more or less closely, the law of common embryonic resemblance. Cirripedes
afford a good instance of this: even the illustrious Cuvier did not
perceive that a barnacle was a crustacean: but a glance at the larva shows
this in an unmistakable manner. So again the two main divisions of
cirripedes, the pedunculated and sessile, though differing widely in
external appearance, have larvae in all their stages barely
distinguishable.

The embryo in the course of development generally rises in organisation. I
use this expression, though I am aware that it is hardly possible to
define clearly what is meant by organisation being higher or lower. But no
one probably will dispute that the butterfly is higher than the
caterpillar. In some cases, however, the mature animal must be considered
as lower in the scale than the larva, as with certain parasitic
crustaceans. To refer once again to cirripedes: the larvae in the first
stage have three pairs of locomotive organs, a simple single eye, and a
probosciformed mouth, with which they feed largely, for they increase much
in size. In the second stage, answering to the chrysalis stage of
butterflies, they have six pairs of beautifully constructed natatory legs,
a pair of magnificent compound eyes, and extremely complex antennae; but
they have a closed and imperfect mouth, and cannot feed: their function at
this stage is, to search out by their well-developed organs of sense, and
to reach by their active powers of swimming, a proper place on which to
become attached and to undergo their final metamorphosis. When this is
completed they are fixed for life: their legs are now converted into
prehensile organs; they again obtain a well-constructed mouth; but they
have no antennae, and their two eyes are now reconverted into a minute,
single, simple eye-spot. In this last and complete state, cirripedes may
be considered as either more highly or more lowly organised than they were
in the larval condition. But in some genera the larvae become developed
into hermaphrodites having the ordinary structure, or into what I have
called complemental males; and in the latter the development has assuredly
been retrograde; for the male is a mere sack, which lives for a short time
and is destitute of mouth, stomach, and every other organ of importance,
excepting those for reproduction.

We are so much accustomed to see a difference in structure between the
embryo and the adult, that we are tempted to look at this difference as in
some necessary manner contingent on growth. But there is no reason why,
for instance, the wing of a bat, or the fin of a porpoise, should not have
been sketched out with all their parts in proper proportion, as soon as
any part became visible. In some whole groups of animals and in certain
members of other groups this is the case, and the embryo does not at any
period differ widely from the adult: thus Owen has remarked in regard to
cuttle-fish, "there is no metamorphosis; the cephalopodic character is
manifested long before the parts of the embryo are completed." Land-shells
and fresh-water crustaceans are born having their proper forms, while the
marine members of the same two great classes pass through considerable and
often great changes during their development. Spiders, again, barely
undergo any metamorphosis. The larvae of most insects pass through a
worm-like stage, whether they are active and adapted to diversified
habits, or are inactive from being placed in the midst of proper
nutriment, or from being fed by their parents; but in some few cases, as
in that of Aphis, if we look to the admirable drawings of the development
of this insect, by Professor Huxley, we see hardly any trace of the
vermiform stage.

Sometimes it is only the earlier developmental stages which fail. Thus,
Fritz Muller has made the remarkable discovery that certain shrimp-like
crustaceans (allied to Penoeus) first appear under the simple
nauplius-form, and after passing through two or more zoea-stages, and then
through the mysis-stage, finally acquire their mature structure: now in
the whole great malacostracan order, to which these crustaceans belong, no
other member is as yet known to be first developed under the
nauplius-form, though many appear as zoeas; nevertheless Muller assigns
reasons for his belief, that if there had been no suppression of
development, all these crustaceans would have appeared as nauplii.

How, then, can we explain these several facts in embryology—namely,
the very general, though not universal, difference in structure between
the embryo and the adult; the various parts in the same individual embryo,
which ultimately become very unlike, and serve for diverse purposes, being
at an early period of growth alike; the common, but not invariable,
resemblance between the embryos or larvae of the most distinct species in
the same class; the embryo often retaining, while within the egg or womb,
structures which are of no service to it, either at that or at a later
period of life; on the other hand, larvae which have to provide for their
own wants, being perfectly adapted to the surrounding conditions; and
lastly, the fact of certain larvae standing higher in the scale of
organisation than the mature animal into which they are developed? I
believe that all these facts can be explained as follows.

It is commonly assumed, perhaps from monstrosities affecting the embryo at
a very early period, that slight variations or individual differences
necessarily appear at an equally early period. We have little evidence on
this head, but what we have certainly points the other way; for it is
notorious that breeders of cattle, horses and various fancy animals,
cannot positively tell, until some time after birth, what will be the
merits and demerits of their young animals. We see this plainly in our own
children; we cannot tell whether a child will be tall or short, or what
its precise features will be. The question is not, at what period of life
any variation may have been caused, but at what period the effects are
displayed. The cause may have acted, and I believe often has acted, on one
or both parents before the act of generation. It deserves notice that it
is of no importance to a very young animal, as long as it is nourished and
protected by its parent, whether most of its characters are acquired a
little earlier or later in life. It would not signify, for instance, to a
bird which obtained its food by having a much-curved beak whether or not
while young it possessed a beak of this shape, as long as it was fed by
its parents.

I have stated in the first chapter, that at whatever age any variation
first appears in the parent, it tends to reappear at a corresponding age
in the offspring. Certain variations can only appear at corresponding
ages; for instance, peculiarities in the caterpillar, cocoon, or imago
states of the silk-moth; or, again, in the full-grown horns of cattle. But
variations which, for all that we can see might have appeared either
earlier or later in life, likewise tend to reappear at a corresponding age
in the offspring and parent. I am far from meaning that this is invariably
the case, and I could give several exceptional cases of variations (taking
the word in the largest sense) which have supervened at an earlier age in
the child than in the parent.

These two principles, namely, that slight variations generally appear at a
not very early period of life, and are inherited at a corresponding not
early period, explain, as I believe, all the above specified leading facts
in embryology. But first let us look to a few analogous cases in our
domestic varieties. Some authors who have written on Dogs maintain that
the greyhound and bull-dog, though so different, are really closely allied
varieties, descended from the same wild stock, hence I was curious to see
how far their puppies differed from each other. I was told by breeders
that they differed just as much as their parents, and this, judging by the
eye, seemed almost to be the case; but on actually measuring the old dogs
and their six-days-old puppies, I found that the puppies had not acquired
nearly their full amount of proportional difference. So, again, I was told
that the foals of cart and race-horses—breeds which have been almost
wholly formed by selection under domestication—differed as much as
the full-grown animals; but having had careful measurements made of the
dams and of three-days-old colts of race and heavy cart-horses, I find
that this is by no means the case.

As we have conclusive evidence that the breeds of the Pigeon are descended
from a single wild species, I compared the young pigeons within twelve
hours after being hatched. I carefully measured the proportions (but will
not here give the details) of the beak, width of mouth, length of nostril
and of eyelid, size of feet and length of leg, in the wild parent species,
in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now,
some of these birds, when mature, differ in so extraordinary a manner in
the length and form of beak, and in other characters, that they would
certainly have been ranked as distinct genera if found in a state of
nature. But when the nestling birds of these several breeds were placed in
a row, though most of them could just be distinguished, the proportional
differences in the above specified points were incomparably less than in
the full-grown birds. Some characteristic points of difference—for
instance, that of the width of mouth—could hardly be detected in the
young. But there was one remarkable exception to this rule, for the young
of the short-faced tumbler differed from the young of the wild
rock-pigeon, and of the other breeds, in almost exactly the same
proportions as in the adult stage.

These facts are explained by the above two principles. Fanciers select
their dogs, horses, pigeons, etc., for breeding, when nearly grown up.
They are indifferent whether the desired qualities are acquired earlier or
later in life, if the full-grown animal possesses them. And the cases just
given, more especially that of the pigeons, show that the characteristic
differences which have been accumulated by man's selection, and which give
value to his breeds, do not generally appear at a very early period of
life, and are inherited at a corresponding not early period. But the case
of the short-faced tumbler, which when twelve hours old possessed its
proper characters, proves that this is not the universal rule; for here
the characteristic differences must either have appeared at an earlier
period than usual, or, if not so, the differences must have been
inherited, not at a corresponding, but at an earlier age.

Now, let us apply these two principles to species in a state of nature.
Let us take a group of birds, descended from some ancient form and
modified through natural selection for different habits. Then, from the
many slight successive variations having supervened in the several species
at a not early age, and having been inherited at a corresponding age, the
young will have been but little modified, and they will still resemble
each other much more closely than do the adults, just as we have seen with
the breeds of the pigeon. We may extend this view to widely distinct
structures and to whole classes. The fore-limbs, for instance, which once
served as legs to a remote progenitor, may have become, through a long
course of modification, adapted in one descendant to act as hands, in
another as paddles, in another as wings; but on the above two principles
the fore-limbs will not have been much modified in the embryos of these
several forms; although in each form the fore-limb will differ greatly in
the adult state. Whatever influence long continued use or disuse may have
had in modifying the limbs or other parts of any species, this will
chiefly or solely have affected it when nearly mature, when it was
compelled to use its full powers to gain its own living; and the effects
thus produced will have been transmitted to the offspring at a
corresponding nearly mature age. Thus the young will not be modified, or
will be modified only in a slight degree, through the effects of the
increased use or disuse of parts.

With some animals the successive variations may have supervened at a very
early period of life, or the steps may have been inherited at an earlier
age than that at which they first occurred. In either of these cases the
young or embryo will closely resemble the mature parent-form, as we have
seen with the short-faced tumbler. And this is the rule of development in
certain whole groups, or in certain sub-groups alone, as with cuttle-fish,
land-shells, fresh-water crustaceans, spiders, and some members of the
great class of insects. With respect to the final cause of the young in
such groups not passing through any metamorphosis, we can see that this
would follow from the following contingencies: namely, from the young
having to provide at a very early age for their own wants, and from their
following the same habits of life with their parents; for in this case it
would be indispensable for their existence that they should be modified in
the same manner as their parents. Again, with respect to the singular fact
that many terrestrial and fresh-water animals do not undergo any
metamorphosis, while marine members of the same groups pass through
various transformations, Fritz Muller has suggested that the process of
slowly modifying and adapting an animal to live on the land or in fresh
water, instead of in the sea, would be greatly simplified by its not
passing through any larval stage; for it is not probable that places well
adapted for both the larval and mature stages, under such new and greatly
changed habits of life, would commonly be found unoccupied or ill-occupied
by other organisms. In this case the gradual acquirement at an earlier and
earlier age of the adult structure would be favoured by natural selection;
and all traces of former metamorphoses would finally be lost.

If, on the other hand, it profited the young of an animal to follow habits
of life slightly different from those of the parent-form, and consequently
to be constructed on a slightly different plan, or if it profited a larva
already different from its parent to change still further, then, on the
principle of inheritance at corresponding ages, the young or the larvae
might be rendered by natural selection more and more different from their
parents to any conceivable extent. Differences in the larva might, also,
become correlated with successive stages of its development; so that the
larva, in the first stage, might come to differ greatly from the larva in
the second stage, as is the case with many animals. The adult might also
become fitted for sites or habits, in which organs of locomotion or of the
senses, etc., would be useless; and in this case the metamorphosis would
be retrograde.

From the remarks just made we can see how by changes of structure in the
young, in conformity with changed habits of life, together with
inheritance at corresponding ages, animals might come to pass through
stages of development, perfectly distinct from the primordial condition of
their adult progenitors. Most of our best authorities are now convinced
that the various larval and pupal stages of insects have thus been
acquired through adaptation, and not through inheritance from some ancient
form. The curious case of Sitaris—a beetle which passes through
certain unusual stages of development—will illustrate how this might
occur. The first larval form is described by M. Fabre, as an active,
minute insect, furnished with six legs, two long antennae, and four eyes.
These larvae are hatched in the nests of bees; and when the male bees
emerge from their burrows, in the spring, which they do before the
females, the larvae spring on them, and afterwards crawl on to the females
while paired with the males. As soon as the female bee deposits her eggs
on the surface of the honey stored in the cells, the larvae of the Sitaris
leap on the eggs and devour them. Afterwards they undergo a complete
change; their eyes disappear; their legs and antennae become rudimentary,
and they feed on honey; so that they now more closely resemble the
ordinary larvae of insects; ultimately they undergo a further
transformation, and finally emerge as the perfect beetle. Now, if an
insect, undergoing transformations like those of the Sitaris, were to
become the progenitor of a whole new class of insects, the course of
development of the new class would be widely different from that of our
existing insects; and the first larval stage certainly would not represent
the former condition of any adult and ancient form.

On the other hand it is highly probable that with many animals the
embryonic or larval stages show us, more or less completely, the condition
of the progenitor of the whole group in its adult state. In the great
class of the Crustacea, forms wonderfully distinct from each other,
namely, suctorial parasites, cirripedes, entomostraca, and even the
malacostraca, appear at first as larvae under the nauplius-form; and as
these larvae live and feed in the open sea, and are not adapted for any
peculiar habits of life, and from other reasons assigned by Fritz Muller,
it is probable that at some very remote period an independent adult
animal, resembling the Nauplius, existed, and subsequently produced, along
several divergent lines of descent, the above-named great Crustacean
groups. So again, it is probable, from what we know of the embryos of
mammals, birds, fishes and reptiles, that these animals are the modified
descendants of some ancient progenitor, which was furnished in its adult
state with branchiae, a swim-bladder, four fin-like limbs, and a long
tail, all fitted for an aquatic life.

As all the organic beings, extinct and recent, which have ever lived, can
be arranged within a few great classes; and as all within each class have,
according to our theory, been connected together by fine gradations, the
best, and, if our collections were nearly perfect, the only possible
arrangement, would be genealogical; descent being the hidden bond of
connexion which naturalists have been seeking under the term of the
Natural System. On this view we can understand how it is that, in the eyes
of most naturalists, the structure of the embryo is even more important
for classification than that of the adult. In two or more groups of
animals, however much they may differ from each other in structure and
habits in their adult condition, if they pass through closely similar
embryonic stages, we may feel assured that they are all descended from one
parent-form, and are therefore closely related. Thus, community in
embryonic structure reveals community of descent; but dissimilarity in
embryonic development does not prove discommunity of descent, for in one
of two groups the developmental stages may have been suppressed, or may
have been so greatly modified through adaptation to new habits of life as
to be no longer recognisable. Even in groups, in which the adults have
been modified to an extreme degree, community of origin is often revealed
by the structure of the larvae; we have seen, for instance, that
cirripedes, though externally so like shell-fish, are at once known by
their larvae to belong to the great class of crustaceans. As the embryo
often shows us more or less plainly the structure of the less modified and
ancient progenitor of the group, we can see why ancient and extinct forms
so often resemble in their adult state the embryos of existing species of
the same class. Agassiz believes this to be a universal law of nature; and
we may hope hereafter to see the law proved true. It can, however, be
proved true only in those cases in which the ancient state of the
progenitor of the group has not been wholly obliterated, either by
successive variations having supervened at a very early period of growth,
or by such variations having been inherited at an earlier age than that at
which they first appeared. It should also be borne in mind, that the law
may be true, but yet, owing to the geological record not extending far
enough back in time, may remain for a long period, or for ever, incapable
of demonstration. The law will not strictly hold good in those cases in
which an ancient form became adapted in its larval state to some special
line of life, and transmitted the same larval state to a whole group of
descendants; for such larval state will not resemble any still more
ancient form in its adult state.

Thus, as it seems to me, the leading facts in embryology, which are second
to none in importance, are explained on the principle of variations in the
many descendants from some one ancient progenitor, having appeared at a
not very early period of life, and having been inherited at a
corresponding period. Embryology rises greatly in interest, when we look
at the embryo as a picture, more or less obscured, of the progenitor,
either in its adult or larval state, of all the members of the same great
class.

RUDIMENTARY, ATROPHIED, AND ABORTED ORGANS.

Organs or parts in this strange condition, bearing the plain stamp of
inutility, are extremely common, or even general, throughout nature. It
would be impossible to name one of the higher animals in which some part
or other is not in a rudimentary condition. In the mammalia, for instance,
the males possess rudimentary mammae; in snakes one lobe of the lungs is
rudimentary; in birds the "bastard-wing" may safely be considered as a
rudimentary digit, and in some species the whole wing is so far
rudimentary that it cannot be used for flight. What can be more curious
than the presence of teeth in foetal whales, which when grown up have not
a tooth in their heads; or the teeth, which never cut through the gums, in
the upper jaws of unborn calves?

Rudimentary organs plainly declare their origin and meaning in various
ways. There are beetles belonging to closely allied species, or even to
the same identical species, which have either full-sized and perfect
wings, or mere rudiments of membrane, which not rarely lie under
wing-covers firmly soldered together; and in these cases it is impossible
to doubt, that the rudiments represent wings. Rudimentary organs sometimes
retain their potentiality: this occasionally occurs with the mammae of
male mammals, which have been known to become well developed and to
secrete milk. So again in the udders of the genus Bos, there are normally
four developed and two rudimentary teats; but the latter in our domestic
cows sometimes become well developed and yield milk. In regard to plants,
the petals are sometimes rudimentary, and sometimes well developed in the
individuals of the same species. In certain plants having separated sexes
Kolreuter found that by crossing a species, in which the male flowers
included a rudiment of a pistil, with an hermaphrodite species, having of
course a well-developed pistil, the rudiment in the hybrid offspring was
much increased in size; and this clearly shows that the rudimentary and
perfect pistils are essentially alike in nature. An animal may possess
various parts in a perfect state, and yet they may in one sense be
rudimentary, for they are useless: thus the tadpole of the common
salamander or water-newt, as Mr. G.H. Lewes remarks, "has gills, and
passes its existence in the water; but the Salamandra atra, which lives
high up among the mountains, brings forth its young full-formed. This
animal never lives in the water. Yet if we open a gravid female, we find
tadpoles inside her with exquisitely feathered gills; and when placed in
water they swim about like the tadpoles of the water-newt. Obviously this
aquatic organisation has no reference to the future life of the animal,
nor has it any adaptation to its embryonic condition; it has solely
reference to ancestral adaptations, it repeats a phase in the development
of its progenitors."

An organ, serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose, and remain perfectly
efficient for the other. Thus, in plants, the office of the pistil is to
allow the pollen-tubes to reach the ovules within the ovarium. The pistil
consists of a stigma supported on the style; but in some Compositae, the
male florets, which of course cannot be fecundated, have a rudimentary
pistil, for it is not crowned with a stigma; but the style remains well
developed and is clothed in the usual manner with hairs, which serve to
brush the pollen out of the surrounding and conjoined anthers. Again, an
organ may become rudimentary for its proper purpose, and be used for a
distinct one: in certain fishes the swim-bladder seems to be rudimentary
for its proper function of giving buoyancy, but has become converted into
a nascent breathing organ or lung. Many similar instances could be given.

Useful organs, however little they may be developed, unless we have reason
to suppose that they were formerly more highly developed, ought not to be
considered as rudimentary. They may be in a nascent condition, and in
progress towards further development. Rudimentary organs, on the other
hand, are either quite useless, such as teeth which never cut through the
gums, or almost useless, such as the wings of an ostrich, which serve
merely as sails. As organs in this condition would formerly, when still
less developed, have been of even less use than at present, they cannot
formerly have been produced through variation and natural selection, which
acts solely by the preservation of useful modifications. They have been
partially retained by the power of inheritance, and relate to a former
state of things. It is, however, often difficult to distinguish between
rudimentary and nascent organs; for we can judge only by analogy whether a
part is capable of further development, in which case alone it deserves to
be called nascent. Organs in this condition will always be somewhat rare;
for beings thus provided will commonly have been supplanted by their
successors with the same organ in a more perfect state, and consequently
will have become long ago extinct. The wing of the penguin is of high
service, acting as a fin; it may, therefore, represent the nascent state
of the wing: not that I believe this to be the case; it is more probably a
reduced organ, modified for a new function: the wing of the Apteryx, on
the other hand, is quite useless, and is truly rudimentary. Owen considers
the simple filamentary limbs of the Lepidosiren as the "beginnings of
organs which attain full functional development in higher vertebrates;"
but, according to the view lately advocated by Dr. Gunther, they are
probably remnants, consisting of the persistent axis of a fin, with the
lateral rays or branches aborted. The mammary glands of the
Ornithorhynchus may be considered, in comparison with the udders of a cow,
as in a nascent condition. The ovigerous frena of certain cirripedes,
which have ceased to give attachment to the ova and are feebly developed,
are nascent branchiae.

Rudimentary organs in the individuals of the same species are very liable
to vary in the degree of their development and in other respects. In
closely allied species, also, the extent to which the same organ has been
reduced occasionally differs much. This latter fact is well exemplified in
the state of the wings of female moths belonging to the same family.
Rudimentary organs may be utterly aborted; and this implies, that in
certain animals or plants, parts are entirely absent which analogy would
lead us to expect to find in them, and which are occasionally found in
monstrous individuals. Thus in most of the Scrophulariaceae the fifth
stamen is utterly aborted; yet we may conclude that a fifth stamen once
existed, for a rudiment of it is found in many species of the family, and
this rudiment occasionally becomes perfectly developed, as may sometimes
be seen in the common snap-dragon. In tracing the homologies of any part
in different members of the same class, nothing is more common, or, in
order fully to understand the relations of the parts, more useful than the
discovery of rudiments. This is well shown in the drawings given by Owen
of the leg bones of the horse, ox, and rhinoceros.

It is an important fact that rudimentary organs, such as teeth in the
upper jaws of whales and ruminants, can often be detected in the embryo,
but afterwards wholly disappear. It is also, I believe, a universal rule,
that a rudimentary part is of greater size in the embryo relatively to the
adjoining parts, than in the adult; so that the organ at this early age is
less rudimentary, or even cannot be said to be in any degree rudimentary.
Hence rudimentary organs in the adult are often said to have retained
their embryonic condition.

I have now given the leading facts with respect to rudimentary organs. In
reflecting on them, every one must be struck with astonishment; for the
same reasoning power which tells us that most parts and organs are
exquisitely adapted for certain purposes, tells us with equal plainness
that these rudimentary or atrophied organs are imperfect and useless. In
works on natural history, rudimentary organs are generally said to have
been created "for the sake of symmetry," or in order "to complete the
scheme of nature." But this is not an explanation, merely a restatement of
the fact. Nor is it consistent with itself: thus the boa-constrictor has
rudiments of hind limbs and of a pelvis, and if it be said that these
bones have been retained "to complete the scheme of nature," why, as
Professor Weismann asks, have they not been retained by other snakes,
which do not possess even a vestige of these same bones? What would be
thought of an astronomer who maintained that the satellites revolve in
elliptic courses round their planets "for the sake of symmetry," because
the planets thus revolve round the sun? An eminent physiologist accounts
for the presence of rudimentary organs, by supposing that they serve to
excrete matter in excess, or matter injurious to the system; but can we
suppose that the minute papilla, which often represents the pistil in male
flowers, and which is formed of mere cellular tissue, can thus act? Can we
suppose that rudimentary teeth, which are subsequently absorbed, are
beneficial to the rapidly growing embryonic calf by removing matter so
precious as phosphate of lime? When a man's fingers have been amputated,
imperfect nails have been known to appear on the stumps, and I could as
soon believe that these vestiges of nails are developed in order to
excrete horny matter, as that the rudimentary nails on the fin of the
manatee have been developed for this same purpose.

On the view of descent with modification, the origin of rudimentary organs
is comparatively simple; and we can understand to a large extent the laws
governing their imperfect development. We have plenty of cases of
rudimentary organs in our domestic productions, as the stump of a tail in
tailless breeds, the vestige of an ear in earless breeds of sheep—the
reappearance of minute dangling horns in hornless breeds of cattle, more
especially, according to Youatt, in young animals—and the state of
the whole flower in the cauliflower. We often see rudiments of various
parts in monsters; but I doubt whether any of these cases throw light on
the origin of rudimentary organs in a state of nature, further than by
showing that rudiments can be produced; for the balance of evidence
clearly indicates that species under nature do not undergo great and
abrupt changes. But we learn from the study of our domestic productions
that the disuse of parts leads to their reduced size; and that the result
is inherited.

It appears probable that disuse has been the main agent in rendering
organs rudimentary. It would at first lead by slow steps to the more and
more complete reduction of a part, until at last it became rudimentary—as
in the case of the eyes of animals inhabiting dark caverns, and of the
wings of birds inhabiting oceanic islands, which have seldom been forced
by beasts of prey to take flight, and have ultimately lost the power of
flying. Again, an organ, useful under certain conditions, might become
injurious under others, as with the wings of beetles living on small and
exposed islands; and in this case natural selection will have aided in
reducing the organ, until it was rendered harmless and rudimentary.

Any change in structure and function, which can be effected by small
stages, is within the power of natural selection; so that an organ
rendered, through changed habits of life, useless or injurious for one
purpose, might be modified and used for another purpose. An organ might,
also, be retained for one alone of its former functions. Organs,
originally formed by the aid of natural selection, when rendered useless
may well be variable, for their variations can no longer be checked by
natural selection. All this agrees well with what we see under nature.
Moreover, at whatever period of life either disuse or selection reduces an
organ, and this will generally be when the being has come to maturity and
to exert its full powers of action, the principle of inheritance at
corresponding ages will tend to reproduce the organ in its reduced state
at the same mature age, but will seldom affect it in the embryo. Thus we
can understand the greater size of rudimentary organs in the embryo
relatively to the adjoining parts, and their lesser relative size in the
adult. If, for instance, the digit of an adult animal was used less and
less during many generations, owing to some change of habits, or if an
organ or gland was less and less functionally exercised, we may infer that
it would become reduced in size in the adult descendants of this animal,
but would retain nearly its original standard of development in the
embryo.

There remains, however, this difficulty. After an organ has ceased being
used, and has become in consequence much reduced, how can it be still
further reduced in size until the merest vestige is left; and how can it
be finally quite obliterated? It is scarcely possible that disuse can go
on producing any further effect after the organ has once been rendered
functionless. Some additional explanation is here requisite which I cannot
give. If, for instance, it could be proved that every part of the
organisation tends to vary in a greater degree towards diminution than
toward augmentation of size, then we should be able to understand how an
organ which has become useless would be rendered, independently of the
effects of disuse, rudimentary and would at last be wholly suppressed; for
the variations towards diminished size would no longer be checked by
natural selection. The principle of the economy of growth, explained in a
former chapter, by which the materials forming any part, if not useful to
the possessor, are saved as far as is possible, will perhaps come into
play in rendering a useless part rudimentary. But this principle will
almost necessarily be confined to the earlier stages of the process of
reduction; for we cannot suppose that a minute papilla, for instance,
representing in a male flower the pistil of the female flower, and formed
merely of cellular tissue, could be further reduced or absorbed for the
sake of economising nutriment.

Finally, as rudimentary organs, by whatever steps they may have been
degraded into their present useless condition, are the record of a former
state of things, and have been retained solely through the power of
inheritance—we can understand, on the genealogical view of
classification, how it is that systematists, in placing organisms in their
proper places in the natural system, have often found rudimentary parts as
useful as, or even sometimes more useful than, parts of high physiological
importance. Rudimentary organs may be compared with the letters in a word,
still retained in the spelling, but become useless in the pronunciation,
but which serve as a clue for its derivation. On the view of descent with
modification, we may conclude that the existence of organs in a
rudimentary, imperfect, and useless condition, or quite aborted, far from
presenting a strange difficulty, as they assuredly do on the old doctrine
of creation, might even have been anticipated in accordance with the views
here explained.

SUMMARY.

In this chapter I have attempted to show that the arrangement of all
organic beings throughout all time in groups under groups—that the
nature of the relationships by which all living and extinct organisms are
united by complex, radiating, and circuitous lines of affinities into a
few grand classes—the rules followed and the difficulties
encountered by naturalists in their classifications—the value set
upon characters, if constant and prevalent, whether of high or of the most
trifling importance, or, as with rudimentary organs of no importance—the
wide opposition in value between analogical or adaptive characters, and
characters of true affinity; and other such rules—all naturally
follow if we admit the common parentage of allied forms, together with
their modification through variation and natural selection, with the
contingencies of extinction and divergence of character. In considering
this view of classification, it should be borne in mind that the element
of descent has been universally used in ranking together the sexes, ages,
dimorphic forms, and acknowledged varieties of the same species, however
much they may differ from each other in structure. If we extend the use of
this element of descent—the one certainly known cause of similarity
in organic beings—we shall understand what is meant by the Natural
System: it is genealogical in its attempted arrangement, with the grades
of acquired difference marked by the terms, varieties, species, genera,
families, orders, and classes.

On this same view of descent with modification, most of the great facts in
Morphology become intelligible—whether we look to the same pattern
displayed by the different species of the same class in their homologous
organs, to whatever purpose applied, or to the serial and lateral
homologies in each individual animal and plant.

On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being inherited
at a corresponding period, we can understand the leading facts in
embryology; namely, the close resemblance in the individual embryo of the
parts which are homologous, and which when matured become widely different
in structure and function; and the resemblance of the homologous parts or
organs in allied though distinct species, though fitted in the adult state
for habits as different as is possible. Larvae are active embryos, which
have become specially modified in a greater or less degree in relation to
their habits of life, with their modifications inherited at a
corresponding early age. On these same principles, and bearing in mind
that when organs are reduced in size, either from disuse or through
natural selection, it will generally be at that period of life when the
being has to provide for its own wants, and bearing in mind how strong is
the force of inheritance—the occurrence of rudimentary organs might
even have been anticipated. The importance of embryological characters and
of rudimentary organs in classification is intelligible, on the view that
a natural arrangement must be genealogical.

Finally, the several classes of facts which have been considered in this
chapter, seem to me to proclaim so plainly, that the innumerable species,
genera and families, with which this world is peopled, are all descended,
each within its own class or group, from common parents, and have all been
modified in the course of descent, that I should without hesitation adopt
this view, even if it were unsupported by other facts or arguments.



CHAPTER XV. RECAPITULATION AND CONCLUSION.


 Recapitulation of the objections to the theory of Natural
 Selection—Recapitulation of the general and special circumstances
 in its favour—Causes of the general belief in the immutability
 of species—How far the theory of Natural Selection may be
 extended—Effects of its adoption on the study of Natural
 History—Concluding remarks.

As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.

That many and serious objections may be advanced against the theory of
descent with modification through variation and natural selection, I do
not deny. I have endeavoured to give to them their full force. Nothing at
first can appear more difficult to believe than that the more complex
organs and instincts have been perfected, not by means superior to, though
analogous with, human reason, but by the accumulation of innumerable
slight variations, each good for the individual possessor. Nevertheless,
this difficulty, though appearing to our imagination insuperably great,
cannot be considered real if we admit the following propositions, namely,
that all parts of the organisation and instincts offer, at least
individual differences—that there is a struggle for existence
leading to the preservation of profitable deviations of structure or
instinct—and, lastly, that gradations in the state of perfection of
each organ may have existed, each good of its kind. The truth of these
propositions cannot, I think, be disputed.

It is, no doubt, extremely difficult even to conjecture by what gradations
many structures have been perfected, more especially among broken and
failing groups of organic beings, which have suffered much extinction; but
we see so many strange gradations in nature, that we ought to be extremely
cautious in saying that any organ or instinct, or any whole structure,
could not have arrived at its present state by many graduated steps. There
are, it must be admitted, cases of special difficulty opposed to the
theory of natural selection; and one of the most curious of these is the
existence in the same community of two or three defined castes of workers
or sterile female ants; but I have attempted to show how these
difficulties can be mastered.

With respect to the almost universal sterility of species when first
crossed, which forms so remarkable a contrast with the almost universal
fertility of varieties when crossed, I must refer the reader to the
recapitulation of the facts given at the end of the ninth chapter, which
seem to me conclusively to show that this sterility is no more a special
endowment than is the incapacity of two distinct kinds of trees to be
grafted together; but that it is incidental on differences confined to the
reproductive systems of the intercrossed species. We see the truth of this
conclusion in the vast difference in the results of crossing the same two
species reciprocally—that is, when one species is first used as the
father and then as the mother. Analogy from the consideration of dimorphic
and trimorphic plants clearly leads to the same conclusion, for when the
forms are illegitimately united, they yield few or no seed, and their
offspring are more or less sterile; and these forms belong to the same
undoubted species, and differ from each other in no respect except in
their reproductive organs and functions.

Although the fertility of varieties when intercrossed, and of their
mongrel offspring, has been asserted by so many authors to be universal,
this cannot be considered as quite correct after the facts given on the
high authority of Gartner and Kolreuter. Most of the varieties which have
been experimented on have been produced under domestication; and as
domestication (I do not mean mere confinement) almost certainly tends to
eliminate that sterility which, judging from analogy, would have affected
the parent-species if intercrossed, we ought not to expect that
domestication would likewise induce sterility in their modified
descendants when crossed. This elimination of sterility apparently follows
from the same cause which allows our domestic animals to breed freely
under diversified circumstances; and this again apparently follows from
their having been gradually accustomed to frequent changes in their
conditions of life.

A double and parallel series of facts seems to throw much light on the
sterility of species, when first crossed, and of their hybrid offspring.
On the one side, there is good reason to believe that slight changes in
the conditions of life give vigour and fertility to all organic beings. We
know also that a cross between the distinct individuals of the same
variety, and between distinct varieties, increases the number of their
offspring, and certainly gives to them increased size and vigour. This is
chiefly owing to the forms which are crossed having been exposed to
somewhat different conditions of life; for I have ascertained by a
labourious series of experiments that if all the individuals of the same
variety be subjected during several generations to the same conditions,
the good derived from crossing is often much diminished or wholly
disappears. This is one side of the case. On the other side, we know that
species which have long been exposed to nearly uniform conditions, when
they are subjected under confinement to new and greatly changed
conditions, either perish, or if they survive, are rendered sterile,
though retaining perfect health. This does not occur, or only in a very
slight degree, with our domesticated productions, which have long been
exposed to fluctuating conditions. Hence when we find that hybrids
produced by a cross between two distinct species are few in number, owing
to their perishing soon after conception or at a very early age, or if
surviving that they are rendered more or less sterile, it seems highly
probable that this result is due to their having been in fact subjected to
a great change in their conditions of life, from being compounded of two
distinct organisations. He who will explain in a definite manner why, for
instance, an elephant or a fox will not breed under confinement in its
native country, whilst the domestic pig or dog will breed freely under the
most diversified conditions, will at the same time be able to give a
definite answer to the question why two distinct species, when crossed, as
well as their hybrid offspring, are generally rendered more or less
sterile, while two domesticated varieties when crossed and their mongrel
offspring are perfectly fertile.

Turning to geographical distribution, the difficulties encountered on the
theory of descent with modification are serious enough. All the
individuals of the same species, and all the species of the same genus, or
even higher group, are descended from common parents; and therefore, in
however distant and isolated parts of the world they may now be found,
they must in the course of successive generations have travelled from some
one point to all the others. We are often wholly unable even to conjecture
how this could have been effected. Yet, as we have reason to believe that
some species have retained the same specific form for very long periods of
time, immensely long as measured by years, too much stress ought not to be
laid on the occasional wide diffusion of the same species; for during very
long periods there will always have been a good chance for wide migration
by many means. A broken or interrupted range may often be accounted for by
the extinction of the species in the intermediate regions. It cannot be
denied that we are as yet very ignorant as to the full extent of the
various climatical and geographical changes which have affected the earth
during modern periods; and such changes will often have facilitated
migration. As an example, I have attempted to show how potent has been the
influence of the Glacial period on the distribution of the same and of
allied species throughout the world. We are as yet profoundly ignorant of
the many occasional means of transport. With respect to distinct species
of the same genus, inhabiting distant and isolated regions, as the process
of modification has necessarily been slow, all the means of migration will
have been possible during a very long period; and consequently the
difficulty of the wide diffusion of the species of the same genus is in
some degree lessened.

As according to the theory of natural selection an interminable number of
intermediate forms must have existed, linking together all the species in
each group by gradations as fine as our existing varieties, it may be
asked, Why do we not see these linking forms all around us? Why are not
all organic beings blended together in an inextricable chaos? With respect
to existing forms, we should remember that we have no right to expect
(excepting in rare cases) to discover DIRECTLY connecting links between
them, but only between each and some extinct and supplanted form. Even on
a wide area, which has during a long period remained continuous, and of
which the climatic and other conditions of life change insensibly in
proceeding from a district occupied by one species into another district
occupied by a closely allied species, we have no just right to expect
often to find intermediate varieties in the intermediate zones. For we
have reason to believe that only a few species of a genus ever undergo
change; the other species becoming utterly extinct and leaving no modified
progeny. Of the species which do change, only a few within the same
country change at the same time; and all modifications are slowly
effected. I have also shown that the intermediate varieties which probably
at first existed in the intermediate zones, would be liable to be
supplanted by the allied forms on either hand; for the latter, from
existing in greater numbers, would generally be modified and improved at a
quicker rate than the intermediate varieties, which existed in lesser
numbers; so that the intermediate varieties would, in the long run, be
supplanted and exterminated.

On this doctrine of the extermination of an infinitude of connecting
links, between the living and extinct inhabitants of the world, and at
each successive period between the extinct and still older species, why is
not every geological formation charged with such links? Why does not every
collection of fossil remains afford plain evidence of the gradation and
mutation of the forms of life? Although geological research has
undoubtedly revealed the former existence of many links, bringing numerous
forms of life much closer together, it does not yield the infinitely many
fine gradations between past and present species required on the theory,
and this is the most obvious of the many objections which may be urged
against it. Why, again, do whole groups of allied species appear, though
this appearance is often false, to have come in suddenly on the successive
geological stages? Although we now know that organic beings appeared on
this globe, at a period incalculably remote, long before the lowest bed of
the Cambrian system was deposited, why do we not find beneath this system
great piles of strata stored with the remains of the progenitors of the
Cambrian fossils? For on the theory, such strata must somewhere have been
deposited at these ancient and utterly unknown epochs of the world's
history.

I can answer these questions and objections only on the supposition that
the geological record is far more imperfect than most geologists believe.
The number of specimens in all our museums is absolutely as nothing
compared with the countless generations of countless species which have
certainly existed. The parent form of any two or more species would not be
in all its characters directly intermediate between its modified
offspring, any more than the rock-pigeon is directly intermediate in crop
and tail between its descendants, the pouter and fantail pigeons. We
should not be able to recognise a species as the parent of another and
modified species, if we were to examine the two ever so closely, unless we
possessed most of the intermediate links; and owing to the imperfection of
the geological record, we have no just right to expect to find so many
links. If two or three, or even more linking forms were discovered, they
would simply be ranked by many naturalists as so many new species, more
especially if found in different geological substages, let their
differences be ever so slight. Numerous existing doubtful forms could be
named which are probably varieties; but who will pretend that in future
ages so many fossil links will be discovered, that naturalists will be
able to decide whether or not these doubtful forms ought to be called
varieties? Only a small portion of the world has been geologically
explored. Only organic beings of certain classes can be preserved in a
fossil condition, at least in any great number. Many species when once
formed never undergo any further change but become extinct without leaving
modified descendants; and the periods during which species have undergone
modification, though long as measured by years, have probably been short
in comparison with the periods during which they retained the same form.
It is the dominant and widely ranging species which vary most frequently
and vary most, and varieties are often at first local—both causes
rendering the discovery of intermediate links in any one formation less
likely. Local varieties will not spread into other and distant regions
until they are considerably modified and improved; and when they have
spread, and are discovered in a geological formation, they appear as if
suddenly created there, and will be simply classed as new species. Most
formations have been intermittent in their accumulation; and their
duration has probably been shorter than the average duration of specific
forms. Successive formations are in most cases separated from each other
by blank intervals of time of great length, for fossiliferous formations
thick enough to resist future degradation can, as a general rule, be
accumulated only where much sediment is deposited on the subsiding bed of
the sea. During the alternate periods of elevation and of stationary level
the record will generally be blank. During these latter periods there will
probably be more variability in the forms of life; during periods of
subsidence, more extinction.

With respect to the absence of strata rich in fossils beneath the Cambrian
formation, I can recur only to the hypothesis given in the tenth chapter;
namely, that though our continents and oceans have endured for an enormous
period in nearly their present relative positions, we have no reason to
assume that this has always been the case; consequently formations much
older than any now known may lie buried beneath the great oceans. With
respect to the lapse of time not having been sufficient since our planet
was consolidated for the assumed amount of organic change, and this
objection, as urged by Sir William Thompson, is probably one of the
gravest as yet advanced, I can only say, firstly, that we do not know at
what rate species change, as measured by years, and secondly, that many
philosophers are not as yet willing to admit that we know enough of the
constitution of the universe and of the interior of our globe to speculate
with safety on its past duration.

That the geological record is imperfect all will admit; but that it is
imperfect to the degree required by our theory, few will be inclined to
admit. If we look to long enough intervals of time, geology plainly
declares that species have all changed; and they have changed in the
manner required by the theory, for they have changed slowly and in a
graduated manner. We clearly see this in the fossil remains from
consecutive formations invariably being much more closely related to each
other than are the fossils from widely separated formations.

Such is the sum of the several chief objections and difficulties which may
justly be urged against the theory; and I have now briefly recapitulated
the answers and explanations which, as far as I can see, may be given. I
have felt these difficulties far too heavily during many years to doubt
their weight. But it deserves especial notice that the more important
objections relate to questions on which we are confessedly ignorant; nor
do we know how ignorant we are. We do not know all the possible
transitional gradations between the simplest and the most perfect organs;
it cannot be pretended that we know all the varied means of Distribution
during the long lapse of years, or that we know how imperfect is the
Geological Record. Serious as these several objections are, in my judgment
they are by no means sufficient to overthrow the theory of descent with
subsequent modification.

Now let us turn to the other side of the argument. Under domestication we
see much variability, caused, or at least excited, by changed conditions
of life; but often in so obscure a manner, that we are tempted to consider
the variations as spontaneous. Variability is governed by many complex
laws, by correlated growth, compensation, the increased use and disuse of
parts, and the definite action of the surrounding conditions. There is
much difficulty in ascertaining how largely our domestic productions have
been modified; but we may safely infer that the amount has been large, and
that modifications can be inherited for long periods. As long as the
conditions of life remain the same, we have reason to believe that a
modification, which has already been inherited for many generations, may
continue to be inherited for an almost infinite number of generations. On
the other hand we have evidence that variability, when it has once come
into play, does not cease under domestication for a very long period; nor
do we know that it ever ceases, for new varieties are still occasionally
produced by our oldest domesticated productions.

Variability is not actually caused by man; he only unintentionally exposes
organic beings to new conditions of life and then nature acts on the
organisation and causes it to vary. But man can and does select the
variations given to him by nature, and thus accumulates them in any
desired manner. He thus adapts animals and plants for his own benefit or
pleasure. He may do this methodically, or he may do it unconsciously by
preserving the individuals most useful or pleasing to him without any
intention of altering the breed. It is certain that he can largely
influence the character of a breed by selecting, in each successive
generation, individual differences so slight as to be inappreciable except
by an educated eye. This unconscious process of selection has been the
great agency in the formation of the most distinct and useful domestic
breeds. That many breeds produced by man have to a large extent the
character of natural species, is shown by the inextricable doubts whether
many of them are varieties or aboriginally distinct species.

There is no reason why the principles which have acted so efficiently
under domestication should not have acted under nature. In the survival of
favoured individuals and races, during the constantly recurrent Struggle
for Existence, we see a powerful and ever-acting form of Selection. The
struggle for existence inevitably follows from the high geometrical ratio
of increase which is common to all organic beings. This high rate of
increase is proved by calculation—by the rapid increase of many
animals and plants during a succession of peculiar seasons, and when
naturalised in new countries. More individuals are born than can possibly
survive. A grain in the balance may determine which individuals shall live
and which shall die—which variety or species shall increase in
number, and which shall decrease, or finally become extinct. As the
individuals of the same species come in all respects into the closest
competition with each other, the struggle will generally be most severe
between them; it will be almost equally severe between the varieties of
the same species, and next in severity between the species of the same
genus. On the other hand the struggle will often be severe between beings
remote in the scale of nature. The slightest advantage in certain
individuals, at any age or during any season, over those with which they
come into competition, or better adaptation in however slight a degree to
the surrounding physical conditions, will, in the long run, turn the
balance.

With animals having separated sexes, there will be in most cases a
struggle between the males for the possession of the females. The most
vigorous males, or those which have most successfully struggled with their
conditions of life, will generally leave most progeny. But success will
often depend on the males having special weapons or means of defence or
charms; and a slight advantage will lead to victory.

As geology plainly proclaims that each land has undergone great physical
changes, we might have expected to find that organic beings have varied
under nature, in the same way as they have varied under domestication. And
if there has been any variability under nature, it would be an
unaccountable fact if natural selection had not come into play. It has
often been asserted, but the assertion is incapable of proof, that the
amount of variation under nature is a strictly limited quantity. Man,
though acting on external characters alone and often capriciously, can
produce within a short period a great result by adding up mere individual
differences in his domestic productions; and every one admits that species
present individual differences. But, besides such differences, all
naturalists admit that natural varieties exist, which are considered
sufficiently distinct to be worthy of record in systematic works. No one
has drawn any clear distinction between individual differences and slight
varieties; or between more plainly marked varieties and subspecies and
species. On separate continents, and on different parts of the same
continent, when divided by barriers of any kind, and on outlying islands,
what a multitude of forms exist, which some experienced naturalists rank
as varieties, others as geographical races or sub species, and others as
distinct, though closely allied species!

If, then, animals and plants do vary, let it be ever so slightly or
slowly, why should not variations or individual differences, which are in
any way beneficial, be preserved and accumulated through natural
selection, or the survival of the fittest? If man can by patience select
variations useful to him, why, under changing and complex conditions of
life, should not variations useful to nature's living products often
arise, and be preserved or selected? What limit can be put to this power,
acting during long ages and rigidly scrutinising the whole constitution,
structure, and habits of each creature, favouring the good and rejecting
the bad? I can see no limit to this power, in slowly and beautifully
adapting each form to the most complex relations of life. The theory of
natural selection, even if we look no further than this, seems to be in
the highest degree probable. I have already recapitulated, as fairly as I
could, the opposed difficulties and objections: now let us turn to the
special facts and arguments in favour of the theory.

On the view that species are only strongly marked and permanent varieties,
and that each species first existed as a variety, we can see why it is
that no line of demarcation can be drawn between species, commonly
supposed to have been produced by special acts of creation, and varieties
which are acknowledged to have been produced by secondary laws. On this
same view we can understand how it is that in a region where many species
of a genus have been produced, and where they now flourish, these same
species should present many varieties; for where the manufactory of
species has been active, we might expect, as a general rule, to find it
still in action; and this is the case if varieties be incipient species.
Moreover, the species of the larger genera, which afford the greater
number of varieties or incipient species, retain to a certain degree the
character of varieties; for they differ from each other by a less amount
of difference than do the species of smaller genera. The closely allied
species also of a larger genera apparently have restricted ranges, and in
their affinities they are clustered in little groups round other species—in
both respects resembling varieties. These are strange relations on the
view that each species was independently created, but are intelligible if
each existed first as a variety.

As each species tends by its geometrical rate of reproduction to increase
inordinately in number; and as the modified descendants of each species
will be enabled to increase by as much as they become more diversified in
habits and structure, so as to be able to seize on many and widely
different places in the economy of nature, there will be a constant
tendency in natural selection to preserve the most divergent offspring of
any one species. Hence during a long-continued course of modification, the
slight differences characteristic of varieties of the same species, tend
to be augmented into the greater differences characteristic of the species
of the same genus. New and improved varieties will inevitably supplant and
exterminate the older, less improved and intermediate varieties; and thus
species are rendered to a large extent defined and distinct objects.
Dominant species belonging to the larger groups within each class tend to
give birth to new and dominant forms; so that each large group tends to
become still larger, and at the same time more divergent in character. But
as all groups cannot thus go on increasing in size, for the world would
not hold them, the more dominant groups beat the less dominant. This
tendency in the large groups to go on increasing in size and diverging in
character, together with the inevitable contingency of much extinction,
explains the arrangement of all the forms of life in groups subordinate to
groups, all within a few great classes, which has prevailed throughout all
time. This grand fact of the grouping of all organic beings under what is
called the Natural System, is utterly inexplicable on the theory of
creation.

As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modifications; it
can act only by short and slow steps. Hence, the canon of "Natura non
facit saltum," which every fresh addition to our knowledge tends to
confirm, is on this theory intelligible. We can see why throughout nature
the same general end is gained by an almost infinite diversity of means,
for every peculiarity when once acquired is long inherited, and structures
already modified in many different ways have to be adapted for the same
general purpose. We can, in short, see why nature is prodigal in variety,
though niggard in innovation. But why this should be a law of nature if
each species has been independently created no man can explain.

Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of a woodpecker, should prey on
insects on the ground; that upland geese, which rarely or never swim,
would possess webbed feet; that a thrush-like bird should dive and feed on
sub-aquatic insects; and that a petrel should have the habits and
structure fitting it for the life of an auk! and so in endless other
cases. But on the view of each species constantly trying to increase in
number, with natural selection always ready to adapt the slowly varying
descendants of each to any unoccupied or ill-occupied place in nature,
these facts cease to be strange, or might even have been anticipated.

We can to a certain extent understand how it is that there is so much
beauty throughout nature; for this may be largely attributed to the agency
of selection. That beauty, according to our sense of it, is not universal,
must be admitted by every one who will look at some venomous snakes, at
some fishes, and at certain hideous bats with a distorted resemblance to
the human face. Sexual selection has given the most brilliant colours,
elegant patterns, and other ornaments to the males, and sometimes to both
sexes of many birds, butterflies and other animals. With birds it has
often rendered the voice of the male musical to the female, as well as to
our ears. Flowers and fruit have been rendered conspicuous by brilliant
colours in contrast with the green foliage, in order that the flowers may
be easily seen, visited and fertilised by insects, and the seeds
disseminated by birds. How it comes that certain colours, sounds and forms
should give pleasure to man and the lower animals, that is, how the sense
of beauty in its simplest form was first acquired, we do not know any more
than how certain odours and flavours were first rendered agreeable.

As natural selection acts by competition, it adapts and improves the
inhabitants of each country only in relation to their co-inhabitants; so
that we need feel no surprise at the species of any one country, although
on the ordinary view supposed to have been created and specially adapted
for that country, being beaten and supplanted by the naturalised
productions from another land. Nor ought we to marvel if all the
contrivances in nature be not, as far as we can judge, absolutely perfect;
as in the case even of the human eye; or if some of them be abhorrent to
our ideas of fitness. We need not marvel at the sting of the bee, when
used against the enemy, causing the bee's own death; at drones being
produced in such great numbers for one single act, and being then
slaughtered by their sterile sisters; at the astonishing waste of pollen
by our fir-trees; at the instinctive hatred of the queen-bee for her own
fertile daughters; at ichneumonidae feeding within the living bodies of
caterpillars; and at other such cases. The wonder, indeed, is, on the
theory of natural selection, that more cases of the want of absolute
perfection have not been detected.

The complex and little known laws governing the production of varieties
are the same, as far as we can judge, with the laws which have governed
the production of distinct species. In both cases physical conditions seem
to have produced some direct and definite effect, but how much we cannot
say. Thus, when varieties enter any new station, they occasionally assume
some of the characters proper to the species of that station. With both
varieties and species, use and disuse seem to have produced a considerable
effect; for it is impossible to resist this conclusion when we look, for
instance, at the logger-headed duck, which has wings incapable of flight,
in nearly the same condition as in the domestic duck; or when we look at
the burrowing tucu-tucu, which is occasionally blind, and then at certain
moles, which are habitually blind and have their eyes covered with skin;
or when we look at the blind animals inhabiting the dark caves of America
and Europe. With varieties and species, correlated variation seems to have
played an important part, so that when one part has been modified other
parts have been necessarily modified. With both varieties and species,
reversions to long-lost characters occasionally occur. How inexplicable on
the theory of creation is the occasional appearance of stripes on the
shoulders and legs of the several species of the horse-genus and of their
hybrids! How simply is this fact explained if we believe that these
species are all descended from a striped progenitor, in the same manner as
the several domestic breeds of the pigeon are descended from the blue and
barred rock-pigeon!

On the ordinary view of each species having been independently created,
why should specific characters, or those by which the species of the same
genus differ from each other, be more variable than the generic characters
in which they all agree? Why, for instance, should the colour of a flower
be more likely to vary in any one species of a genus, if the other species
possess differently coloured flowers, than if all possessed the same
coloured flowers? If species are only well-marked varieties, of which the
characters have become in a high degree permanent, we can understand this
fact; for they have already varied since they branched off from a common
progenitor in certain characters, by which they have come to be
specifically distinct from each other; therefore these same characters
would be more likely again to vary than the generic characters which have
been inherited without change for an immense period. It is inexplicable on
the theory of creation why a part developed in a very unusual manner in
one species alone of a genus, and therefore, as we may naturally infer, of
great importance to that species, should be eminently liable to variation;
but, on our view, this part has undergone, since the several species
branched off from a common progenitor, an unusual amount of variability
and modification, and therefore we might expect the part generally to be
still variable. But a part may be developed in the most unusual manner,
like the wing of a bat, and yet not be more variable than any other
structure, if the part be common to many subordinate forms, that is, if it
has been inherited for a very long period; for in this case it will have
been rendered constant by long-continued natural selection.

Glancing at instincts, marvellous as some are, they offer no greater
difficulty than do corporeal structures on the theory of the natural
selection of successive, slight, but profitable modifications. We can thus
understand why nature moves by graduated steps in endowing different
animals of the same class with their several instincts. I have attempted
to show how much light the principle of gradation throws on the admirable
architectural powers of the hive-bee. Habit no doubt often comes into play
in modifying instincts; but it certainly is not indispensable, as we see
in the case of neuter insects, which leave no progeny to inherit the
effects of long-continued habit. On the view of all the species of the
same genus having descended from a common parent, and having inherited
much in common, we can understand how it is that allied species, when
placed under widely different conditions of life, yet follow nearly the
same instincts; why the thrushes of tropical and temperate South America,
for instance, line their nests with mud like our British species. On the
view of instincts having been slowly acquired through natural selection,
we need not marvel at some instincts being not perfect and liable to
mistakes, and at many instincts causing other animals to suffer.

If species be only well-marked and permanent varieties, we can at once see
why their crossed offspring should follow the same complex laws in their
degrees and kinds of resemblance to their parents—in being absorbed
into each other by successive crosses, and in other such points—as
do the crossed offspring of acknowledged varieties. This similarity would
be a strange fact, if species had been independently created and varieties
had been produced through secondary laws.

If we admit that the geological record is imperfect to an extreme degree,
then the facts, which the record does give, strongly support the theory of
descent with modification. New species have come on the stage slowly and
at successive intervals; and the amount of change after equal intervals of
time, is widely different in different groups. The extinction of species
and of whole groups of species, which has played so conspicuous a part in
the history of the organic world, almost inevitably follows from the
principle of natural selection; for old forms are supplanted by new and
improved forms. Neither single species nor groups of species reappear when
the chain of ordinary generation is once broken. The gradual diffusion of
dominant forms, with the slow modification of their descendants, causes
the forms of life, after long intervals of time, to appear as if they had
changed simultaneously throughout the world. The fact of the fossil
remains of each formation being in some degree intermediate in character
between the fossils in the formations above and below, is simply explained
by their intermediate position in the chain of descent. The grand fact
that all extinct beings can be classed with all recent beings, naturally
follows from the living and the extinct being the offspring of common
parents. As species have generally diverged in character during their long
course of descent and modification, we can understand why it is that the
more ancient forms, or early progenitors of each group, so often occupy a
position in some degree intermediate between existing groups. Recent forms
are generally looked upon as being, on the whole, higher in the scale of
organisation than ancient forms; and they must be higher, in so far as the
later and more improved forms have conquered the older and less improved
forms in the struggle for life; they have also generally had their organs
more specialised for different functions. This fact is perfectly
compatible with numerous beings still retaining simple and but little
improved structures, fitted for simple conditions of life; it is likewise
compatible with some forms having retrograded in organisation, by having
become at each stage of descent better fitted for new and degraded habits
of life. Lastly, the wonderful law of the long endurance of allied forms
on the same continent—of marsupials in Australia, of edentata in
America, and other such cases—is intelligible, for within the same
country the existing and the extinct will be closely allied by descent.

Looking to geographical distribution, if we admit that there has been
during the long course of ages much migration from one part of the world
to another, owing to former climatical and geographical changes and to the
many occasional and unknown means of dispersal, then we can understand, on
the theory of descent with modification, most of the great leading facts
in Distribution. We can see why there should be so striking a parallelism
in the distribution of organic beings throughout space, and in their
geological succession throughout time; for in both cases the beings have
been connected by the bond of ordinary generation, and the means of
modification have been the same. We see the full meaning of the wonderful
fact, which has struck every traveller, namely, that on the same
continent, under the most diverse conditions, under heat and cold, on
mountain and lowland, on deserts and marshes, most of the inhabitants
within each great class are plainly related; for they are the descendants
of the same progenitors and early colonists. On this same principle of
former migration, combined in most cases with modification, we can
understand, by the aid of the Glacial period, the identity of some few
plants, and the close alliance of many others, on the most distant
mountains, and in the northern and southern temperate zones; and likewise
the close alliance of some of the inhabitants of the sea in the northern
and southern temperate latitudes, though separated by the whole
intertropical ocean. Although two countries may present physical
conditions as closely similar as the same species ever require, we need
feel no surprise at their inhabitants being widely different, if they have
been for a long period completely sundered from each other; for as the
relation of organism to organism is the most important of all relations,
and as the two countries will have received colonists at various periods
and in different proportions, from some other country or from each other,
the course of modification in the two areas will inevitably have been
different.

On this view of migration, with subsequent modification, we see why
oceanic islands are inhabited by only few species, but of these, why many
are peculiar or endemic forms. We clearly see why species belonging to
those groups of animals which cannot cross wide spaces of the ocean, as
frogs and terrestrial mammals, do not inhabit oceanic islands; and why, on
the other hand, new and peculiar species of bats, animals which can
traverse the ocean, are often found on islands far distant from any
continent. Such cases as the presence of peculiar species of bats on
oceanic islands and the absence of all other terrestrial mammals, are
facts utterly inexplicable on the theory of independent acts of creation.

The existence of closely allied representative species in any two areas,
implies, on the theory of descent with modification, that the same
parent-forms formerly inhabited both areas; and we almost invariably find
that wherever many closely allied species inhabit two areas, some
identical species are still common to both. Wherever many closely allied
yet distinct species occur, doubtful forms and varieties belonging to the
same groups likewise occur. It is a rule of high generality that the
inhabitants of each area are related to the inhabitants of the nearest
source whence immigrants might have been derived. We see this in the
striking relation of nearly all the plants and animals of the Galapagos
Archipelago, of Juan Fernandez, and of the other American islands, to the
plants and animals of the neighbouring American mainland; and of those of
the Cape de Verde Archipelago, and of the other African islands to the
African mainland. It must be admitted that these facts receive no
explanation on the theory of creation.

The fact, as we have seen, that all past and present organic beings can be
arranged within a few great classes, in groups subordinate to groups, and
with the extinct groups often falling in between the recent groups, is
intelligible on the theory of natural selection with its contingencies of
extinction and divergence of character. On these same principles we see
how it is that the mutual affinities of the forms within each class are so
complex and circuitous. We see why certain characters are far more
serviceable than others for classification; why adaptive characters,
though of paramount importance to the beings, are of hardly any importance
in classification; why characters derived from rudimentary parts, though
of no service to the beings, are often of high classificatory value; and
why embryological characters are often the most valuable of all. The real
affinities of all organic beings, in contradistinction to their adaptive
resemblances, are due to inheritance or community of descent. The Natural
System is a genealogical arrangement, with the acquired grades of
difference, marked by the terms, varieties, species, genera, families,
etc.; and we have to discover the lines of descent by the most permanent
characters, whatever they may be, and of however slight vital importance.

The similar framework of bones in the hand of a man, wing of a bat, fin of
the porpoise, and leg of the horse—the same number of vertebrae
forming the neck of the giraffe and of the elephant—and innumerable
other such facts, at once explain themselves on the theory of descent with
slow and slight successive modifications. The similarity of pattern in the
wing and in the leg of a bat, though used for such different purpose—in
the jaws and legs of a crab—in the petals, stamens, and pistils of a
flower, is likewise, to a large extent, intelligible on the view of the
gradual modification of parts or organs, which were aboriginally alike in
an early progenitor in each of these classes. On the principle of
successive variations not always supervening at an early age, and being
inherited at a corresponding not early period of life, we clearly see why
the embryos of mammals, birds, reptiles, and fishes should be so closely
similar, and so unlike the adult forms. We may cease marvelling at the
embryo of an air-breathing mammal or bird having branchial slits and
arteries running in loops, like those of a fish which has to breathe the
air dissolved in water by the aid of well-developed branchiae.

Disuse, aided sometimes by natural selection, will often have reduced
organs when rendered useless under changed habits or conditions of life;
and we can understand on this view the meaning of rudimentary organs. But
disuse and selection will generally act on each creature, when it has come
to maturity and has to play its full part in the struggle for existence,
and will thus have little power on an organ during early life; hence the
organ will not be reduced or rendered rudimentary at this early age. The
calf, for instance, has inherited teeth, which never cut through the gums
of the upper jaw, from an early progenitor having well-developed teeth;
and we may believe, that the teeth in the mature animal were formerly
reduced by disuse owing to the tongue and palate, or lips, having become
excellently fitted through natural selection to browse without their aid;
whereas in the calf, the teeth have been left unaffected, and on the
principle of inheritance at corresponding ages have been inherited from a
remote period to the present day. On the view of each organism with all
its separate parts having been specially created, how utterly inexplicable
is it that organs bearing the plain stamp of inutility, such as the teeth
in the embryonic calf or the shrivelled wings under the soldered
wing-covers of many beetles, should so frequently occur. Nature may be
said to have taken pains to reveal her scheme of modification, by means of
rudimentary organs, of embryological and homologous structures, but we are
too blind to understand her meaning.

I have now recapitulated the facts and considerations which have
thoroughly convinced me that species have been modified, during a long
course of descent. This has been effected chiefly through the natural
selection of numerous successive, slight, favourable variations; aided in
an important manner by the inherited effects of the use and disuse of
parts; and in an unimportant manner, that is, in relation to adaptive
structures, whether past or present, by the direct action of external
conditions, and by variations which seem to us in our ignorance to arise
spontaneously. It appears that I formerly underrated the frequency and
value of these latter forms of variation, as leading to permanent
modifications of structure independently of natural selection. But as my
conclusions have lately been much misrepresented, and it has been stated
that I attribute the modification of species exclusively to natural
selection, I may be permitted to remark that in the first edition of this
work, and subsequently, I placed in a most conspicuous position—namely,
at the close of the Introduction—the following words: "I am
convinced that natural selection has been the main but not the exclusive
means of modification." This has been of no avail. Great is the power of
steady misrepresentation; but the history of science shows that
fortunately this power does not long endure.

It can hardly be supposed that a false theory would explain, in so
satisfactory a manner as does the theory of natural selection, the several
large classes of facts above specified. It has recently been objected that
this is an unsafe method of arguing; but it is a method used in judging of
the common events of life, and has often been used by the greatest natural
philosophers. The undulatory theory of light has thus been arrived at; and
the belief in the revolution of the earth on its own axis was until lately
supported by hardly any direct evidence. It is no valid objection that
science as yet throws no light on the far higher problem of the essence or
origin of life. Who can explain what is the essence of the attraction of
gravity? No one now objects to following out the results consequent on
this unknown element of attraction; notwithstanding that Leibnitz formerly
accused Newton of introducing "occult qualities and miracles into
philosophy."

I see no good reasons why the views given in this volume should shock the
religious feelings of any one. It is satisfactory, as showing how
transient such impressions are, to remember that the greatest discovery
ever made by man, namely, the law of the attraction of gravity, was also
attacked by Leibnitz, "as subversive of natural, and inferentially of
revealed, religion." A celebrated author and divine has written to me that
"he has gradually learned to see that it is just as noble a conception of
the Deity to believe that He created a few original forms capable of
self-development into other and needful forms, as to believe that He
required a fresh act of creation to supply the voids caused by the action
of His laws."

Why, it may be asked, until recently did nearly all the most eminent
living naturalists and geologists disbelieve in the mutability of species?
It cannot be asserted that organic beings in a state of nature are subject
to no variation; it cannot be proved that the amount of variation in the
course of long ages is a limited quantity; no clear distinction has been,
or can be, drawn between species and well-marked varieties. It cannot be
maintained that species when intercrossed are invariably sterile and
varieties invariably fertile; or that sterility is a special endowment and
sign of creation. The belief that species were immutable productions was
almost unavoidable as long as the history of the world was thought to be
of short duration; and now that we have acquired some idea of the lapse of
time, we are too apt to assume, without proof, that the geological record
is so perfect that it would have afforded us plain evidence of the
mutation of species, if they had undergone mutation.

But the chief cause of our natural unwillingness to admit that one species
has given birth to other and distinct species, is that we are always slow
in admitting any great changes of which we do not see the steps. The
difficulty is the same as that felt by so many geologists, when Lyell
first insisted that long lines of inland cliffs had been formed, and great
valleys excavated, by the agencies which we still see at work. The mind
cannot possibly grasp the full meaning of the term of even a million
years; it cannot add up and perceive the full effects of many slight
variations, accumulated during an almost infinite number of generations.

Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of facts
all viewed, during a long course of years, from a point of view directly
opposite to mine. It is so easy to hide our ignorance under such
expressions as the "plan of creation," "unity of design," etc., and to
think that we give an explanation when we only restate a fact. Any one
whose disposition leads him to attach more weight to unexplained
difficulties than to the explanation of a certain number of facts will
certainly reject the theory. A few naturalists, endowed with much
flexibility of mind, and who have already begun to doubt the immutability
of species, may be influenced by this volume; but I look with confidence
to the future, to young and rising naturalists, who will be able to view
both sides of the question with impartiality. Whoever is led to believe
that species are mutable will do good service by conscientiously
expressing his conviction; for thus only can the load of prejudice by
which this subject is overwhelmed be removed.

Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but that
other species are real, that is, have been independently created. This
seems to me a strange conclusion to arrive at. They admit that a multitude
of forms, which till lately they themselves thought were special
creations, and which are still thus looked at by the majority of
naturalists, and which consequently have all the external characteristic
features of true species—they admit that these have been produced by
variation, but they refuse to extend the same view to other and slightly
different forms. Nevertheless, they do not pretend that they can define,
or even conjecture, which are the created forms of life, and which are
those produced by secondary laws. They admit variation as a vera causa in
one case, they arbitrarily reject it in another, without assigning any
distinction in the two cases. The day will come when this will be given as
a curious illustration of the blindness of preconceived opinion. These
authors seem no more startled at a miraculous act of creation than at an
ordinary birth. But do they really believe that at innumerable periods in
the earth's history certain elemental atoms have been commanded suddenly
to flash into living tissues? Do they believe that at each supposed act of
creation one individual or many were produced? Were all the infinitely
numerous kinds of animals and plants created as eggs or seed, or as full
grown? and in the case of mammals, were they created bearing the false
marks of nourishment from the mother's womb? Undoubtedly some of these
same questions cannot be answered by those who believe in the appearance
or creation of only a few forms of life or of some one form alone. It has
been maintained by several authors that it is as easy to believe in the
creation of a million beings as of one; but Maupertuis' philosophical
axiom "of least action" leads the mind more willingly to admit the smaller
number; and certainly we ought not to believe that innumerable beings
within each great class have been created with plain, but deceptive, marks
of descent from a single parent.

As a record of a former state of things, I have retained in the foregoing
paragraphs, and elsewhere, several sentences which imply that naturalists
believe in the separate creation of each species; and I have been much
censured for having thus expressed myself. But undoubtedly this was the
general belief when the first edition of the present work appeared. I
formerly spoke to very many naturalists on the subject of evolution, and
never once met with any sympathetic agreement. It is probable that some
did then believe in evolution, but they were either silent or expressed
themselves so ambiguously that it was not easy to understand their
meaning. Now, things are wholly changed, and almost every naturalist
admits the great principle of evolution. There are, however, some who
still think that species have suddenly given birth, through quite
unexplained means, to new and totally different forms. But, as I have
attempted to show, weighty evidence can be opposed to the admission of
great and abrupt modifications. Under a scientific point of view, and as
leading to further investigation, but little advantage is gained by
believing that new forms are suddenly developed in an inexplicable manner
from old and widely different forms, over the old belief in the creation
of species from the dust of the earth.

It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct
the forms are which we consider, by so much the arguments in favour of
community of descent become fewer in number and less in force. But some
arguments of the greatest weight extend very far. All the members of whole
classes are connected together by a chain of affinities, and all can be
classed on the same principle, in groups subordinate to groups. Fossil
remains sometimes tend to fill up very wide intervals between existing
orders.

Organs in a rudimentary condition plainly show that an early progenitor
had the organ in a fully developed condition, and this in some cases
implies an enormous amount of modification in the descendants. Throughout
whole classes various structures are formed on the same pattern, and at a
very early age the embryos closely resemble each other. Therefore I cannot
doubt that the theory of descent with modification embraces all the
members of the same great class or kingdom. I believe that animals are
descended from at most only four or five progenitors, and plants from an
equal or lesser number.

Analogy would lead me one step further, namely, to the belief that all
animals and plants are descended from some one prototype. But analogy may
be a deceitful guide. Nevertheless all living things have much in common,
in their chemical composition, their cellular structure, their laws of
growth, and their liability to injurious influences. We see this even in
so trifling a fact as that the same poison often similarly affects plants
and animals; or that the poison secreted by the gall-fly produces
monstrous growths on the wild rose or oak-tree. With all organic beings,
excepting perhaps some of the very lowest, sexual reproduction seems to be
essentially similar. With all, as far as is at present known, the germinal
vesicle is the same; so that all organisms start from a common origin. If
we look even to the two main divisions—namely, to the animal and
vegetable kingdoms—certain low forms are so far intermediate in
character that naturalists have disputed to which kingdom they should be
referred. As Professor Asa Gray has remarked, "the spores and other
reproductive bodies of many of the lower algae may claim to have first a
characteristically animal, and then an unequivocally vegetable existence."
Therefore, on the principle of natural selection with divergence of
character, it does not seem incredible that, from some such low and
intermediate form, both animals and plants may have been developed; and,
if we admit this, we must likewise admit that all the organic beings which
have ever lived on this earth may be descended from some one primordial
form. But this inference is chiefly grounded on analogy, and it is
immaterial whether or not it be accepted. No doubt it is possible, as Mr.
G.H. Lewes has urged, that at the first commencement of life many
different forms were evolved; but if so, we may conclude that only a very
few have left modified descendants. For, as I have recently remarked in
regard to the members of each great kingdom, such as the Vertebrata,
Articulata, etc., we have distinct evidence in their embryological,
homologous, and rudimentary structures, that within each kingdom all the
members are descended from a single progenitor.

When the views advanced by me in this volume, and by Mr. Wallace or when
analogous views on the origin of species are generally admitted, we can
dimly foresee that there will be a considerable revolution in natural
history. Systematists will be able to pursue their labours as at present;
but they will not be incessantly haunted by the shadowy doubt whether this
or that form be a true species. This, I feel sure and I speak after
experience, will be no slight relief. The endless disputes whether or not
some fifty species of British brambles are good species will cease.
Systematists will have only to decide (not that this will be easy) whether
any form be sufficiently constant and distinct from other forms, to be
capable of definition; and if definable, whether the differences be
sufficiently important to deserve a specific name. This latter point will
become a far more essential consideration than it is at present; for
differences, however slight, between any two forms, if not blended by
intermediate gradations, are looked at by most naturalists as sufficient
to raise both forms to the rank of species.

Hereafter we shall be compelled to acknowledge that the only distinction
between species and well-marked varieties is, that the latter are known,
or believed to be connected at the present day by intermediate gradations,
whereas species were formerly thus connected. Hence, without rejecting the
consideration of the present existence of intermediate gradations between
any two forms, we shall be led to weigh more carefully and to value higher
the actual amount of difference between them. It is quite possible that
forms now generally acknowledged to be merely varieties may hereafter be
thought worthy of specific names; and in this case scientific and common
language will come into accordance. In short, we shall have to treat
species in the same manner as those naturalists treat genera, who admit
that genera are merely artificial combinations made for convenience. This
may not be a cheering prospect; but we shall at least be freed from the
vain search for the undiscovered and undiscoverable essence of the term
species.

The other and more general departments of natural history will rise
greatly in interest. The terms used by naturalists, of affinity,
relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, etc., will cease to be
metaphorical and will have a plain signification. When we no longer look
at an organic being as a savage looks at a ship, as something wholly
beyond his comprehension; when we regard every production of nature as one
which has had a long history; when we contemplate every complex structure
and instinct as the summing up of many contrivances, each useful to the
possessor, in the same way as any great mechanical invention is the
summing up of the labour, the experience, the reason, and even the
blunders of numerous workmen; when we thus view each organic being, how
far more interesting—I speak from experience—does the study of
natural history become!

A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation, on the effects of use and
disuse, on the direct action of external conditions, and so forth. The
study of domestic productions will rise immensely in value. A new variety
raised by man will be a far more important and interesting subject for
study than one more species added to the infinitude of already recorded
species. Our classifications will come to be, as far as they can be so
made, genealogies; and will then truly give what may be called the plan of
creation. The rules for classifying will no doubt become simpler when we
have a definite object in view. We possess no pedigree or armorial
bearings; and we have to discover and trace the many diverging lines of
descent in our natural genealogies, by characters of any kind which have
long been inherited. Rudimentary organs will speak infallibly with respect
to the nature of long-lost structures. Species and groups of species which
are called aberrant, and which may fancifully be called living fossils,
will aid us in forming a picture of the ancient forms of life. Embryology
will often reveal to us the structure, in some degree obscured, of the
prototypes of each great class.

When we can feel assured that all the individuals of the same species, and
all the closely allied species of most genera, have, within a not very
remote period descended from one parent, and have migrated from some one
birth-place; and when we better know the many means of migration, then, by
the light which geology now throws, and will continue to throw, on former
changes of climate and of the level of the land, we shall surely be
enabled to trace in an admirable manner the former migrations of the
inhabitants of the whole world. Even at present, by comparing the
differences between the inhabitants of the sea on the opposite sides of a
continent, and the nature of the various inhabitants of that continent in
relation to their apparent means of immigration, some light can be thrown
on ancient geography.

The noble science of geology loses glory from the extreme imperfection of
the record. The crust of the earth, with its embedded remains, must not be
looked at as a well-filled museum, but as a poor collection made at hazard
and at rare intervals. The accumulation of each great fossiliferous
formation will be recognised as having depended on an unusual occurrence
of favourable circumstances, and the blank intervals between the
successive stages as having been of vast duration. But we shall be able to
gauge with some security the duration of these intervals by a comparison
of the preceding and succeeding organic forms. We must be cautious in
attempting to correlate as strictly contemporaneous two formations, which
do not include many identical species, by the general succession of the
forms of life. As species are produced and exterminated by slowly acting
and still existing causes, and not by miraculous acts of creation; and as
the most important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism—the improvement
of one organism entailing the improvement or the extermination of others;
it follows, that the amount of organic change in the fossils of
consecutive formations probably serves as a fair measure of the relative,
though not actual lapse of time. A number of species, however, keeping in
a body might remain for a long period unchanged, whilst within the same
period, several of these species, by migrating into new countries and
coming into competition with foreign associates, might become modified; so
that we must not overrate the accuracy of organic change as a measure of
time.

In the future I see open fields for far more important researches.
Psychology will be securely based on the foundation already well laid by
Mr. Herbert Spencer, that of the necessary acquirement of each mental
power and capacity by gradation. Much light will be thrown on the origin
of man and his history.

Authors of the highest eminence seem to be fully satisfied with the view
that each species has been independently created. To my mind it accords
better with what we know of the laws impressed on matter by the Creator,
that the production and extinction of the past and present inhabitants of
the world should have been due to secondary causes, like those determining
the birth and death of the individual. When I view all beings not as
special creations, but as the lineal descendants of some few beings which
lived long before the first bed of the Cambrian system was deposited, they
seem to me to become ennobled. Judging from the past, we may safely infer
that not one living species will transmit its unaltered likeness to a
distinct futurity. And of the species now living very few will transmit
progeny of any kind to a far distant futurity; for the manner in which all
organic beings are grouped, shows that the greater number of species in
each genus, and all the species in many genera, have left no descendants,
but have become utterly extinct. We can so far take a prophetic glance
into futurity as to foretell that it will be the common and widely spread
species, belonging to the larger and dominant groups within each class,
which will ultimately prevail and procreate new and dominant species. As
all the living forms of life are the lineal descendants of those which
lived long before the Cambrian epoch, we may feel certain that the
ordinary succession by generation has never once been broken, and that no
cataclysm has desolated the whole world. Hence, we may look with some
confidence to a secure future of great length. And as natural selection
works solely by and for the good of each being, all corporeal and mental
endowments will tend to progress towards perfection.

It is interesting to contemplate a tangled bank, clothed with many plants
of many kinds, with birds singing on the bushes, with various insects
flitting about, and with worms crawling through the damp earth, and to
reflect that these elaborately constructed forms, so different from each
other, and dependent upon each other in so complex a manner, have all been
produced by laws acting around us. These laws, taken in the largest sense,
being Growth with reproduction; Inheritance which is almost implied by
reproduction; Variability from the indirect and direct action of the
conditions of life, and from use and disuse; a Ratio of Increase so high
as to lead to a Struggle for Life, and as a consequence to Natural
Selection, entailing Divergence of Character and the Extinction of less
improved forms. Thus, from the war of nature, from famine and death, the
most exalted object which we are capable of conceiving, namely, the
production of the higher animals, directly follows. There is grandeur in
this view of life, with its several powers, having been originally
breathed by the Creator into a few forms or into one; and that, whilst
this planet has gone circling on according to the fixed law of gravity,
from so simple a beginning endless forms most beautiful and most wonderful
have been, and are being evolved.



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

(I am indebted to the kindness of Mr. W.S. Dallas for this Glossary, which
has been given because several readers have complained to me that some of
the terms used were unintelligible to them. Mr. Dallas has endeavoured to
give the explanations of the terms in as popular a form as possible.)

ABERRANT.—Forms or groups of animals or plants which deviate in
important characters from their nearest allies, so as not to be easily
included in the same group with them, are said to be aberrant.

ABERRATION (in Optics).—In the refraction of light by a convex lens
the rays passing through different parts of the lens are brought to a
focus at slightly different distances—this is called SPHERICAL
ABERRATION; at the same time the coloured rays are separated by the
prismatic action of the lens and likewise brought to a focus at different
distances—this is CHROMATIC ABERRATION.

ABNORMAL.—Contrary to the general rule.

ABORTED.—An organ is said to be aborted, when its development has
been arrested at a very early stage.

ALBINISM.—Albinos are animals in which the usual colouring matters
characteristic of the species have not been produced in the skin and its
appendages. Albinism is the state of being an albino.

ALGAE.—A class of plants including the ordinary sea-weeds and the
filamentous fresh-water weeds.

ALTERNATION OF GENERATIONS.—This term is applied to a peculiar mode
of reproduction which prevails among many of the lower animals, in which
the egg produces a living form quite different from its parent, but from
which the parent-form is reproduced by a process of budding, or by the
division of the substance of the first product of the egg.

AMMONITES.—A group of fossil, spiral, chambered shells, allied to
the existing pearly Nautilus, but having the partitions between the
chambers waved in complicated patterns at their junction with the outer
wall of the shell.

ANALOGY.—That resemblance of structures which depends upon
similarity of function, as in the wings of insects and birds. Such
structures are said to be ANALOGOUS, and to be ANALOGUES of each other.

ANIMALCULE.—A minute animal: generally applied to those visible only
by the microscope.

ANNELIDS.—A class of worms in which the surface of the body exhibits
a more or less distinct division into rings or segments, generally
provided with appendages for locomotion and with gills. It includes the
ordinary marine worms, the earth-worms, and the leeches.

ANTENNAE.—Jointed organs appended to the head in Insects, Crustacea
and Centipedes, and not belonging to the mouth.

ANTHERS.—The summits of the stamens of flowers, in which the pollen
or fertilising dust is produced.

APLACENTALIA, APLACENTATA or APLACENTAL MAMMALS.—See MAMMALIA.

ARCHETYPAL.—Of or belonging to the Archetype, or ideal primitive
form upon which all the beings of a group seem to be organised.

ARTICULATA.—A great division of the Animal Kingdom characterised
generally by having the surface of the body divided into rings called
segments, a greater or less number of which are furnished with jointed
legs (such as Insects, Crustaceans and Centipedes).

ASYMMETRICAL.—Having the two sides unlike.

ATROPHIED.—Arrested in development at a very early stage.

BALANUS.—The genus including the common Acorn-shells which live in
abundance on the rocks of the sea-coast.

BATRACHIANS.—A class of animals allied to the Reptiles, but
undergoing a peculiar metamorphosis, in which the young animal is
generally aquatic and breathes by gills. (Examples, Frogs, Toads, and
Newts.)

BOULDERS.—Large transported blocks of stone generally embedded in
clays or gravels.

BRACHIOPODA.—A class of marine Mollusca, or soft-bodied animals,
furnished with a bivalve shell, attached to submarine objects by a stalk
which passes through an aperture in one of the valves, and furnished with
fringed arms, by the action of which food is carried to the mouth.

BRANCHIAE.—Gills or organs for respiration in water.

BRANCHIAL.—Pertaining to gills or branchiae.

CAMBRIAN SYSTEM.—A series of very ancient Palaeozoic rocks, between
the Laurentian and the Silurian. Until recently these were regarded as the
oldest fossiliferous rocks.

CANIDAE.—The Dog-family, including the Dog, Wolf, Fox, Jackal, etc.

CARAPACE.—The shell enveloping the anterior part of the body in
Crustaceans generally; applied also to the hard shelly pieces of the
Cirripedes.

CARBONIFEROUS.—This term is applied to the great formation which
includes, among other rocks, the coal-measures. It belongs to the oldest,
or Palaeozoic, system of formations.

CAUDAL.—Of or belonging to the tail.

CEPHALOPODS.—The highest class of the Mollusca, or soft-bodied
animals, characterised by having the mouth surrounded by a greater or less
number of fleshy arms or tentacles, which, in most living species, are
furnished with sucking-cups. (Examples, Cuttle-fish, Nautilus.)

CETACEA.—An order of Mammalia, including the Whales, Dolphins, etc.,
having the form of the body fish-like, the skin naked, and only the fore
limbs developed.

CHELONIA.—An order of Reptiles including the Turtles, Tortoises,
etc.

CIRRIPEDES.—An order of Crustaceans including the Barnacles and
Acorn-shells. Their young resemble those of many other Crustaceans in
form; but when mature they are always attached to other objects, either
directly or by means of a stalk, and their bodies are enclosed by a
calcareous shell composed of several pieces, two of which can open to give
issue to a bunch of curled, jointed tentacles, which represent the limbs.

COCCUS.—The genus of Insects including the Cochineal. In these the
male is a minute, winged fly, and the female generally a motionless,
berry-like mass.

COCOON.—A case usually of silky material, in which insects are
frequently enveloped during the second or resting-stage (pupa) of their
existence. The term "cocoon-stage" is here used as equivalent to
"pupa-stage."

COELOSPERMOUS.—A term applied to those fruits of the Umbelliferae
which have the seed hollowed on the inner face.

COLEOPTERA.—Beetles, an order of Insects, having a biting mouth and
the first pair of wings more or less horny, forming sheaths for the second
pair, and usually meeting in a straight line down the middle of the back.

COLUMN.—A peculiar organ in the flowers of Orchids, in which the
stamens, style and stigma (or the reproductive parts) are united.

COMPOSITAE or COMPOSITOUS PLANTS.—Plants in which the inflorescence
consists of numerous small flowers (florets) brought together into a dense
head, the base of which is enclosed by a common envelope. (Examples, the
Daisy, Dandelion, etc.)

CONFERVAE.—The filamentous weeds of fresh water.

CONGLOMERATE.—A rock made up of fragments of rock or pebbles,
cemented together by some other material.

COROLLA.—The second envelope of a flower usually composed of
coloured, leaf-like organs (petals), which may be united by their edges
either in the basal part or throughout.

CORRELATION.—The normal coincidence of one phenomenon, character,
etc., with another.

CORYMB.—A bunch of flowers in which those springing from the lower
part of the flower stalks are supported on long stalks so as to be nearly
on a level with the upper ones.

COTYLEDONS.—The first or seed-leaves of plants.

CRUSTACEANS.—A class of articulated animals, having the skin of the
body generally more or less hardened by the deposition of calcareous
matter, breathing by means of gills. (Examples, Crab, Lobster, Shrimp,
etc.)

CURCULIO.—The old generic term for the Beetles known as Weevils,
characterised by their four-jointed feet, and by the head being produced
into a sort of beak, upon the sides of which the antennae are inserted.

CUTANEOUS.—Of or belonging to the skin.

DEGRADATION.—The wearing down of land by the action of the sea or of
meteoric agencies.

DENUDATION.—The wearing away of the surface of the land by water.

DEVONIAN SYSTEM or FORMATION.—A series of Palaeozoic rocks,
including the Old Red Sandstone.

DICOTYLEDONS, or DICOTYLEDONOUS PLANTS.—A class of plants
characterised by having two seed-leaves, by the formation of new wood
between the bark and the old wood (exogenous growth) and by the
reticulation of the veins of the leaves. The parts of the flowers are
generally in multiples of five.

DIFFERENTATION.—The separation or discrimination of parts or organs
which in simpler forms of life are more or less united.

DIMORPHIC.—Having two distinct forms.—DIMORPHISM is the
condition of the appearance of the same species under two dissimilar
forms.

DIOECIOUS.—Having the organs of the sexes upon distinct individuals.

DIORITE.—A peculiar form of Greenstone.

DORSAL.—Of or belonging to the back.

EDENTATA.—A peculiar order of Quadrupeds, characterised by the
absence of at least the middle incisor (front) teeth in both jaws.
(Examples, the Sloths and Armadillos.)

ELYTRA.—The hardened fore-wings of Beetles, serving as sheaths for
the membranous hind-wings, which constitute the true organs of flight.

EMBRYO.—The young animal undergoing development within the egg or
womb.

EMBRYOLOGY.—The study of the development of the embryo.

ENDEMIC.—Peculiar to a given locality.

ENTOMOSTRACA.—A division of the class Crustacea, having all the
segments of the body usually distinct, gills attached to the feet or
organs of the mouth, and the feet fringed with fine hairs. They are
generally of small size.

EOCENE.—The earliest of the three divisions of the Tertiary epoch of
geologists. Rocks of this age contain a small proportion of shells
identical with species now living.

EPHEMEROUS INSECTS.—Insects allied to the May-fly.

FAUNA.—The totality of the animals naturally inhabiting a certain
country or region, or which have lived during a given geological period.

FELIDAE.—The Cat-family.

FERAL.—Having become wild from a state of cultivation or
domestication.

FLORA.—The totality of the plants growing naturally in a country, or
during a given geological period.

FLORETS.—Flowers imperfectly developed in some respects, and
collected into a dense spike or head, as in the Grasses, the Dandelion,
etc.

FOETAL.—Of or belonging to the foetus, or embryo in course of
development.

FORAMINIFERA.—A class of animals of very low organisation and
generally of small size, having a jelly-like body, from the surface of
which delicate filaments can be given off and retracted for the prehension
of external objects, and having a calcareous or sandy shell, usually
divided into chambers and perforated with small apertures.

FOSSILIFEROUS.—Containing fossils.

FOSSORIAL.—Having a faculty of digging. The Fossorial Hymenoptera
are a group of Wasp-like Insects, which burrow in sandy soil to make nests
for their young.

FRENUM (pl. FRENA).—A small band or fold of skin.

FUNGI (sing. FUNGUS).—A class of cellular plants, of which
Mushrooms, Toadstools, and Moulds, are familiar examples.

FURCULA.—The forked bone formed by the union of the collar-bones in
many birds, such as the common Fowl.

GALLINACEOUS BIRDS.—An order of birds of which the common Fowl,
Turkey, and Pheasant, are well-known examples.

GALLUS.—The genus of birds which includes the common Fowl.

GANGLION.—A swelling or knot from which nerves are given off as from
a centre.

GANOID FISHES.—Fishes covered with peculiar enamelled bony scales.
Most of them are extinct.

GERMINAL VESICLE.—A minute vesicle in the eggs of animals, from
which the development of the embryo proceeds.

GLACIAL PERIOD.—A period of great cold and of enormous extension of
ice upon the surface of the earth. It is believed that glacial periods
have occurred repeatedly during the geological history of the earth, but
the term is generally applied to the close of the Tertiary epoch, when
nearly the whole of Europe was subjected to an arctic climate.

GLAND.—An organ which secretes or separates some peculiar product
from the blood or sap of animals or plants.

GLOTTIS.—The opening of the windpipe into the oesophagus or gullet.

GNEISS.—A rock approaching granite in composition, but more or less
laminated, and really produced by the alteration of a sedimentary deposit
after its consolidation.

GRALLATORES.—The so-called wading-birds (storks, cranes, snipes,
etc.), which are generally furnished with long legs, bare of feathers
above the heel, and have no membranes between the toes.

GRANITE.—A rock consisting essentially of crystals of felspar and
mica in a mass of quartz.

HABITAT.—The locality in which a plant or animal naturally lives.

HEMIPTERA.—An order or sub-order of insects, characterised by the
possession of a jointed beak or rostrum, and by having the fore-wings
horny in the basal portion and membranous at the extremity, where they
cross each other. This group includes the various species of bugs.

HERMAPHRODITE.—Possessing the organs of both sexes.

HOMOLOGY.—That relation between parts which results from their
development from corresponding embryonic parts, either in different
animals, as in the case of the arm of man, the fore-leg of a quadruped,
and the wing of a bird; or in the same individual, as in the case of the
fore and hind legs in quadrupeds, and the segments or rings and their
appendages of which the body of a worm, a centipede, etc., is composed.
The latter is called serial homology. The parts which stand in such a
relation to each other are said to be homologous, and one such part or
organ is called the homologue of the other. In different plants the parts
of the flower are homologous, and in general these parts are regarded as
homologous with leaves.

HOMOPTERA.—An order or sub-order of insects having (like the
Hemiptera) a jointed beak, but in which the fore-wings are either wholly
membranous or wholly leathery, The Cicadae, frog-hoppers, and Aphides, are
well-known examples.

HYBRID.—The offspring of the union of two distinct species.

HYMENOPTERA.—An order of insects possessing biting jaws and usually
four membranous wings in which there are a few veins. Bees and wasps are
familiar examples of this group.

HYPERTROPHIED.—Excessively developed.

ICHNEUMONIDAE.—A family of hymenopterous insects, the members of
which lay their eggs in the bodies or eggs of other insects.

IMAGO.—The perfect (generally winged) reproductive state of an
insect.

INDIGENES.—The aboriginal animal or vegetable inhabitants of a
country or region.

INFLORESCENCE.—The mode of arrangement of the flowers of plants.

INFUSORIA.—A class of microscopic animalcules, so called from their
having originally been observed in infusions of vegetable matters. They
consist of a gelatinous material enclosed in a delicate membrane, the
whole or part of which is furnished with short vibrating hairs (called
cilia), by means of which the animalcules swim through the water or convey
the minute particles of their food to the orifice of the mouth.

INSECTIVOROUS.—Feeding on insects.

INVERTEBRATA, or INVERTEBRATE ANIMALS.—Those animals which do not
possess a backbone or spinal column.

LACUNAE.—Spaces left among the tissues in some of the lower animals
and serving in place of vessels for the circulation of the fluids of the
body.

LAMELLATED.—Furnished with lamellae or little plates.

LARVA (pl. LARVAE).—The first condition of an insect at its issuing
from the egg, when it is usually in the form of a grub, caterpillar, or
maggot.

LARYNX.—The upper part of the windpipe opening into the gullet.

LAURENTIAN.—A group of greatly altered and very ancient rocks, which
is greatly developed along the course of the St. Laurence, whence the
name. It is in these that the earliest known traces of organic bodies have
been found.

LEGUMINOSAE.—An order of plants represented by the common peas and
beans, having an irregular flower in which one petal stands up like a
wing, and the stamens and pistil are enclosed in a sheath formed by two
other petals. The fruit is a pod (or legume).

LEMURIDAE.—A group of four-handed animals, distinct from the monkeys
and approaching the insectivorous quadrupeds in some of their characters
and habits. Its members have the nostrils curved or twisted, and a claw
instead of a nail upon the first finger of the hind hands.

LEPIDOPTERA.—An order of insects, characterised by the possession of
a spiral proboscis, and of four large more or less scaly wings. It
includes the well-known butterflies and moths.

LITTORAL.—Inhabiting the seashore.

LOESS.—A marly deposit of recent (Post-Tertiary) date, which
occupies a great part of the valley of the Rhine.

MALACOSTRACA.—The higher division of the Crustacea, including the
ordinary crabs, lobsters, shrimps, etc., together with the woodlice and
sand-hoppers.

MAMMALIA.—The highest class of animals, including the ordinary hairy
quadrupeds, the whales and man, and characterised by the production of
living young which are nourished after birth by milk from the teats
(MAMMAE, MAMMARY GLANDS) of the mother. A striking difference in embryonic
development has led to the division of this class into two great groups;
in one of these, when the embryo has attained a certain stage, a vascular
connection, called the PLACENTA, is formed between the embryo and the
mother; in the other this is wanting, and the young are produced in a very
incomplete state. The former, including the greater part of the class, are
called PLACENTAL MAMMALS; the latter, or APLACENTAL MAMMALS, include the
Marsupials and Monotremes (ORNITHORHYNCHUS).

MAMMIFEROUS.—Having mammae or teats (see MAMMALIA).

MANDIBLES.—in insects, the first or uppermost pair of jaws, which
are generally solid, horny, biting organs. In birds the term is applied to
both jaws with their horny coverings. In quadrupeds the mandible is
properly the lower jaw.

MARSUPIALS.—An order of Mammalia in which the young are born in a
very incomplete state of development, and carried by the mother, while
sucking, in a ventral pouch (marsupium), such as the kangaroos, opossums,
etc. (see MAMMALIA).

MAXILLAE.—in insects, the second or lower pair of jaws, which are
composed of several joints and furnished with peculiar jointed appendages
called palpi, or feelers.

MELANISM.—The opposite of albinism; an undue development of
colouring material in the skin and its appendages.

METAMORPHIC ROCKS.—Sedimentary rocks which have undergone
alteration, generally by the action of heat, subsequently to their
deposition and consolidation.

MOLLUSCA.—One of the great divisions of the animal kingdom,
including those animals which have a soft body, usually furnished with a
shell, and in which the nervous ganglia, or centres, present no definite
general arrangement. They are generally known under the denomination of
"shellfish"; the cuttle-fish, and the common snails, whelks, oysters,
mussels, and cockles, may serve as examples of them.

MONOCOTYLEDONS, or MONOCOTYLEDONOUS PLANTS.—Plants in which the seed
sends up only a single seed-leaf (or cotyledon); characterised by the
absence of consecutive layers of wood in the stem (endogenous growth), by
the veins of the leaves being generally straight, and by the parts of the
flowers being generally in multiples of three. (Examples, grasses, lilies,
orchids, palms, etc.)

MORAINES.—The accumulations of fragments of rock brought down by
glaciers.

MORPHOLOGY.—The law of form or structure independent of function.

MYSIS-STAGE.—A stage in the development of certain crustaceans
(prawns), in which they closely resemble the adults of a genus (Mysis)
belonging to a slightly lower group.

NASCENT.—Commencing development.

NATATORY.—Adapted for the purpose of swimming.

NAUPLIUS-FORM.—The earliest stage in the development of many
Crustacea, especially belonging to the lower groups. In this stage the
animal has a short body, with indistinct indications of a division into
segments, and three pairs of fringed limbs. This form of the common
fresh-water CYCLOPS was described as a distinct genus under the name of
NAUPLIUS.

NEURATION.—The arrangement of the veins or nervures in the wings of
insects.

NEUTERS.—Imperfectly developed females of certain social insects
(such as ants and bees), which perform all the labours of the community.
Hence, they are also called WORKERS.

NICTITATING MEMBRANE.—A semi-transparent membrane, which can be
drawn across the eye in birds and reptiles, either to moderate the effects
of a strong light or to sweep particles of dust, etc., from the surface of
the eye.

OCELLI.—The simple eyes or stemmata of insects, usually situated on
the crown of the head between the great compound eyes.

OESOPHAGUS.—The gullet.

OOLITIC.—A great series of secondary rocks, so called from the
texture of some of its members, which appear to be made up of a mass of
small EGG-LIKE calcareous bodies.

OPERCULUM.—A calcareous plate employed by many Molluscae to close
the aperture of their shell. The OPERCULAR VALVES of Cirripedes are those
which close the aperture of the shell.

ORBIT.—The bony cavity for the reception of the eye.

ORGANISM.—An organised being, whether plant or animal.

ORTHOSPERMOUS.—A term applied to those fruits of the Umbelliferae
which have the seed straight.

OSCULANT.—Forms or groups apparently intermediate between and
connecting other groups are said to be osculant.

OVA.—Eggs.

OVARIUM or OVARY (in plants).—The lower part of the pistil or female
organ of the flower, containing the ovules or incipient seeds; by growth
after the other organs of the flower have fallen, it usually becomes
converted into the fruit.

OVIGEROUS.—Egg-bearing.

OVULES (of plants).—The seeds in the earliest condition.

PACHYDERMS.—A group of Mammalia, so called from their thick skins,
and including the elephant, rhinoceros, hippopotamus, etc.

PALAEOZOIC.—The oldest system of fossiliferous rocks.

PALPI.—Jointed appendages to some of the organs of the mouth in
insects and Crustacea.

PAPILIONACEAE.—An order of plants (see LEGUMINOSAE), The flowers of
these plants are called PAPILIONACEOUS, or butterfly-like, from the
fancied resemblance of the expanded superior petals to the wings of a
butterfly.

PARASITE.—An animal or plant living upon or in, and at the expense
of, another organism.

PARTHENOGENESIS.—The production of living organisms from
unimpregnated eggs or seeds.

PEDUNCULATED.—Supported upon a stem or stalk. The pedunculated oak
has its acorns borne upon a footstool.

PELORIA or PELORISM.—The appearance of regularity of structure in
the flowers of plants which normally bear irregular flowers.

PELVIS.—The bony arch to which the hind limbs of vertebrate animals
are articulated.

PETALS.—The leaves of the corolla, or second circle of organs in a
flower. They are usually of delicate texture and brightly coloured.

PHYLLODINEOUS.—Having flattened, leaf-like twigs or leafstalks
instead of true leaves.

PIGMENT.—The colouring material produced generally in the
superficial parts of animals. The cells secreting it are called
PIGMENT-CELLS.

PINNATE.—Bearing leaflets on each side of a central stalk.

PISTILS.—The female organs of a flower, which occupy a position in
the centre of the other floral organs. The pistil is generally divisible
into the ovary or germen, the style and the stigma.

PLACENTALIA, PLACENTATA.—or PLACENTAL MAMMALS, See MAMMALIA.

PLANTIGRADES.—Quadrupeds which walk upon the whole sole of the foot,
like the bears.

PLASTIC.—Readily capable of change.

PLEISTOCENE PERIOD.—The latest portion of the Tertiary epoch.

PLUMULE (in plants).—The minute bud between the seed-leaves of
newly-germinated plants.

PLUTONIC ROCKS.—Rocks supposed to have been produced by igneous
action in the depths of the earth.

POLLEN.—The male element in flowering plants; usually a fine dust
produced by the anthers, which, by contact with the stigma effects the
fecundation of the seeds. This impregnation is brought about by means of
tubes (POLLEN-TUBES) which issue from the pollen-grains adhering to the
stigma, and penetrate through the tissues until they reach the ovary.

POLYANDROUS (flowers).—Flowers having many stamens.

POLYGAMOUS PLANTS.—Plants in which some flowers are unisexual and
others hermaphrodite. The unisexual (male and female) flowers, may be on
the same or on different plants.

POLYMORPHIC.—Presenting many forms.

POLYZOARY.—The common structure formed by the cells of the Polyzoa,
such as the well-known seamats.

PREHENSILE.—Capable of grasping.

PREPOTENT.—Having a superiority of power.

PRIMARIES.—The feathers forming the tip of the wing of a bird, and
inserted upon that part which represents the hand of man.

PROCESSES.—Projecting portions of bones, usually for the attachment
of muscles, ligaments, etc.

PROPOLIS.—A resinous material collected by the hivebees from the
opening buds of various trees.

PROTEAN.—Exceedingly variable.

PROTOZOA.—The lowest great division of the animal kingdom. These
animals are composed of a gelatinous material, and show scarcely any trace
of distinct organs. The Infusoria, Foraminifera, and sponges, with some
other forms, belong to this division.

PUPA (pl. PUPAE).—The second stage in the development of an insect,
from which it emerges in the perfect (winged) reproductive form. In most
insects the PUPAL STAGE is passed in perfect repose. The CHRYSALIS is the
pupal state of butterflies.

RADICLE.—The minute root of an embryo plant.

RAMUS.—One half of the lower jaw in the Mammalia. The portion which
rises to articulate with the skull is called the ASCENDING RAMUS.

RANGE.—The extent of country over which a plant or animal is
naturally spread. RANGE IN TIME expresses the distribution of a species or
group through the fossiliferous beds of the earth's crust.

RETINA.—The delicate inner coat of the eye, formed by nervous
filaments spreading from the optic nerve, and serving for the perception
of the impressions produced by light.

RETROGRESSION.—Backward development. When an animal, as it
approaches maturity, becomes less perfectly organised than might be
expected from its early stages and known relationships, it is said to
undergo a RETROGRADE DEVELOPMENT or METAMORPHOSIS.

RHIZOPODS.—A class of lowly organised animals (Protozoa), having a
gelatinous body, the surface of which can be protruded in the form of
root-like processes or filaments, which serve for locomotion and the
prehension of food. The most important order is that of the Foraminifera.

RODENTS.—The gnawing Mammalia, such as the rats, rabbits, and
squirrels. They are especially characterised by the possession of a single
pair of chisel-like cutting teeth in each jaw, between which and the
grinding teeth there is a great gap.

RUBUS.—The bramble genus.

RUDIMENTARY.—Very imperfectly developed.

RUMINANTS.—The group of quadrupeds which ruminate or chew the cud,
such as oxen, sheep, and deer. They have divided hoofs, and are destitute
of front teeth in the upper jaw.

SACRAL.—Belonging to the sacrum, or the bone composed usually of two
or more united vertebrae to which the sides of the pelvis in vertebrate
animals are attached.

SARCODE.—The gelatinous material of which the bodies of the lowest
animals (Protozoa) are composed.

SCUTELLAE.—The horny plates with which the feet of birds are
generally more or less covered, especially in front.

SEDIMENTARY FORMATIONS.—Rocks deposited as sediments from water.

SEGMENTS.—The transverse rings of which the body of an articulate
animal or annelid is composed.

SEPALS.—The leaves or segments of the calyx, or outermost envelope
of an ordinary flower. They are usually green, but sometimes brightly
coloured.

SERRATURES.—Teeth like those of a saw.

SESSILE.—Not supported on a stem or footstalk.

SILURIAN SYSTEM.—A very ancient system of fossiliferous rocks
belonging to the earlier part of the Palaeozoic series.

SPECIALISATION.—The setting apart of a particular organ for the
performance of a particular function.

SPINAL CORD.—The central portion of the nervous system in the
Vertebrata, which descends from the brain through the arches of the
vertebrae, and gives off nearly all the nerves to the various organs of
the body.

STAMENS.—The male organs of flowering plants, standing in a circle
within the petals. They usually consist of a filament and an anther, the
anther being the essential part in which the pollen, or fecundating dust,
is formed.

STERNUM.—The breast-bone.

STIGMA.—The apical portion of the pistil in flowering plants.

STIPULES.—Small leafy organs placed at the base of the footstalks of
the leaves in many plants.

STYLE.—The middle portion of the perfect pistil, which rises like a
column from the ovary and supports the stigma at its summit.

SUBCUTANEOUS.—Situated beneath the skin.

SUCTORIAL.—Adapted for sucking.

SUTURES (in the skull).—The lines of junction of the bones of which
the skull is composed.

TARSUS (pl. TARSI).—The jointed feet of articulate animals, such as
insects.

TELEOSTEAN FISHES.—Fishes of the kind familiar to us in the present
day, having the skeleton usually completely ossified and the scales horny.

TENTACULA or TENTACLES.—Delicate fleshy organs of prehension or
touch possessed by many of the lower animals.

TERTIARY.—The latest geological epoch, immediately preceding the
establishment of the present order of things.

TRACHEA.—The windpipe or passage for the admission of air to the
lungs.

TRIDACTYLE.—Three-fingered, or composed of three movable parts
attached to a common base.

TRILOBITES.—A peculiar group of extinct crustaceans, somewhat
resembling the woodlice in external form, and, like some of them, capable
of rolling themselves up into a ball. Their remains are found only in the
Palaeozoic rocks, and most abundantly in those of Silurian age.

TRIMORPHIC.—Presenting three distinct forms.

UMBELLIFERAE.—An order of plants in which the flowers, which contain
five stamens and a pistil with two styles, are supported upon footstalks
which spring from the top of the flower stem and spread out like the wires
of an umbrella, so as to bring all the flowers in the same head (UMBEL)
nearly to the same level. (Examples, parsley and carrot.)

UNGULATA.—Hoofed quadrupeds.

UNICELLULAR.—Consisting of a single cell.

VASCULAR.—Containing blood-vessels.

VERMIFORM.—Like a worm.

VERTEBRATA or VERTEBRATE ANIMALS.—The highest division of the animal
kingdom, so called from the presence in most cases of a backbone composed
of numerous joints or VERTEBRAE, which constitutes the centre of the
skeleton and at the same time supports and protects the central parts of
the nervous system.

WHORLS.—The circles or spiral lines in which the parts of plants are
arranged upon the axis of growth.

WORKERS.—See neuters.

ZOEA-STAGE.—The earliest stage in the development of many of the
higher Crustacea, so called from the name of ZOEA applied to these young
animals when they were supposed to constitute a peculiar genus.

ZOOIDS.—In many of the lower animals (such as the Corals, Medusae,
etc.) reproduction takes place in two ways, namely, by means of eggs and
by a process of budding with or without separation from the parent of the
product of the latter, which is often very different from that of the egg.
The individuality of the species is represented by the whole of the form
produced between two sexual reproductions; and these forms, which are
apparently individual animals, have been called ZOOIDE.



INDEX.


  Aberrant groups
  Abyssinia, plants of
  Acclimatisation
  Adoxa
  Affinities of extinct species
 —of organic beings
  Agassiz on Amblyopsis
 —on groups of species suddenly appearing
 —on prophetic forms
 —on embryological succession
 —on the Glacial period
 —on embryological characters
 —on the latest tertiary forms
 —on parallelism of embryological development and geological succession
 —Alex., on pedicellariae
  Algae of New Zealand
  Alligators, males, fighting
  Alternate generations
  Amblyopsis, blind fish
  America, North, productions allied to those of Europe
 —boulders and glaciers of
 —South, no modern formations on west coast
  Ammonites, sudden extinction of
  Anagallis, sterility of
  Analogy of variations
  Andaman Islands inhabited by a toad
  Ancylus
  Animals, not domesticated from being variable
 —domestic; descended from several stocks
 —acclimatisation of
  Animals of Australia
 —with thicker fur in cold climates
 —blind, in caves
 —extinct, of Australia
  Anomma
  Antarctic islands, ancient flora of
  Antechinus
  Ants attending aphides
 —slave-making instinct
 —neuters, structure of
  Apes, not having acquired intellectual powers
  Aphides attended by ants
  Aphis, development of
  Apteryx
  Arab horses
  Aralo-Caspian Sea
  Archeopteryx
  Archiac, M. de, on the succession of species
  Artichoke, Jerusalem
  Ascension, plants of
  Asclepias, pollen of
  Asparagus
  Aspicarpa
  Asses, striped
 —improved by selection
  Ateuchus
  Aucapitaine, on land-shells
  Audubon, on habits of frigate-bird
 —on variation in birds' nests
 —on heron eating seeds
  Australia, animals of
 —dogs of
 —extinct animals of
 —European plants in
 —glaciers of
  Azara, on flies destroying cattle
  Azores, flora of
  Babington, Mr., on British plants
  Baer, Von, standard of Highness
 —comparison of bee and fish
 —embryonic similarity of the Vertebrata
  Baker, Sir S., on the giraffe
  Balancement of growth
  Baleen
  Barberry, flowers of
  Barrande, M., on Silurian colonies
 —on the succession of species
 —on parallelism of palaeozoic formations
 —on affinities of ancient species
  Barriers, importance of
  Bates, Mr., on mimetic butterflies
  Batrachians on islands
  Bats, how structure acquired
 —distribution of
  Bear, catching water-insects
  Beauty, how acquired
  Bee, sting of
 —queen, killing rivals
 —Australian, extermination of
  Bees, fertilizing flowers
 —hive, not sucking the red clover
 —hive, cell-making instinct
 —Ligurian
 —variation in habits
  Bees, parasitic
 —humble, cells of
  Beetles, wingless, in Madeira
 —with deficient tarsi
  Bentham, Mr., on British plants
 —on classification
  Berkeley, Mr., on seeds in salt-water
  Bermuda, birds of
  Birds acquiring fear
 —beauty of
 —annually cross the Atlantic
 —colour of, on continents
 —footsteps, and remains of, in secondary rocks
 —fossil, in caves of Brazil
 —of Madeira, Bermuda, and Galapagos
 —song of males
 —transporting seeds
 —waders
 —wingless
  Bizcacha, affinities of
  Bladder for swimming, in fish
  Blindness of cave animals
  Blyth, Mr., on distinctness of Indian cattle
 —on striped Hemionus
 —on crossed geese
  Borrow, Mr., on the Spanish pointer
  Bory St. Vincent, on Batrachians
  Bosquet, M., on fossil Chthamalus
  Boulders, erratic, on the Azores
  Branchiae
 —of crustaceans
  Braun, Prof., on the seeds of Fumariaceae
  Brent, Mr., on house-tumblers
  Britain, mammals of
  Broca, Prof., on Natural Selection
  Bronn, Prof., on duration of specific forms
 —various objections by
  Brown, Robert, on classification
  Brown-Sequard, on inherited mutilations
  Busk, Mr., on the Polyzoa
  Butterflies, mimetic
  Buzareingues, on sterility of varieties
  Cabbage, varieties of, crossed
  Calceolaria
  Canary-birds, sterility of hybrids
  Cape de Verde Islands, productions of
 —plants of, on mountains
  Cape of Good Hope, plants of
  Carpenter, Dr., on foraminifera
  Carthemus
  Catasetum
  Cats, with blue eyes, deaf
 —variation in habits of
 —curling tail when going to spring
  Cattle destroying fir-trees
 —destroyed by flies in Paraguay
 —breeds of, locally extinct
 —fertility of Indian and European breeds
 —Indian
  Cave, inhabitants of, blind
  Cecidomyia
  Celts, proving antiquity of man
  Centres of creation
  Cephalopodae, structures of eyes
 —development of
  Cercopithecus, tail of
  Ceroxylus laceratus
  Cervulus
  Cetacea, teeth and hair
 —development of the whalebone
  Cetaceans
  Ceylon, plants of
  Chalk formation
  Characters, divergence of
 —sexual, variable
 —adaptive or analogical
  Charlock
  Checks to increase
 —mutual
  Chelae of Crustaceans
  Chickens, instinctive tameness of
  Chironomus, its asexual reproduction
  Chthamalinae
  Chthamalus, cretacean species of
  Circumstances favourable to selection of domestic products
 —to natural selection
  Cirripedes capable of crossing
 —carapace aborted
 —their ovigerous frena
 —fossil
 —larvae of
  Claparede, Prof., on the hair-claspers of the Acaridae
  Clarke, Rev. W.B., on old glaciers in Australia
  Classification
  Clift, Mr., on the succession of types
  Climate, effects of, in checking increase of beings
 —adaptation of, to organisms
  Climbing plants
 —development of
  Clover visited by bees
  Cobites, intestine of
  Cockroach
  Collections, palaeontological, poor
  Colour, influenced by climate
 —in relation to attacks by flies
  Columba livia, parent of domestic pigeons
  Colymbetes
  Compensation of growth
  Compositae, flowers and seeds of
 —outer and inner florets of
 —male flowers of
  Conclusion, general
  Conditions, slight changes in, favourable to fertility
  Convergence of genera
  Coot
  Cope, Prof., on the acceleration or retardation of the period of
  reproduction
  Coral-islands, seeds drifted to
 —reefs, indicating movements of earth
  Corn-crake
  Correlated variation in domestic productions
  Coryanthes
  Creation, single centres of
  Crinum
  Croll, Mr., on subaerial denudation
 —on the age of our oldest formations
 —on alternate Glacial periods in the North and South
  Crosses, reciprocal
  Crossing of domestic animals, importance in altering breeds
 —advantages of
 —unfavourable to selection
  Cruger, Dr., on Coryanthes
  Crustacea of New Zealand
  Crustacean, blind
  air-breathers
  Crustaceans, their chelae
  Cryptocerus
  Ctenomys, blind
  Cuckoo, instinct of
  Cunningham, Mr., on the flight of the logger-headed duck
  Currants, grafts of
  Currents of sea, rate of
  Cuvier on conditions of existence
 —on fossil monkeys
  Cuvier, Fred., on instinct
  Cyclostoma, resisting salt water
  Dana, Prof., on blind cave-animals
 —on relations of crustaceans of Japan
 —on crustaceans of New Zealand
  Dawson, Dr., on eozoon
  De Candolle, Aug. Pyr., on struggle for existence
 —on umbelliferae
 —on general affinities
  De Candolle, Alph., on the variability of oaks
 —on low plants, widely dispersed
 —on widely-ranging plants being variable
 —on naturalisation
 —on winged seeds
 —on Alpine species suddenly becoming rare
 —on distribution of plants with large seeds
 —on vegetation of Australia
 —on fresh-water plants
 —on insular plants
  Degradation of rocks
  Denudation, rate of
 —of oldest rocks
 —of granite areas
  Development of ancient forms
  Devonian system
  Dianthus, fertility of crosses
  Dimorphism in plants
  Dirt on feet of birds
  Dispersal, means of
 —during Glacial period
  Distribution, geographical
 —means of
  Disuse, effect of, under nature
  Diversification of means for same general purpose
  Division, physiological, of labour
  Divergence of character
  Dog, resemblance of jaw to that of the Thylacinus
  Dogs, hairless, with imperfect teeth
 —descended from several wild stocks
 —domestic instincts of
 —inherited civilisation of
 —fertility of breeds together
 —of crosses
 —proportions of body in different breeds, when young
  Domestication, variation under
  Double flowers
  Downing, Mr., on fruit-trees in America
  Dragon-flies, intestines of
  Drift-timber
  Driver-ant
  Drones killed by other bees
  Duck, domestic, wings of, reduced
 —beak of
 —logger-headed
  Duckweed
  Dugong, affinities of
  Dung-beetles with deficient tarsi
  Dyticus
  Earl, Mr., W., on the Malay Archipelago
  Ears, drooping, in domestic animals
 —rudimentary
  Earth, seeds in roots of trees
 —charged with seeds
  Echinodermata, their pedicellariae
  Eciton
  Economy of organisation
  Edentata, teeth and hair
 —fossil species of
  Edwards, Milne, on physiological division of labour
 —on gradations of structure
  Edwards, on embryological characters
  Eggs, young birds escaping from
  Egypt, productions of, not modified
  Electric organs
  Elephant, rate of increase
 —of Glacial period
  Embryology
  Eozoon Canadense
  Epilipsy inherited
  Existence, struggle for
 —condition of
  Extinction, as bearing on natural selection
 —of domestic varieties
  Eye, structure of
 —correction for aberration
  Eyes, reduced, in moles
  Fabre, M., on hymenoptera fighting
 —on parasitic sphex
 —on Sitaris
  Falconer, Dr., on naturalisation of plants in India
 —on elephants and mastodons
 —and Cautley on mammals of sub-Himalayan beds
  Falkland Islands, wolf of
  Faults
  Faunas, marine
  Fear, instinctive, in birds
  Feet of birds, young molluscs adhering to
  Fertilisation variously effected
  Fertility of hybrids
 —from slight changes in conditions
 —of crossed varieties
  Fir-trees destroyed by cattle
 —pollen of
  Fish, flying
 —teleostean, sudden appearance of
 —eating seeds
 —fresh-water, distribution of
  Fishes, ganoid, now confined to fresh water
 —ganoid, living in fresh water
 —electric organs of
 —of southern hemisphere
  Flight, powers of, how acquired
  Flint-tools, proving antiquity of man
  Flower, Prof., on the larynx
 —on Halitherium
 —on the resemblance between the jaws of the dog and Thylacinus
 —on the homology of the feet of certain marsupials
  Flowers, structure of
 —in relation to crossing
 —of composite and umbelliferae
 —beauty of
 —double
  Flysch formation, destitute of organic remains
  Forbes, Mr. D., on glacial action in the Andes
  Forbes, E., on colours of shells
 —on abrupt range of shells in depth
 —on poorness of palaeontological collections
 —on continuous succession of genera
 —on continental extensions
 —on distribution during Glacial period
 —on parallelism in time and space
  Forests, changes in, in America
  Formation, Devonian
 —Cambrian
 —intermittent
 —thickness of, in Britain
  Formica
 —rufescens
 —sanguinea
 —flava, neuter of
  Forms, lowly organised, long enduring
  Frena, ovigerous, of cirripedes
  Fresh-water productions, dispersal of
  Fries on species in large genera being closely allied to other species
  Frigate-bird
  Frogs on islands
  Fruit-trees, gradual improvement of
 —in United States
 —varieties of, acclimatised in United States
  Fuci, crossed
  Fur, thicker in cold climates
  Furze
  Galapagos Archipelago, birds of
 —productions of
  Galaxias, its wide range
  Galeopithecus
  Game, increase of, checked by vermin
  Gartner on sterility of hybrids
 —on reciprocal crosses
 —on crossed maize and verbascum
 —on comparison of hybrids and mongrels
  Gaudry, Prof., on intermediate genera of fossil mammals in Attica
  Geese, fertility when crossed
 —upland
  Geikie, Mr., on subaerial denudation
  Genealogy, important in classification
  Generations, alternate
  Geoffroy St. Hilaire, on balancement
 —on homologous organs
  Geoffroy St. Hilaire, Isidore, on variability of repeated parts
 —on correlation, in monstrosities
 —on correlation
 —on variable parts being often monstrous
  Geographical distribution
  Geography, ancient
  Geology, future progress of
 —imperfection of the record
  Gervais, Prof., on Typotherium
  Giraffe, tail of
 —structure of
  Glacial period
 —affecting the North and South
  Glands, mammary
  Gmelin, on distribution
  Godwin-Austin, Mr., on the Malay Archipelago
  Goethe, on compensation of growth
  Gomphia
  Gooseberry, grafts of
  Gould, Dr. Aug. A., on land-shells
  Gould, Mr., on colours of birds
 —on instincts of cuckoo
 —on distribution of genera of birds
  Gourds, crossed
  Graba, on the Uria lacrymans
  Grafting, capacity of
  Granite, areas of denuded
  Grasses, varieties of
  Gray, Dr. Asa, on the variability of oaks
 —on man not causing variability
 —on sexes of the holly
 —on trees of the United States
 —on naturalised plants in the United States
 —on aestivation
 —on rarity of intermediate varieties
 —on Alpine plants
  Gray, Dr. J.E., on striped mule
  Grebe
  Grimm, on asexual reproduction
  Groups, aberrant
  Grouse, colours of
 —red, a doubtful species
  Growth, compensation of
  Gunther, Dr., on flat-fish
 —on prehensile tails
 —on the fishes of Panama
 —on the range of fresh-water fishes
 —on the limbs of Lepidosiren
  Haast, Dr., on glaciers of New Zealand
  Habit, effect of, under domestication
 —effect of, under nature
 —diversified, of same species
  Hackel, Prof., on classification and the lines of descent
  Hair and teeth, correlated
  Halitherium
  Harcourt, Mr. E.V., on the birds of Madeira
  Hartung, M., on boulders in the Azores
  Hazel-nuts
  Hearne, on habits of bears
  Heath, changes in vegetation
  Hector, Dr., on glaciers of New Zealand
  Heer, Oswald, on ancient cultivated plants
 —on plants of Madeira
  Helianthemum
  Helix pomatia, resisting salt water
  Helmholtz, M., on the imperfection of the human eye
  Helosciadium
  Hemionus, striped
  Hensen, Dr., on the eyes of Cephalopods
  Herbert, W., on struggle for existence
 —on sterility of hybrids
  Hermaphrodites crossing
  Heron eating seed
  Heron, Sir R., on peacocks
  Heusinger, on white animals poisoned by certain plants
  Hewitt, Mr., on sterility of first crosses
  Hildebrand, Prof., on the self-sterility of Corydalis
  Hilgendorf, on intermediate varieties
  Himalaya, glaciers of
 —plants of
  Hippeastrum
  Hippocampus
  Hofmeister, Prof., on the movements of plants
  Holly-trees, sexes of
  Hooker, Dr., on trees of New Zealand
 —on acclimatisation of Himalayan trees
 —on flowers of umbelliferae
 —on the position of ovules
 —on glaciers of Himalaya
 —on algae of New Zealand
 —on vegetation at the base of the Himalaya
 —on plants of Tierra del Fuego
 —on Australian plants
 —on relations of flora of America
 —on flora of the Antarctic lands
 —on the plants of the Galapagos
 —on glaciers of the Lebanon
 —on man not causing variability
 —on plants of mountains of Fernando Po
  Hooks on palms
 —on seeds, on islands
  Hopkins, Mr., on denudation
  Hornbill, remarkable instinct of
  Horns, rudimentary
  Horse, fossil in La Plata
 —proportions of, when young
  Horses destroyed by flies in Paraguay
 —striped
  Horticulturists, selection applied by
  Huber on cells of bees
  Huber, P., on reason blended with instinct
 —on habitual nature of instincts
 —on slave-making ants
 —on Melipona domestica
  Hudson, Mr., on the Ground-woodpecker of La Plata
 —on the Molothrus
  Humble-bees, cells of
  Hunter, J., on secondary sexual characters
  Hutton, Captain, on crossed geese
  Huxley, Prof., on structure of hermaphrodites
 —on the affinities of the Sirenia
 —on forms connecting birds and reptiles
 —on homologous organs
 —on the development of aphis
  Hybrids and mongrels compared
  Hybridism
  Hydra, structure of
  Hymenoptera, fighting
  Hymenopterous insect, diving
  Hyoseris
  Ibla
  Icebergs transporting seeds
  Increase, rate of
  Individuals, numbers favourable to selection
 —many, whether simultaneously created
  Inheritance, laws of
 —at corresponding ages
  Insects, colour of, fitted for their stations
 —sea-side, colours of
 —blind, in caves
 —luminous
 —their resemblance to certain objects
 —neuter
  Instinct, not varying simultaneously with structure
  Instincts, domestic
  Intercrossing, advantages of
  Islands, oceanic
  Isolation favourable to selection
  Japan, productions of
  Java, plants of
  Jones, Mr. J.M., on the birds of Bermuda
  Jordain, M., on the eye-spots of star fishes
  Jukes, Prof., on subaerial denudation
  Jussieu on classification
  Kentucky, caves of
  Kerguelen-land, flora of
  Kidney-bean, acclimatisation of
  Kidneys of birds
  Kirby, on tarsi deficient in beetles
  Knight, Andrew, on cause of variation
  Kolreuter, on intercrossing
 —on the barberry
 —on sterility of hybrids
 —on reciprocal crosses
 —on crossed varieties of nicotiana
 —on crossing male and hermaphrodite flowers
  Lamarck, on adaptive characters
  Lancelet, eyes of
  Landois, on the development of the wings of insects
  Land-shells, distribution of
 —of Madeira, naturalised
 —resisting salt water
  Languages, classification of
  Lankester, Mr. E. Ray, on longevity
 —on homologies
  Lapse, great, of time
  Larvae
  Laurel, nectar secreted by the leaves
  Laurentian formation
  Laws of variation
  Leech, varieties of
  Leguminosae, nectar secreted by glands
  Leibnitz, attack on Newton
  Lepidosiren, limbs in a nascent condition
  Lewes, Mr. G.H., on species not having changed in Egypt
 —on the Salamandra atra
 —on many forms of life having been at first evolved
  Life, struggle for
  Lingula, Silurian
  Linnæus, aphorism of
  Lion, mane of
 —young of, striped
  Lobelia fulgens
  Lobelia, sterility of crosses
  Lockwood, Mr., on the ova of the Hippocampus
  Locusts transporting seeds
  Logan, Sir W., on Laurentian formation
  Lowe, Rev. R.T., on locusts visiting Madeira
  Lowness, of structure connected with variability
 —related to wide distribution
  Lubbock, Sir J., on the nerves of coccus
 —on secondary sexual characters
 —on a diving hymenopterous insect
 —on affinities
 —on metamorphoses
  Lucas, Dr. P., on inheritance
 —on resemblance of child to parent
  Lund and Clausen, on fossils of Brazil
  Lyell, Sir C., on the struggle for existence
 —on modern changes of the earth
 —on terrestrial animals not having been developed on islands
 —on a carboniferous land-shell
 —on strata beneath Silurian system
 —on the imperfection of the geological record
 —on the appearance of species
 —on Barrande's colonies
 —on tertiary formations of Europe and North America
 —on parallelism of tertiary formations
 —on transport of seeds by icebergs
 —on great alternations of climate
 —on the distribution of fresh-water shells
 —on land-shells of Madeira
  Lyell and Dawson, on fossilized trees in Nova Scotia
  Lythrum salicaria, trimorphic
  Macleay, on analogical characters
  Macrauchenia
  McDonnell, Dr., on electric organs
  Madeira, plants of
 —beetles of, wingless
 —fossil land-shells of
 —birds of
  Magpie tame in Norway
  Males, fighting
  Maize, crossed
  Malay Archipelago, compared with Europe
 —mammals of
  Malm, on flat-fish
  Malpighiaceae, small imperfect flowers of
  Mammae, their development
 —rudimentary
  Mammals, fossil, in secondary formation
 —insular
  Man, origin of
  Manatee, rudimentary nails of
  Marsupials, fossil species of
  Marsupials of Australia, structure of their feet
  Martens, M., experiment on seeds
  Martin, Mr. W.C., on striped mules
  Masters, Dr., on Saponaria
  Matteucci, on the electric organs of rays
  Matthiola, reciprocal crosses of
  Maurandia
  Means of dispersal
  Melipona domestica
  Merrill, Dr., on the American cuckoo
  Metamorphism of oldest rocks
  Mice destroying bees
 —acclimatisation of
 —tails of
  Miller, Prof., on the cells of bees
  Mirabilis, crosses of
  Missel-thrush
  Mistletoe, complex relations of
  Mivart, Mr., on the relation of hair and teeth
 —on the eyes of cephalopods
 —various objections to Natural Selection
 —on abrupt modifications
 —on the resemblance of the mouse and antechinus
  Mocking-thrush of the Galapagos
  Modification of species, not abrupt
  Moles, blind
  Molothrus, habits of
  Mongrels, fertility and sterility of
 —and hybrids compared
  Monkeys, fossil
  Monachanthus
  Mons, Van, on the origin of fruit-trees
  Monstrosities
  Moquin-Tandon, on sea-side plants
  Morphology
  Morren, on the leaves of Oxalis
  Moths, hybrid
  Mozart, musical powers of
  Mud, seeds in
  Mules, striped
  Muller, Adolph, on the instincts of the cuckoo
  Muller, Dr. Ferdinand, on Alpine Australian plants
  Muller, Fritz, on dimorphic crustaceans
 —on the lancelet
 —on air-breathing crustaceans
 —on the self-sterility of orchids
 —on embryology in relation to classification
 —on the metamorphoses of crustaceans
 —on terrestrial and fresh-water organisms not undergoing any metamorphosis
 —on climbing plants
  Multiplication of species not indefinite
  Murchison, Sir, R., on the formations of Russia
 —on azoic formations
 —on extinction
  Murie, Dr., on the modification of the skull in old age
  Murray, Mr. A., on cave-insects
  Mustela vison
  Myanthus
  Myrmecocystus
  Myrmica, eyes of
  Nageli, on morphological characters
  Nails, rudimentary
  Nathusius, Von, on pigs
  Natural history, future progress of
 —selection
 —system
  Naturalisation of forms distinct from the indigenous species
 —in New Zealand
  Naudin, on analagous variations in gourds
 —on hybrid gourds
 —on reversion
  Nautilus, Silurian
  Nectar of plants
  Nectaries, how formed
  Nelumbium luteum
  Nests, variation in
  Neuter insects
  New Zealand, productions of, not perfect
 —naturalised products of
 —fossil birds of
 —glaciers of
 —crustaceans of
 —algae of
 —number of plants of
 —flora of
  Newman, Col., on humble-bees
  Newton, Prof., on earth attached to a partridge's foot
  Newton, Sir I., attacked for irreligion
  Nicotiana, crossed varieties of
 —certain species very sterile
  Nitsche, Dr., on the Polyzoa
  Noble, Mr., on fertility of Rhododendron
  Nodules, phosphatic, in azoic rocks
  Oak, varieties of
  Onites apelles
  Orchids, fertilisation of
 —the development of their flowers
 —forms of
  Orchis, pollen of
  Organisation, tendency to advance
  Organs of extreme perfection
 —electric, of fishes
 —of little importance
 —homologous
 —rudiments of, and nascent
  Ornithorhynchus, mammae of
  Ostrich not capable of flight
 —habit of laying eggs together
 —American, two species of
  Otter, habits of, how acquired
  Ouzel, water
  Owen, Prof., on birds not flying
 —on vegetative repetition
 —on variability of unusually developed parts
 —on the eyes of fishes
 —on the swim-bladder of fishes
 —on fossil horse of La Plata
 —on generalised form
 —on relation of ruminants and pachyderms
 —on fossil birds of New Zealand
 —on succession of types
 —on affinities of the dugong
 —on homologous organs
 —on the metamorphosis of cephalopods
  Pacific Ocean, faunas of
  Pacini, on electric organs
  Paley, on no organ formed to give pain
  Pallas, on the fertility of the domesticated descendants of wild stocks
  Palm with hooks
  Papaver bracteatum
  Paraguay, cattle destroyed by flies
  Parasites
  Partridge, with ball of dirt attached to foot
  Parts greatly developed, variable
  Parus major
  Passiflora
  Peaches in United States
  Pear, grafts of
  Pedicellariae
  Pelargonium, flowers of
 —sterility of
  Peloria
  Pelvis of women
  Period, glacial
  Petrels, habits of
  Phasianus, fertility of hybrids
  Pheasant, young, wild
  Pictet, Prof., on groups of species suddenly appearing
 —on rate of organic change
 —on continuous succession of genera
 —on close alliance of fossils in consecutive formations
 —on change in latest tertiary forms
 —on early transitional links
  Pierce, Mr., on varieties of wolves
  Pigeons with feathered feet and skin between toes
 —breeds described, and origin of
 —breeds of, how produced
 —tumbler, not being able to get out of egg
 —reverting to blue colour
 —instinct of tumbling
 —young of
  Pigs, black, not affected by the paint-root
 —modified by want of exercise
  Pistil, rudimentary
  Plants, poisonous, not affecting certain coloured animals
 —selection, applied to
 —gradual improvement of
 —not improved in barbarous countries
 —dimorphic
 —destroyed by insects
 —in midst of range, have to struggle with other plants
 —nectar of
 —fleshy, on sea-shores
 —climbing
 —fresh-water, distribution of
 —low in scale, widely distributed
  Pleuronectidae, their structure
  Plumage, laws of change in sexes of birds
  Plums in the United States
  Pointer dog, origin of
 —habits of
  Poison not affecting certain coloured animals
  Poison, similar effect of, on animals and plants
  Pollen of fir-trees
 —transported by various means
  Pollinia, their development
  Polyzoa, their avicularia
  Poole, Col., on striped hemionus
  Potemogeton
  Pouchet, on the colours of flat-fish
  Prestwich, Mr., on English and French eocene formations
  Proctotrupes
  Proteolepas
  Proteus
  Psychology, future progress of
  Pyrgoma, found in the chalk
  Quagga, striped
  Quatrefages, M., on hybrid moths
  Quercus, variability of
  Quince, grafts of
  Rabbits, disposition of young
  Races, domestic, characters of
  Race-horses, Arab
 —English
  Radcliffe, Dr., the electrical organs of the torpedo
  Ramond, on plants of Pyrenees
  Ramsay, Prof., on subaerial denudation
 —on thickness of the British formations
 —on faults
  Ramsay, Mr., on instincts of cuckoo
  Ratio of increase
  Rats, supplanting each other
 —acclimatisation of
 —blind, in cave
  Rattle-snake
  Reason and instinct
  Recapitulation, general
  Reciprocity of crosses
  Record, geological, imperfect
  Rengger, on flies destroying cattle
  Reproduction, rate of
  Resemblance, protective, of insects
 —to parents in mongrels and hybrids
  Reversion, law of inheritance
 —in pigeons, to blue colour
  Rhododendron, sterility of
  Richard, Prof., on Aspicarpa
  Richardson, Sir J., on structure of squirrels
 —on fishes of the southern hemisphere
  Robinia, grafts of
  Rodents, blind
  Rogers, Prof., Map of N. America
  Rudimentary organs
  Rudiments important for classification
  Rutimeyer, on Indian cattle
  Sageret, on grafts
  Salamandra atra
  Saliva used in nests
  Salmons, males fighting, and hooked jaws of
  Salt-water, how far injurious to seeds
 —not destructive to land-shells
  Salter, Mr., on early death of hybrid embryos
  Salvin, Mr., on the beaks of ducks
  Saurophagus sulphuratus
  Schacht, Prof., on Phyllotaxy
  Schiodte, on blind insects
 —on flat-fish
  Schlegel, on snakes
  Schobl, Dr., on the ears of mice
  Scott, Mr. J., on the self-sterility of orchids
 —on the crossing of varieties of verbascum
  Sea-water, how far injurious to seeds
 —not destructive to land-shells
  Sebright, Sir J., on crossed animals
  Sedgwick, Prof., on groups of species suddenly appearing
  Seedlings destroyed by insects
  Seeds, nutriment in
 —winged
 —means of dissemination
 —power of resisting salt-water
 —in crops and intestines of birds
 —eaten by fish
 —in mud
 —hooked, on islands
  Selection of domestic products
 —principle not of recent origin
 —unconscious
 —natural
 —sexual
 —objections to term
 —natural, has not induced sterility
  Sexes, relations of
  Sexual characters variable
 —selection
  Sheep, Merino, their selection
 —two sub-breeds, unintentionally produced
 —mountain, varieties of
  Shells, colours of, littoral
 —hinges of
 —seldom embedded
  Shells, fresh-water, long retain the same forms
 —fresh-water, dispersal of
 —of Madeira
 —land, distribution of
 —land, resisting salt water
  Shrew-mouse
  Silene, infertility of crosses
  Silliman, Prof., on blind rat
  Sirenia, their affinities
  Sitaris, metamorphosis of
  Skulls of young mammals
  Slave-making instinct
  Smith, Col. Hamilton, on striped horses
  Smith, Dr., on the Polyzoa
  Smith, Mr. Fred., on slave-making ants
 —on neuter ants
  Snake with tooth for cutting through egg-shell
  Somerville, Lord, on selection of sheep
  Sorbus, grafts of
  Sorex
  Spaniel, King Charles' breed
  Specialisation of organs
  Species, polymorphic
 —dominant
 —common, variable
 —in large genera variable
 —groups of, suddenly appearing
 —beneath Silurian formations
 —successively appearing
 —changing simultaneously throughout the world
  Spencer, Lord, on increase in size of cattle
  Spencer, Mr. Herbert, on the first steps in differentiation
 —on the tendency to an equilibrium in all forces
  Sphex, parasitic
  Spiders, development of
  Sports in plants
  Sprengel, C.C., on crossing
 —on ray-florets
  Squalodon
  Squirrels, gradations in structure
  Staffordshire, heath, changes in
  Stag-beetles, fighting
  Star fishes, eyes of
 —their pedicellariae
  Sterility from changed conditions of life
 —of hybrids
 —laws of
 —causes of
 —from unfavourable conditions
 —not induced through natural selection
  St. Helena, productions of
  St. Hilaire, Aug., on variability of certain plants
 —on classification
  St. John, Mr., on habits of cats
  Sting of bee
  Stocks, aboriginal, of domestic animals
  Strata, thickness of, in Britain
  Stripes on horses
  Structure, degrees of utility of
  Struggle for existence
  Succession, geological
 —of types in same areas
  Swallow, one species supplanting another
  Swaysland, Mr., on earth adhering to the feet of migratory birds
  Swifts, nests of
  Swim-bladder
  Switzerland, lake inhabitants of
  System, natural
  Tail of giraffe
 —of aquatic animals
 —prehensile
 —rudimentary
  Tanais, dimorphic
  Tarsi deficient
  Tausch, Dr., on umbelliferae
  Teeth and hair correlated
 —rudimentary, in embryonic calf
  Tegetmeier, Mr., on cells of bees
  Temminck, on distribution aiding classification
  Tendrils, their development
  Thompson, Sir W., on the age of the habitable world
 —on the consolidation of the crust of the earth
  Thouin, on grafts
  Thrush, aquatic species of
 —mocking, of the Galapagos
 —young of, spotted
 —nest of
  Thuret, M., on crossed fuci
  Thwaites, Mr., on acclimatisation
  Thylacinus
  Tierra del Fuego, dogs of
 —plants of
  Timber-drift
  Time, lapse of
 —by itself not causing modification
  Titmouse
  Toads on islands
  Tobacco, crossed varieties of
  Tomes, Mr., on the distribution of bats
  Transitions in varieties rare
  Traquair, Dr., on flat-fish
  Trautschold, on intermediate varieties
  Trees on islands belong to peculiar orders
 —with separated sexes
  Trifolium pratense
 —incarnatum
  Trigonia
  Trilobites
 —sudden extinction of
  Trimen, Mr., on imitating-insects
  Trimorphism in plants
  Troglodytes
  Tuco-tuco, blind
  Tumbler pigeons, habits of, hereditary
 —young of
  Turkey-cock, tuft of hair on breast
  Turkey, naked skin on head
 —young of, instinctively wild
  Turnip and cabbage, analogous variations of
  Type, unity of
  Types, succession of, in same areas
  Typotherium
  Udders enlarged by use
 —rudimentary
  Ulex, young leaves of
  Umbelliferae, flowers and seeds of
 —outer and inner florets of
  Unity of type
  Uria lacrymans
  Use, effects of
 —under domestication
 —in a state of nature
  Utility, how far important in the construction of each part
  Valenciennes, on fresh-water fish
  Variability of mongrels and hybrids
  Variation, under domestication
 —caused by reproductive system being affected by conditions of life
 —under nature
 —laws of
 —correlated
  Variations appear at corresponding ages
 —analogous in distinct species
  Varieties, natural
 —struggle between
 —domestic, extinction of
 —transitional, rarity of
  Varieties, when crossed
 —fertile
 —sterile
 —classification of
  Verbascum, sterility of
 —varieties of, crossed
  Verlot, M., on double stocks
  Verneuil, M. de, on the succession of species
  Vibracula of the Polyzoa
  Viola, small imperfect flowers of
 —tricolor
  Virchow, on the structure of the crystalline lens
  Virginia, pigs of
  Volcanic islands, denudation of
  Vulture, naked skin on head
  Wading-birds
  Wagner, Dr., on Cecidomyia
  Wagner, Moritz, on the importance of isolation
  Wallace, Mr., on origin of species
 —on the limit of variation under domestication
 —on dimorphic lepidoptera
 —on races in the Malay Archipelago
 —on the improvement of the eye
 —on the walking-stick insect
 —on laws of geographical distribution
 —on the Malay Archipelago
 —on mimetic animals
  Walsh, Mr. B.D., on phytophagic forms
 —on equal variability
  Water, fresh, productions of
  Water-hen
  Waterhouse, Mr., on Australian marsupials
 —on greatly developed parts being variable
 —on the cells of bees
 —on general affinities
  Water-ouzel
  Watson, Mr. H.C., on range of varieties of British plants
 —on acclimatisation
 —on flora of Azores
 —on rarity of intermediate varieties
 —on Alpine plants
 —on convergence
 —on the indefinite multiplication of species
  Weale, Mr., on locusts transporting seeds
  Web of feet in water-birds
  Weismann, Prof., on the causes of variability
 —on rudimentary organs
  West Indian islands, mammals of
  Westwood, on species in large genera being closely allied to others
 —on the tarsi of Engidae
 —on the antennae of hymenopterous insects
  Whales
  Wheat, varieties of
  White Mountains, flora of
  Whittaker, Mr., on lines of escarpment
  Wichura, Max, on hybrids
  Wings, reduction of size
 —of insects homologous with branchiae
 —rudimentary, in insects
  Wolf crossed with dog
 —of Falkland Isles
  Wollaston, Mr., on varieties of insects
 —on fossil varieties of shells in Madeira
  Wollaston, Mr., on colours of insects on sea-shore
 —on wingless beetles
 —on rarity of intermediate varieties
 —on insular insects
 —on land-shells of Madeira naturalised
  Wolves, varieties of
  Woodcock with earth attached to leg
  Woodpecker, habits of
 —green colour of
  Woodward, Mr., on the duration of specific forms
 —on Pyrgoma
 —on the continuous succession of genera
 —on the succession of types
  World, species changing simultaneously throughout
  Wright, Mr. Chauncey, on the giraffe
 —on abrupt modifications
  Wrens, nest of
  Wyman, Prof., on correlation of colour and effects of poison
 —on the cells of the bee
  Youatt, Mr., on selection
 —on sub-breeds of sheep
 —on rudimentary horns in young cattle
  Zanthoxylon
  Zebra, stripes on
  Zeuglodons





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