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“ 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 intel-
ligent agent to render it so, z.¢., to effect it continually or at stated times,
as what is supernatural or miraculous does to eftect 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.
Down, Beckenham, Kent,
First Edition, November 24th, 1859.
Siath Edition, Jan. 1872.
aa
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.BS, &e.
SIXTH EDITION, WITH ADDITIONS AND CORRECTIONS.
(ELEVENTH THOUSAND.)
LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1872.
The right of Translation ts reserved.
BY THE SAME AUTHOR,
THE DESCENT OF MAN AND SELECTION IN RELA-
TION TO SEX. Fighth Thousand. 2 vols. 8vo. 248. Morray, 1871,
THE VARIATION OF ANIMALS AND PLANTS UNDER
DOMESTICATION. With Illustrations. Third Thousand. 2 vols. 8vo. 28s.
Morray, 1868,
A NATURALIST’S VOYAGE ROUND THE WORLD; or,
A JOURNAL OF RESEARCHES INTO THE NATURAL HISTORY AND GEOLOGY OF THE
Countrtss visited during the voyage of H.MLS. ‘ Beagle,’ under the command of
Captain FirzRoy, R.N. Eleventh Thousand. Post 8vo. 9s. Morray.
ON THE VARIOUS CONTRIVANCES sy waice BRITISH
AND FOREIGN ORCHIDS ARE FERTILISED BY INSECTS; and on the
Goop EFFEcts OF CrossInc. With Woodcuts. Post 8vo. 9s. Murray.
ON THE STRUCTURE AND DISTRIBUTION OF CORAL
REEFS. SmirH, ELper, & Co.
GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.
SmitH, ELper, & Co.
GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA.
Smrtu, Exper, & Co.
A MONOGRAPH OF THE CIRRIPEDIA. With numerous
Illustrations. 2 vols. 8vo. Ray Soctety. HarDWICKE.
ON THE MOVEMENTS AND HABITS OF CLIMBING
PLANTS. With Woodcuts. Witurams & NorGATE,
FACTS AND ARGUMENTS FOR DARWIN. By Frirz
Mutter. From the German, with Additions by the Author, Translated by
W. S. Datias, F.L.S. With Illustrations. Post 8vo. 6s. Mougray.
LONDON: PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET,
AND CHARING CROSS.
a
CONTENTS.
PW
ADDITIONS AND CORRECTIONS, TO THE SIXTH EDITION .. Page xi-xii
HISTORICAL SKETCH ..we ce ee ewe ee wee XIX XI
INTRODUCTION ee oe ee oo oe oe ee ee oe ee ee 1—4
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 fol-
lowed, their Effects — Methodical and Unconscious Selection — Un-
known Origin of our Domestic Productions — Circumstances favour-
able to Man’s power of Selection i rr rs)
NTS CHAPTER IL
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,
wepT\G and in having restricted ranges... $8 se we cork?
CHAPTER III.
m STRUGGLE FOR EXISTENCE.
a att Its bearing on natural selection — The term used in a wide sense—Geome-
trical 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 rela-
tionS «nue ueeeteeneelnlw 48-61
¥
vi CONTENTS,
CHAPTER IV.
NaTurAL SELECTION; OR THE SURVIVAL OF THE FITTEsT,
z
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 unfa-
vourable 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 ek ews eee eee” | Pare 62-105
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 .. .. ~- 106-132
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 .. 2. «2 « «+ 133-167
CONTENTS. Vil.
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 incom-
petence 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 disbeliev-
ing in great and abrupt modifications .. .. .. ~. Page 168-204
CHAPTER VIII.
INSTINCT.
Instincts comparable with habits, but different in their origin — Instincts
graduated — Aphides and ants — Instincts variable — Domestic in-
stincts, their origin — Natural instincts of the cuckoo, molothrus,
ostrich, and parasitic bees —Slave-making ants — Hive-hee, 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 oe ee 205-234
CHAPTER IX.
HyYBRIDISM.
Distinction between the sterility of first crosses and of hybrids — Sterility
various in degree, not universal, affected by close interbreeding, re-
moved 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 «6 «6 oe oe wee we we 234-263
Will CONTENTS.
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 by years— On the
poorness of our paleontological 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 oe wee oe ewe we we we Swe Page 264-289
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 develop-
ment of ancient forms — On the succession of the same types within
the same areas — Summary of preceding and present chapter 290-315
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 ne ee we we we we we ee 816-342
CHAPTER XIII.
GEOGRAPHICAL DIstrinutTion—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 main-
land — On colonization from the nearest source with subsequent modi-
fication — Summary of the.last and present chapter .. +. 343-362
a —_™
CONTENTS. 1x
CHAPTER XIV.
MouruaL AFFINITIES oF OrGANIC Bernas: MorPHoLoeY :
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 — MorpPHoLocy, between members of the same class, between
parts of the same individual — Empryonoey, laws of, explained by
variations not supervening at an early age, and being inherited at a
corresponding age — RUDIMENTARY ORGANS; their origin explained —
Summary ve ee ee we ee we Tee Page 863-403
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 — Con-
cluding remarks...) ww) wee ewe nee we 404-429
GLOSSARY OF SCIENTIFIC TERMS
INDUS “ae we oo sc rf Gh ws 6 oF 6% 45 . 443
INSTRUCTION TO BINDER.
en oO eeeaearas
The Diagram to front page 90, and to face the latter part of the Volume.
ADDITIONS AND CORRECTIONS
TO THE SIXTH EDITION.
———$*Oo
Numerous small corrections have been made in the last
and present editions on various subjects, according as the
evidence has become somewhat stronger or weaker. The
more important corrections and some additions in the pre-
sent volume are tabulated on the following page, for the
convenience of those interested in the subject, and who
possess the fifth edition. The second edition was little
more than a reprint of the first. The third edition was
largely corrected and added to, and the fourth and fifth
still more largely. As copies of the present work will
be sent abroad, it may be of use if I specify the state of
the foreign editions. The third French and second Ger-
man editions were from the third English, with some few
of the additions given in the fourth edition. A new fourth
French edition has been translated by Colonel Moulinié ;
of which the first half is from the fifth English, and the
latter half from the present edition. A third German
edition, under the superintendence of Professor Victor
Carus, was from the fourth English edition ; a fifth is now
preparing by the same author from the present volume.
The second American edition was from the English second,
with a few of the additions given in the third; and a
third American edition has been printed from the fifth
English edition. The Italian is from the third, the Dutch
and three Russian editions from the second English edition,
and the Swedish from the fifth English edition.
Xil
Additions and Corrections.
ean, he wae: Chief Additions and Corrections.
Page Page
100 68 | Influence of fortuitous destruction on natural selection.
158 101 | On the convergence of specific forms.
220 | 142 | Account of the Ground-Woodpecker of La Plata modified.
225 | 145 | On the modification of the eye.
230 149 | Transitions through the acceleration or retardation of the
period of reproduction.
231 150 | The account of the electric organ of fishes added to.
233 151 | Analogical resemblance between the eyes of Cephalopods
and Vertebrates.
234 | 153 | Claparéde on the analogical resemblance of the hair-claspers
of the Acaride.
248 | 162 | The probable use of the rattle to the Rattle-snake.
248 | 163 | Helmholtz on the imperfection of the human eye.
255 | 168 | The first part of this new chapter consists of portions, in a
much modified state, taken from chap. iv. of the former
editions. The latter and larger part is new, and relates
chiefly to the supposed incompetency of natural selection
to account for the incipient stages of useful structures.
There is also a discussion on the causes which prevent
in many cases the acquisition through natural selection
of useful structures. Lastly, reasons are given for dis-
believing in great and sudden modifications. Gradations
of character, often accompanied by changes of function,
are likewise here incidentally considered.
268 | 214 | The statement with respect to young cuckoos ejecting their
foster-brothers confirmed.
270 | 215 | On the cuckoo-like habits of the Molothrus.
307 | 240 | On fertile hybrid moths.
319 | 248 | The discussion on the fertility of hybrids not having been ac-
quired through natural selection condensed and modified.
326 | 252 | On the causes of sterility of hybrids, added to and corrected.
377 284 | Pyrgoma found in the chalk.
402 | 801 | Extinct forms serving to connect existing groups.
440 | 328 | On earth adhering to the feet of migratory birds.
463 | 343 | On the wide geographical range of a species of Galaxias,
a fresh-water fish.
505 | 873 | Discussion on analogical resemblances, enlarged and modified.
516 | 382 | Homological structure of the feet of certain marsupial
animals.
518 | 384 | On serial homologies, corrected.
520 | 3885 | Mr. E. Ray Lankester on morphology.
521 | 387 | On the asexual reproduction of Chironomus.
541 | 401 | On the origin of rudimentary parts, corrected.
547 | 405 | Recapitulation on the sterility of hybrids, corrected.
552 | 409 | Recapitulation on the absence of fossils beneath the Cam-
brian system, corrected.
568 | 421 | Natural selection not the exclusive agency in the modi-
fication of species, as always maintained in this work.
572 | 424 | The belief in the separate creation of species generally held
by naturalists, until a recent period.
a
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 witt here a 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,* 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 transforma-
tion 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 pub-
lished his views in 1801; he much enlarged them in 1809 in his
‘Philosophie Zoologique,’ and subsequently, in 1815, in the Intro-
duction to his ‘ Hist. Nat. des Animaux sans Vertébres.’ In these
* Aristotle, in his ‘ Physice 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 mas-
ticating 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 the other parts in
which there appears to exist an adaptation toanend. Wheresoever, there-
fore, 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,
Xiv . EHlistorical Sketch.
ce Re
works he upholds 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 preduc-
tions. 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. *
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 modifica-
tion; and, as his son adds, “ C’est donc un probleme & réserver
entigrement & Vavenir, supposé méme que l’avenir doive avoir prise
sur lui.”
* I have taken the date of the first publication of Lamarck from Isid.
Geoffroy Saint Hilaire’s (‘ Hist. Nat. Generale,’ tom. ii. p. 405, 1859) excel-
lent 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. pp. 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 Intro-
duction to a work written in 1794 and 1795, but not published till long
afterwards: 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 17 94-5.
ne Sa le aa
a
ee | oe tea tea omens army
Historical Sketch. XV
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 ‘ 'l'wo
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 acci-
dental varieties of man, which would occur among the first few and
scattered inhabitants of the middle regiotts of Africa, some one
would be better fitted than the others to bear the diseases of the
country. ‘This race would consequently multiply, while the others
would decrease ; not only from their inability 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 leneth
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 atten-
tion, through Mr. Brace, to the above passage in Dr. Well’s work.
The Hon. and Rev. W. Herbert, afterwards Dean of Manchester,
in the fourth volume of the ‘ Horticultural Transactions,’ 1822, and
in his work on the ‘ Amaryllidacez ’ (1887, p. 19, 889), declares that
“horticultural experiments have established, beyond the possibility
of refutation, that botanical species are only a higher and more per-
manent 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 intercrossing, but likewise by variation, all
our existing species.
In 1826 Professor Grant, in the concluding paragraph in his
well known paper (‘Edinburgh Philosophical J ournal,’ vol. xiv.
p. 283) on the Spongilla, clearly declares his belief that species are
ES
Xvi Historical Sketch.
descended from other species, and that they become improved in
the course of modification. This same view was given in his 55th
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 7th, 1860. The differences of Mr.
Matthew’s view from mine are not of much importance: he seems
to consider that the world was nearly depopulated at successive
periods, and then re-stocked; and he gives as an alternative, that
new forms may be generated “ without the presence of any mould
or germ of former aggregates.” JI 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, how-
ever, the full force of the principle of natural selection.
The celebrated geologist and naturalist, Von Buch, in his ex-
cellent ‘Description Physique des Isles Canaries’ (18386, p. 147),
clearly expresses his belief that varieties slowly become changed
into permanent species, which are no longer capable of inter-
crossing.
Rafinesque, in his‘ New Flora of North America,’ published in
1836, wrote (p. 6) as follows:—“ All species might have been
varieties once, and many varieties are gradually becoming species
by assuming constant and peculiar characters;” but farther on
(p. 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. p. 468) has ably given the arguments for and
against the hypothesis of the development and modification of
species: he seems to lean towards the side of change. |
The ‘ Vestiges of Creation’ appeared in 1844. In the tenth
and much improved edition (1853) the anonymous author says
_(p. 155):—The proposition determined on after much considera- |
tion is, that the several series of animated beings, from the simplest ?
and oldest up to the highest and most recent, are, under the provi- :
dence 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 ,
a
Htistorical Sketch. XVII
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 accor-
dance 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 co-
adaptations 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 earlier editions little
accurate knowledge and a great want of scientific caution, imme-
diately had a very wide circulation. In my opinion it has done
excellent service in this country in calling attention to the sub-
ject, 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 pub-
lished in an excellent though short paper (‘ Bulletins de l’Acad. Roy.
Bruxelles, tom. xiii. p. 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,’ p. 86), wrote as
follows :—“ 'he 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 (p. li.) of “the axiom of the continuous
Operation of creative power, or of the ordained becoming of living
things.” Farther on (p. xc.), after referring to geographical distri-
bution, 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 respec-
tively. Always, also, it may be well to bear in mind that by the
word ‘creation’ the zoologist means ‘a process he knows not
b
xVili ‘Historical Sketch.
what.” He amplifies this idea by adding, that when such cases
as that of the Red Grouse are ‘enumerated by the zoologist ag
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 con-
fidence 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 paleontologists
as being firmly convinced of the immutability of species; but it
appears (‘ Anat. of Vertebrates,’ vol. iii. p. 796) that this was on
my part a preposterous error. In the last edition of this work I
inferred, and the inference still seems to me perfectly just, from a
passage beginning with the words “ no doubt the type-form,” &c.
(ibid. vol. i. p. xxxv.), that Professor Owen admitted that natural
selection may have done something in the formation of new
species; but this it appears (Ibid. vol. iii. p. 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. iil.
p- 798), | have either partially or wholly again fallen into error.
It is consolatory to me that others find Professor Owen's controver-
sial writings as difficult to understand and to reconcile with each
other, as Ido. 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 wRich a Résumé appeared in the ‘ Revue et Mag. de
ce oo, Se TH Se Oe ee ae ae ay ey
Do @ Fa eM So ™m™
Se fS-
Historical Sketch. xix
Zoolog.,’ Jan. 1851), briefly gives his reason for believing that
specific characters “ sont fixés, pour chaque espéce, tant quelle se
perpétue au milieu des mémes circonstances: ils se modifient, si
les circonstances ambiantes viennent & changer.” ‘“ En résumé,
lobservation des animaux sauvages démontre déja la variabilité
limitée des especes. Les expériences sur les animaux sauvages
devenus domestiques, et sur les animaux domestiques redevenus
sauvages, la démontrent plus clairement encore. Ces mémes expé-
riences prouvent, de plus, que les différences produites peuvent étre
de valeur générique.” In his ‘ Hist. Nat. Générale’ (tom. ii. p.
430, 1859) he amplifies analogous conclusions.
From a circular lately issued it appears that Dr. Freke, in 1851
(‘ Dublin Medical Press,’ p. 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 diffi-
cult 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 dis-
tinguishing 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,’
p. 102; since partly republished in the ‘Nouvelles Archives du
Muséum,’ tom. i. p. 171), his belief that species are formed in
an analogous manner as varieties are under cultivation; and the
latter process he attributes to man’s power of selection. But he
does not show how selection acts under nature. He believes, like
Dean Herbert, that species, when nascent, were more plastic than at
present. He lays weight on what he calls the principle of finality,
“puissance mystérieuse, indéterminée; fatalité pour les uns ; pour
les autres, volonté providentielle, dont l’action incessante sur les
€tres vivants détermine, & toutes les époques de Vexistence du
monde, la forme, le volume, et la durée de chacun d’eux, en raison
b 2
xX Historical Sketch.
—n
“de sa destinée dans ordre de choses dont il fait partie. C'est cette
puissance qui harmonise chaque membre a Vensemble en l’appro-
priant a la fonction qwil doit remplir dans l’organisme général de
la nature, fonction qui est pour lui sa raison d’étre.” *
In 1853 a celebrated geologist, Count Keyserling (‘Bulletin de
la Soc. Geolog.,’ 2nd Ser., tom. x. p. 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 circum-
ambient molecules of a particular nature, and thus have given
rise to new forms.
In this same year, 1853, Dr. Schaaffhausen published an ex-
‘cellent pamphlet (‘Verhand. des Naturhist. Vereins der Preuss.
Rheinlands,’ &c.), in which he maintains the progressive develop-
ment of organic forms on the earth. He infers that many species
have kept true for long periods, whereas a few have become modi-
‘fied. 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. Lecog, writes in 1854
(‘Etudes sur Géograph. Bot.,’ tom. i. p. 250), “On voit que nos
‘recherches sur la fixité ou la variation de l’espéce, nous conduisent
directement aux idées émises, par deux hommes justement célébres,
“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,
* From references in Bronn’s ‘ Untersuchungen iiber die Entwickelungs-
Gesetze,’ it appears that the celebrated botanist and paleontologist Unger
published, in 1852, his belief that species undergo development and modifi-
cation. D’Alton, 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 |’Espéce,’ it seems that Bory St. Vincent,
Burdach, Poiret, and Fries, have all admitted that new species are continu-
ally 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. oo
ee eae cps eae cr rr
bento
| = i ~ ee
Historical Sketch. xxi
not a casual phenomenon,” or, as Sir John Herschel expresses it,
‘“‘o natural in contradistinction to a miraculous process.”
The third volume of the ‘ Journal of the Linnean Society ’ con-
tains papers, read July Ist, 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, towards whom all zoologists feel so profound a respect,
expressed about the year 1859 (see Prof. Rudolph Wagner, ‘ Zoolo-
gisch-Anthropologische Untersuchungen,’ 1861, s. 51) his convic-
tion, 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 'l'ypes 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 24th,
1859, and the second edition on January 7th, 1860,
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 inhabit-
ing 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 toa
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 manu-
scripts.
This Abstract, which I now publish, must necessarily be im-
perfect. I cannot here give references and authorities for my
B
2 Introduction.
several statements; and I must trust to the reader reposing some
confidence in my accuracy. No doubt errors will 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 1 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 argu-
ments on both sides of each question; and this is here impossible.
I much regret that want of space prevents my having the satis-
faction of acknowledging the generous assistance which I have
received from very many naturalists, some of them personally un-
known 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 con-
clusion that species had not been independently created, but had
descended, like varieties, from other species. Nevertheless, such a
conclusion, even if well founded, would be unsatisfactory, until it
could be shown how the innumerable species inhabiting this world
have been modified, so as to acquire that perfection of structure
and coadaptation which justly excites our admiration. Naturalists
continually refer to external conditions, such as climate, food, &c.,
as the only possible cause of variation. In one limited sense, as
we shall hereafter see, this may be true; but it is preposterous to
attribute to mere external conditions, the structure, for instance, of
the woodpecker, with its feet, tail, beak, and tongue, so admirably
adapted to catch insects under the bark of trees. In the case of the
mistletoe, which draws its nourishment from certain trees, which
has seeds that must be transported by certain birds, and which has
flowers with separate sexes absolutely requiring the agency of
certain insects to bring pollen from one flower to the other, it is
equally preposterous to account for the structure of this parasite,
with its relations to several distinct organic beings, by the effects
Liutroductton. 3
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 commence-
ment 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 amongst 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
an 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 com-
plex 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 Diver-
gence 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
BQ
4 Introduction.
—————
simple being or a simple organ can be changed and perfected into:
a highly developed being or into an elaborately constructed organ s.
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 geo-.
logical 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
cive a brief recapitulation of the whole work, and a few concluding
remarks.
No one ought to feel surprise at much remaining as yet un-
explained 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 ex-
plain 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.
Cuap. 1. Variation under Domestication. 5
a te me
ae me ee ee ene
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 Varicties ana Species — Origin
of Domestic Varieties from one or more Species — Domestic Pigeons,
their Differences and Origin — Principles of Selection, anciently fol-
lowed, their Effects — Methodical and Unconscious Selection — Un-
known Origin of our Domestic Productions — Circumstances favour-
able to Man’s power of Selection.
Causes of Variability.
\WueEN 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 treat-
ment, we are driven to concludo 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
oenerations to new conditions to cause any great amount of varia-
tion; and that, when the organisation has once begun to vary, it
cenerally 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 varie-
ties: our oldest domesticated animals are still capable of rapid
improvement or modification.
As faras 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 indirectly by
affecting the reproductive system. With respect to the direct
© Variation under Domestication. Crap. I.
action, we must bear in mind that in every case, as Professor
Weismann has lately insisted, and as I have incidentally 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 indefinite. They
may be considered as definite when all or nearly all the offspring of
individuals exposed to certain conditions during several generations
are modified in the same manner. It is extremely difficult to come
to any conclusion in regard to the extent of the changes which have
been thus definitely induced. There can, however, be little doubt
about many slight changes,—such as size from the amount of food,
colour from the nature of the food, thickness of the skin and hair
from climate, &c. Each of the endless variations which we see in
the plumage of our fowls must have had some efficient cause ; and
if the same cause were to act uniformly during a long series of
cenerations on many individuals, all probably would be modified in
the same manner. Such facts as the complex and extraordinary
out-growths which invariably follow from the insertion of a minute
drop of poison by a gall-producing insect, show us what singular
modifications might result in the case of plants from a chemical
change in the nature of the sap. .
Indefinite 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:
indefinite variability in the endless slight peculiarities which dis-
tinguish 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 amongst many individuals living together, may be
considered as the indefinite effects of the conditions of life on each
individual organism, in nearly the same manner as a chill affects.
different men in an indefinite manner, according to their state:
Cuar. I. Variation under Domestication. f
of body or constitution, causing coughs or colds, rheumatism, or
inflammations of various organs.
With respect to what I have called the indirect action of changed
conditions, namely, through the reproductive system 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 con-
ditions, and partly from the similarity, as Koireuter 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 reproduc-
tive 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! his 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
1 have collected and elsewhere published on this curious subject ;
but to show how singular the laws are which determine the repro-
duction of animals under confinement, J may mention that car-
nivorous animals, even from the tropics, breed in this country
pretty freely under confinement, with the exception of the planti-
grades or bear family, which seldom produce young; whereas
carnivorous birds, with the rarest exceptions, hardly ever lay fertile
evos. 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
cive 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
8 Variation under Domestication. Crap. I.
or cultivation, and vary very slightly—perhaps hardly more than in
a state of nature. oo
Some naturalists have maintained that all variations are con-
nected 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 pro-
pagated by grafts, offsets, &c., and sometimes by seed. They occur
rarely under nature, but are far from rare under culture. As a
single bud out of the 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
sreat 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 the 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
Cuap. I. Variation under Domestication. 9
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
maturé 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 amongst animals
and plants. From facts collected by Heusinger, it appears that
white sheep and pigs are injured by certain plants, whilst dark-
coloured individuals escape: Professor Wyman has recently com-
municated 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.” Hair-
less dogs have imperfect teeth: long-haired and coarse-haired
animals are apt to have, as is asserted, long or many horns; pigeons
with feathered feet have skin between their outer toes; pigeons
with short beaks have small feet, and those with long beaks large
feet. Hence if man goes on selecting, and thus augmenting, any
peculiarity, he will almost certainly modify unintentionally other —
parts of the structure, owing to the mysterious laws of correlation.
The results of the various, unknown, or but dimly understood
laws of variation are infinitely complex and diversified. It is well
worth while carefully to study the several treatises on some of
our old cultivated plants, as on the hyacinth, potato, even the
dahlia, &c.; and it is really surprising to note the endless points of
structure and constitution in which the varieties and sub-varieties
differ slightly from each other. The whole organisation seems to
have become plastic, and departs in a slight degree from that of the
parental type.
Any variation which is not inherited is unimportant for us.
But the number and diversity of inheritable deviations of structure,
both those of slight and those of considerable physiological impor-
tance, are endless. Dr. Prosper Lucas’s treatise, in two large
volumes, is the fullest and the best on this subject. No brecder
IO Variation under Domestication. CHAP. 1.
doubts how strong is the tendency to inheritance ; that like pro-
duces like is kis 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 amongst individuals, apparently exposed
to the same conditions, any very rare deviation, due to some
extraordinary combination of circumstances, appears in the parent
—say, once amongst several million individuals—and it reappears
in the child, the mere doctrine of chances almost compels us to.
attribute its reappearance to inheritance. Every one must have
heard of cases of albinism, prickly skin, hairy bodies, &c., appearing.
in several members of the same family. If strange and rare
deviations of structure are really inherited, less strange and com-
moner deviations may be frecly 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
characters to its grandfather or grandmother or more remote ances-.
tor; 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 re-appear in the offspring at a corresponding age, though some-
times 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:
Cuap. I. Variation under Domestiu ation. It
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 ¢ ne 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 charac-
ter 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 varie-
ties 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 have been turned loose in its new home. N everthe-
less, 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, whilst kept
under the same conditions, and whilst 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.
12 Character of Domestic Varteties. Cuap. I,
Character of Domestic Varieties: difficulty of distinguishing
between Varieties und Species; origin of Domestic Varicties
Srom one or more Species.
When we look to the hereditary varicties 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 trtie,
for the domestic races of many animals and plants have been
ranked by some competent judges as the descendants of aborigi-
nally 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 different quarters of the
world. Ido not believe, as we shall presently see, that the whole
e
Cuar. 1 Character of Domestic Varieties. 13,
——
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 domestica-
tion 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
eenerations, 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 exist-
ing. But this only throws far backwards the history of civilisation,
and shows that animals were domesticated at a much earlier period
than has hitherto been supposed. ‘The lake-inhabitants of Swit--
zerland 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..
‘ince the discovery of flint tools in the superficial formations of
many parts of the world, all geologists believe that barbarian man:
14 Character of Domestic Varieties. Cuap. I,
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 for
ever 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 Canide 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, con-
stitution, 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 re-
searches of Professor Riitimeyer. With respect to horses, from
reasons which I cannot here give, [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 wild duck and 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 proto-
type. At this rate there must have existed at least a score of
species of wild cattle, as many sheep, and several goats, in Europe
alone, and several even within Great Britain. One author believes
that there formerly existed eleven wild species of sheep peculiar to
Great Britain! When we bear in mind that Britain has now not
one peculiar mammal, and France but few distinct from those of
Germany, and so with Hungary, Spain, &c., but that each of these
kingdoms possesses several peculiar breeds of cattle, sheep, &c., we
must admit that many domestic breeds must have originated in
Europe; for whence otherwise could they have been derived? So it is
Cua. I. Domestic Pigeons. 1m
in India. Even in the case of the breeds of the domestic dog through-
out the world, which I admit are descended from several wild spe-
cies, it cannot be doubted that there has been an immense amount
of inherited variation; for who will believe that animals closely
resembling the Italian greyhound, the bloodhound, the bull-dog,
pug-dog, or Blenheim spaniel, &c.—so unlike all wild Canida—
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 grey-
hound, bloodhound, bull-dog, &c., in the wild state. Moreover,
the possibility of making distinct races by crossing has been greatly
exaggerated. Many cases are on record, showing that a race may
be modified by occasional crosses, if aided by the careful selection
of the individuals which present the desired character; but to
obtain a race intermediate between two quite distinct races, would
be very difficult. Sir J. Sebright expressly experimented with this
object, and failed. The offspring from the first cross between two
pure breeds is tolerably and sometimes (as I have found with
pigeons) quite uniform in character, and everything 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
creatly elongated eyelids, very large external orifices to the nostrils,
10 Domestic Pigeons. Cuap. |,
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. ‘Ihe 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 cesophacus. he Jacobin has the feathers so much reversed
along the back of the neck that they form a hood; and it has, pro-
portionally 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 enor-
mously. ‘he 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 vertebra 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
cesophagus ; 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 hp body ; the
relative length of the lez and foot; the number of scutella 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
SS
“2 be iat td
Cuap. I Domestic Pigeons. 17
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,
T 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 wiil 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 domestie 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 gecgraphical 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 remark-
able characters, seems improbable; or they must have become
extinct in the wild state. But birds breeding on precipices, and
sood fliers, 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 sores of the Mediterranean. Hence the supposed exter-
—_ st ¥ . Se
risers ok fate many species hay Ing eee habits with the rock-
pig Sa very rash assumption. Morever, the several above-
re a enone oh a eal not
) tore, em must have been carried back
C
18 Domestic Pigeons. Cuap. Ii
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-civilised man, as to be quite prolific under con-
finernent.
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 abnor-
mal in other parts; we may look in vain through the whole great
family of Columbide 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-civilised 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
Cua. I. Domestic Pigeons, — IQ
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 pro-
duced 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 im-
probable suppositions, Either, first, that all the several imacined
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: 1 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 succeed-
ing 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 obser-
vations, 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. Sorne
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 pro-
bably quite correct, if applied to species closely related to each
other. But to extend it so far as to suppose that species, aborizi-
nally as distinct as carriers, tumblers, pouters, and fantails now are,
should yield offspring perfectly fertile inter sé, would be rash in the
extreme.
From these several reasons, namely,—the improbability of man
having formerly made seven or eight supposed species of pigeons to
c 2
20 Domestic Pigeons. Cuap. I,
—
breed freely under domestication ;— these supposed species being
quite unknown in a wild state, and their not having become any-
where feral ;—these species presenting certain very abnormal cha-
yacters, as compared with all other Columbide, though so like the
yock-pigeon in most respects ;—the occasional re-appearance of
the blue colour and various black marks in all the breeds, both
when kept pure and when crossed ;—and lastly, the mongrel off-
spring 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 geogra-
phical 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 struc-
ture 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 dis-
tinctive 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 Selec-
tion. Fourthly, pigeons have been watched and tended with the
utmost care, and loved by many people. They have been domesti-
cated for thousands of years in several quarters of the world; the
earliest known record of pigeons is in the fifth Augyptian dynasty,
about 38000 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 vari-
ation which pigeons have undergone, will likewise be obvious when
Cuap. L Domestic Pigeons. 21
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 dis-
tinct 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 at-
tended, 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
@ common parent-stock, and he will laugh you to scorn. 1 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 dis-
tinct 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 eene-
rations. 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 descendant3
of other species ?
a :
22 Selection by Man. Car. I,
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 car-
rier 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 cannot be rivalled by any mechanical con-
trivance, is only a variety of the wild Dipsacus; and this amount
of change may have suddenly arisen in a seedling. So it has pro-
bably 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 quar-
relsome, 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 ditferent purposes, or so
beautiful in his eyes, we must, I think, look further than to mere
variability. We cannot 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 succes-
sive 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 sub-—
ject, and to inspect the animals. Breeders habitually speak of an
animal’s organisation as something plastic, which they can model
al’
Cuar. L Selection by Man. 23
almost as they please. If I had space I could quote numerous pas-
saces to this effect from highly competent authorities, Youatt,
who was probably better acquainted with the works of agriculturists
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 amongst 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 sepa-
rating some very distinct variety, and breeding from it, the principle
would be so obvious as hardly to be worth notice; but its import-
ance 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 perse-
verance, 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 vari-
ations 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 has not been so in
-several cases in which exact records have been kept; thus, to give u
24 Methodical Selection. Cuap. §,
es
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 com-
pared with drawings made only twenty or thirty years ago, | When
arace 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 ag 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 diver-
sity 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 diffe-
rences,. 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 dif~
fering 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 im-
ported, 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 prin-
ciple of selection I find distinctly given in an ancient Chinese ency~
Cuap. I. Unconscious Selection. 25,
a ee
clopedia. 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 associ-
ated with Europeans. ome 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 pur--
pose, 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. Neverthe-
less we imay infer that this process, continued during centuries,
would improve and modify any breed, in the same way as Bake--
well, Collins, &e., 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 can 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, un--
changed, 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's spaniel has been un-
consciously 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 ::
26 — Unconsctous Selection. Crap. I,
but what concerns us is, that the change has been effected uncon-
sciously 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
recehorses 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 bé 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 two 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.
‘here 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
cuap. I. Unconscious Selection. 27
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 hearsease or dahlia from
the sced of a wild plant. No one would expect to raise a first-rate
melting pear from the seed of the wild pear, though he might
succeed from a poor seedling growing wild, if it had come from a
carden-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 horti-
cultural works at the wonderful skill of gardeners, in having pro-
duced such splendid results from such poor maicrials ; 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 on-
wards. 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 ac-
cumulated, 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 cul!ti-
vated 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 use-
ful 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
lor 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
oiten 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
28 Unconscious Selection. Cuap. 5
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 1s 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 unustial 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 descend-:
ants 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 ceso-
phagus,—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 ex-
tremely 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 1S 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 devia-
tions from the standard of perfection in each breed. The commom
goose has not given rise to any marked varieties; hence the Tou-
Jouse and the common breed, which differ only in colour, that mos’
cuar.I. Circumstances favourable to Selection. 29
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 improvea
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 would be a slow process. As soon as the points of value
are once acknowledged, the principle, as I have called it, of un-
conscious 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 fuvourable to Mun’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 vari-
ability 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 parts of
Yorkshire, “as they generally belong to poor people, and are mostly
am small lots, they never can be improved.” On the other hand,
hurserymen, from keeping large stocks of the same plant, are gener-
ally far more successful than amateurs in raising new and valuable
varieties, A large number of individuals of an animal or plant can
30 Circumstances favourable to Selection, Car. 1.
be reared only where the conditions for its propagation are favour-
able. 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. Uniess 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. Ag:
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 ap-
peared 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 en-
closure of the land plays a part. Wandering savages or the in-
habitants 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 cir-
cumstance 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, cannot be easily matched, and, although
so much valued by women and children, we rarely see a distinct
breed long kept up; such breeds as we do sometimes see are almost
always imported from some other country. Although I do not
doubt that some domestic animals vary less than others, yet the
rarity or absence of distinct breeds of the cat, the donkey, peacock,
goose, &c., may be attributed in main part to selection not having
been brought into play: in cats, from the difficulty in pairing them ;
m 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
cua. Circumstances favourable to Selection. 31
aving been felt in the display of distinct:
onditions to which it is exposed
ngularly inflexible organisa-
extent, as I have elsewhere:
especially from no pleasure h
breeds; but the goose, under the cond!
when domesticated, seems to have a 51
tion, though it has varied to a slight
described. tee
Some authors have maintained that the amount of variation in
our domestic productions is soon reached, and can never afterwards.
pe 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 body to be carried, and the power of contraction in the muscular
fibres. But what concerns us is that the domestic varicties 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, Kclipse was far fleeter, and a dray-horse is incomparably
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 rever-
Sion determine whether variations shall endure. Variability is.
governed by many unknown laws, of which correiated 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
32 | ~— Summary of Selection. Crap. I,
increased use or disuse of parts. The final resuit 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 selec-
tion, 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 pro-
yagated by seed. With plants which are temporarily propagated
by cuttings, buds, &c., the importance of crossing is immense; for
the cultivator may here disregard the extreme variability both of
hybrids and of mongrels, and the sterility of hybrids; but plants
not propagated by seed are of little importance to us, for their
endurance is only temporary. Over all these causes of Change, the
accumulative action of Selection, whether applied methodicaily and
quickly, or unconsciously and slowly but more efficiently, seems to
shave been the predominant Power.
Cuap. Il. Variation under Nature. 33
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.
Lrrorn 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
& 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
D
Individual Differences. Cuap. II,
«(34
improbable that any part should have been suddenly produced
perfect, as that a complex machine should have been invented by
Under domestication monstrosities some-
man in a perfect state. :
times 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 5 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 in-
habiting 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 physio-
logical or classificatory point of view, sometimes vary i 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 arets 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
Cuap, IL Individual Differences. 38
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 beer
effected only by slow degrees; yet Sir J. Lubbock has shown a
decree 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 larve 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 the species present an inor-
dinate 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 amongst
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 like-
wise, 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 amongst insects, and in the immature and
larval states of many of the lower animals. There are, also, cases
i" dimorphism and trimorphism, both with animals and plants.
has shown ee wae eas ate’y called attention to the subject,
Malaves _ , a ele an certain species of butterflies, in the
yan archipelago, recularly appear under two or even three
Pi ase fos eb conneed by trate varie
as des analogous but more extraordinary cases
D 2
/
36 Doubtful Species. Cuap. Il.
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
antennss 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 probable that they were once thus connected.
Mr. Wallace, for instance, describes a certain butterfly which pre-
sents 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 tegether 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 pro-
ducing 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 cha-
racter 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 2
variety of another, even when they are closely connected by inter-
mediate links; nor will the commonly-assumed hybrid nature of
A
Cup. IL. Doubtful Species. 37
the intermediate forms always remove the difficulty. In very many
cases, however, one form 1s ranked ag 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 judg-
ment 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
rance, 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
cenerally 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!
Amongst 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 Lepi-
doptera, inhabiting the islands of the great Malayan archipelago,
shows that they may be classed under four heads, namely, as vari-
uble forms, as local forms, as geographical races or sub-species, and
s true representative species. The first or variable forms vary
much within the limits of the same island. he local forms
we moderately constant and distinct in each separate island; but
Vaen all from the several islands are compared together, the dif-
38 , Doubtful Species. Cap, 13,
ferences 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. Never-
theless, no certain criterion can possibly be given by which variable
forms, local forms, sub-species, and representative species can be
recognised. | co
Many years ago, when comparing, and seeing others compare, the
birds from the closely neighbouring islands of the Galapagos archi-
pelago, one with another, and with those from the American main-
land, 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 charac-
terized 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 distanée
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 Phy-
tophagic 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
/
)
Crs
Cuap, II. Doubtful Species. :
ne -
nature of their secretions. Tn some instances the males aione, 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 varietics. Mr. Walsh ranks the forms which it may be
supposed would freely intercross, as varieties 5 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 inks connecting the several forms should
now be found. The naturalist thus loses his best guide in deter-
mining whether to rank doubtful forms as varieties or species. This
likewise necessarily occurs with closely allied organisms, which
‘nhabit 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 varie-
tics; 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, 1m-
plying and assuming a separate act of creation. It is certain that
many forms, considered by highly-competent judges to be varieties,
resemble species so completely in character, that they have been thus
ranked by other highly-competent judges. But to discuss whether
they ought to be called species or varieties, before any definition of
these terms has been generally accepted, is vainly to beat the air.
Many of the cases of strongly-marked varieties or doubtful species
well deserve consideration ; for several interesting lines of argument,
from geographical distribution, analogical variation, hybridism, &c.,
have been brought to bear in the attempt to determine their rank ;
but space does not here permit me to discuss them. Close investi-
gation, 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.
T have been struck with the fact, that if any animal or plant ina
state of nature be highly useful to man, or from any cause closely
40 a Doubtful Species. Cuap. II.
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 specieg |
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 con-
nected 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 spon-.
taneous varieties and sub-varieties. Thus Quercus robur has twenty- —
eight varieties, all of which, excepting six, are clustered round three
Be panes, namely, Q. pedunculata, sessiliflora, and pubescens.
he 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 wholly 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 sur-
round the typical Quercus robur, Finally, De Candolle admits
that out of the 800 species, which will be enumerated in his Pro-
dromus as belonging to the oak family, at least two-thirds are
SL
™
(uae. IL Doubtful Spectres. AI
se
provisional species, that is, are not known strictly to fulfil the defi-
nition 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 palxontology, geo-
eraphical botany and zoology, of anatomical structure and classifi-
cation.”
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
cenerally 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
croups and in other countries, by which to correct his first impres-
sions. 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 dif-
ierences. ‘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 steps 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
42 Dominant Species vary mosi. Cuap, I.
towards more strongly-marked and permanent varieties ; and at the
latter, as leading to sub-species, and then to species. ‘The passage
from one stage of difference to another may, in many cases, be the
simple result of the nature of the organism and of the different
physical conditions to which it has long been exposed; but with
respect to the more important and adaptive characters, the passage
from one stage of difference to another, may be safely attributed to |
the cumulative action of natural selection, hereafter to be explained, ;
and to the effects of the increased use or disuse of parts. A well-
marked variety may therefore be called an incipient species; but
whether this belief is justifiable must be judged by the weight
of the various facts and considerations to be given throughout this
work.
It need not be supposed that all varieties or incipient species
attain the rank of species. They may become extinct, or they may
endure as varieties for very long periods, as has been shown to be
the case by Mr. Wollaston with the varieties of certain fossil land-
shells in Madeira, and with plants by Gaston de Saporta. Ifa
variety were to flourish so as to exceed in numbers the parent
‘species, it would them 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 arbi-
trarily, for convenience’ sake.
an
Wide-ranging, much-diffused, and common Species vary most.
Guided by theoretical considerations, I thought that some in-
teresting 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 —
Cua. Il. Dominant Species vary most. 43
(
tables, he thinks that the following statements are fairly well esta-
plished. ‘Che whole subject, however, treated as 1b necessarily here
‘s 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 wach
have very wide ranges generally present varieties ; and this might
have been expected, as they are exposed to diverse physical condi-
tions, and as they come into competition (which, as we shall here-
after see, is an equally or 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 com-
monness), oftenest give rise to varicties 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 predominance, it should be
understood that reference is made only to the forms which come
into competition with each other, and more especially to the mem-
bers 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
a in the above respects, it will then be dominant within its own
Clg
ds»,
44 Species of Larger Genera variable, Cuar. Il,
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 antici-
pated ; for the mere fact of many species of the same genus inhabit-
ing 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 ranves 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.¢.,
‘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, cir-
cumstances have been favourable for variation ; and hence we might
expect that the circumstances would generally be still 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
chaving 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
Cusr. Ul. Species of Larger Genera variable. AS.
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
cenera which present any varieties, invariably present a larger
averaze 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 3.
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 gencra 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
Vurieties 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.
46 Species of Larger Genera Cuap, Ii,
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 gencra 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 varie-
ties or incipient species greater than the average are now manufac-
turing, many of the species already manufactured still to a certain
extent resemble varieties, for they differ from each other by 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
elated 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 varie-
ties, 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 gene-
rally 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 denomi-
nations 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) 68 plants which are therein ranked
as species, but which he considers as so clasely allied to other
species as to be of doubtful value: these 63 reputed species range
on an average over 6°9 of the provinces into which Mt. Watson has
divided Great Britain. Now, in this same Catalogue, 53 acknow-
ledged varieties are recorded, and these range over 7'7 provinces;
Cup. IL. vesemble Variettes. 47
whereas, the species to which these varieties belong range over 14°3
provinces. So that the acknowledged varieties have 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, not-
withstanding 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 through-
out 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
cencra also tend to break up into smaller genera. And thus, the
forms of life throughout the universe become divided into groups
subordinate to groups.
48 Struggle for Existence. Crap. Ul.
CHAPTER III.
STRUGGLE FoR EXISTENCE.
Its bearing on natural selection — The term used in a wide sense—Geome-
trical 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,
BeroRE 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
amongst 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 misletoe ; 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
Guar. IIL Struggle for Existence. AQ
senera, 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 terrn 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, as we shall hereafter see, is a power inces-
santly ready for action, and is as immeasurably superior to man’s
iceble 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 have
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, extinc-
tion, 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 ecgs, or their nestlings, are destroyed by birds
and beasts of prey; we do not always bear in mind, that, thouch
food may be now superabundant, it is not so at all seasons of each
recurring year,
E
50 Geometrical Ratio of [ncrease. _ Cup. IIE.
The Term, Struggle for Huistence, used in a large sense.
I should premise that I use this term in a large and metaphorical
sense including dependence of cone being on: another, and including
(which is more important) not only the life of the individual, but
success in leaving progeny. ‘l'wo canine animals, in a time of
dearth, may be truly said to struggle with each other which shall
get food and live. Buta 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 on 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
misletoe 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 misletoes, growing close together on
the same branch, may more truly be said to struggle with each
other. As the misletoe 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 sufier
destruction during some period of its life, and during some seasot
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, oF
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.
.
Cuap. III. Geometrical Ratio of [ncrease. 51
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. lLinneus 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 astonish-
ingly rapid increase of various animals in a state or 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 ofthe plants,
such as the cardoon and a tall thistle, which are now the com-
monest 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 dis-
covery. 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.
E 2
52 Geometrical Ratio of Increase. © Case. Il,
In a state of nature almost every full-grown plant annually
produces seed, and amongst animals there are very al which do
not annually pair. Hence we may confidently asset t, 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 famiharity with
the larger domestic animals tends, J think, to mislead us : We see
no great destruction falling on them, but 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 fluctua-
ting 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
Cuar. Ul Nature of the Checks to [ucrease. 53
— —_—_-—
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. Eges or very young
animals seem generally to suffer most, but this is not invariably the
case. With plants there is a vast destruction of secds, 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
inewn, 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 growing 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 numbers 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
hot 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. Cn
the other hand, in some cases, aS with the elephant, none are
54 Nature of the Checks to Increase. Cuap. II,
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 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 inhabi-
tants, the other species must decrease. When we travel south-
ward 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 northwards ; hence in going
northwards, 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 southwards 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.
Cua. IL. Struggle for Existence. 55
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 amongst the crowded
animals, been disproportionally favoured : and here comes in a sort
of struggle between the parasite and its prey.
On the other hand, in many cases, a large stock of individuals
of the same species, relatively to the numbers of its enemies, is
absolutely necessary for its preservation. ‘Thus we can easily raise
plenty of corn and rape-seed, &c., in our fields, because the seeds
are in great excess compared with the number of birds which feed
on them; nor can the birds, though having a superabundance of
food at this one season, increase in number proportionally to the
supply of seed, as their numbers are checked during 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
tozether, 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 Stafford-
shire, 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
bi a
56 Struggle for Existence, Cuap, IL.
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 sur-
prised 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
Cnap. III. Struggle for Existence. 57
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 obser-
ved in parts of South America) the vegetation: this again would
largely affect the insects; and this, as we have just seen in Stafford-
shire, the insectivorous birds, and so onwards in ever-increasing
circles of complexity. Not that under nature the relations will
ever be as simple ag this. Battle within battle must be con-
tinually 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. Never-
theless, 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!
Iam 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. T
find from experiments that humble-bees are almost indispensable to
the fertilisation of the heartsease (Viola tricolor), for other bees do
hot visit this flower. Ihave also found that the visits of bees arc
necessary for the fertilisation of some kinds of clover : for instances.
20 heads of Dutch clover (‘Trifolium repens) yielded 2,290 seeds, but
20 other heads protected from bees produced not one. Again, 100
heads of red clover (‘T. pratense) produced 2,700 sceds, but the same
number of protected heads produced not a single seed. Humble-bees
alone visit ted clover, as other bees cannot reach fhe 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 creat measure on the
number of field-mice, which destroy their combs and nests ; and
Col. Newman, who has long attended to the habits of humble-bees,
*
58 Struggle for Existence. Cup. HL,
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 Col. Newman says,
“Near villages and small towns I have found the nests of humble-
bees more numerous than elsewhere, which I attribute to the num-
ber 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
dotermine, 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, pro-
bably 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 pro-
portional 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 surround-
ing virgin forest. 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 cen-
turies, 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 tliose which may
‘be strictly said to struggle with each other for existence, as in the
ease 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
Guar. IL. Struggle for Existence.
Cry
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,
-¢ several varieties of wheat be sown together, and the mixed seed be
regown, 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 varie-
ties. ‘To keep up a mixed stock of even such extremely close varic-
ties 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
cenerations, 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 sce 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 char-
lock 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
oie gaestas ‘melee m no one case could we precisely say why
1g victorious over another in the great battle of life.
60 Struggle for Existence. Crap. IIE,
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 the 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 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 fal]
on unoccupied ground. In the water-beetle, the structure of itg
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 else-
where it ranges into slightly hotter or colder, damper or drier dis-
tricts. In this case we can clearly see that if we wish in imagination
to give the plant the power of increasing in number, 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 amongst new Competitors, the conditions of its life will
generally be changed in an essential manner, although the. climate
Cwar, IL. Struggle for Existence. 61
—
may be exactly the same as in its former home. If its average num-
bers are to increase in its new home, we should have to modify it in
9 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 to 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 geometri-
cal 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.
62 Natural Selection. Cuap, IV,
CHAPTER IV.
NATURAL SELECTION; OR THE SURVIVAL OF THE FITrest,
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 unfa-
vourable 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 de-
scendants 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 be truly
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
Z
Natural Selection. 63
Cuap. iV.
Oe
undoubtedly occurred, that other variations useful in some way to
cach 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 of
procreating their kind? On the other hand, we may feel sure that
any variation in the least degree injurious would be rigidly de-
stroyed. 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 selec-
tion, 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 con-
ditions 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 voli-
tion, 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 oravity
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
+
64 Natural Selection. Cuap, IV,
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 modifica-
tions, 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 have changed,
and this would manifestly be favourable to natural selection, by
aifording 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 indivi- -
dual differences are included. As man can produce a great result
with his domestic animals and piants 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 un-
usual degree of isolation to check immigration, is necessary in order
that new and unoccupied places should be left, for natural selec-
tion to fill up by improving some of the varying inhabitants. For
as all the inhabitants of each country are strugeling together with
nicely balanced forces, extremely slight modifications in the struc-
ture 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
cpap. IV Natural Selection. 65
—__.
the native inhabitants are now so periectly 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,
en the whole machinery of life. Man selects only for his own
good: Nature only for that of the being which she tends. Every
sclected 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 anda 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. He 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 t
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
F
Le stamp
66: Natural Selection. Cnap, IV.
—
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 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 be again pre-
served, and so onwards 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
humbers; 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 the 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 con-
sidered 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
Cuap. IV. Natural Selection. 67
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 differ-
ences 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 differ-
ences would effectually settle which variety, whether a smooth or
downy, a yellow or 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, &c., 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 diffi-
culty 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 fhe 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 strue-
ture of one Species, without giving it any advantage, for the good of
F 2
68 Natural Selection. Cuap. LV,
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 egg. 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 advan-
tage, 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. or 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
aud plants, whether or not they be the best adapted to their con-
ditions, 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 egos 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, sup-
posing 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
Cap. LV. Serualt Selection. 69
underco modification and improvement at the same time in the
: oO
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 com-
petitor, 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, as on having special weapons, confined to the male
sex, A hornless stag or spurless cock would have a poor chance of
Icaving numerous offspring. Sexual selection, by always allowing
the victor to breed might surely give indomitable courage, length
to the spur, and strength to the wing to strike in the spurred leg, in
nearly the same manner as does the brutal cocktighter by the care-
ful 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 to the lion, and the hooked jaw to the male salmon;
lor the shield may be as important for victory, as the sword or
spear.
Amongst birds, the contest is often of a more peaceful charaeter,
70 Sexual Selection. Cuap. IV,
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 plu-
mage; 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 ex-
plained 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 suc-
vessive 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 domestica-
tion, 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 5
Cua. IV. | Natural Selection. 7T
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 would have the best chance of surviv-
ing, 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
seo ho more reason to doubt that this would be the result, than that
man should be able to improve the fleetness of his greyhouuds 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 grey-
hound-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, 1 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 subse-
quent 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
80 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 te
survive and to breed, and that half its young inherited the favour-
able variation ; still, as the Reviewer goes on to show, the young
72 Illustrations of the Action of CHap, 2)
eee
would have only a slightly better chance of surviving and breeding ;
and this chance would go on decreasing in the succeeding Senerar-
tions. 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 perpetuat-
ing 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 cou'd 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 thas
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:
eacn newly-formed variety would generally be at first local, as seems
to be the common tule 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
snecessful in its battle for life, it would slowly spread from a centiab
snap. IV. Natural Selection. 73,
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 illus~
tration of the action of natural selection. Certain plants excrete-
sweet juice, apparently for the sake of eliminating something in-
jurious from the sap: this is effected, for instance, by glands at the
pase of the stipules in some Leguminose, 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,,.
vives 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 in-
stead 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 stiil 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, recularly carry pollen from flower to flower; and that
they do this effectually, I could easily show by many striking facts.
twill give only one, as likewise illustrating one step in the separa--
nen of the sexes of plants. Some holly-trees bear only male flowers,.
vole nave ao Stamens oan. a rather small quantity of
flowers; these have ; / ae a ter oly-trees peut only female:
shriveled anthers, is ahd SizeC ae u, and four sraNons with
8, 1 not a grain of pollen can be detected.
Illustrations of the Action of Cuap. IV.
a
74
Having fund 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 get
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 “ physio-
logical 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 sepa-
ration of the sexes of our plant would be advantageous on the prin-
ciple 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 diceciously 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, &¢c., 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
Cua. IV. Natural Selection. 75
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 and
incarnate clovers (Trifolium pratense and incarnatum ) do not ona
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 accu-
rate; 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 con-
tinued preservation of all the individuals which presented slight
deviations of structure mutually favourable to each other.
Tam well aware that this doctrine of natural selection, exempli-
fied 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 or insignificant, when used in explaining the excavation
of the deepest valleys or the formation of long lines of inland
clifls, Natural selection acts only by the preservation and accumu-
‘ation of small inherited modifications, each profitable to the pre-
‘served being; and ag modern geology has almost banished such
76 On the Intercrossing of Individuals. Cuar. iv,
LT
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 Kélreuter. 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 dis-
cussion. 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 hermaphro-
dites. 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 off-
spring; 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, under=
stand several large classes of facts, such as the following, whicli
on any other view are inexplicable, Every hybridizer knows how
unfavourable exposure to wet is to the fertilisation of a flower, yet ;
Cap. IV. On the Intercrossing of Individuals. 77
what a multitude of flowers have their anthets ue seigmas “uly
exposed to the weather! If an occasional ine Te wae spensable,
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 dimi-
nished 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 Girtner, 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 Kélreuter 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-allicd 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 effecixally
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
humerous po.len-eranules 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
sarden, by insects, it never sets a seed, though by placing pollen
trom one flower on the stigma of another, I raised plenty of seed-
lings. Another species of Lobelia, which is visited by bees, seeds
78 On the Intercrossing of Individuals. Cuar.1V
freely in my garden. In very many other cases, though there is no
special mechanical contrivance to prevent the stigma receiving
pollen from the sume 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 ex-
plained 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 have found, mongrels: for
instance, I raised 283 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 own 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 mon-
grelized? 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 almost
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 t0
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
_ are separated, although the male and female flowers may be
produced on the same tree, pollen must be regularly carried from
Cuar.IV. On the Intercrossing of Individuals. 79
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 offerg
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 Ine 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-fertilisng 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
1S 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,
1t appears that with animals and plants an occasional intercross
between distinct individuals is a very general, if not universal, law
of nature,
80 Circumstances favourable to the — Car. IV,
Circumstances favourable for the proaenan of new forms
through Natural Selection.
This is an extremely intricate subject. A great amount ot varia-
bility, under which term individual differences are always included,
will evidently be favourable. A large number of individuals, by
civing a better chance within any given period for ue appearance
of profitable variations, will compensate for a lesser amount ot
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 modi-
fied 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 modifi-
cation 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 »
Joug 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
xate. Hence with animals of thi
varieties, will generally be confined t
I find to be the case.
only occasionally,
inten
sink ait Mite ibis
a very quick
S nature, for instance, birds,
0 separated countries; and this
With hermaphrodite organisms which cross
and likewise with animals which unite for each
Cap. IV. Results of Natural Selection. 8i
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 afterwards
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.
Evey with animals which unite for each birth, and which do not
] ropagate 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
tovether.
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 inter-
vals 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 cha-
racter can be retained by them under the same conditions of life,
only through the principle of inheritance, and through natural selec-
tion which will destroy any individuals departing from the proper™
type. Ifthe conditions of life change and the form undergoes modifi-
cation, uniformity of character can be given to the modified oftspring,
solely by natural selection preserving similar favourable variations.
Isolation, also, is an important element in the modification of
Species through natural selection. Ina confined or isolated area, if
hot very large, the organic and inorganic conditions of life will
generally be almost uniform; so that natural selection will tend to
modily all the varying individuals of the same species in the same
manner, Intercrossing with the inhabitants of the surrounding dis-
tricts Will, also, be thus prevented. Moritz Wagner has lately pub-
lished an interesting essay on this subject, and has shown that the
S-rvice rendered by isolation in preventing crosses between newly-
G
82 Circumstances favourable to the Cuar. IV,
——
ably greater even than I supposed. But
d I can by no means agree with this
d isolation are necessary elements for
, i The importance of isolation is like
~ ee of new agen any ph sical change in the conditions
wise great in preventing, alter any pay che immiorat
euch as of climate, elevation of the land, &c., the Immigration of
Detter adapted orgaitisms ; and thus new places nn natural
economy of the district will be left open to be ed 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 some-
times 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 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 importaint, especially for the production of species which
shall prove capable of enduring for a long period, and of spreading
widely. ‘T'hroughout a great and open area, not only will there be@
formed varieties is prob
from reasons already assigne
naturalist, that migration an
sas
Cnap. IV. Results of Natural Selection. 83
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 @ 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
4 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
Kuropxo-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 then more slowly
produced, and old forms more slowly exterminated. And it is in
iresh-water basins that we find seven genera of Ganoid fishes,
remnants of a once preponderant order: and in fresh water we find
nithrbyches an Lape eee the wel th
certain ectent a epi OsHnER, which, like fossils, connect to a
— These someone fome be, te os - ne md
have endured a ms mos may de called living fossils ; they
» present day, from having inhabited a confined
G 2
Circumstunces favourable to the Crap. IV.
ee
84
area, and from having been exposed to less varied, and therefore less
severe, competition.
: intri subject its,
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 pro-
duction of many new forms of life, fitted to endure for a long time
and to spread widely. Whilst the area existed as a continent, the
inhabitants will have bee 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 compevition: 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 propor
tional 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 acts 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 pre-
vented. 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 indi-
viduals of the same species differ in some slight degree from each
other, wb 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
Crap. 1Y, Results of Natural Selection. 85
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 in-
habitants 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 con-
ditions of life, which may have been effected in the long course of
time through nature’s power of selection, that is by the survival
af 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 con-
nected 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 ali organic beings, each area is already fully stocked with inhabit-
ants; and it follows from this, that as the favoured forms increase in
number, so, generally, will the less favoured decrease and become
are. 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 of 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 indefi-
nitely 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 indi-
viduals 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,
86. Extinction by Natural Selection. Car. IV,
—_—
that as new species in the course of time are formed nove natural
selection, others will become rarer and rarer, and nally 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 of 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 amongst 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. Neverthe-
less, 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? ‘Uhat this does habitu-
ally 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 v
i ariety again to differ
from its parent in the very s bee
ame character and in a ereater degree ;
but this alone would never account for go habitual and large
a degree of difference as that between the species of the same
genus.
As has always been my practice, I have soucht light on this
ee er eat ee
Cap. LV. Divergence of Character. 87
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, &c., 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, whilst 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 interme-
diate characters, being neither very swift nor very strong, would
not have been used for breeding, and will thus have tended to dis-
appear. 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
$8 Divergence of Character. Crap, IV,
——
—-:
‘nstance, being enabled to feed on new kinds of prey, either dead
or alive; some: inhabiting new stations, climbing trees, frequenting
od live: °.
or alive ; ing less carnivorous. The more
. ‘haps becom
water, and some perhap Jovte af a
diversified in habits and structure the descendants of our carnivo-
yous 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 experi~
mentally 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
herbace 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
varicties, 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 indivi-
dual 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
most 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
Cuap. IV. Divergence of Character. 89
-
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
jand 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 indi-
genous, 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 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
«loubts that a stomach 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
Results of the Action of Cuap, IV,
gO
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 Austra-
lian 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 mam-
mals, 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 Natwral 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 the 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 simultancously, 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 (repre=
oe
Lol Rar eed
By oe
a ahd
Cuap. IV. Natural Selection. Or
Pe
sented by the outer dotted lines) being preserved and accumulated
by natural selection. When a dotted line reaches one of the hori-
zontal 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 a! and m4. 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 heredi-
tary; 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 a! is supposed in the
diagram to have produced variety a?, which will, owing to the prin-
ciple of divergence, differ more from (A) than did variety a}.
Variety m' is supposed to have produced two varieties, namely m?
and s*, 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 modifiéd 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 unal-
tered, and then again undergoes modification. Nor do I stippose
ri
Q2 Results of the Action of Cua® LV,
—
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
an. 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 num-
bered 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 inter-
vals 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
ef the same advantages which made their parent successful in
life, they will generally go on muitiplying 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
ancdification wili 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 he represented in the diagram, if all the lines pro-
ceeding from (A) were removed, excepting that from a'toa™, In
the same way the English race-horse and English pointer have appa-
rently 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, a, f/?, and m, 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 be-
tween 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
amore numerous or greater in amount, to convert these three forms
_
Crap. IV. Natural Selection. 93
‘nto doubtful or at last into well-defined species. Thus the diagram.
illustrates the steps by which the small differences distinguishing
varieties are increased into the larger differences distinguishing spe-
cies. By continuing the same process for a greater number of gene-
rations (as shown in the diagram in a condensed and simplified
manner), we get eight species, marked by the letters between a”
and m4, 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 (1) has
produced, by analogous steps, after ten thousand generations, either
two well-marked varieties (w" and 21°) or two species, according to:
the amount of change supposed to be represented between the hori-
zontal lines. After fourteen thousand generations, six new species,
marked by the letters 1 to z'4, are supposed to have been produced.
In any genus, the species which are already very different in cha--
racter 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 (1), 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 de-
scendants; and this is shown in the diagram by the dotted lines.
unequally prolonged upwards.
But during the process of modification, represented in the dia-
gram, 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 advan-
tage in the struggle for life over other forms, there will be a constant
tendency in the improved descendants of any one species to sup-
plant 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 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
94 Results of the Action of Cap. IV,
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 (a™ to
m) ; and species (1) will be replaced by six (** to z*) 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, OC, and D, than to the other species ; and species (1)
more to G, H, K, L, than to the others. These two species (A) and
(1) 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 (1), but likewise
some of the original species which were most nearly related to their
parents. Hence very few of the original species will have trans-
mitted 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 trans-
mitted 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 diver-
gent tendency of natural selection, the extreme amount of difference
in character between species a and z' 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 dif-
ferent manner. Of the eight descendants from (A) the three marked
a’, q*, p“', will be nearly related from having recently branched off
from a”; 64, and f'4, from having diverged at an earlier period from
a”, will be in some degree distinct from the three first-named. species;
and lastly, o', e', and m*, 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 (1) will form two sub-genera or genera.
But as the original species (I) differed largely from (A), standing
Cuap. IV. Natural Selection. 95
nearly at the extreme end of the original genus, the SIX descendants
from (1) 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 considera~
tion), which connected the original species (A) and (1), have all
become, excepting (F), extinct, and have left no descendants.
Hence the six new species descended from (I), and the eight de-
scended 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 dia-
gram, 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 fora moment on the character of the
new species F’4, 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 which stood between the parent-species
(A) and (1), 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 (¥™) will not be directly intermediate between them,
but rather between types of the two groups; and every naturalist
will be able to call such cages 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 sce that the diagram
throws light on the affinities of extinct beings, which, though gene-
rally 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,
96 Results of the Action of Crap,
——
I see no reason to limit the process of modification, as now ex-
plained, 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 a" to p'4,
those marked b™ and /4, and these marked o to m™, will form
three very distinct genera. We shall also have two very distinct;
genera descended from (1), differing widely from the descendants
of (A). These two groups of genera will tius form two distinct
families, or orders, according to the amount of divergent modifica-
tion 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 numbers, 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 whichsgroups 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
Onap. IV. Natural Selection. 97
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 exists 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 time.
On the Degree to which Organisation tends to advance.
Natural Selection acts exclusively by the preservation and accu-
mulation 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 im-
provement 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. Amongst 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
the 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 specialisa-
tion 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,
amongst which some naturalists rank those as highest which, like
the sharks, approach nearest to amphibians ; whilst 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, amongst which the standard of intel-
lect 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,
H
98 On the Degree to which. Cuap. IV.
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 selec-
tion. 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 everywhere 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 spon-
taneous 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 com-
plex 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 in-
testinal worm—or even to an earth-worm, to be highly organised.
If it were no advantage, these forms would be left, by natural selec-
tion, 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,
tae
Cuap. IV. Organisation tends to aavance. — 99
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
crades of organisation within the same great group; for instance,
in the vertebrata, to the co-existence of mammals and fish— amongst
mammalia, to the co-existence of man and the ornithorhynchus—
amongst 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 advance-
ment 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 aérial respiration ;
so that warm-blooded mammals when inhabiting the water lie
under a disadvantage in having to come continually to the sur-
face to breathe. With fishes, members of the shark family would
not tend to supplant the lancelet; for the lancelet, as I hear
from Fritz Miiller, 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 nume-
rous 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 pre-
served 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 favour-
» able variations arising,
Finally, I believe that many lowly organised forms now exist
throughout the world, from various causes. In some cases varia-
tions 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
H 2
100 Convergence of Character. Cuap. IV, i
development. In some few cases there has been what we must call
retrogression of 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 steps in the advancement or differentiation of
parts have arisen? Mr. Herbert Spencer would probably answer
that, as soon as simple unicellular organism 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 inci-
dent forces’ become different” would come into action, But as we
have no facts to guide us, speculation on the subiect 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 igno-
vance 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 diver-
gent 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 con-
verge intoone. Butit would in most cases be extremely rash to attri-
bute 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
~~ OS Vesa CO!
Cuap. IV. Convergence of Character. IOI
—_
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 surround-
ing physical conditions, and in a still higher degree on the sur-
rounding organisms with which each being has come Into competi-
tion,—and lastly, on inheritance Gn itself a fluctuating element) from
innumerable progenitors, all of which have had their forms deter-
mined 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 organi-
sation. If this had occurred, we should meet with the same form,
independently of genetic connection, recurring in widely separated
cseological 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 con-
ditions are concerned, it seems probable that a sufficient number of
species would soon become adapted to all considerable diversities
of heat, moisture, &c.; 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 sight 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
80 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 extermi-
nation in such cases would be rapid, whereas the production of new
102 Natural Selection. Cuap. IV.
————.
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 principle 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 Scot-
land, and of Bears in Norway, &c. 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, vill 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 exter-
minate 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 8.E. corner of Australia, where, appa-
rently, 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 indivi-
dual 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, con-
stitution, 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 pre-
Cuap. IV. Summary. 103
ee
al of the fittest, I have called Natural
Selection. It leads to the improvement of each creature 1n 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. - a
Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young, as easily |
as the adult. Amongst many animals, sexual selection will have
civen 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 tenor 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 same 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 de-
scendants become, the better will be their chance cf 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
oy sbring that superiority which now makes them dominant in
i" - jn countries, Natural selection, as has just been remarked,
improved and ints of character and to much extinction of the less
saturs of er aeirmae forms of life. On these principles, the
between the oie the generally well-defined distinctions
organic beings in each class throughout
servation, or the surviv
104 Natural Selection. - Cuap. LY,
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 connection 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 geolo-
gical 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
Cuap. IV. Summary. 10S.
_ —
occasionally see an animal like the Ornithorhynchus or Lepidosiren,
which in some small degree connects by its affinities two large
pranches 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 @ feebler branch, so by generation I
pelieve 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.
106 Laws of Variation. Cuap. V.
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 acknow-
ledge plainly our ignorance of the cause of each particular variation.
Some authors believe it to be as much the function of the repro-
ductive 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 condi-
tions 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 organisa-
tion 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 organi-
sation seems to become plastic, and we have much fluctuating vari-
ability. 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, &¢., have acted in a definite manner. There is
Cuar. V. Laws of Vartation. 107
reason to believe that in the course of time the effects have been
ereater 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 : i,
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 atmos-
phere, 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 else-
where 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 indivi-
duals 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 domes-
tic 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 con-
ditions, than on a tendency to vary, due to causes of which we are
quite ignorant,
In one sense the conditions of life ma
variability, either directly or indirect]
natural selection ; for the conditions det
variety shall survive,
clearly see that the two
y be said, not only to cause
y, but likewise to include
ermine whether this or that
But when man is the selecting agent, we
elements of change are distinct ; variability —
108 Effects of Use and Disuse. Cuap. V:
is in some manner excited, but it is the will of man which accumu-.
lates the variations in certain directions ; 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 thirtk 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. Cunning-
ham, 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 beast 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 proge-
nitor of the ostrich genus had habits like those of the bustard, and.
that, as the size and weight of its body were increased during suc-
cessive 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-Séquard in guinea-pigs, of the
inherited effects of operations, should make us cautious in denying
Cuar. V. Effects of Use and Disuse. 109
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,
put as due to the effects of long-continued disuse ; for he Et y
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. a
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 frequently blown to sea
and perish; that the bectles 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 extra-
ordinary fact, so strongly insisted on by Mr. Woilaston, 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 ¢he island, the tendency of natural
selection to enlarge or to reduce the Wings, would depend on
Netra « greater number of individuals were saved by successfully
a i . . +s ; 1
-_ - with the w inds, or by giving up the attempt and rareiy or
avetbe ing. As with mariners shipwrecked near a coast, it would
have been be ; ne
sui eal : ae for the sood Swimmers if they had been able to
: urther, whereas it would have been better for the bad
110 ' Effects of Use and Disuse. Cuap, V,
—-—
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 bur-
rowing rodent, the tuco-tuco, or Ctenomys, is even more subter-
ranean 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 of Ken-
tucky, 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 there-
fore not in the profoundest depths, the eyes were lustrous and of
large size; and these animals, as I am informed by Professor Silli-
man, 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; and with respect to the insects alone, Schiddte 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 thé
Mammoth cave (in Kentucky) and the caves in Carniola, otherwise
than as a very plain expression of that analogy which subsists
Cuap. V. Effects of Use and Disuse. LII
generally between the fauna of Europe and of x oh america”
On my view we must suppose that American an _ ae
most cases ordinary powers of vision, slowly migrate rd veooee ee.
generations from the outer world into the deeper an eeper me
cesses of the Kentucky caves, as did European animals into the
caves of Europe. We have some evidence of this gradation of
habit ; for, as Schiddte remarks, “ We accordingly look upon the
subterranean faunas as small ramifications which have penetrated
into the earth from the geographically limited faunas of the adja-
cent 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 twi-
light ; 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
antenne or palpi, as a compensation for blindness. Notwithstand-
ing such modifications, we might expect still to see in the cave-
animals of America, affinities to the other inhabitants of that con-
tinent, 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
* blind species of Bathyscia is found in abundance on shady rocks
ar from caves, the loss of vision in the cave-species of this one
Sate probably had no relation to its dark habitation ; for it
becotne “ . a aot already deprived of vision should readily
thalmus) o dark caverns. Another blind genus (Anoph-
ollers this remarkable peculiarity, that the species, as
er Mr. Mu
| 7 saves observes, have not as yet been found anywhere except
Arnerica oat 1m the several caves of Europe and
ible that the progenitors of these
ished with eyes, may formerly
II2 Acclimatisation. Cuap. V,
have ranged over both continents, and then have become extinct,
excepting in their present secluded abodes. Far from feeling sur-
prise 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 germi-
nate, &c., 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 in-
ability to predict whether or not an imported plant will endure our
climate, and from the number of plants and animals brought front
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 Himalaya, 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
Kuropean 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 cooler latitudes,
LS —“—i—s al
io]
Acclimatisation. 113
Cuar. V.
ene
but we do not positively know that these animals
were strictly adapted to their native climate, though 2 all prenary
cases we assume such to be the case; nor do we know an they
have subsequently become specially acclimatised a” t a 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 ex-
traordinary capacity in our domestic animals of not only withstand-
ing 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, how-
ever, push the foregoing argument too far, on account of the pro-
bable 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 trans-
ported 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
ws 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 18 due to mere habit, and how much to the natural selection
both ae paving ae innate constitutions, and how much to
has some influence T mn st real roth fenandoce aia f custom
incessant advice a st believe, both from analogy and from the
Encyclopedias of “China, to be vers a a y tnane ie ancien
mals from one district to another And az it isn Pikes tio nan
should have succeeded in selecting HS EOS not Likely tat man
witl § so many breeds and sub-breéds
f 1 constitutions specially fitted for their own districts, the result
must, I think :
ust, I think, be due to habit. On the other hand, natural selec-
I
and conversely 5
114 Correlated Variation. Czar. V.
tion would inevitably tend to preserve those individuals which were
porn with constitutions best adapted to any country which they
inhabited. In treatises on many kinds of cultivated plants, certain
varicties 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 recom-
mended 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 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 im-
portant 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 larve naturally
tend to affect the structure of the mature animal. The several
parts of the body which are homologous, and which, at an early
embryonic period, are identical in structure, and which are neces-
Cuap. V. Correlated Variation. 115
sarily exposed to similar conditions, seem eminently liable to vary
ina 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 selection.
Homélogous 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 their 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 form 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 malcon-
formations frequently, and that others rarely, co-exist, 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 5
orin 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 ;
Tertoh are pation pehween fia hair and teeth in the naked
With ree ai . A pre no doubt homology comes into play?
be nentdentet 4 . ‘ er case of correlation, I think it can hardly
abnormal. in ‘ a ; e two orders of mammals which are most
Edentata (armadill aml OVENS: VIE Menace, (whales) and
whole the most ulloes, scaly ant-eaters, &c.), are likewise on the
abnormal in their teeth ; but there are so many
exceptions t i é oe
F ae. dalue. 0 this rule, as Mr. Mivart has remarked, that it has
I know of no case bet
laws of correlation an
therefore of y
ter adapted to show the importance cf the
ve d Variation, independently of utility and
atural selection, than that of the difference between
I 2
116 Correlated Variation. Cuar. V2
the outer and inner flowers in some Compositous and Umbelliferous:
plants. Every one 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 Composite countenances this idea; but with the
Umbellifere, 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 cause, for in some Composite 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 differ-
ences in shape, which are not always correlated with any difference
in the corolla, can be in any way beneficial: yet in the Umbelli-
feree these differences are of such apparent importance—the seeds
being sometimes orthospermous in the exterior flowers and ccelo-
spermous 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
cuar.V. Compensation and Economy of Growth. 117
are simply due to inheritance ; for an ancient Pee may fave
acquired through natural selection some one Moa cation ae
ture, and, after thousands of generations, some other an " e-
yendent modification ; and these two modifications, having een,
transmitted to a whole group of descendants with diverse habits,
would naturally be thought to be in some necessary Manner CO
related. 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
elfects, on the one hand, of a part being largely developed through
natural selection and another and adjoining part being reduced by
this 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.
1 suspect, also, that some of the cases of compensation which
have been advanced, and likewise some other facts, may be merged
ue ie general principle, namely, that natural selection is
ani che, ed ne ° eoononiise every part of the organisation. If
ang ditions of life a structure, b8fore useful, becomes
118 Multiple and Rudimentary Cuar, V.
less useful, its diminution will be favoured, for it will profit the
individual not to have its nutriment wasted in building up an
useless structure. I can thus only understand a fact with which
I was much struck when examining cirripedes, and of which many
analogous 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 Proteo-
lepas: for the carapace in all other cirripedes consists of the three
highly-important anterior segments of the head enormously deve-
loped, 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 antenne. 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 com-
pensation 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 vertebre in
snakes, and the stamens in polyandrous flowers) the number is
variable ; whereas 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 Prof. 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,.
&&&=Qa—