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Charles Darwin.

On the origin of species by means of natural selection; or, The preservation of favoured races in the struggle for life online

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* * * * *


ON THE

ORIGIN OF SPECIES.

* * * * *

"But with regard to the material world, we can at least go so far as
this - we can perceive that events are brought about not by insulated
interpositions of Divine power, exerted in each particular case, but by the
establishment of general laws."

WHEWELL: _Bridgewater Treatise_.

"The only distinct meaning of the word 'natural' is _stated_, _fixed_, or
_settled_; since what is natural as much requires and presupposes an
intelligent agent to render it so, _i.e._ to effect it continually or at
stated times, as what is supernatural or miraculous does to effect it for
once."

BUTLER: _Analogy of Revealed Religion_.

"To conclude, therefore, let no man out of a weak conceit of sobriety, or
an ill-applied moderation, think or maintain, that a man can search too far
or be too well studied in the book of God's word, or in the book of God's
works; divinity or philosophy; but rather let men endeavour an endless
progress or proficience in both."

BACON: _Advancement of Learning_.

* * * * *

_Down, Bromley, Kent,_
_October 1st, 1859._ (_1st Thousand_).

* * * * *


ON

THE ORIGIN OF SPECIES

BY MEANS OF NATURAL SELECTION,

OR THE

PRESERVATION OF FAVOURED RACES IN THE STRUGGLE
FOR LIFE.

BY CHARLES DARWIN, M.A.,

FELLOW OF THE ROYAL, GEOLOGICAL, LINNEAN, ETC., SOCIETIES;

AUTHOR OF 'JOURNAL OF RESEARCHES DURING H. M. S. BEAGLE'S VOYAGE
ROUND THE WORLD.'

_FIFTH THOUSAND._

LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1860.

_The right of Translation is reserved._

* * * * *

LONDON: PRINTED BY W. CLOWES AND SONS, STAMFORD STREET,
AND CHARING CROSS.

* * * * *


{v}

CONTENTS.

* * * * *

INTRODUCTION

Page 1

CHAPTER I.

VARIATION UNDER DOMESTICATION.

Causes of Variability - Effects of Habit - Correlation of
Growth - 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 - Principle of Selection anciently followed, its Effects - Methodical
and Unconscious Selection - Unknown Origin of our Domestic
Productions - Circumstances favourable to Man's power of Selection

7-43

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 any
country vary more 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

44-59

{vi}

CHAPTER III.

STRUGGLE FOR EXISTENCE.

Its bearing on natural selection - The term used in a wide
sense - Geometrical powers 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

60-79

CHAPTER IV.

NATURAL SELECTION.

Natural Selection - its power compared with man's selection - its power on
characters of trifling importance - its power at all ages and on both
sexes - Sexual Selection - On the generality of intercrosses between
individuals of the same species - Circumstances favourable and unfavourable
to 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

80-130

CHAPTER V.

LAWS OF VARIATION.

Effects of external conditions - Use and disuse, combined with natural
selection; organs of flight and of vision - Acclimatisation - Correlation of
growth - 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

131-170

{vii}

CHAPTER VI.

DIFFICULTIES ON THEORY.

Difficulties on the theory of descent with
modification - Transitions - 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 - Means 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

171-206

CHAPTER VII.

INSTINCT.

Instincts comparable with habits, but different in their origin - Instincts
graduated - Aphides and ants - Instincts variable - Domestic instincts, their
origin - Natural instincts of the cuckoo, ostrich, and parasitic
bees - Slave-making ants - Hive-bee, its cell-making instinct - Difficulties
on the theory of the Natural Selection of instincts - Neuter or sterile
insects - Summary

207-244

CHAPTER VIII.

HYBRIDISM.

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

245-278

{viii}

CHAPTER IX.

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 vast lapse of
time, as inferred from the rate of deposition and of denudation - On the
poorness of our palæontological collections - On the intermittence of
geological formations - 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

279-311

CHAPTER X.

ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.

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

312-345

CHAPTER XI.

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 co-extensive with the world

346-382

CHAPTER XII.

GEOGRAPHICAL DISTRIBUTION - _continued_.

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

383-410

CHAPTER XIII.

MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY
ORGANS.

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

411-458

CHAPTER XIV.

RECAPITULATION AND CONCLUSION.

Recapitulation of the difficulties on the theory of Natural
Selection - Recapitulation of the general and special circumstances in its
favour - Causes of the general belief in the immutability of species - How
far the theory of natural selection may be extended - Effects of its
adoption on the study of Natural history - Concluding remarks

459-490

* * * * *


{1}

ON THE 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 inhabitants of South America, and
in the geological relations of the present to the past inhabitants of that
continent. These facts seemed to me to throw some light on the origin of
species - that mystery of mysteries, as it has been called by one of our
greatest philosophers. On my return home, it occurred to me, in 1837, that
something might perhaps be made out on this question by patiently
accumulating and reflecting on all sorts of facts which could possibly have
any bearing on it. After five years' work I allowed myself to speculate on
the subject, and drew up some short notes; these I enlarged in 1844 into a
sketch of the conclusions, which then seemed to me probable: from that
period to the present day I have steadily pursued the same object. I hope
that I may be excused for entering on these personal details, as I give
them to show that I have not been hasty in coming to a decision.

My work is now nearly finished; but as it will take me two or three 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 {2} natural history of the
Malay archipelago, has arrived at almost exactly the same general
conclusions that I have on the origin of species. Last year he sent me a
memoir on this subject, with a request that I would forward it to Sir
Charles Lyell, who sent it to the Linnean Society, and it is published in
the third volume of the Journal of that Society. Sir C. Lyell and Dr.
Hooker, who both knew of my work - the latter having read my sketch of
1844 - honoured me by thinking it advisable to publish, with Mr. Wallace's
excellent memoir, some brief extracts from my manuscripts.

This Abstract, which I now publish, must necessarily be imperfect. I cannot
here give references and authorities for my several statements; and I must
trust to the reader reposing some confidence in my accuracy. No doubt
errors 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 I have arrived, with a few facts in illustration, but
which, I hope, in most cases will suffice. No one can feel more sensible
than I do of the necessity of hereafter publishing in detail all the facts,
with references, on which my conclusions have been grounded; and I hope in
a future work to do this. For I am well aware that scarcely a single point
is discussed in this volume on which facts cannot be adduced, often
apparently leading to conclusions directly opposite to those at which I
have arrived. A fair result can be obtained only by fully stating and
balancing the facts and arguments on both sides of each question; and this
cannot possibly be here done.

I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very many
naturalists, some of them personally unknown to me. I cannot, however, {3}
let this opportunity pass without expressing my deep obligations to Dr.
Hooker, who for the last fifteen years has aided me in every possible way
by his large stores of knowledge and his excellent judgment.

In considering the Origin of Species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings, on their
embryological relations, their geographical distribution, geological
succession, and other such facts, might come to the conclusion that each
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 most justly
excites our admiration. Naturalists continually refer to external
conditions, such as climate, food, &c., as the only possible cause of
variation. In one very 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 misseltoe, which draws its nourishment from certain
trees, which has seeds that must be transported by certain birds, and which
has flowers with separate sexes absolutely requiring the agency of certain
insects to bring pollen from one flower to the other, it is equally
preposterous to account for the structure of this parasite, with its
relations to several distinct organic beings, by the effects of external
conditions, or of habit, or of the volition of the plant itself.

The author of the 'Vestiges of Creation' would, I presume, say that, after
a certain unknown number of {4} generations, some bird had given birth to a
woodpecker, and some plant to the missletoe, and that these had been
produced perfect as we now see them; but this assumption seems to me to be
no explanation, for it leaves the case of the coadaptations of organic
beings to each other and to their physical conditions of life, untouched
and unexplained.

It is, therefore, of the highest importance to gain a clear insight into
the means of modification and coadaptation. At the commencement of my
observations it seemed to me probable that a careful study of domesticated
animals and of cultivated plants would offer the best chance of making out
this obscure problem. Nor have I been disappointed; in this and in all
other perplexing cases I have invariably found that our knowledge,
imperfect though it be, of variation under domestication, afforded the best
and safest clue. I may venture to express my conviction of the high value
of such studies, although they have been very commonly neglected by
naturalists.

From these considerations, I shall devote the first chapter of this
Abstract to Variation under Domestication. We shall thus see that a large
amount of hereditary modification is at least possible; and, what is
equally or more important, we shall see how great is the power of man in
accumulating by his Selection successive slight variations. I will then
pass on to the variability of species in a state of nature; but I shall,
unfortunately, be compelled to treat this subject far too briefly, as it
can be treated properly only by giving long catalogues of facts. We shall,
however, be enabled to discuss what circumstances are most favourable to
variation. In the next chapter the Struggle for Existence amongst all
organic beings throughout the world, which inevitably follows from the high
geometrical ratio of their {5} increase, will be treated of. This is the
doctrine of Malthus, applied to the whole animal and vegetable kingdoms. As
many more individuals of each species are born than can possibly survive;
and as, consequently, there is a frequently recurring struggle for
existence, it follows that any being, if it vary however slightly in any
manner profitable to itself, under the complex and sometimes varying
conditions of life, will have a better chance of surviving, and thus be
_naturally selected_. From the strong principle of inheritance, any
selected variety will tend to propagate its new and modified form.

This fundamental subject of Natural Selection will be treated at some
length in the fourth chapter; and we shall then see how Natural Selection
almost inevitably causes much Extinction of the less improved forms of
life, and leads to what I have called Divergence of Character. In the next
chapter I shall discuss the complex and little known laws of variation and
of correlation of growth. In the four succeeding chapters, the most
apparent and gravest difficulties on the theory will be given: namely,
first, the difficulties of transitions, or in understanding how a simple
being or a simple organ can be changed and perfected into a highly
developed being or elaborately constructed organ; secondly, the subject of
Instinct, or the mental powers of animals; thirdly, Hybridism, or the
infertility of species and the fertility of varieties when intercrossed;
and fourthly, the imperfection of the Geological Record. In the next
chapter I shall consider the geological succession of organic beings
throughout time; in the eleventh and twelfth, their geographical
distribution throughout space; in the thirteenth, their classification or
mutual affinities, both when mature and in an embryonic condition. In the
last chapter I shall give a {6} brief recapitulation of the whole work, and
a few concluding remarks.

No one ought to feel surprise at much remaining as yet unexplained in
regard to the origin of species and varieties, if he makes due allowance
for our profound ignorance in regard to the mutual relations of all the
beings which live around us. Who can explain why one species ranges widely
and is very numerous, and why another allied species has a narrow range and
is rare? Yet these relations are of the highest importance, for they
determine the present welfare, and, as I believe, the future success and
modification of every inhabitant of this world. Still less do we know of
the mutual relations of the innumerable inhabitants of the world during the
many past geological epochs in its history. Although much remains obscure,
and will long remain obscure, I can entertain no doubt, after the most
deliberate study and dispassionate judgment of which I am capable, that the
view which most naturalists entertain, 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
main but not exclusive means of modification.

* * * * *


{7}

CHAPTER I.

VARIATION UNDER DOMESTICATION.

Causes of Variability - Effects of Habit - Correlation of
Growth - 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 - Principle of Selection anciently followed, its
Effects - Methodical and Unconscious Selection - Unknown Origin of our
Domestic Productions - Circumstances favourable to Man's power of
Selection.

When we look to 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. When we
reflect on the vast diversity of the plants and animals which have been
cultivated, and which have varied during all ages under the most different
climates and treatment, I think we are driven to conclude that this great
variability is simply 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 have been exposed under nature. There is
also, I think, some probability in the view propounded by Andrew Knight,
that this variability may be partly connected with excess of food. It seems
pretty clear that organic beings must be exposed during several generations
to the new conditions of life to cause any appreciable amount of variation;
and that when the organisation has once begun to vary, it generally
continues to vary for many generations. {8} No case is on record of a
variable being ceasing to be variable under cultivation. Our oldest
cultivated plants, such as wheat, still often yield new varieties: our
oldest domesticated animals are still capable of rapid improvement or
modification.

It has been disputed at what period of life the causes of variability,
whatever they may be, generally act; whether during the early or late
period of development of the embryo, or at the instant of conception.
Geoffroy St. Hilaire's experiments show that unnatural treatment of the
embryo causes monstrosities; and monstrosities cannot be separated by any
clear line of distinction from mere variations. But I am strongly inclined
to suspect that the most frequent cause of variability may be attributed to
the male and female reproductive elements having been affected prior to the
act of conception. Several reasons make me believe in this; but the chief
one is the remarkable effect which confinement or cultivation has on the
function of the reproductive system; this system appearing to be far more
susceptible than any other part of the organisation, to the action of any
change in the conditions of life. Nothing is more easy than to tame an
animal, and few things more difficult than to get it to breed freely under
confinement, even in the many cases when the male and female unite. How
many animals there are which will not breed, though living long under not
very close confinement in their native country! This is generally
attributed to vitiated instincts; but how many cultivated plants display
the utmost vigour, and yet rarely or never seed! In some few such cases it
has been discovered that very trifling changes, such as a little more or
less water at some particular period of growth, will determine whether or
not the plant sets a seed. I cannot here enter on the copious details which
I have collected on {9} this curious subject; but to show how singular the
laws are which determine the reproduction of animals under confinement, I
may just mention that carnivorous animals, even from the tropics, breed in
this country pretty freely under confinement, with the exception of the
plantigrades or bear family; whereas carnivorous birds, with the rarest
exceptions, hardly ever lay fertile eggs. Many exotic plants have pollen
utterly worthless, in the same exact condition as in the most sterile
hybrids. When, on the one hand, we see domesticated animals and plants,
though often weak and sickly, yet breeding quite freely under confinement;
and when, on the other hand, we see individuals, though taken young from a
state of nature, perfectly tamed, long-lived, and healthy (of which I could
give numerous instances), yet having their reproductive system so seriously
affected by unperceived causes as to fail in acting, we need not be
surprised at this system, when it does act under confinement, acting not



Online LibraryCharles DarwinOn the origin of species by means of natural selection; or, The preservation of favoured races in the struggle for life → online text (page 1 of 39)