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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.” W.
WHEWELL: Bridgewater Treatise. |
|
“To
conclude, therefore, let no man out of a weak conceit of BACON: Advancement
of Learning. |
Down, Bromley,
Kent,
October 1st,
1859.
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, LINNAEAN, ETC., SOCIETIES;
AUTHOR OF ‘JOURNAL OF RESEARCHES DURING H.M.S. BEAGLE’S VOYAGE ROUND THE WORLD.’
LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1859.
CONTENTS. 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 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 Bears 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 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
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 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 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. 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. 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. 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 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 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 MUTUAL
AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: 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 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 INSTRUCTION
TO BINDER. The
Diagram to front page 117, and to face the latter part of the Volume.
EMBRYOLOGY:
RUDIMENTARY ORGANS.
|
ON THE ORIGIN
OF SPECIES. 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 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 to 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, 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 generations, some bird had given birth to a woodpecker, and
some plant to the misseltoe, 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 their high geometrical powers of 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 induces 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 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. |