The Origin of Species
Introduction
by Charles Darwin
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 judgement.
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
co-adaptation 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 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 judgement 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.