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On the generality of intercrosses between individuals of the same species--Cir.
LAWS OF VARIATION.
organs of flight and of vision-Acclimatisation-Correlation of growth-Compen-
DIFFICULTIES ON THEORY.
rarity of transitional varieties-Transitions in habits of life-Diversified habits in
Aphides and ants-Instincts variable-Domestic instincts, their origin-Natural
Distinction between the sterility of first crosses and of hybrids--Sterility various in
degree, not universal, affected by close interbreeding, removed by domestication-
. . . . . . . 217
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
taly . . . . . . . . . . . 245
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
change-Species once lost do not reappear-Groups of species follow the same gen.
portance of barriers-Affinity of the productions of the same continent-Centres
GEOGRAPHICAL DISTRIBUTION— Continued.
sence of Batrachians and of terrestrial mammals-On the relation of the inhabit.
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY : RUDI.
CLASSIFICATION, groups subordinate to groups-Natural system-Rules and difficul.
ties in classification, explained on the theory of descent with modification Classi.
· · · · · 358
RECAPITULATION AND CONCLUSION.
of the general and special circumstances in its favour-Causes of the general
· · · · · · · · · · 398
Instruction to Binder. The Diagram to front page 138.
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 coulclusions 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 de