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On the generality of intercrosses between individuals of the same species--Cir.
cumstances favourable and unfavourable to Natural Selection, namely, intercross.
ing, 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, . . . . . . . 77

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LAWS OF VARIATION.
Effects of external conditions-Use and disuse, combined with natural selection ;

organs of flight and of vision-Acclimatisation-Correlation of growth-Compen-
sation 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-Re-
versions to long lost characters-Summary,

. 120

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, .

154

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,

• 185

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 crosbe
ing-Fertility of varieties when crossed and of their mongrel offspring not uni.
versal-Hybrids and mongrels compared independently of their fertility-Sum.
mary,

. . . . . . . 217

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 palæonto-
logical collections-On the intermittence of geological formations-On the absenco
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,

taly . . . . . . . . . . . 245

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 gen.
eral rules in their appearance and disappearance as do single species-On Extinc-
tion-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 de.
velopment of ancient forms-On the succession of the same types within the same
areas-Summary of preceding and present chapters, . . .

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CHAPTER XI.

GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in physical conditions-Im.

portance 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, . . . . . . . . . . 80%

CHAPTER XII.

GEOGRAPHICAL DISTRIBUTIONContinued.
Distribution of fresh-water productions-On the inhabitants of oceanic islands--Abo

sence of Batrachians and of terrestrial mammals-On the relation of the inhabit.
ants of islands to those of the nearest mainland-On colonisation from the nearest
source with subsequent modification-Summary of the last and present chap-
ters, . . . . . . . . .

.834

CHAPTER XIII.

MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY : RUDI.

MENTARY ORGANS.

CLASSIFICATION, groups subordinate to groups-Natural system-Rules and difficul.

ties in classification, explained on the theory of descent with modification Classi.
fication of varieties-Descent always used in classification-Analogical or adaptivo
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 super.
vening at an early age, and being inherited at a corresponding age-RUDIMENTARY
ORGANS; their origin explained-Summary,

· · · · · 358

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,

· · · · · · · · · · 398

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Instruction to Binder. The Diagram to front page 138.

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 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

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