conditions--Use and disuse, combined with natural selection; organs of flight and of vision--Acclimatisation--Correlated variation--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. CHAPTER VI. DIFFICULTIES OF THE THEORY. Difficulties of the theory of descent with modification--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--Modes 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. CHAPTER VII. MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION. Longevity--Modifications not necessarily simultaneous--Modifications apparently of no direct service--Progressive development--Characters of 7 small functional importance, the most constant--Supposed incompetence of natural selection to account for the incipient stages of useful structures--Causes which interfere with the acquisition through natural selection of useful structures--Gradations of structure with changed functions--Widely different organs in members of the same class, developed from one and the same source--Reasons for disbelieving in great and abrupt modifications. CHAPTER VIII. 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, molothrus, ostrich, and parasitic bees--Slave-making ants--Hivebee, its cell-making instinct--Changes of instinct and structure not necessarily simultaneous--Difficulties on the theory of the Natural Selection of instincts--Neuter or sterile insects--Summary. CHAPTER IX. 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, not accumulated by natural selection--Causes of the sterility of first crosses and of hybrids--Parallelism between the effects of changed conditions of life and of crossing--Dimorphism and Trimorphism--Fertility of varieties when crossed and of their mongrel offspring not universal--Hybrids and mongrels compared independently of their fertility--Summary. CHAPTER X. 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 lapse of time, as inferred from the rate of denudation and of deposition--On the lapse of time as estimated in years--On the poorness of our palaeontological collections--On the intermittence of geological formations--On the denudation of granitic areas--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--Antiquity of the habitable earth. CHAPTER XI. 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 chapter. CHAPTER XII. 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--Alternate Glacial periods in the north and south. CHAPTER XIII. 8 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 chapter. CHAPTER XIV. 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. CHAPTER XV. RECAPITULATION AND CONCLUSION. Recapitulation of the objections to 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. GLOSSARY OF SCIENTIFIC TERMS. INDEX. 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 organic beings inhabiting South America, and in the geological relations of the present to the past inhabitants of that continent. These facts, as will be seen in the latter chapters of this volume, seemed 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 (1859) nearly finished; but as it will take me many 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. In 1858 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. 9 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 may 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 is here impossible. 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 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
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