On the generality of intercrosses between individuals of the same species-Cir cumstances 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, 77 CHAPTER V. 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 aro 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, CHAPTER VI. DIFFICULTIES ON THEORY. 120 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 alliesOrgans 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 graduatedAphides 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, CHAPTER VIII. HYBRIDISM. 185 Distinction between the sterility of first crosses and of hybrids-Sterility various in degree, not universal, affected by close interbreeding, removed by domesticationLaws 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 cross 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 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, 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 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, CHAPTER XI. GEOGRAPHICAL DISTRIBUTION. 273 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, 302 CHAPTER XII. 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, 334 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 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, 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, INDEX, 398 425 Instruction to Binler.-The Diagram to front page 108 ON THE ORIGIN OF SPECIES. INTRODUCTION. WHEN on board H. M. S. 'Beagle' as naturalist, I was My work is now nearly finished; but as it will take Charles Lyell, who sent it to the Linnean Society, and it This Abstract, which I now publish, must necessarily I much regret that want of space prevents my having In considering the Origin of Species, it is quite con- |