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Show 6 CONTENTS. On the generality of intercrosses between individuals of the sam~ species-Circumstances 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-ActionofNatural Selection, through Divergence of Character and Extinction, on the descendants from a common parent-Explains the Grouping of all organic beings~ 77 CHAPTER V. L..A.WS OF V..A.RI..A.TION. Effects of external conditions-Use and disuse, combined with natural selection; organs of flight and of vision-Acclimatisation-Correlation of growth-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, . 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 alliesOrgans of extreme perfection-Means of transition-Cases of difficulty-Natura non facit saltum-Organs of small importanC'C-O~gans·rrot 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~ p~ides and ants-Instincts variable-Domestic instincts, their origin-Natural mstmcts 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. HYBRIDIS.M. Distinction betwe~n the sterility of first crosses and of hybrids-Sterility various in degree, not U~lVersal, affected by close interbreeding, removed by domesticationLa~ s govermng 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- CONTENTS. 7 ing-Fertility of varieties when crossed and of their mongrel offspring not universal- Hybrids and mongrels compared independently of their fertility-Summary, . 217 CHAPTER IX. ON THE IMPERFECTION OF TIIE 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 palreontological 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 tho 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, . 273 CHAPTER XI. 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 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. CLABSIFIOATION, groups subordinate to groups-Natural system-Rules and di:fD.cul· ties in classification, explained on the theory of descent with modification-Olassi· |