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.
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.
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.
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.
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.
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.
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.