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CONTENTS OF VOL. II.
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 . . . . . . . Page 1
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 by 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 . . . . 48 W
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 . . Page 89
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 . . 129
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 . • - * * * * - - • . 171