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--Causes of the sterility of first crosses and of hybrids--Parallelism between the effects of changed conditions of life and crossing--Fertility of varieties when crossed and of their mongrel offspring not universal--Hybrids and mongrels compared independently of their fertility--Summary 245-278 {viii} 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 279-311 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 312-345 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 346-382 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 383-410 CHAPTER XIII. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY ORGANS.