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CONTENTS OF VOL. II. 



CHAPTER rX- 



Hybridism. 



Distinction between the sterility of first crosses and of hybri'ls — 

 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 

 hfe 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 



CHAPTER X. 



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 lajise of time as estimated by years 

 — On the poorness of our paL-eoutological 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 fossiUferous 

 strata — Antiquity of the habitable earth 48 



