CONTENTS 



CHAPTER IX 



HYBRIDISM 



'Distinction between the sterility of first crosses and of hybrids — Sterility 

 various in degree, not universal, affected by close interbreeding, re- 

 moved 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 con- 

 ditions of life and of crossing — Dimorphism and trimorphism — Fertility 

 of varieties when crossed and of their mongrel offspring not universal — 

 Hybrids and mongrels compared mdependenlly of their fertility — 

 Summary I 



CHAPTER X ' 



ON THE IMPERFECTION OF THE GEOLOGICAL RECORD 



On the absence of mtermediate 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 paleontolog- 

 ical 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 tlie lowest known fossiliferous strata — 

 Antiquity of the habitable earth .... ... 54 



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