CONTENTS 



Volume II 



PAGX 



CHAPTER IX 



HYBRIDISM 



Distinction between the sterility of first crosses and of hybrids — 

 Sterility various in degree, not universal, affected by close inter- 

 breeding, 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 steriUty of first crosses and of hybrids — Parallelism between 

 the effects of changed conditions of life and of crossing — Dimor- 

 phism and Trimorphism — Fertility of varieties when crossed and 

 of their mongrel offspring not universal — Hybrids and mongrels 

 compared independently of their fertility — Summary .... 239 



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 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 fossiUferous strata — ^Antiquity of 

 the habitable earth 269 



CHAPTER XI 



ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 



On the slow and successive appearance of new species — On their dif- 

 ferent 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 simulta- 

 neous changes in the forms of life throughout the world — On the 



