CHAP. X.] IMPERFECTION OF GEOLOGICAL RECORD. 247 



of fertility, in all other respects there is the closest general 

 resemblance between hybrids and mongrels, in their variability, 

 in their power of absorbing each other by repeated crosses, and in 

 their inheritance of characters from both parent-forms. Finally, 

 then, although we are as ignorant of the precise cause of the 

 sterility of first crosses and of hybrids as we are why animals and 

 plants removed from their natural conditions become sterile, yet 

 the facts given in this chapter do not seem to me opposed to the 

 belief that species aboriginally existed as varieties. 



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 palaoontological col- 

 lections On the intermittence of geological formations On the denuda- 

 tion 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. 



IN the sixth chapter I enumerated the chief objections which 

 might be justly urged against the views maintained in this volume. 

 Most of them have now been discussed. One, namely the distinct- 

 ness of specific forms, and their not being blended together by 

 innumerable transitional links, is a very obvious difficulty. I 

 assigned reasons why such links do not commonly occur at the 

 present day under the circumstances apparently most favourable 

 for their presence, namely on an extensive and continuous area 

 with graduated physical conditions. I endeavoured to show, that 

 the life of each species depends in a more important manner on 

 the presence of other already defined organic forms, than oa 

 climate, and, therefore, that the really governing conditions of life 

 do not graduate away quite insensibly like heat or moisture. I 

 endeavoured, also, to show that intermediate varieties, from exist- 

 ing in lesser numbers than the forms which they connect, will 

 generally be beaten out and exterminated during the course ot 

 further modification and improvement. The main cause, however, 

 of innumerable intermediate links not now occurring everywhere 

 throughout nature, depends on the very process of natural selec- 

 tion, through which new varieties continually take the places of 

 and supplant their parent-forms. But just in proportion as this 



