312 IMPEBPEGTION OF THE 



CHAPTBE X. 



OTS THE IMPEEFECTION OF THE GEOLOGICAL BECOED. 



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 palseontological 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 knovpn 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 distinctness 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 favorable for their pres- 

 ence, namely, on an extensive and continuous area with 

 graduated physical conditions. I endeavored to show, that 

 the life of each species depends in a more important manner 

 on the presence of other already defined organic forms, than 

 on climate, and, therefore, that the really governing condi- 

 tions of life do not graduate away quite insensibly like 

 heat or moisture. I endeavored, also, to show that inter- 

 mediate varieties, from existing in lesser numbers than 

 the forms which they connect, will generally be beaten out 

 and exterminated during the course of further modifica- 

 tion and improvement. The main cause, however, of 

 innumerable intermediate links not now occurring every- 

 where throughout nature, depends on the very process of 

 natural selection, through which new varieties continually 

 take the places of and supplant their parent-forms. But 



