Chap. X.] GEOLOGICAL KECOED. 49 



that intermediate varieties, from existing in lesser 

 numbers than the forms which they connect, will 

 generally be beaten out and exterminated during the 

 course of 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 selection, through 

 which new varieties continually take the places of and 

 supplant their parent-forms. But just in proportion as 

 this process of extermination has acted on an enormous 

 scale, so must the number of intermediate varieties, 

 which have formerly existed, be truly enormous. Why 

 then is not every geological formation and every stratum 

 full of such intermediate links ? Geology assuredly does 

 not reveal any such finely-graduated organic chain ; and 

 this, perhaps, is the most obvious and serious objection 

 which can be urged against the theory. The explanation 

 lies, as I believe, in the extreme imperfection of the 

 geological record. 



In the first place, it should always be borne in mind 

 what sort of intermediate forms must, on the theory, 

 have formerly existed. I have found it difficult, when 

 looking at any two species, to avoid picturing to 

 myself forms directly intermediate between them. But 

 this is a wholly false view ; we should always look for 

 forms intermediate between each species and a common 

 but unknown progenitor ; and the progenitor will gen- 

 erally have differed in some respects from all its 

 modified descendants. To give a simple illustration: 

 the fantail and pouter pigeons are both descended from 

 the rock-pigeon ; if we possessed all the intermediate 

 varieties which have ever existed, we should have an 

 extremely close series between both and the rock- 



