IMPERFECTION OF GEOLOGICAL RECORD 251 



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 

 exterminate 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 on the earth, 

 be truly enormous. Why then is not every geo- 

 logical formation and every stratum full of such inter- 

 mediate links ? Geology assuredly does not reveal any 

 such finely graduated organic chain ; and this, perhaps, 

 is the most obvious and gravest objection which can be 

 urged against my 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 my 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 generally 

 have differed in some respects from all its modified 

 descendants. To give a simple illustration : the fantail 

 and pouter pigeons have 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-pigeon ; but we 

 should have no varieties directly intermediate between 

 the fantail and pouter ; none, for instance, combining 

 a tail somewhat expanded with a crop somewhat en- 

 larged, the characteristic features of these two breeds. 

 These two breeds, moreover, have become so much 

 modified, that if we had no historical or indirect 

 evidence regarding their origin, it would not have been 

 possible to have determined from a mere comparison of 

 their structure with that of the rock-pigeon, whether 



