IMPERFECTION OF GEOLOGICAL RECORD 267 



nise their relationship, and should consequently be 

 compelled to rank them all as distinct species. 



It is notorious on what excessively slight differenc&<? 

 many palaeontologists have founded their species ; and 

 they do this the more readily if the specimens come 

 from different sub-stages of the same formation. Some 

 experienced conchologists are now sinking many of the 

 very fine species of D'Orbigny and others into the rank 

 of varieties ; and on this view we do find the kind of 

 evidence of change which on my theory we ought to 

 find. Moreover, if we look to rather wider intervals, 

 namely, to distinct but consecutive stages of the same 

 great formation, we find that the embedded fossils, 

 though almost universally ranked as specifically dif- 

 ferent, yet are far more closely allied to each other 

 than are the species found in more widely separated 

 formations ; but to this subject I shall have to return 

 in the following chapter. 



One other consideration is worth notice : with animals 

 and plants that can propagate rapidly and are not 

 highly locomotive, there is reason to suspect, as we 

 have formerly seen, that their varieties are generally at 

 first local ; and that such local varieties do not spread 

 widely and supplant their parent-forms until they have 

 been modified and perfected in some considerable 

 degree. According to this view, the chance of discover- 

 ing in a formation in any one country all the early stages 

 of transition between any two forms, is small, for the 

 successive changes are supposed to have been local or 

 confined to some one spot. Most marine animals have 

 a wide range ; and we have seen that with plants it is 

 those which have the widest range, that oftenest present 

 varieties ; so that with shells and other marine animals, 

 it is probably those which have had the widest range, 

 far exceeding the limits of the known geological forma- 

 tions of Europe, which have oftenest given rise, first 

 to local varieties and ultimately to new species ; and 

 this again would greatly lessen the chance of our being 

 able to trace the stages of transition in any one 

 geological formation. 



