IMPERFECTION OF OEOLOGIQAL RECORD 77 



before explained, that A might be the actual progenitor 

 of B and C, and yet would not necessarily be strictly 

 intermediate between them in all respects. So that we 

 might obtain tlie parent-species and its several modified 

 descendants from the lower and upper beds of the same 

 formation, and unless we obtained numerous transi- 

 tional gradations, we should not recognize their blood- 

 relationship, and should consequently rank them as 

 distinct species. 



It is notorious on what excessively slight differences 

 many paleontologists 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 the theory we ought to 

 find. Look again at the later tertiary deposits, which 

 include many shells believed by the majority of natural- 

 ists to be identical with existing species; but some excel- 

 lent naturalists, as Agassiz and Pictet, maintain that all 

 these tertiary species are specifically distinct, though the 

 distinction is admitted to be very slight; so that here, 

 unless we believe that these eminent naturalists have 

 been misled by their imaginations, and that these late 

 tertiary species really present no difference whatever from 

 their living representatives, or unless we admit, in oppo- 

 sition to the Judgment of most naturalists, that these 

 tertiary species are all truly distinct from the recent, we 

 have evidence of the frequent occurrence of slight modifi- 

 cations of the kind required. If we look to rather wider 

 intervals of time, namely, to distinct but consecutive 



