Chap. X.] IN ANY SINGLE FORMATION. 71 



them in all respects. So that we might obtain the 

 parent-species and its several modified descendants 

 from the lower and upper beds of the same formation, 

 and unless we obtained numerous transitional grada- 

 tions, we should not recognise their blood-relationship, 

 and should consequently rank them as distinct species. 

 It is notorious on what excessively slight differences 

 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 the theory we ought to 

 find. Look again at the later tertiary deposits, which 

 include many shells believed by the majority of natu- 

 ralists to be identical with existing species ; but some 

 excellent 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 opposition 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 modifications of the kind 

 required. If we look to rather wider intervals of time, 

 namely, to distinct but consecutive stages of the same 

 great formation, we find that the embedded fossUs, 

 though universally ranked as specifically different, yet 

 are far more closely related to each other than are the 



