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 un- 

 less we obtained numerous transitional gradations, 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 natu- 

 ralists have been misled by their imaginations, and 

 that these late tertiary species really present no dif- 

 ference whatever from their living representatives, or 

 unless we admit, in opposition to the judgment of most 

 naturalists, that these tertiary species are all truly dis- 

 tinct 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 fossils, though 

 universally ranked as specifically different, yet are far 

 more closely related to each other than are the species 



