IN AN T SINGLE FORMA TION. 339 



intermediate gradations; and this, from the reasons just 

 assigned, we can seldom hope to effect in any one geolog- 

 ical section. Supposing B and C to be two species, and a 

 third. A, to be found in an older and underlying bed; even 

 if A. were strictly intermediate between B and C, it would 

 simply be ranked as a" third and distinct species, unless at 

 the same time it could be closely connected by -intermedi- 

 ate varieties with either one or both forms. ISTor should it 

 be forgotten, as 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 the 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-rela- 

 tionship, and should consequently rank them as distinct 

 species. 



It is notorious on what excessively slight difEerences 

 many palaeontologists have founded their species; and they 

 do this the more readily if the specimens come from diiier- 

 ent 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 naturalists to be identical with existing 

 species; but some excellent naturalists, as Agassiz and 

 Pictet, maintain that all these tertiary species are specific- 

 ally 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 

 tliat 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 consecu- 

 tive stages of the same great formation, we find that the 

 embedded fossils, though universally ranked as specific- 

 ally different, yet are far more closely related to each other 



