Chap. IX. GEOLOGICAL RECORD. 297 



no golden rule by which to distinguish species and va- 

 rieties ; they grant some little variability to each species, 

 but when they meet with a somewhat greater amount 

 of difference between any two forms, they rank both as 

 species, unless they are enabled to connect them to- 

 gether by close intermediate gradations. And this from 

 the reasons just assigned we can seldom hope to effect 

 in any one geological section. Supposing B and C to 

 be two species, and a third, A, to be found in an 

 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 

 most closely connected with either one or both forms by 

 intermediate varieties. Nor should it be forgotten, as 

 before explained, that A might be the actual progenitor 

 of B and C, and yet might not at all necessarily be 

 strictly intermediate between them in all points of struc- 

 ture. So that we might obtain the parent-species and 

 its several modified descendants from the lower and 

 upper beds of a formation, and unless we obtained 

 numerous transitional gradations, we should not recog- 

 nise their relationship, and should consequently be com- 

 pelled to rank them all 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 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 different, 



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