THE ORIGIN' OF SPECIES. 49 



and between that period and the present; also that 

 the later gradations are finer, so as to leave it doubt- 

 ful whether the succession is one of species — believed 

 on the one theory to be independent, on the other, 

 derivative — or of varieties, which are confessedly deriv- 

 ative. The proof of the finer gradation appears to 

 be forthcoming. Des Hayes and Lyell have concluded 

 that many of the middle Tertiary and a large pro- 

 portion of the later Tertiary mollusca are specifically 

 identical with living species; and this is still the 

 almost universally prevalent view. But Air. Agassiz 

 states that, " in every instance wdiere he had sufficient 

 materials, he had found that the species of the two 

 epochs supposed to be identical by Des Hayes and 

 Lyell were in reality distinct, although closely allied 

 species." ' Moreover, he is now satisfied, as we under- 

 stand, that the same gradation is traceable not merely 

 in each great division of the Tertiary, but in particular 

 deposits or successive beds, each answering to a great 

 number of years ; where what have passed unques- 

 tioned as members of one species, upon closer examina- 

 tion of numerous specimens exhibit differences which 

 in his opinion entitle them to be distinguished into 

 two, three, or more species. It is plain, therefore, that 

 whatever conclusions can be fairly drawn from the 

 present animal and vegetable kingdoms in favor of a 

 gradation of varieties into species, or into what may 

 be regarded as such, the same may be extended to the 

 Tertiary period. In both cases, what some call species 

 others call varieties ; and in the later Tertiary shells 



1 " Proceedings of the American Academy of Arts and Sciences," 

 vol. iv., p. 178. 



