244 



Seeing, then, that faunae are not unfrequently mingled in con- 

 tiguous formations, that they do not hold the same precise geo- 

 logical level or vertical distribution at different localities, that they 

 may even become inverted in relation to each other, offering in 

 this and other cases evidence of emigration and remigration in 

 successive times, Prof. Rogers maintained that the precise and 

 absolute limitation of faunae to formations, as advocated by Prof. 

 Agassiz, if true in any case, is at best but of local and par- 

 tial application, and can not be the basis of a paleontological 

 arrangement of formations. Even according to the ordinary and 

 much less stringent view of geological faunte, it is found that 

 only the few great divisional lines of the geological column are 

 persistent over extensive areas, while the numerous subordinate 

 ones, however distinct at the typical locality, lose themselves as 

 they are traced, to give place to other modes of subdivision. 



As regards the comparison of the tertiary fossils with corre- 

 sponding modern forms. Prof. Rogers said that we had the author- 

 ity of Say, Conrad, Deshayes, and indeed paleontologists in gen- 

 eral, for the conclusion that a large number of the fossils found 

 in the tertiary deposits are identical with existing species. Even 

 supposing, however, that the fauna of the tertiary contains no 

 such identical forms, it will be admitted that the likeness becomes 

 extremely close as we approach the modern epoch, and this 

 would seem to lend support to Mr. Darwin's doctrine of modifi- 

 cation by natural selection. In regard to the discrimination of 

 species, the question at last must come to this : What is the limit 

 of specific difference ? who shall be the arbiter ? what the princi- 

 ple of distinction between species and variety, and what the guide 

 in drawing the lines of demarcation of the successive faunae ? 



Prof. Agassiz remarked that he did not expect the immediate 

 reception of his views, though convinced that they were true, but 

 believed that after mature examination of his facts they would be 

 generally received. He mentioned the fossils of the Jura as an 

 instance of the great change in the views on such subjects ; ac- 

 cording to Goldfuss, they are identical throughout the whole 

 series, but they are now admitted to be different. He did not 

 think that the increasing acknowledged differences point in any 

 way to a gradation of species or to a confusion of animal forms, 



