H THE DEFTHS OF THE SEA. [chap. i. 



Eastern Fa-jna. Western Fauna. 



Hellita hexapora, A. Ag. Mellita pacifica, Vek. 



Encope michelini, Ag. Encope grandis, Ag. 



„ eiaarginata, Ag. „ micropora, Ag. 



Rhyncholampas caribbcearum, Ehynclwlampas pacificus, A. Ag. 



A. Ag. 



Brissus columbarls, Ag. Brissus obesus, Ver. 



jYeoma ventricosa, LiJTK. Meoma grandis, Gray. 



Plagionotics pieetoralis, Ag. Plagionotus nobilis, A. Ag. 



Agassizia excentrica, A. Ag. Agassizia scrobiculata, Val. 



ifem atropos, Mich. J/<»m cZofAo, Mich. 



Supposing species to be constant, this singular 

 chain of resemblances would indicate simply the 

 special creation on the two sides of the Isthmus of 

 two groups of species closely resembling one another, 

 because the circumstances under which they were 

 placed were so very similar ; but admitting ' descent 

 with modification,' while gladly availing ourselves 

 of the convenient term ' representation,' we at once 

 come to the conclusion that these nearly allied ' re- 

 presentative species ' must have descended from a com- 

 mon stock, and we look for the cause of their diver- 

 gence. Now on examining the Isthmus of Panama we 

 find that a portion of it consists of cretaceous beds 

 containing fossils un distinguishable from fossils from 

 the cretaceous beds of Europe ; the Isthmus must 

 therefore have been raised into dry land in tertiary or 

 post-tertiary times. It is difiicult to doubt that the 

 rising of this natural l^arrier isolated two portions 

 of a shallow-water fauna which have since slightly 

 diverged under slightly different conditions. I quote 

 Alexander Agassiz : — *' The question naturally arises, 

 have we not in the different Faunae on both sides of 

 the Isthmus a standard by which to measure the 



