14 THE DEPTHS OF THE SEA. [CHAP. I. 
Eastern Fauna. WESTERN Fauna. 
Mellita hecapora, A. AG. Mellita pacifica, Vmr. 
Encope michelini, AG. Encope grandis, Ac. 
- emarginata, AG. »  micropora, AG. 
Rhyncholampas caribbearum, Rhynchclampas pacificus, A. Aa. 
A, Aa. 
Brissus columbaris, Aa. Brissus obesus, VER. 
Meoma ventricosa, LUTK. Meoma grandis, GRAY. 
Plagionotus pectoralis, Ac. Plagionotus nobilis, A. Ac. 
Agassizia excentrica, A. AG. Agassizia scrobiculata, V Au. 
Mera atropos, Micu. Mera clotho, Micu. 
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 undistinguishable 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’ difficult to doubt that the 
rising of this natural barrier 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 Fauneze on both sides of 
the Isthmus a standard by which to measure the 
