166 THE ORIGIN OF SPECIES 



Worlds, we find very few identical species (though Asa 

 Gray has lately shown that more plants are identical 

 than was formerly supposed), but we find in every great 

 class many forms, which some naturalists rank as geo- 

 graphical races, and others as distinct species; and a host 

 of closely allied or representative forms which are ranked 

 by all naturalists as specifically distinct. 



As on the land, so in the waters of the sea, a slow 

 southern migration of a marine fauna, which, during the 

 Pliocene or even a somewhat earlier period, was nearly 

 uniform along the continuous shores of the Polar Circle, 

 will account, on the theory of modification, for many 

 closely allied forms now living in marine areas com- 

 pletely sundered. Thus, I think, we can understand the 

 presence of some closely allied, still existing and extinct 

 tertiary forms, on the eastern and western shores of tem- 

 perate North America; and the still more striking fact of 

 many closely allied crustaceans (as described in Dana's 

 admirable work), some fish and other marine animals, 

 inhabiting the Mediterranean and the seas of Japan — these 

 two areas being now completely separated by the breadth 

 of a whole continent and by wide spaces of ocean. 



These cases of close relationship in species either now 

 or formerly inhabiting the seas on the eastern and west- 

 ern shores of North America, the Mediterranean and 

 Japan, and the temperate lands of North America and 

 Europe, are inexplicable on the theory of creation. We 

 cannot maintain that such species have been created 

 alike, in correspondence with the nearly similar phys- 

 ical conditions of the areas; for if we compare, for in- 

 stance, certain parts of South America with parts of 

 South Africa or Australia, we see countries . closely 



