334 ON THE ORIGIN OF SPECIES 



the arctic lands of the two Worlds. Hence it has 

 come, that when we compare the now living produc- 

 tions of the temperate regions of the New and Old 

 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 geographical 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 modifica- 

 tion, for many closely allied forms now living in areas 

 completely sundered. Thus, I think, we can under- 

 stand the presence of many existing and tertiary repre- 

 sentative forms on the eastern and western shores of 

 temperate North America ; and the still more striking 

 case of many closely allied crustaceans (as described in 

 Dana's admirable work), of some fish and other marine 

 animals, in the Mediterranean and in the seas of Japan, 

 — areas now separated by a continent and by nearly a 

 hemisphere of equatorial ocean. 



These cases of relationship, without identity, of the 

 inhabitants of seas now disjoined, and likewise of the 

 past and present inhabitants of the temperate lands of 

 North America and Europe, are inexplicable on the 

 theory of creation. We cannot say that they have 

 been created alike, in correspondence with the nearly 

 similar physical conditions of the areas ; for if we com- 

 pare, for instance, certain parts of South America with 

 the southern continents of the Old World, we see 

 countries closely corresponding in all their physical 

 conditions, but with their inhabitants utterly dissimilar. 



But we must return to our more immediate subject, 

 the Glacial period. I am convinced that Forbes' s view 

 may be largely extended. In Europe we have the 



