372 GEOGEAPHICAL DISTRIBUTION, Chap. XI. 



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 dis- 

 tinct species ; and a host of closely allied or represen- 

 tative 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 areas completely 

 sundered. Thus, I think, we can understand the pre- 

 sence of many existing and tertiary representative 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 ad- 

 mirable 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 hemi- 

 sphere 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 



