372 
GEOGRAPHICAL DISTRIBUTION, 
Chap. XI. 
the now living productions 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 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 admir¬ 
able 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 compare, for 
instance, certain parts of South America with the south¬ 
ern continents of the Old World, we see countries closely 
corresponding in all their physical conditions, but with 
their inhabitants utterly dissimilar. 
