394 msPBRSAL DXTRma 



little identity, between the productions of ISTorth America 

 and Europe — a relationship which is highly remarkable, 

 considering the distance of the two areas, and their sepa- 

 ration by the whole Atlantic Ocean. "We can further under- 

 stand the singular fact remarked on by several observers 

 that the productions of Europe and America during, the 

 later tertiary stages were more closely related to each other 

 than they are at the present time; for during t^ese warmer 

 periods the northern jjarts of the Old and New Worlds will 

 have been almost continuously united by land, serving as a 

 bridge, since rendered impassible by cold, for iutermigra- 

 tion of their inhabitants. 



During the slowly decreasing warmth of the Pliocene 

 period, as soon as the species in common, which inhabited 

 the New and Old Worlds, migrated south of the Polar Circle, 

 they will have been completely cut ofE from each other. 

 This separation, as far as the more temperate productions 

 are concerned, must have taken place long ages ago. As 

 the plants and animals migrated southward, they will 

 have become mingled in the one great region with the 

 native American productions, and would have had to com- 

 pete with them; and in the other great region, with those 

 of the Old World. Consequently we have here everything 

 favorable for much modification — for far more modifica- 

 tion than with the Alpine productions, left isolated, within 

 a much more recent period, on the several mountain ranges 

 and on the arctic lands of Europe and North America. 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 for- 

 merly 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 south- 

 • ern 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 completely sundered. Thus, I 



