Chap. XI. DURING TIIP: GLACIAL PEKIOD. 34I 



North America and Europe — a relationship whicli is highly 

 romarkai)lc, considering the distance of the two areas, and 

 tlieir separation hy tlie whole Atlantic Ocean. We can fur- 

 ther understand 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 tliese 

 warmer periods the northern parts of the Old and New Worlds 

 will have been almost conthiuously united by land, serving as 

 a bridge, since rendered impassable by cold, for the intermi- 

 gration of their inhabitants. 



During the slowly-decreasing warmth of the Pliocene pe- 

 riod, as soon as the species in common, which inliabitcd the 

 New and 0!d Worlds, migrated south of the Polar Circle, they 

 would be completely cut off 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 would become mingled in the one 

 great region with the native American productions, and would 

 have had to compete with them ; and, in the other great region, 

 with those of the Old World. Consequently we have here 

 every tiling favorable for much modification — for far more 

 modification than with the Alpine productions, left isolated, 

 within a much more recent period, on tlie several mountain- 

 ranges and on the arctic lands of Europe and North America. 

 Hence it has come that, when we compare the now living pro- 

 ductions of the temperate regions of the New and Old Worlds, 

 we find very few identical species (though Asa Gray has late- 

 ly shown that more plants are identical than was formerly sup- 

 j)Osed), 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 representative 

 forms which are ranked by all naturalists as specifically dis- 

 tinct. 



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 pcried, was nearly imiform along the 

 continuous shores of tlie Pohir Circle, will account, on tlie the- 

 ory of modification, for many closely-allied forms now living 

 in marine areas completely sundered. Thus, I tliink, we can 

 imderstand the ])resence of some still existing and of some ter- 

 tiary closely-allied fijrms on the eastern anti western shores of 

 temperate North America ; and the still more striking fact of 



