THE GLACIAL PERIOD. 369 



At this, the newer Pliocene period, the majority of the in- 

 habitants of the world were specifically the same as now, and 

 we have good reason to believe that the climate was warmer 

 than at the present day. Hence we may suppose that the 

 organisms which now live under latitude 60 degrees, lived 

 during the Pliocene period further north, under the Polar 

 Circle, in latitude 66-67 degrees ; and that the present arctic 

 productions then lived on the broken land still nearer to the 

 pole. Now, if we look at a terrestrial globe, we see undei 

 the Polar Circle that there is almost continuous land from 

 Western Europe througn Siberia, to Eastern America. And 

 this continuity of the circumpolar land, with the conse- 

 quent freedom under a more favorable climate for inter- 

 migration, will account for the supposed uniformity of the 

 sub-arctic and temperate productions of the Old and New 

 Worlds, at a period anterior to the Glacial epoch. 



Believing, from reasons before alluded to, that our con- 

 tinents have long remained in nearly the same relative posi- 

 tion, though subjected to great oscillations of level, I am 

 strongly inclined to extend the above view, and to infer that 

 during some still earlier and still warmer period, such as the 

 older Pliocene period, a large number of the same plants and 

 animals inhabited the almost continuous circumpolar land ; 

 and that these plants and animals, both in the Old and New 

 Worlds, began slowly to migrate southward as the climate 

 became less warm, long before the commencement of the 

 Glacial period. We now see, as I believe, their descendants 

 mostly in a modified condition, in the central parts of 

 Europe and the United States. On this view we can under- 

 stand the relationship, with very little identity, between the 

 productions of North America and Europe, — a relationship 

 which is highly remarkable, considering the distance of the 

 two areas, and their separation by the whole Atlantic Ocean. 

 We can further understand the singular fact remarked on by 

 several observers, that the productions of Europe and Amer 

 ica during the later tertiary stages were more closely relatec ( 

 to each other than they are at the present time ; for during 

 these warmer periods the northern parts of the Old and New 

 Worlds will have been almost continuously united by land, 

 serving as a bridge, since rendered impassable by cold, for 

 inter-migration 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 Circlei 



