THE GLACIAL PERIOD. 393 



the commencement of the real Glacial period. At the 

 present day, the sub-arctic and northern temperate pro- 

 ductions of the Old and New Worlds are separated from 

 each other by the whole Atlantic Ocean and by the north- 

 ern part of the Pacific. During the Glacial period, when 

 the inhabitants of the Old and New Worlds lived further 

 southward then they do at present, they must have been 

 still more ^mpletely sepai-ated from each other by wider 

 spaces of ocean; so that it may well be asked how the same 

 species could then or previously have entered the two con- 

 tinents. The explanation, I believe, lies in the nature of 

 the climate before the commencement of the Glacial period. 

 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 tlie 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 

 Oircle, 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 under 

 the Polar Circle that there is almost continuous land from 

 western Europe through 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 

 position, 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 cir- 

 cumpolar land; and that these plants and animals, both in 

 the Old and New Worlds, begun slowly to migrate south- 

 ward 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 understand the relationship with very 



