DISPERSAL DURING GLACIAL PERIOD 415 



asked how I account for this degree of uniformity in the 

 sub-arctic and temperate forms round the world, at the com- 

 mencement of the real Glacial period. At the present day, 

 the sub-arctic and northern temperate productions of the Old 

 and New Worlds are separated from each other by the whole 

 Atlantic Ocean and by the northern part of the Pacific. 

 During the Glacial period, when the inhabitants of the Old 

 and New Worlds lived farther southwards than they do at 

 present, they must have been still more completely separated 

 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 continents. 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 inhabitants 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 lati- 

 tude 60°. lived during the Pliocene period farther north 

 under the Polar Circle, in latitude 66°-6y* ; 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 Amer- 

 ica. And this continuity of the circumpolar land, with the 

 consequent freedom under a more favourable climate for 

 intermigration, 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 conti- 

 nents 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 southwards as the climate 

 became less warm, long before the commencement of the 

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



