340 DISPERSAL Chaj-. XI. 



North America and Europe ; and it may be asked how I ac- 

 count for tliis degree of uniformity in the sub-arctic and temper- 

 ate forms round tlie world, at the commencement of the real 

 Glacial period. At the present day, the sub-arctic and northern 

 temperate productions of the Old and New Worlds are sepa- 

 rated from each other by the whole Atlantic Ocean and by the 

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

 the inhabitants of the Old and New Worlds lived farther south- 

 ward 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 have 

 entered the two continents then so widely separated. The ex- 

 planation, I believe, lies in the nature of the climate before the 

 commencement of the Glacial period. At this, the newer Plio- 

 cene 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 un- 

 der latitude 60°, lived during the Pliocene jieriod farther north 

 under the Polar Circle, in latitude CG°-G(° ; and that the pres- 

 ent arctic productions then lived on the broken land still nearer 

 to the pole. Now, if we look at a terrestrial globe, we see un- 

 der the Polar Circle that there is almost continuous land from 

 western Europe, through Siberia, to eastern America. And 

 this ccHitinuity of the circumpolar land, with the consequent 

 freedom imder a more favorable climate for intermigration, will 

 account for the supposed uniformity of the sub-arctic and tem- 

 perate 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 position, 

 though sul)jected to large but partial oscillations of level, I 

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

 that during some still earlier and still vrarmer 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 be- 

 came 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. ( )n this A-iew we can understand tlie relation- 

 shi]), ^^itll very little identity, l)etween the productions of 



