332 ON THE ORIGIN OF SPECIES 



But the foregoing- remarks on distribution apply not 

 only to strictly arctic forms, but also to many sub- 

 arctic and to some few northern temperate forms, for 

 some of these are the same on the lower mountains 

 and on the plains of North America and Europe ; and 

 it may be reasonably asked how I account for the 

 necessary degree of uniformity of the sub-arctic and 

 northern temperate forms round the world, at the 

 commencement of the Glacial period. At the present 

 day, the sub-arctic and northern temperate produc- 

 tions of the Old and New Worlds are separated from 

 each other by the Atlantic Ocean and by the extreme 

 northern part of the Pacific. During the Glacial 

 period, when the inhabitants of the Old and New 

 Worlds lived further southwards than at present, they 

 must have been still more completely separated by 

 wider spaces of ocean. I believe the above difficulty 

 may be surmounted by looking to still earlier changes 

 of climate of an opposite nature. We have good reason 

 to believe that during the newer Pliocene period, be- 

 fore the Glacial epoch, and whilst the majority of the 

 inhabitants of the world were specifically the same 

 as now, the climate was warmer than at the present 

 day. Hence we may suppose that the organisms now 

 living under the climate of latitude 60°, during the 

 Pliocene period lived further north under the Polar 

 Circle, in latitude 66°-67° ; and that the strictly arctic 

 productions then lived on the broken land still nearer 

 to the pole. Now if we look at a globe, we shall see 

 that under the Polar Circle there is almost continuous 

 land from western Europe, through Siberia, to eastern 

 America. And to this continuity of the circumpolar 

 land, and to the consequent freedom for intermigra- 

 tion under a more favourable climate, I attribute the 

 necessary amount of uniformity in the sub-arctic and 

 northern temperate productions of the Old and New 

 Worlds, at a period anterior to the Glacial epoch. 



Believing, from reasons before alluded to, that our 

 continents have long remained in nearly the same 

 relative position, though subjected to large, but partial 



