164 THE ORIOII^ OF SPECIES 



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 latitude 60°, lived 

 during the Pliocene period further north under the Polar 

 Circle, in latitude 66°-67°; 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 contin- 

 uous land from western Europe, through Siberia, to east- 

 ern America. And this continuity of the circumpolar 

 land, with the consequent freedom under a more favor- 

 able climate for intermigration, will account for the sup 

 posed uniformity of the sub-arctic and temperate produc- 

 tions 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 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 num- 

 ber 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 



I 



