The relationship between the palaearctic and nearctic faunas 



3" 



DiAGR. II. Three main elements of the Pliocene-Lower Pleistocene flora of SW 

 Polonia. 



Black dots = the American element. Open dots = the East Asia element. 



Crosses = the Eurasian element 

 Elements of a generally Holarctic or more southern character omitted. 

 (After Szafer, 1954.) 



types of organisation. And the species arisen in these mountains had comparatively 

 too short time to spread. 



Older mountains, of Palaeozoic (Carboniferous-Permian) age, the Hercynians 

 of Europe, the Appalachians, &c., have been more important centres of evolution. 

 Apparently it was the fauna of such mountains, in northeastern Asia, that provided 

 the main source of circumpolar animals. Considering that the Bering Strait during 

 the entire Tertiary and Quaternary periods has formed only a temporary obstacle 

 to dispersal, the fauna (and flora) of northeastern Asia was in the best imaginable 

 position, in the centre of the Holarctic region, for sending branches in both direc- 

 tions, towards Europe and towards North America. 



But this is not sufficient explanation. Why, for instance, did the Appalachians 



