312 CHAPTER III 



not provide a similar old faunal element, spreading west across the Bering land 

 bridge? 



The most acceptable hypothesis seems to be that the old mountains of north- 

 eastern Asia, simply because they were situated farther north, the Hercynian ranges 

 reaching at least 60° north in the Jablonoj Mountains, and probably farther 

 northeast (Obrutschew, 1926, p. 445), and because they probably always had a 

 more continental climate, gave rise to a Tertiary fauna with a high amount of 

 hardy species^ which, more than any other contemporaneous fauna, were fit for 

 subsequent dispersal across two continents, and later to surv'^ive the critical glacial 

 periods. Thus they became circumpolar. 



The original faunas of Europe and North America— and it should be realized 

 that these regions were certainly never "empty" — were less adapted to arctic- 

 subarctic conditions, were perhaps largely inhabitants of mountain woods, and 

 therefore had no chance of becoming members of a cold-adapted Holarctic fauna. 

 It is possible, though, that the Ural Mountains once served as a centre similar to 

 that of northeastern Asia but, if so, the following Pleistocene glaciations swept 

 away the traces of their original fauna. 



^ Kusnezov (1935, p. 130) tries to explain this phenomenon by proposing a Miocene 

 glaciation of Alaska and NE Siberia. 



