294 





CHAPTER III 



^^,^^ 



:^5;i. 













FIG. 52. Pleistocene glaciation of North America regardless of time. 



(Reproduced by permission from "Textbook of Geology", Part i, by Longwell, Knopf, & 



Flint, published by John Wiley & Sons.) 



tion of arctic-subarctic species are mainly hypothetical but are mentioned as a 

 possible incentive for further research. 



(e) Arctic animals are more "hardy" than others, especially as regards low 

 temperature. They may therefore be more liable to survive passive transport, 

 above all as "aerial plankton" at high altitude. There would be no difficulty in 

 testing this experimentally. 



(f) Arctic (and subarctic) animals may generally be older as species and thus 

 have had more time at their disposal for dispersal. This may be a direct conse- 

 quence of climate (Van Dyke, 1939, p. 258), either because the rate of mutation 

 decreases with temperature, as shown in Drosophila (Timofeeff-Ressovsky, 1947, 

 p. 243), or because the number of generations per unit of time is smaller. The 

 latter suggestion is more than a hypothesis: it is a common experience among 



