Comparative Evolution of Biomes 305 



have occurred only when tropical climates extended north to lati- 

 tudes of the Bering bridge. Estimates concerning climates at the 

 Bering bridge indicate that the latest tropical conditions occurred 

 during Upper Cretaceous time (Emerson, 1952) and the latest 

 subtropical conditions during Eocene time (Durham, 1950). Since 

 then climates in that area have become gradually cooler, and the 

 climate during the latest probable dates of bridging in the Pleisto- 

 cene probably ranged between temperate and subarctic. If all 

 these premises were true, we would expect the similarity between 

 existing Eurasian and North American species to be greatest to 

 the north and to decrease towards the south. In his analysis of the 

 insects of the order Collembola, Mills (1939) pointed out that 

 this proportionality does actually exist. 



The north and south movement of temperature bands coupled 

 with intercontinental connections at only the polar end of the 

 continental systems in the northern hemisphere explains the move- 

 ments of many tropical groups. Three other dynamic sets of fac- 

 tors produce climatic zonation which may be at right angles to 

 these temperature rings and hence cause drastic deviations from 

 a simple annular pattern of dispersal. These three sets of factors 

 are rainfall patterns, mountain systems, and ocean currents. 



RAINFALL PATTERNS 



The relatively high relief of existing mountain systems and other 

 factors influencing world wind directions together have resulted 

 in great inequality of rainfall on different parts of the earth. This 

 rainfall pattern ( Fig. 132 ) , does not coincide with the temperature 

 bands, more often than not forming arid and humid bands run- 

 ning generally from north to south. Because organisms are usually 

 adapted to live under only a narrow set of rainfall conditions, these 

 rainfall bands act as rigid barriers to the free movement of or- 

 ganisms. These bands may therefore be just as important a factor 

 as temperature banding in limiting opportunities for interconti- 

 nental dispersal. 



MOUNTAIN SYSTEMS 



Mountain systems have a unique effect in relation to interconti- 

 nental connections in that they may negate the restricting effects 

 of other factors. Mountain systems trending north and south have 

 cooler climates at higher elevations, and toward the equator afford 

 avenues of dispersal for cool-adapted forms across regions which 

 at lower levels would be too warm for the species to exist. Cool- 

 adapted terrestrial organisms have seldom dispersed across the 



