■7 Phylogeny and dispersal of the mosquito genus Culex (Ross 1953). 



developed in tropical regions : all are definitely handi- 

 capped in dispersing into temperate or cold regions by 

 the climatic barriers. Fresh-water fishes have evolved 

 a richer north temperate and arctic fauna than am- 

 phibians and reptiles, since their aquatic habitat pro- 

 tects them better against extreme cold than does the 

 terrestrial or semi-terrestrial habitat of the other two 

 groups. Amphibians extend farther north than do 

 reptiles. Evaluating all types of evidence, Darlington 

 (1957) concludes that these three groups of cold- 

 blooded vertebrates, and probably also the warm- 

 blooded groups, reached their greatest taxonomic di- 

 versification during the Cretaceous and early Tertiary, 

 and not in temperate Eurasia but in the Old World 

 tropics, especially in the Orient. Uniform, warm, 

 humid climates and a great variety of available niches, 

 along with many possibilities for geographic isolation, 

 appear to have induced evolution in these groups, 

 stimuli quite different than those suggested by Mat- 



thew for the warm-blooded mammals. From the Old 

 World tropics, dispersal proceeded into Africa, into 

 Eurasia, across the Bering land bridge into North 

 America, and finally across the Panama land bridge 

 into South America. Subsequent evolutionary radia- 

 tion of new forms occurred in each continent. 



A northern route between Asia and North Amer- 

 ica by way of the Bering land bridge as a route of 

 dispersal for tropical species presents problems in re- 

 spect to climate. We know, however, that during the 

 Cretaceous and early Tertiary the climate in these 

 northern regions was much warmer than it is now. 

 We may suppose that the bridge shut off the cold 

 Arctic Sea from the Pacific Ocean, and that the south- 

 ern shores of the land bridge were washed by the 

 warm Japanese current. This would have made it 

 possible for warm temperate species to use the bridge, 

 but probably not tropical species, unless we suppose 

 that the tolerance to cold of the ancestral stock of our 



Dispersal, migration, and ecesis 153 



