312 



Comparative Evolution of Biomes 



Fig. 136. The phylogenetic tree of the caddisfly genus Wonnaldia subgenus 

 Doloclanes superimposed on the known distribution of the species. The circle 

 in Japan represents W. kisoensis, that in eastern North America W . inohri. 



The sawfly genus Taxonus shows the same features but with 

 dispersal in the opposite direction. The genus apparently evohed 

 in the eastern deciduous forest of North America where sexeral 

 species now occur. A single species occurs in Europe, and it is a 

 member of one of the more highly specialized North American 

 branches of the genus. Kinsey (1930, 1936) postulated that a single 

 species of the gall wasp genus Cynips dispersed from western 

 North America to Eurasia and became the parent of the Palearctic 

 species of the genus. 



This parallel between present conditions and probable past oc- 

 currences, especially remarkable in examples concerning the Bering 

 bridge region, suggests that only slight ecological changes accom- 

 panied the formation of many connections between the continents, 

 and the bridges usually were of a narrowly restricted ecological 

 type. 



The correlation of several "one-per-genus" dispersals suggests 

 the importance of peripheral communities in biome dispersal. 



In his remarkable studies on termite distribution, Emerson (1952) 



