Comparative Evolution of Biomes 319 



REGIONAL EVOLUTION 



Because the factors affecting the evokition of communities are 

 extremely numerous and complex, it is reasonable to suppose that 

 the total of these factors would never produce the same results in 

 any two isolated segments of a biome. The truth of this postulate 

 becomes evident when parts of a biome on different continents are 

 compared with respect to relative increase and extinction of species. 



Extinction 



According to Chancy (1940), the holarctic and continuous tem- 

 perate deciduous forest had a fairly uniform taxonomic composi- 

 tion during middle Cenozoic time. The existing large isolated seg- 

 ments of this forest are now different from each other. In the west- 

 ern European segment many plant genera are no longer present, 

 whereas in both the Asiatic and the eastern North American seg- 

 ments most of the earlier genera have survived. This difference 

 reflects a differential regional extinction of certain ancestral ele- 

 ments. 



In the mammals this sort of regional extinction has been com- 

 monplace (Simpson, 1947). In many examples such as the horses, 

 the representatives in one region of the biome became extinct, 

 then colonization from the other region repopulated the first region 

 with close relatives of the extinct forms, and these colonists in 

 turn became extinct. 



Increase in Number of Species 



Differential evolution of this type is exemplified by certain insects 

 of the temperate deciduous forest. In the European and Asiatic 

 segments of this biome the leafhoppers comprising the genus Ery- 

 throneura have a known fauna of about 100 distinctive species, all 

 belonging to a closely knit series of species groups. Members of 

 two of the groups apparently dispersed into North America in 

 middle Cenozoic time, and these have since evolved into several 

 distinctive groups together containing over 400 species. Obviously 

 some special set of circumstances favoring an unusual increase in 

 the number of species (probably a host-transfer mechanism, p. 187) 

 occurred in the North American region and not in the other conti- 

 nents. Why this happened in only one region is not known, but 

 it does illustrate the point that many evolutionary developments 



