Comparative Evolution of Biomes 315 



their closest relatives in Europe and Asia (Ross, 1929; Benson, 

 1952). 



The grass genera Danthonia and Aristida and the genus Dolerus 

 are subclimax organisms, and illustrate the wide geographic range 

 commonly found in the components of subclimax communities. 

 The distribution of these organisms is sufficiently extensive to high- 

 light an interesting aspect of biome evolution, namely, that species 

 of the subclimax communities have had a greater amount of inter- 

 continental mixing than have species restricted to the climax com- 

 munities. Among invertebrates inhabiting temperate climates, two 

 insect genera afford excellent examples. In the leaf hopper genus 

 Macrosteles, many species feed on subclimax herbs such as Urtica 

 and subclimax grasses, and much intercontinental mixing has oc- 

 curred (Fig. 139) (Moore and Ross, 1957). However, in the sawfly 

 genus Neodiprion, confined to climax or near-climax coniferous 

 hosts, present evidence indicates only a single intercontinental dis- 

 persal and in addition only little inter-regional dispersal between 

 the eastern and western parts of North America (Fig. 140) (Ross, 

 1955). 



Information concerning the insect fauna of the grasses Aristida 

 and Danthonia indicates that even with subclimax species we can- 

 not draw too wide a generalization. Although the two grass genera 

 have dispersed between North America and Australia, all their 

 North American leafhopper fauna has not. Certain members of the 

 North American leafhopper genera Unoka and Flexamia feed on 

 species of these two grass genera but have not been found on any 

 other continent. Either the grasses dispersed before the leafhopper 

 species began living on these hosts, or at the time of dispersal the 

 grasses had either a greater vagility or wider tolerances than the 

 leaf hoppers. 



Three genera of grass-feeding leafhoppers abundant in subclimax 

 communities of the eastern deciduous forest of North America 

 exemplify the dispersal effects of differences in ecological toler- 

 ances. The genus Folyamia occurs almost wholly in communities 

 in or near the range of the eastern deciduous forest and apparently 

 has never dispersed to any other continent. The tribe Balcluthini 

 (in reality only one well-marked genus or supergenus) and the 

 genus Exitianus also occur in these same eastern communities but, 

 like the grasses Aristida and Danthonia, occur also in a wide array 

 of central and western communities in both arid country and at 

 elevations up to 10,000 feet. Unlike Folyamia, the Balcluthini and 



