314 



Comparative Evolution of Biomes 



TermitomimuU//^^ Termitoptocnus 



Fig. 138. Distribution of the genera of the subtribe Corotocina of the family 

 Staphyhnidae. Lines indicate the phylogeny and paths of dispersal. (After 

 See vers. ) 



Subclimax versus Climax Dispersal 



Because most subclimax species occur throughout every associa- 

 tion of the biome and many occur in the subclimax communities 

 of several biomes, subclimax species in general have an inordinately 

 greater chance of being adjacent to land bridges than the climax 

 species restricted to only a few associations of a single biome. This 

 circumstance has resulted in many apparent paradoxes of distribu- 

 tion which have puzzled students of biogeography. For example, 

 the scrub malices of Australian deserts are dominated by certain 

 species of Eucalyptus and other indigenous shrubs and trees which 

 occur only in Australia, whereas their counterparts, the semi-desert 

 scrub of southwestern North America, are dominated by New World 

 shrubs and trees. In contrast to this taxonomic divergence, genera 

 such as the grasses Danthonia and Aristida are abundant in both 

 places (Coaldrake, 1951). 



Sawflies of the genus Dolerus which feed on Equisetum and 

 grasses provide another example of paradoxical distribution. Many 

 species of Dolerus abound in the temperate deciduous forest of 

 eastern North America, but unlike many leafhoppers, grasshoppers, 

 and other insects endemic in the same areas, many of these Dolerus 

 species extend across North America and Eurasia, and others have 



