Comparative Epolution of Biomes 321 



long, 1948; Ribault, 1952; Oman, 1949) which are Hkewise mem- 

 bers of the same prairie biomes. The fact that these xeric forms 

 occur only in North America suggests that the grassland biomes 

 to which they belong have had only incomplete intercontinental 

 mixing, if any, since these leafhoppers became established in the 

 more xeric grassland communities. In the grasshoppers (Orthop- 

 tera) many similar examples from many continents (Roberts, 1941; 

 Uvarov, 1943; Rehn, 1958) emphasize the importance of coloniza- 

 tion in contributing to regional diversity. 



Conservative Elements 



In contrast to the examples above, many plants and invertebrate 

 animals seem to exhibit a pronounced evolutionary conservatism 

 throughout widely separated regions. In the cool, temperate, rapid 

 stream biome, the caddisfly genus Sortosa dispersed into all the 

 continents, apparently during Cretaceous time, and has existed 

 subsequently as isolated geographic units. Since that time each 

 separate branch has evolved into a distinctive subgenus. A daugh- 

 ter genus Wormaldia of nearly similar ecological traits dispersed 

 throughout the northern hemisphere, probably in early Cenozoic 

 time, and the isolated branches of this group have evolved into 

 moderately distinctive species groups. In spite of a great difference 

 in age and the widespread nature of the regional units of this 

 biome, the geographic segregates of these genera have undergone 

 little change either ecologically or in number of species. 



Within the temperate deciduous forest biome many of the ferns 

 have been an equally conservative element. The fern components 

 of the different regions of the biome are strikingly similar, and this 

 would seem to be due to small amounts of change since the regions 

 became separated rather than to fairly recent dispersal. 



PHYLOGENY AND AGE OF BIOMES 



The alternation of intercontinental or inter-regional mixing and the 

 evolution of the isolated regional segments produce biomes having 

 a reticulate phylogeny. Typical stages in this phylogenetic pattern 

 are available for study because biomes are of different ages and 

 exhibit different stages of development. 



The oldest existing biomes are those of the sea. Peterson ( 1914, 

 1918) and Ekman (1953) presented a great deal of information 

 concerning marine communities and ecological and geographic 



