306 



Comparative Evolution of Biomes 



1^^^ Dry subhumid 

 l:-:--'-4 Semiarid 

 IZZl And 



Fig. 132. Moisture bands in the United States. (Adapted from U.S.D.A.) 



equator (de Beaufort, 1951; Darlington, 1957), but when they 

 have done so it has undoubtedly been through the cool habitats 

 of mountain chains ( Ross, 1956^ ) . 



Mountains have been of especial importance in the interconti- 

 nental mixing of freshwater communities. Many of these occur only 

 in swift, cold streams associated with mountains. The interconti- 

 nental and inter-regional dispersal of these communities is there- 

 fore dependent on chains of hill or montane country connecting 

 various areas. That such terrain has at times connected all the 

 major portions of the earth is well shown by certain insect com- 

 ponents of these communities. The net-winged midges comprising 

 the family Blepharoceridae occur only in cold, rapid streams but 

 do not extend into truly arctic regions. The present range of all 

 species is restricted to widely separated mountain systems (Alex- 

 ander, 1958), yet each subfamily has a wide distribution (Fig. 

 133). Past conditions in the more northern moimtains must have 

 approximated those in the more southern mountains now occupied 

 by Blepharoceridae and thus provided intercontinental and inter- 

 regional avenues of dispersal for these forms. 



