Comparative Evolution of Biomes 



307 



Fig. 133, Geographic distribution of seven genera of net-winged midges of 

 the family Blepharoceridae. (After Alexander.) 



OCEAN CURRENTS 



These play the same modifying role in providing avenues of dis- 

 persal across temperature zones as do mountain systems but are 

 of especial importance in the dispersal of marine forms. Cool and 

 warm ocean currents traverse practically all parts of the ocean and 

 connect many of the major oceanic basins. It is certain that similar 

 ocean currents have existed since the beginning of life. As a result 

 the widespread dispersal of marine organisms has always been 

 more probable than comparable dispersals of terrestrial organisms. 



Dispersal Opportunities 



Changes in geomorphology and climatic zonation thus provide op- 

 portunities for members of different biomes to spread to various 

 parts of the world at different times. Representatives of marine 

 biomes have greater opportunities for dispersal than do those of 

 terrestrial biomes. The latter are frequently rigidly restricted geo- 

 graphically by ecological factors. As a result the changes in land 

 bridges and in climatic zonation produce a curious, intermittent, 



