326 Comparative Evolution of Biomes 



The Age of the Process 



The alternation of intercontinental mixing and regional isolation 

 has undoubtedly been a major factor in biome evolution over many 

 geologic periods. Indirect evidence can be found dating back as 

 early as Pennsylvanian and Mississippian times of the Paleozoic. 

 Schuchert ( 1924 ) mentioned that the tree species of the coal 

 measures (Pennsylvanian in age) were remarkably alike in distant 

 locations such as central North America and Europe. These Paleo- 

 zoic floras probably represented the same well-mixed stage of biome 

 phylogeny as the present-day temperate deciduous forest or the 

 taiga. Various elements had undoubtedly evolved in isolated regions 

 of the tropical biome and then intermingled across bridging areas. 

 Because these processes are automatic when a dynamic biota oc- 

 curs in an area of dynamic geomorphology, it is reasonable to as- 

 sume that they have been important in biome evolution ever since 

 the most primitive biomes came into existence perhaps two billion 

 years ago. 



