308 



Comparative Evolution of Biomes 



gatelike set of dispersal opportunities which have allowed first one 

 terrestrial biome then another to disperse between continents 



(Fig. 134). 



Cool areas in 

 mountain choins 

 through tropics 



Warm cycle 



Fig. 134. Differential dispersal opportunities for temperate versus tropical 

 biotas during cold and warm periods of the earth. Arrows in the top drawing 

 indicate dispersal paths of cool-adapted species, and the arrows in the figure at 

 the bottom indicate dispersal paths of warm-adapted species. Note the restric- 

 tion of dispersal of tropical species between the New and Old Worlds during 

 cold periods and the isolation of temperate species during warm periods. 



Large segments of terrestrial and fresh water biomes are sep- 

 arated from each other by land areas which form dividing agents 

 as effective as oceans. In the case of the mountain stream biome, 

 the western North American segment is as effectively separated 



