ECOLOGY 149 



the one in salt-marshes the other upon sand-dunes. 

 Many species have a circumpolar distribution* and often 

 extend southward along the mountain ranges. Poa 

 alpina, found at sea-level within the arctic circle, extends 

 southward in the Rocky Mountains to Colorado where it 

 is found on alpine summits. 



190. Circumpolar distribution. — Those species that 

 are indigenous to North America and Eurasia usually 

 show evidence, by a present circumpolar distribution, 

 such as that of Poa alpina and many others, of a common 

 origin in polar regions; or they suggest the probability 

 of such distribution in the past. During a preceding 

 warm epoch, when vegetation zones lay farther north 

 than now, many species were circumpolar that afterward 

 were driven south by the succeeding ice period. These 

 species survived only where they found conditions suited 

 to their requirements. Some were driven along the moun- 

 tain ranges; others were driven along the coastal regions. 

 As the climates of the northeastern coasts of North 

 America and Asia are similar, there are many cases where 

 the same or similar species of plants inhabit both 

 regions. t Among grasses one notes the genera Diarrhena 

 and Zizania, each represented by similar species in the 

 two regions and not found elsewhere. 



191. Generic distribution. — Sometimes large genera 

 show a special development in certain areas although 

 there may be scattering species in regions remote from 

 the areas of greatest development. The genera Bouteloua 

 and Muhlenbergia, mentioned above, are represented by 

 numerous species on the tableland of Mexico, although 

 certain species of the former are found as far south as 



♦Hooker, "Distribution of Arctic Plants." 



tGray, "Analogy between the Flora of Japan and that of the United States." 



