GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 233 



flora is characteristically northern. The regions bor- 

 dering on the Mediterranean, of course, show many 

 forms related to the adjacent regions of northern Africa 

 and western Asia. 



In eastern Asia the conditions are very similar to 

 those in eastern North America. In both regions the 

 main trend of the mountains is north and south, so that 

 there is direct communication with the tropics, and 

 in both the climatic conditions are remarkably similar, 

 showing great extremes of heat and cold in the northern 

 portions, the characteristics of a continental climate. 

 In both regions a very large area lies much further 

 south than Europe, and the flora is much richer, 

 this being especially noticeable in the much larger 

 number of forest trees. While in Europe the trees 

 are few in number, probably not more than a third or 

 fourth as many as in the United States or eastern 

 Asia, in the two latter regions there is a remarkably 

 large number of types, both of trees and herbaceous 

 and shrubby plants, which are absent from Europe, 

 and what is perhaps most unexpected, absent also from 

 the Pacific coast of North America. In both eastern 

 Asia and North America, the number of tropical types 

 is very much larger than in Europe, where very few 

 of these exist. 



The interior of all the great continents except Europe 

 is more or less arid, and in some cases extensive deserts 

 occur with a very peculiar flora adapted to the desert con- 

 ditions. Similar arid conditions prevail in the warmer 

 parts of western Asia and America, but western Europe, 

 owing to the invasion of the land by branches of the 

 sea, and the influence of the Gulf Stream, has an insular 



