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 
