n] FEATURES OF ENVIRONMENT 49 



and arable land ; abundance of mostly deciduous 

 trees and shrubs. 



4. A cold belt across Canada, Scandinavia, 

 Northern Russia and Siberia, which belt includes 

 the northern tree-line, to the south of which are 

 huge timber forests, composed chiefly of conifers. 

 Enforced hibernation during the severe winter. 



5. The arctic, circumpolar region, with pre- 

 valence of ice and snow ; the permanent vegetation 

 represented mainly by mosses on the Siberian 

 * tundras,' by lichens on the North American * barren 

 lands.' 



Because of this agreement the study of certain 

 climatic effects upon the ' Biota,' i.e. flora and fauna, 

 can be made on a complete mountain within a small 

 compass. We can also investigate the effect of the 

 change of habitat. 



Let us take the simplest case, a mountain with its 

 whole base in tropical lowland. Its whole Biota may 

 then be assumed (excepting the few instances of 

 conveyance by wind, etc.) to have ascended from the 

 plains, and if the top species are different from those 

 at the bottom, they probably have been modified by 

 the conditions prevailing on the mountain. 



The case is more complicated if the mountain 

 has two bases, one in the tropical lowland, the other 

 a high temperate plateau, which probably has an 

 endemic Biota of its own, differing from that of the 



G. 4 



