MOUNTAIN FAUNAS 73 



the exposed rocks are at most scantily clad in mosses 

 and lichens. 



Without discussing in detail the characters of moun- 

 tain climates, we may note that it seems probable that 

 the upper limit of the forest corresponds roughly to 

 the zone of maximum precipitation. In ascending 

 a mountain the rainfall increases up to a dertain point, 

 beyond which it begins to diminish. With certain 

 modifications, due to inversions, &c., the temperature 

 diminishes steadily as we ascend. The relatively 

 narrow alpine zone on, e.g., the Alps is due to the fact 

 that the vertical distance between the zone of maxi- 

 mum precipitation and the snow-line is short. In 

 tropical regions the snow-hne is pushed far upwards, 

 much above the zone of maximum precipitation, and 

 the wide interval thus produced between the forest and 

 the snow allows for the development of a broad band 

 of gradually increasing aridity, where the surface is 

 clad in plants of definitely drought-resisting type, the 

 low temperature increasing the need for xerophytic 

 characters by diminishing the power of absorbing 

 water. 



Plateau regions, such as the great plateau of Tibet 

 and the far less extensive plateau of Spain, may be said 

 to correspond to the steppe zone on an ordinary moun- 

 tain. Plateaux of great altitude and great horizontal 

 extension tend to show some of the characters of a cold 

 desert, thus giving us the special conditions which reign 

 in Tibet, but the conditions tend to be less extreme 

 than in small and isolated areas of great elevation. 

 Plateaux differ from other steppes or deserts in the 

 peculiar nature of the relief, which demands special 

 agihty upon the part of the animals inhabiting them, 

 but gives to those possessing such agihty a certain 



