MOUNTAIN FAUNAS is 
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 certain 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-line 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 
agility upon the part of the animals inhabiting them, 
but gives to those possessing such agility a certain 
