46 THE WANDERINGS OF ANIMALS [cH. 



whilst the mean annual temperature is of little 

 consequence. In many cases the lowest extremes 

 of temperature are also important. If you want 

 to cultivate Indian corn at a certain place its mean 

 annual temperature may be dismissed ; the success 

 depends upon the number of days sufficiently warm 

 to ripen the corn, and we are not concerned about 

 the cold of the winter. The latter may, however, be 

 of great importance to the growth of perennials, 

 shrubs and trees. 



Temperature and humidity seem to be most 

 important factors in distribution. 



High mountains have a more or less permanent 

 belt of clouds ; what determines its height above 

 the neighbouring plain is not known. This belt is 

 naturally the zone of greatest moisture ; although 

 it rains more below, the permanence of such a belt 

 ensures greater effects of the moisture. A mountain 

 standing on the southern edge of a plateau has a 

 higher temperature at a given altitude on its north 

 slope than on its south side, because the sun-warmed 

 surface of the plateau is nearer to the mountain top 

 than is the lower, although hotter, southern plain. 

 This explains why, e.g., in the Himalayas the snow 

 and timber-line on the north side are about 3000 feet 

 higher than they are on the south side. 



The side of a mountain exposed to the prevailing 

 sea wind is moist, whilst the lee side may be arid. 



