RAIN-FOREST AND DESERT MOUNTAINS 141 



as a differential factor in the distribution of the vegetation. In 

 spite of the great seasonal fluctuations of atmospheric humidity 

 in Arizona there is a relatively small difference in the values of 

 this factor for any series of locations or habitats at any given 

 time. The great differences in the evaporative power of the air 

 which are found in different locations and at different altitudes 

 are chiefly attributable to wind and temperature. Atmospheric 

 humidity is not, therefore, a differential factor in the desert 

 mountains of Arizona, although it is the strongest single factor 

 involved in determining the habital distribution of plants in 

 the Blue Mountains of Jamaica. The actual factors which un- 

 derlie the topographic control of the vegetation in the two 

 mountains are therefore diametrically opposed. 



