284 CALIFORNIA MAMMALS. 



effects, the higher area having a similar dimate to that of the 

 lower area situated a certain distance further from the equator. 

 In other words, a traveler passing from the tropics toward the 

 poles at sea level finds the climate steadily becoming colder; in 

 climbing a mountain the same change is observed. 



If the area of high altitude is great it is warmer than a small 

 similar area at the same height and latitude, for the reason that 

 the greater area conserves the greater amount of heat as daily re- 

 ceived from the sun. It sometimes happens that the base level on 

 one side of a mountain range is higher than that on the other side ; 

 in this case the higher level tends to raise the temperature and 

 therefore the life zones on that side. A good illustration is the 

 Himalaya Mountain range. The plain on the south side is sev- 

 eral thousand feet higher than the plateau on the north side; in 

 consequence of this difference of base level on the two sides the 

 timber line and snow line are about three thousand feet higher on 

 the north than on the south side. This is in direct opposition to 

 the efifect of latitude which would tend to lower the snow line on 

 the north side. The Sierra Nevada Mountains are another il- 

 lustration. The plateau on the eastern side is from three to four 

 thousand feet higher than the San Joaquin and Sacramento Val- 

 leys on the west side, and in consequence all the life zones are 

 higher on the east side than on the west. 



Slope exposure is another disturbing cause. A slope directly 

 facing the sun is warmer than one facing away from it. This is 

 very noticeable in many canyons running east and west in semi- 

 arid parts of California, in which case the timber will be found 

 growing considerably lower down on the side receiving the least 

 amount of direct sunshine. 



Prevailing winds coming directly from large bodies of water 

 tend to cool the region continguous and therefore lower the life 

 zones. 



The next most important agent in the distribution of life is 

 moisture. The greater or lesser amount of moisture present in 

 air and soil strongly afifects the vegetable growth of a locality; as 



