434 THE AMERICAN NATURALIST [Vol. LVI 



locally restricted. A forest fire or avalanche, by de- 

 stroying ground shade with the consequent raising of 

 the soil temperature, will usually cause an area to grow 

 up to plants and trees of the zone immediately below, to 

 be gradually restored, in future years, to its original 

 zonal status. Base level has its effect, for the foot of a 

 mountain mass rising from a plain five thousand feet in 

 altitude will have lower zonal tendencies than will the 

 five thousand foot level of a mountain rising from a plain 

 with an elevation of but one thousand feet, because the 

 higher plain accumulates more heat. Similarly, a large 

 mountain mass is less influenced by the conditions which 

 surround it than is an isolated peak. A steep slope will 

 carry a certain zone to a greater height than will a 

 gentle one, because the former will receive, during the 

 day, more of the warm air arising from the lowlands, 

 and the cold air which descends during the night will 

 flow off more rapidly. However, this rule is often nulli- 

 fied by the steep slope being so situated that it receives 

 less sunlight than the more gentle gradient. These 

 points are finely illustrated on most of the mountains of 

 the southwest. Plants and trees of the Transition Zone 

 often flourish on the bottom of a north-facing canyon, 

 while the Sonoran sagebrush extends a couple of thou- 

 sand feet higher upon the steep slopes with southern ex- 

 posures. 



Protective cover is important to most of the more re- 

 tiring forms of active life, and to such it is not only 

 necessary as a screen during their daily foragings, but 

 they must have holes into which they may dart at the 

 approach of danger and safe retreats in which to rear 

 their young. To very few vertebrates is the actual char- 

 aicter of the soil of great moment, but there ^re excep- 

 tions, as instanced by the large kangaroo rat, Dipodomys 

 deserti, the front feet of which are so weak that it seems 

 able to burrow only in deposits of seolian or other loose 

 sand, and it is useless to expect to find this species in 

 hard soil. Needless to say, character of food, both gen- 



