462 WHEELER. 



ent region, the San Francisco Mountains of Arizona (1890). There he 

 found the normal difference in altitude of the same zone on the south- 

 west and northeast slopes to be about 900 feet. After giving numerous 

 examples of this altitudinal distribution on Mt. Shasta, he calls 

 attention to other factors, besides those of insolation, which influence 

 the range of plants and animals: "It is well-known that in ordinary 

 calm weather the air-currents on mountain sides and in deep canyons 

 ascend by day and descend by night. The ascending currents are 

 warm, the descending currents cold. The night current, being in the 

 main free from local influences that affect its temperature, must exert 

 an essentially equal affect on all sides of a mountain ; but the tempera- 

 ture of the ascending day current, being constantly exposed to and 

 in fact created by the influence of the sun, must vary enormously 

 on different slopes. The activity and effectiveness of this current 

 increase with the steepness of the slope and the directness of its 

 exposure to the afternoon sun. Hence the hottest normal slopes — 

 those that face the sun at nearly a right angle during the hottest part 

 of the day — are rendered still more potent by increased steepness, 

 the direct exposure of the sun keeping up the supply of heat while 

 the steepness of the slope accelerates the rate of movement of the 

 diurnal ascending current, carrying the heated air upward a very 

 great distance before it has time to be cooled to the general temperature 

 of the stratum it penetrates. Thus it is that species characteristic 

 of the Transition zone on Shasta — species which on normal south- 

 westernly slopes attain their upper limits at an altitude of 5500 to 

 5700 feet — are in favorable places enabled to live at elevations of 

 7900 or even 8000 feet, considerably more than 2000 feet above their 

 normal limits." 



Every observer in the field must have been impressed with the fact 

 that steepness of slope is an important factor in the local distribution 

 of mountain ants. These insects always greatly prefer the more 

 gradual slopes and alpine meadows, probably because the soil of such 

 places retains a more abundant and more equable supply of moisture 

 and because their surfaces are much less exposed to rapid evaporation 

 both from direct insolation and from air-currents. All of these eco- 

 logical factors demand much more careful study. 



It is, of course, well known that the delimitation of the various 

 life-zones in mountain regions depends not only on slope-exposure but 

 also on latitude. That the upper limit of the zones descends in more 

 northern and ascends in more southern latitudes even within the 

 confines of a single one of our western states is well shown in the fol- 



