432 THE AMERICAN NATURALIST [Vol. LVI 



makes no difference whether it is summer or winter 

 above, and so these squirrels seem to lead an existence 

 closely similar to that of their near kin at the Arctic 

 Circle, but ^vith the probable difference that the northern 

 forms experience an actually greater number of hours 

 of daylight throughout the long arctic summer months. 



In the plains section of the interior, zonal divisions 

 are acted upon by comparatively few modifying agencies, 

 and their boundaries are rather regular and easily de- 

 fined, but in parts of the three Pacific Coast states, whose 

 shores are bathed by w^arm ocean currents, and where 

 the topography is decidedly irregular, the problem of 

 zonal definition is often extremely complicated. In the 

 coast region of northern California, for instance, there 

 is but slight daily and seasonal change of temperature, 

 and a number of Boreal forms are able to occur there 

 because the summers are cool enough for them, while 

 certain Sonoran species are also able to exist because the 

 mean temperature of the breeding season is high enough 

 for their needs. The result is a confusion of zonal in- 

 dices that is extremely puzzling at first glance. 



To these three widely-recognized zonal factors, when 

 operative in certain regions, should undoubtedly be added 

 character of the coastal sea currents— whether warm or 

 cold— and direction of the prevailing winds. 



Faunal conditions depend largely upon humidity as 

 well as upon all zonal factors. The chief cause of a hu- 

 mid climate is, of course, ample precipitation, either 

 rather evenly distributed throughout the year, or else 

 supplemented during the drier season by heavy fogs 

 and dense forests to retard evaporation, w^hile a cool 

 climate is often helpful. Precipitation may be largely 

 dependent upon the position of adjacent mountain masses 

 with respect to the prevailing winds, for, as is well 

 known, moisture-laden air is cooled upon contact with 

 an elevated land mass, and precipitation results; but 

 little moisture will then be left in the clouds for rain in 

 the trans-montanic sections. This fact is beautifully 



