No. 646] THE DISTRIBUTION OF LIFE 433 



shown by tlie humid and heavily forested coast and 

 mountain areas of northwestern Washington, in contrast 

 to the bare, arid plains east of the Cascade range. 



Associational temperature is induced by many causes, 

 and although limited in extent it profoundly influences 

 local zonal boundaries. Even associational factors other 

 than temperature may raise barriers to distribution that 

 are insurmountable to many organisms. 



Insolation, or the relative amounts of sun and shade 

 received by a species in its habitat, is sometimes of 

 paramount importance. This may be influenced by 

 cloudiness, by the amount and density of surrounding 

 vegetation or by the character of the topographical en- 

 vironment. In illustrating this point, we may mention 

 as extremes the bottom of a deep, narrow, forested gulch, 

 and the top of a warm, bare ridge; the face of a steep 

 north slope, and one facing south. A uully on a north 

 slope may be so situated n< never to rf(vi\.' liic ra\- of 

 the sun, while at a certain optinnnii anuh- oiu' faring 

 towai-ds tlie south will receive forty rent, moiv >nlar 

 heat than will a U'vel surface. Hence, zonal lioundaries 

 upon iln'two -lope aspects will be found lo ocrui- at wvy 

 different allitiides. Soil conditions are of ureal ini[)or- 

 tance in iiilhiencinu- the temperature imnietliately above 

 its surface, and its character helps to control both the 

 amount ol' e\ a j )oration and the degree of moisture which 

 it is capable of retaining. A light-colored soil is con- 

 siderably cooler, other things being equal, than a dark, 

 rocky one, which will absorb and retain more heat. The 

 importance of tlie chemical coni])osition as well as the 

 mechanical con(liti(^n, witli amount of hnnuis, acid or 



