336 University of California Publications in Zoology [VOL. 16 



In the life-zone system, humidity is recognized to have considerable 

 influence on distribution, but is held to be always subordinate to the 

 influence of temperature. However, it seems that either of several 

 climatic factors may be of importance in limiting organic distribution. 

 An extreme variation of humidity, or probably of other climatic fac- 

 tors besides temperature, may form a positive barrier to the distribu- 

 tion of species. All the climatic factors are complexly interrelated 

 and a variation of any factor has an influence on the effect of the 

 others. Different organisms are adapted to different climatic com- 

 plexes and react in different manners to different factors and to vary- 

 ing degrees of the same factor. Temperature perhaps often is the 

 most important factor in limiting distribution, but it would seem to 

 be impossible to base a system of distribution on variations in any one 

 climatic factor without obscuring many facts of prime importance. It 

 has not yet been established that small differences of temperature of 

 the degree supposedly separating some of the life-zones are as im- 

 portant barriers to distribution as are some of the more marked dif- 

 ferences due to variations in rainfall and humidity. 



The zones of life which occur in any given locality may be depend- 

 ent in part on temperature, yet there are other factors which evidently 

 have a very strong modifying influence. Differences in rainfall, in 

 the humidity of the air, in slope exposure, or in other factors may 

 greatly modify the position of zones. It may be that differences in 

 some of those factors, other than temperature, might even be the 

 principal cause in the production of certain zones. In each given case 

 it is probably the complex of climatic factors which determines the 

 occurrence of the zone rather than the action of one factor alone. 



Three distinct regions of life may be recognized in North America, 

 the Holarctic (Boreal) region, the Sonoran (Austral) region, and the 

 Neotropical region (Tropical zone) (Lydekker, 1896, frontispiece). 

 The limitation of many characteristic species and genera to each of 

 these regions is probably due principally to the action of temperature 

 as a barrier. In the Holarctic region of North America three trans- 

 continental belts of life, Arctic, Hudsonian, and Canadian, have been 

 recognized by nearly all students of geographical distribution. These 

 belts of life are probably also determined largely by the effect of 

 temperature. However, there is much more difficulty in recognizing 

 transcontinental life belts within the Sonoran region, and in the truly 

 tropical regions zones of distribution corresponding to isotherms have 

 not been recognized. 



