No. 646] 



THE DISTRIBUTION OF LIFE 



429 



In any one realm, or larger region of the earth's sur- 

 face, there are various climatic divisions, the chief of 

 which have been named zones, and these stretch across 

 the continent following isotherms, or mean temperature 

 bands, usually, for our purpose, based upon the average 

 amount of heat present during the three chief reproduc- 

 tive months. Zones are divisible into faunal districts, 

 whose bounds are limited by conditions of humidity, pre- 

 cipitation and a few other causes that may be operative 

 over considerable areas. These are further divisible into 

 associations, an almost limitless number of which may 

 be recognized. Thus, we have littoral, riparian or 

 stream bank, palustral or marshy associations, the latter 

 being capable of still narrower subdivision into tule, 

 arrow-head or salt grass associations, and so on with- 

 out end. 



Associations are sometimes but little considered in 

 parts of the country where climatic conditions are uni- 

 form over a wide extent of territory; but in the moun- 

 tainous parts of the west, where practically every pos- 

 sible local environment from the hottest, most arid des- 

 erts, to arctic-alpine conditions may be encountered 

 within a few miles, the importance of their recognition 

 can hardly be overestimated. 



In considering the agencies governing the range of 

 a form, the question of temperature is undoubtedly of 

 chief importance as a usual thing, but in some cases 

 physical barriers should be given greater weight, for it 

 need hardly be indicated that it is such directly— and 

 temperature only indirectly if at all— that keep many 

 forms of life from greatly increasing their ranges. In 

 studying such barriers, manner of dispersal may be of 

 much importance. In the case of plants more than of 

 vertebrates (with few exceptions), human agency must 

 now be taken into account, for the activities of man, both 

 intentional and unintentional, are responsible in greater 

 degree for the widespread dissemination of seeds and 

 insects over vast stretches of the earth's surface than 



