LIFE AREAS 285 



animal life of a locality is practically dependent on the vegetation 

 it is in that way affected by the proportion O'f moisture present. 

 The amount of moisture of a region is regulated by its distance 

 from large bodies of water, the direction of the prevailing air cur- 

 rents, and the height of intervening obstacles, such as mountain 

 ranges. Most of the moisture present in the air oTiginates in the 

 evaporation of seas and other large bodies of water. The moisture 

 laden air moving inland when cooled is unable to hold up all its 

 moisture, which falls as rain. A high range of mountains will 

 greatly cool the air currents passing over it and the heavy rainfall 

 or snowfall resulting may abstract sO' much of the moisture from' 

 the air, that little is left for the region beyond the mountains, 

 which thus becomes arid. The region of the Colorado and Mo- 

 jave Deserts and the greater part of Nevada is an illustration of 

 the drying influence which the Sierra Nevada Mountains exert 

 on the air currents passing over them. 



The quality of the soil is another factor in the quantity and 

 character of the plant and animal life of a region. The carnivor- 

 ous species of animals of a region subsist on the herbivorous spec- 

 ies ; these subsist on the leaves, stems, seeds or root of plants 

 which draw their nourishment from the soil; therefore a richer 

 or poorer soil has a considerable direct influence on such apparent- 

 ly remotely connected beings as the foxes or hawks that live in a 

 region. 



Dr. C. Hart Merriman has formulated certain laws of the dis- 

 tribution of life which appear to be based on sound reasoning from 

 a sufficient mass of observed facts to assure their correctness. 



"The northward distribution of animals and plants is de- 

 termined by the total amount of heat — the sum of effective tem- 

 peratures. 



The southward distribution of Boreal, Transition zone, and 

 Upper Austral species is determined by the mean temperature of 

 the hottest part oi the year." 



If the North Temperate Realm was composed of sea and 

 level land only, its life zones would nearly follow parallels of 



