Geographical Distribution 



161 



are well nigh as varied as are those of human society. Tem- 

 perature, moisture, wind and light are the four principal 

 parts of the plant's environment, but these in turn are de- 

 pendent on other factors, such as altitude, topography and 

 soil. The structure of the plant itself determines its re- 

 sponse to these factors and its survival or extinction, while 

 the relation of plant to plant in determining such factors 

 as light, food supply, growing space, etc., and the inter- 

 relationship between plants and animals, affecting transport 

 of seeds, and forage for herbivorous animals, all have a life 

 and death meaning in the existence of the plant. 



Not only is the distribution of plants determined in large 

 measure by that of animals, but even more is the occurrence 

 of the latter dependent upon that of the former. Especially 



30 60 30 /fO ISO ISO 



DIAGRAM OF THE Six GREAT ZOOGEOGRAPHICAL EEALMS OF THE EARTH 

 After Sclater and Wallace. 



is this true of herbivorous types, which necessarily are de- 

 pendent upon the presence of their forage plants. The move- 

 ments of grazing animals, such as horses and cattle, are in 

 particular dependent upon the abundance of grasses, and in 

 the early days of the West, Indians and white men alike 

 guided their movements in the search for buffalo largely by 

 the condition of the prairies over which the bison roamed. 

 Not only are the herbivorous types dependent on the vege- 

 tation in their movements, but also the carnivorous animals 

 which prey upon the former. In the northern forests the 

 movements of the deer in winter largely determine those of 

 their enemy, the wolf. During the mouse plague in the Hum- 

 boldt Valley in Nevada in 1907-8 the abundance of the mice 

 attracted thither large numbers of hawks to feed upon them. 

 Based on the distribution of their animal inhabitants the 



