GEOGRAPHICAL DISTRIBUTION OF ANIMALS 291 



The climate, as distinct from the temperature, also 

 greatly influences many species. In the Eastern United 

 States and in the extreme Northwest, as in Europe and 

 much of Asia, the atmosphere is humid all the year long. 

 Eains occur at intervals in the summer, and rain or snow in 

 the winter. The green season is from spring to fall, and the 

 resting of plants is in the winter. To this condition the 

 native animals adapt themselves, and this would seem to 

 be the natural order of things. 



But as we pass the Western plains of Nebraska, Kan- 

 sas, and Texas this condition is materially changed. For 

 part of the year rainfall is practically unknown. The air 

 becomes dry, and under the cloudless sky the greater part 

 of the vegetation ripens its seed and perishes. This is the 

 arid climate. When the rainfall is very scant the region 

 is never covered with verdure, and is known as desert. 

 Such great desert tracts are found in parts of Wyoming, 

 Utah, Nevada, Idaho, Colorado, Arizona, New Mexico, Cali- 

 fornia, as well as in the northern parts of Mexico. In some 

 cases the deserts are exposed to great heat, forming an 

 ultra-torrid region, as in Death Valley in California and in 

 certain parts of Arizona. 



But the arid region is not as a whole desolate. In many 

 parts rain falls more or less heavily for part of the year, 

 bringing a rank growth of annual grasses and of verdure 

 in general. In California this rainfall is in the winter, the 

 coldest part of the year, and the country is green from 

 November or October to June or May. In Mexico and 

 northward to Colorado the chief rainfall is in midsummer, 

 the warmest part of the year, and the summer is the time 

 of verdure. 



To all these conditions the plant life must adapt itself 

 and with this the animal life. But the species that have 

 become fitted to the arid habitat have undergone some 

 change in the process and may have become different spe- 

 cies. It is, then, not easy for them to recross the barrier 



