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. 
Rains 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 
