SPREADING OF ANIMALS. 49 



connected with the respective position of the tract it 

 inhabits ; nor can a sufficient knowledge of an ani-? 

 mal's condition be learned, unless it be considered in 

 its own peculiar and natural situation. The Polar 

 Bear lives only on and between the frozen plains of 

 the North Pole ; the Lion, only in Africa's burning 

 deserts. The inhabitants of the snow-line, between 

 the Tropics, never descend to the warm level ; nor 

 do those of the level ascend to the snow-line: each 

 altitude having its peculiar animals. 



The Mammalia remain always in their native 

 country, during the whole year; but Birds are sub- 

 jected to the change of seasons. They, for the most 

 part, keep at a certain distance from the snow line; 

 and in the Northern Hemisphere, accordingly mi- 

 grate, at the fall of the year, southward ; following 

 the apparent motion of the sun, and flying from 

 the approaching fall of snow ; but they return in the 

 spring as fast as the receding snow-line will permit 

 Subjected to the caprice of nature, their dwelling- 

 place is much limited ; although the very same spe- 

 cies seem to extend over immense districts. A well- 

 informed Naturalist ought not to leave those circum- 

 stances unattended to which are essential to the 

 existence of life on earth ; that is, to Physiology. 

 Nor does Natural History answer its purpose, when 

 c 



