THE DISTRIBUTION OF ANIMALS 439 



as effective a barrier to these animals as a whole sea. 

 High mountain ranges and broad deserts are barriers to 

 many land-animals, partly because of the physical ob- 

 stacles, partly because of the differences in temperature 

 and in food-supply. 



Temperature and climate (as distinct from temperature) 

 and the ocean are the three great barriers when we con- 

 sider the animal kingdom as a whole, and look for the 

 causes which determine the chief zoogeographical divisions 

 of the earth's surface. Most of the tropical animals 

 cannot endure frost, hence the isothermal line of frost is 

 a line across which few tropical animals venture. Most 

 arctic animals are enfeebled by heat, and the isothermal 

 line which marks off the region in which frost occurs the 

 year round is another great zoogeographical boundary. 

 But while these lines are limits for localized species, some 

 animals, as birds, especially, keep within a relatively 

 uniform temperature by migrations across these lines. It 

 should be borne in mind that the gradual decrease in 

 temperature met with in going north or south from the 

 tropics is also met in the ascent of high mountains. The 

 summits of lofty peaks, even in the tropics, are truly 

 arctic in character; they are snow-covered, and the 

 animals and plants on them are truly arctic. Thus in the 

 ascent of a single mountain a whole series of life-zones 

 from tropical to arctic can be traversed. 



Climate, as distinct from temperature, establishes limits 

 of distribution. The animals of Eastern North America 

 accustomed to a humid atmosphere cannot live in the dry 

 plains and deserts of the West. Closely associated with 

 climate is the nature of the plant-growth covering the 

 land ; here are forests and luxuriant meadows, there are 

 sparse tough grasses of the dry plateau. The limits of a 

 special kind of plant-growth often are the limits of distri- 

 bution of certain animals. 



