HOIV ANIMALS ARE DISTRIBUTED OI^ER THE IVORLD 321 



animals of the tropics, we find that most of them can- 

 not endure any frost or freezing at all, the southernmost 

 line of frost being to them a barrier north of which they 

 cannot live. These barriers are raised by temperature. 

 Similarly there are barriers made by differences in rain- 

 fall. The animals of the Eastern United States, accus- 

 tomed to a large amount of moisture and to luxuriant 

 vegetation, could not live on the arid, burning, sterile 

 desert; but the lizards and desert rats and coyotes live 

 there successfully. 



But barriers more marked and more tangible are those 

 such as oceans which surround continents and islands and 

 thus limit the land animals of these regions to their re- 

 spective districts. Similarly the land which surrounds a 

 lake or pond limits the fishes in it to that particular lake 

 or pond, although they would live quite as well in some 

 other. And it is true that many animals could live else- 

 where than in the place to which they are now restricted 

 if they could only get there. Indeed they could live in 

 any other region where the climate and general conditions 

 are like their present home. So we say that the distri- 

 bution of animals over the world is largely determined by 

 barriers — barriers of temperature, of moisture, of water, 

 of land, of high mountains, of deserts, of anything that 

 the animal cannot cross. 



How animals spread. — The ways in which animals 

 spread are mostly easily understood. Birds can fly to new 

 regions ; quadrupeds can travel on foot for long distances ; 

 fishes can swim from one part of a river or lake or ocean 

 to another. But although two rivers may empty into the 

 ocean close together fishes cannot often easily get from 

 one into the other. For most fresh-water fishes cannot 

 live in salt water, so that even a small stretch of ocean 

 is an effective barrier to them. Salmon, some eels, and 

 a few other fishes, however, live part of the time in the 



