408 THE ANIMALS AND MAN 



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 rainfall. The animals of the Eastern United 

 States, accustomed 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 respective 

 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 elsewhere 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 distribution of animals over the 

 world is largely determined by barriers barriers of tem- 

 perature, 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 ocean and 

 part in streams. Many animals are transported long 

 distances involuntarily. Rats and mice invading a ship 

 from wharves at Liverpool sometimes get carried across 



