368 THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS 



Barriers to dispersal. — The barriers that prevent or 

 retard the migration of animals from one region to another, 

 may be roughly classed as land, water, and climatic barriers. 

 Of course, land is a barrier in the strictest sense only to 

 those animals that live in the water. For example, it is 

 easy to see that a fish on the coast of California would be 

 forever debarred from entering the Gulf of Mexico by the 

 Isthmus of Panama, narrow though it be. Likewise, fish 

 living in inland seas without outlets are forever debarred 

 from leaving them of their own free will. In two rivers 

 running side by side, and emptying into the ocean, there 

 may be fish that will never get from one to the other, owing 

 to the narrow strip of land between. 



Deserts are barriers to the dispersal of many animals. It 

 is true that this is due to the absence of water more than to 

 any other cause, yet they are land barriers. 



Falls in streams are barriers to the distribution of fish. 

 For example, eels could never have reached the Great 

 Lakes over Niagara Falls, unless they had been aided by 

 man. Mountains present strong hindrances to the migra- 

 tion of animals. Especially is this true of ranges that are 

 capped with snow. The mammals of California difTer in 

 some species from those on the eastern side of the Rockies. 



Perhaps, on the whole, water is a more serious barrier 

 to a greater number of animals than any other barrier noted. 

 The ocean is a barrier to many birds; and birds are probably 

 best fitted for migration of any animals in existence. A 

 small river will hold a race of monkeys in confinement in a 

 given area. Many animals, however, can swim across wide 

 streams. The effect of water as a barrier is well shown by 

 a study of the animals on an island separated from the 

 mainland by a deep channel. The island of Madagascar is 



