3()S THE GEOGRAPHICAL DISTUIBUTION OE ANIMALS 



Barriers to dispersal. — The barriers that prevent or 

 retard the migration of animals from one region to another 

 ma}' be roughly classed as land, water, and climatic barriers. 

 Of course, land is a Ijarrier 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 entei'ing the Gulf of Mexico by the 

 Isthmus (jf Panama, narrow though it ])e. likewise, fish 

 living in inland seas witliout outlets are fore\'cr debarred 

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

 running side In' side, and emptying into the ocean, there 

 may be fish that w\\\ nvvvr get from one to the other, owing 

 to the narrow sti'ip of land lietwcen. 



Deserts are barriers to the dis]:)ersal of mam' animals. It 

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

 any other cause, yet tluv' are land l)arriei-s. 



Falls in streams are barriers to the distribution of fi.sh. 

 For example, eels could ne\'er ha'S'e reached the Great 

 Lakes over Niagara Falls, unless they had l)een aided by 

 man. Mountains present strong hindi-anccs to the migra- 

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

 capped with snow. The mamuials of California, differ in 

 some s]>ecies from those on the eastei'u si(k^ of the Rockies. 



Perhaps, on the whole, water is a moi-e sei'ious bari'ier 

 to a greater numlier of animals than any other liarrier noted. 

 The ocean is a bari'ier to uiany bu-ds; and birds aic projxibly 

 })est fitted for migration of an>' animals in existence. A 

 small vivvv ^^'ill hold a ra('(> of uioid^eys in confinement in a 

 given area. Many animals, howe\'er, can swiui across witle 

 streams. The effect of watei' as ,'i, barrier is well shown liy 

 a study of the animals on an island scp.ai'ated from the 

 mainland by a deej) channel. 'J1ie island of Madagascar is 



