STRUCTURE OF THE VERTEBRATES 327 



effectively stops east and west migrations, for the urodele can- 

 not go over the mountains. Most groups of animals find high 

 or rocky mountains insuperable barriers. 



Bodies of water are other effective barriers. Few mammals 

 will swim far in salt water, although fresh water is not so effec- 

 tive in stopping them. Recently several deer were found four 

 miles off the coast of New Jersey in an exhausted condition, and 

 when the tide or current is with them these animals have been 

 known to swim twenty miles in the ocean. However, studies of 

 the mammals on islands indicate that only a few miles of salt 

 water acts as a complete barrier. 



Deserts, or a lack of water, are as difficult to cross as a super- 

 abundance of water. Only animals highly adapted to drought 

 can stand the desert. Reptiles succeed better than either amphib- 

 ia or mammals, although certain toads and mammals have 

 become specialized for life in very dry regions. 



Other barriers of physiological nature are less well under- 

 stood. Throughout a large area of central Europe there is a 

 complete lack of urodeles, correlated with a high lime content 

 in the soil. Some mammals seem to avoid areas which lack a 

 supply of salt, and alkalis in the water supply prevent spread 

 into other regions. It is unknown how far the chemical nature of 

 the soil and water has affected migration; but as the soil dic- 

 tates the plant life, herbivorous animals would naturally seek 

 plant food to which they were formerly adapted, and in this 

 indirect way the soil would affect animal life. 



Pathways from one region to another must be present or all 

 islands would be limited to birds and flying mammals. This we 

 know is not true. America is completely cut off from Asia, yet 

 the animal life of North America is very similar to that of 

 northern Europe and Asia. Evidently a land bridge at one time 

 existed between Alaska and Siberia. The long stretch of the 

 Aleutian Islands which leaves a relatively short space of open 

 water between Alaska and Kamchatka would permit the migra- 

 tion of human beings in canoes; and in view of the fact that 

 most of the islands are volcanic, and the region is in a condition 

 of geological flux, the existence of a land bridge in recent times 

 is more than mere speculation. A rise or fall of a few hundred 

 feet in the land surface would make a land bridge; and the ev- 



