THE DISTRIBUTION OF ANIMALS 



461 



another river which may be only a few miles away. Waterfalls often 

 bar the progress of animals. Niagara Falls formerly kept the marine 

 lampreys out of the Great Lakes ; but, when shipping canals were built 

 from the St. Lawrence River, the lampreys entered the Great Lakes and 

 have become a serious menace to the fish. 



By LIFE photographer, George Skadding, Copyright TIME, Inc. 



Fig. 29.1. Lamprey eel attached to a large trout taken from one of the Great Lakes. 



This marine eel has found its way into the Great Lakes through ship canals which 



have been built from the St. Lawrence River. 



Climatic Barriers. Tropical and warm-temperate animals are re- 

 strained from northward movements by the long, cold, and dark arctic 

 winters. All the monkeys are tropical or warm-temperate — none in- 

 habits the United States and only one is found in Europe and that is in 

 southern Spain where a number of subtropical species are found. On 



