THE GEOGRAPHICAL DISTKIBUTION OF ANIMALS 367 



barriers that prevent animals from going from one region to 

 another. Finally, after an animal has reached a region far 

 from its original home, it is liable to become profoundly 

 changed in habits, character, and, possibly, in structure. 



Why is it that certain animals occupy certain areas? 

 How do they pass from one region to another? What 

 retains animals within the boundaries of certain areas? 

 How are animals modified when they migrate to new areas ? 

 These are some of the questions that arise from the study 

 of the distribution of animals. 



Means of dispersal. — One of the most universal means 

 by which animals are distributed from one region to an- 

 other is through the agency of man. For example, the eel 

 has been put into many ponds and streams of the United 

 States that it could not have reached except by the aid of 

 man. The horn fly, a serious pest to cattle, was brought 

 over from Europe, probably in ships with live stock. By 

 the same agency, the English sparrow and even the horse 

 have been brought to America. 



Birds can fly from one region to another, across wide 

 expanses of water or over high mountain ranges. Fish can 

 swim to widely remote parts of the sea. Many mammals 

 can swim across narrow seas, rivers, etc. Some mammals, 

 notably the whales and the seals, can swim for leagues at 

 sea. Mammals are sometimes carried on floating masses 

 of vegetation to islands in the sea. Birds are blown across 

 seas by high winds. The bodies of some animals, as roti- 

 fers and infusorians, may become so dry and light that they 

 are carried by winds. Insects are often carried on birds' 

 feet, blown by the wind, or floated on the water in logs of 

 wood, etc. Worms crawl, frogs and toads leap, snakes 

 swim and crawl. 



