250 A YEAR IN SCIENCE 



differences result primarily from the methods of breath- 

 ing. These two groups can again be divided into the 

 fresh and the salt water forms, and into the land and 

 aerial forms. 



Means of distribution. Animals are distributed in 

 many ways. The chief method for their dispersal is 

 by means of their own powers of locomotion. They 

 wander about to search for food, to escape drouth or a 

 sudden change of temperature, or sometimes to escape 

 an advancing enemy. Water forms are carried about 

 by waves and currents. Some smaller animals are 

 transferred on the bodies of larger ones. Many have 

 been transferred from one place to another by man. In 

 several instances this has proved a great disadvantage 

 to him. The English sparrow, for example, was intro- 

 duced into this country by man. It has been able to 

 thrive here to such an extent that it has become a 

 pest because of its great numbers and because it feeds 

 on grains, seeds, and fruits. The gypsy moth was 

 accidentally introduced into the United States by man. 

 More than a million dollars are now being spent each 

 year trying to control this pest and to protect the trees 

 of Northeastern United States. 



Factors determining distribution. Every kind of 

 animal multiplies and spreads from a given location. It 

 would ultimately be found all over the surface of the 

 earth where conditions suitable for its maintenance 

 exist except for three factors which prevent this uniform 

 distribution : 



