50 ANIMAL COMPETITORS 



kind of mouse which now and then in this 

 country, and still more frequently in eastern 

 Europe, appears suddenly in such vast num- 

 bers as to constitute a veritable plague, ruining 

 the produce of the year in all directions. 



Sometimes wild animals increase in numbers 

 so suddenly that the change has been likened 

 to a tidal wave, and ignorant people have re- 

 garded the invasion as of miraculous origin. 

 The belief that crickets, locusts, frogs, and even 

 mice sometimes fall from the clouds is still held 

 in many countries. 



"The careful observer, however, sees little mystery 

 in the phenomena mentioned. He has studied the 

 general habits of animals their food, their powers 

 of reproduction, their migrations, the checks on their 

 increase due to natural enemies, disease, and varying 

 climate and consequently he attributes sudden 

 changes in their numbers to known causes. In such 

 changes he recognizes, especially, the influence of 

 man, both direct and indirect, and his responsibility 

 for interferences that greatly modify the operations 

 of nature." 



American voles or meadoiv-mice. The mice 

 of the genus Microtus (formerly Arvicola) rep- 

 resent a group which embraces a large number 



