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 meadow-mice. The mice 
of the genus Microtus (formerly Arvicola) rep- 
resent a group which embraces a large number 
