62 ANIMAL COMPETITORS 
‘In the spring,’’ Mr. Piper relates, ‘‘the mice in 
this locality failed to reproduce, while the same spe- 
cies was breeding prolifically in other localities. In 
March several hundred females were examined in 
Humboldt Valley, of which very few were pregnant. 
Moreover, the mice themselves presented a different 
appearance from those seen when the plague was at 
its height—a fact noted by many ranchmen in the 
valley. During the fall of 1907 larger and much 
more vigorous individuals predominated, while in 
the spring of 1908 scarcely any of these remained. 
They continued in destructive numbers until ; 
May. But they did not noticeably breed with the 
return of favorable weather and by August had 
practically disappeared.’’ 
Prevention of plagues of mice. Unfortu- 
nately the liability to such ‘‘plagues’’ in- 
creases with the spread of settlement and 
cultivation. ‘‘Agricultural development dis- 
tinctly increases the danger by furthering the 
destruction of their natural enemies, by furnish- 
ing a great abundance of food, and by increas- 
ing the area in which they find favorable 
homes.’’ On the other hand, the prevention 
of plagues is comparatively easy. Systematic 
poisoning must be relied upon to repress them 
when they are obviously on the increase, but 
