02 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 



