60 ANIMAL COMPETITORS 
ened to develop into a plague as great as any 
recorded, and the facts concerning it have been 
studied and preserved in a pamphlet by Stan- 
-ley E. Piper of the Biological Survey. The 
species was the black or Carson mouse (Micro- 
tus montanus), which is widely prevalent west 
of the Rocky Mountains. 
“‘The greatest loss occurred in the rich fields of 
alfalfa bordering Humboldt River for the last ten 
or twelve miles of its course. Noticeable here through 
gradually increasing damage during 1906, the field- 
mice appeared early in the summer of 1907 in alarm- 
ing numbers. By November they had overrun a 
large part of the cultivated area, and on many large 
ranches were estimated to number from 8,000 to 
12,000 to the acre. Fields were literally honey- 
combed by their holes, which numbered about 24,000 
to the acre. During the summer they ruined one- 
‘third. of the alfalfa, destroyed three-fourths of the 
_potatoes, and severely injured root-crops, as beets and 
carrots. Upon the disappearance of green food in 
the fall they attacked the roots of alfalfa, so as to 
render many alfalfa fields a total loss. They girdled 
and killed most of the young shade-trees planted 
along ditches and about the borders of fields, while 
small orchards suffered severely.’’ 
Decline of the visitation. By January, 1908, 
the ravages had extended over considerably 
