58 ANIMAL COMPETITORS 
These mice, like their European relatives, the 
voles and lemmings, at times increase in num- 
bers abnormally, but the causes are little un- 
derstood. At such times they multiply with 
amazing rapidity, and begin breeding when 
only six months old. 
“Tf a thousand pairs of field-mice,’’ remarks 
Mr. Lantz, ‘‘survive the winter in any neigh- 
borhood, the potential conditions for a vole 
plague are present. If, now, instead of normal 
reproduction, circumstances bring about a con- 
siderable increase both in the number of young 
at ‘a time and in the number of litters in a sea- 
son, the probability of a plague is greatly in- 
creased.’’ 
Plagues of field-mice. Swarms of mice dev- 
astating the fields have been seen by very 
few American farmers, though occasional se- 
vere outbreaks of this kind are on record in 
Nova Scotia and in various eastern states. 
They have been all too frequent in all parts of 
the Old World from the earliest times, and used 
to be regarded superstitiously as punishments 
sent from on high. The valley of the Danube 
and the plains of southern Russia seem to have 
