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. 



"If 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 



