RELATION TO AGRICULTURE 37 



ently, however, a period arrives when the rodents 

 seem to multiply as if by magic. One litter 

 after another is born, and within a few weeks 

 these litters produce new litters. Perhaps birds 

 and vermin have withdrawn from the neighbor- 

 hood, attracted to another locality by an abun- 

 dance of food, and the rodents are free to breed 

 unmolested. Then, without warning, they sweep 

 in countless hordes broadcast over the land. 



In this way the lemmings appear every few 

 years in Norway and Sweden. They sweep in 

 incalculable thousands slowly across the country, 

 devouring all vegetation that stands in their path 

 and leaving a brown, barren stretch behind. They 

 pause for nothing, swim broad rivers and lakes, 

 climb mountains, cross prairies, and finally plunge 

 into the ocean. At their first appearance hosts 

 of predatory animals gather. Foxes, wolves, 

 small vermin, and birds of every description as- 

 semble on the trail of the lemmings and fight the 

 retreating horde until it is swallowed by the sea. 

 Even cattle and horses trample the rodent army 

 under foot when it attempts to cross their pasture. 

 Nature orders all her living forces to prey upon 

 the insurgents. 



The history of Great Britain is filled with men- 

 tionings of "plagues of mice" which from time to 

 time have arisen to destroy the meadows and the 

 root-crops. And her history also is replete with 



