90 ANIMAL LIFE AND SOCIAL GROWTH 



vast numbers, advancing along the ground and 

 ^devouring every green thing.' In Lapland, 

 innumerable bands march from Kolen, through 

 Northland and Finmark, to the Western Ocean, 

 which they immediately enter; and after swim- 

 ming about for some time, perish. Other bands 

 take their route through Swedish Lapland to the 

 Bothnian Gulf, where they are drowned in the 

 same manner. They are followed in their journey 

 by bears, wolves, and foxes which prey upon them 

 incessantly. They generally move in lines which 

 are about three feet from each other and exactly 

 parallel, going straight forward over precipices 

 or through rivers and lakes; when they meet with 

 stacks of hay or corn, they gnaw their way through 

 instead of passing around them. One observer 

 remarked: "There is no sense in this." 



The lemmings march chiefly at night and may 

 cover a hundred miles before reaching the sea 

 into which they plunge in such numbers that their 

 drowned bodies form drifts on the nearby shores. 

 Such migrations are caused primarily by over- 

 population and are a symptom of maximum num- 

 bers which migration and epidemics reduce until 

 the following year relatively few individuals are left. 



Records show that ordinary "lemming years" 

 occur about 3.5 years apart but there is evidence 

 that years of extraordinary abundance show 



