THE GNAWERS 99 



direction despite enemies. They swim rivers, and once in 

 Russia, when the swarm was confronted by the sea, all 

 swam into it, impelled by this strange instinct of unrest, 

 the entire horde being lost. 



This instinct is not peculiar to the lemmings alone. In 

 Brazil a rat plague occurs every thirty or forty years, 

 apparently when a certain seed fails. Then the animals 

 swarm over the country in large numbers. A similar rat 

 migration has been observed in Ceylon, when a certain 

 plant dies out, generally once in seven years ; and in Chile 

 rat plagues have been observed whenever a certain bam- 

 boo fails, generally once in fifteen or twenty years. These 

 movements are caused doubtless by the failure of crops 

 and the unusual increase of the animals, which move on 

 to obtain a better food supply. 



