144 COMMON BRITISH ANIMALS 



For 3^ears voles may exist in any locality in numbers, 

 which do not effect any inordinate damage, but quite 

 suddenly a s&ason especially favourable to them may 

 occur when " they come upon the earth," says Blasius, 

 '' as if by magic in their tens of thousands. A mild 

 winter and an unusual abundance of good grass may 

 perhaps account for it. Many instances are known 

 in which a great part of the harvest has been 

 destroyed over large tracts of country by their 

 inordinate increase, and more than a thousand acres 

 of young birch trees have been destroyed by their 

 gnawing the bark.^^ During the second decade of 

 the nineteenth century several vole plagues visited 

 the Lower Rhine, and Brehm says : '^ The fields 

 were so undermined in places that you could scarcely 

 set foot on the ground without touching a vole-hole, 

 and innumerable paths were deeply trodden between 

 these openings. On fine days it swarmed with voles, 

 Avhich ran about openly and fearlessly. If they were 

 approached from six to ten rushed to the same hole 

 to creep in. It Avas not difficult in the crush to kill 

 half a dozen with one blow from a stick. All seemed 

 to be strong and healthy, but mostly rather small, 

 and for the greater part were probably young ones. 

 Three weeks after I revisited the place. The number 

 of voles had actually increased, but the animals were 

 apparently in a sickly state. Many had mangy places 

 or sores over the whole body, and even in those 

 which appeared sound, the skin was so loose and 

 delicate that it could not be roughly handled with- 

 out destroying it. When I visited the place for the 

 third time, four weeks later, every trace of them had 



