MAMMALS. 375 



are renewed from below. In each incisor two parts can 

 be distinguished: the anterior face of the tooth is covered 

 with a very hard layer (enamel), while the posterior sur- 

 face is composed of a much softer dentine. This dentine 

 wears away much faster than the enamel, and the result 

 is that the teeth are constantly kept at a chisel-edge. 



Lowest of the rodents come those forms familiarly 

 known as hares and rabbits, wdth disproportional hind 

 legs and long ears. The distinction between the two- 

 hares and rabbits is very slight, the true rabbit being a 

 native of southern Europe. All the rest are hares. In 

 America, however, the term rabbit is usually restricted 

 to the small burrowing forms. 



The porcupines, with some of their hair changed to long 

 sharp spines, efficient weapons of defence, come next. 

 These occur in both hemispheres, but the American forms 

 are mostly arboreal, while those of the Old World burrow. 

 Allied to them in structure, but differing in fur, are the 

 chinchilla and the coypu of South America, the latter fur- 

 nishing the well-known 'nutria fur.' The same country 

 furnishes the stupid, so-called guinea-pigs, whose young 

 shed their milk-teeth before birth, and the giants of 

 rodents, the capybara, with a body four feet in length. 



Rats and mice are the great pests of the order. Our 

 common brown rat is a recent immigrant. The early set- 

 tlers brought with them the black rat, the brown rat being 

 then unknown in western Europe, but about 1720-30 the 

 latter came west from the Volga region, and gradually 

 spread all over western Europe and then over America, 

 the black rat disappearing before the invader. There are 

 many rat-like forms, among them the lemmings of the 

 Arctic regions, vast hordes of which occasionally overrun 

 Norway; the dormice, which hibernate in winter; the 



