THE DOCILE MAMMALS. 241 



are very prolific, and would multiply to an enormous extent 

 were they not held in check by hawks and beasts of prey. 

 In Australia, where there are no larger animals to prey 

 upon them, the rabbits introduced there from England 

 have increased so as to threaten the sheep-raising industry 

 of that country, the rabbits cropping the herbage and leav- 

 ing none for the sheep. In one colony, in 1882, 3,000,000 

 sheep died of starvation from this cause. Nearly all the 

 rodents, such as mice, rats, gophers, squirrels, as well as 

 beavers, build nests, and in this respect they are not unlike 

 birds. While the hare and rabbit are the least intelligent. 



Fio. 240. — Skull of the Beaver, t, incisor teeth; m, molars. 



building ho place of refuge, the beaver evinces a high de- 

 gree of intelligence in its architectural habits. 



Beavers build a dam in a running stream so as to create 

 an artificial pond as a refuge when attacked, as well as a 

 subaquatic entrance to their lodges and to their burrows in 

 the banks of the streams they inhabit. Beaver dams are 

 built at first by a single pair or family, and are added to 

 from year to year, and afterwards maintained for centuries 

 by constant repairs. They are built of sticks and mud, 

 and usually curve up stream, with a sloping water-face. 

 Beavers lay up stores of wood for winter use in the autumn ; 

 they can gnaw through trees eighteen inches in diameter ; 

 they work mostly at night. They often construct artificial 

 16 



