The incisors, generally two or four on the front of each jaw, are long, stout, and curved. They are covered with enamel on the front only, and so the back part of the tooth wears away faster than the front part, and thus constantly preserves a sharp, knife-like edge, which is as serviceable as the best tempered chisel in cutting any kind of wood. The incisors never wear out. They grow from the base just as fast as they wear off at the tip. The rodents are furry, four-legged animals, with nails or claws upon their toes. They have large bright eyes, sharp, well-set ears, and all their senses are very acute. They are also distinguished for intelligence, and generally for amiable dispositions. Among them are to be found the rat, mouse, squirrel, rabbit, hare, musk-rat, dormouse, jerboa, chinchilla, porcupine, ground pig, marmot, and many others. The porcupine and his family offer an exception to the other rodents in wearing quills among the fur. The rodents differ widely in their homes; some, as the squirrels, live in trees; others in forms or lairs on the ground, as rabbits and hares; some, as the beaver and musk-rat, are chiefly aquatic. The rodents also differ in their mode of life. The squirrels, rabbits, hares,[90] and some of the mice, work and eat by day and sleep at night; others, as the rats, guinea-pigs, and porcupines, are principally nocturnal animals, coming forth from their homes about sunset. In general the rodents feed on roots, vegetables, seeds, nuts, bark, leaves, or fruit; but some of them, as the rats, are fond of meat, and indeed devour anything in the shape of food, and are as ready to eat carrion as fresh diet. All rodents are easily tamed, and are often kept in captivity. Rabbits, squirrels, and guinea-pigs have always been favorites as pets; even the rats and mice, in their white specimens with red eyes, have not been despised. [Illustration: THE NIBBLERS.] The rodents include some animals with valuable fur, as the gray Siberian squirrel, and the chinchillas of the Peruvian and Chilian mountain ranges. The rodents are generally very prolific, and to the great number of their progeny is due their continuance, in despite of the multitudes of them killed every year by their enemies. The hunter, with his trap and gun, seeks the rodents for fur or for food; hawks, vultures, and owls prey upon them; dogs, cats, and foxes constantly lie in wait for them. Rats also fight among themselves, and prey upon each other, for the victor feasts upon the vanquished. Rats are also hunted for their skins, as thousands of dozens of kid gloves have been first on the backs, not of kids, but of rats. As instances of the rapid increase of rodent families we may note that the mother rat has several litters of young each year, and ten or twelve at a litter. The fieldmouse has twelve little ones a year, but all of the same litter.