The great order Rodentia—rats, mice, rabbits, porcupines, squirrels, beavers, etc., derives its name from the Latin verb rodere, to gnaw, or eat away (something), and is characterized by the great development of the front (incisor) teeth, by means of which rodents get their living by biting off, or gnawing through, the plants and woody stems on which they feed, or which they use in constructing their dwellings. All are primarily vegetable eaters, yet none will refuse a meal of flesh when opportunity offers to get it, and some are decidedly carnivorous, especially as to fish. They are distributed all over the world, including the Australian region. They are chiefly terrestrial, and often burrow or live in ready-made burrows. Some are aquatic, such as the voles; others, like the squirrels, are arboreal. In perhaps a majority of the forms the hind legs are much longer and stronger than the forelegs, giving the animals great leaping power, while the forefeet, with their long and flexible fingers, are constantly used as hands. Many are beautifully marked in varied tints of gray, brown, red, and black, so that their pelts have value in the fur market; and their flesh is an important element in human food. On the other hand the activity of these animals, when numerous, causes serious damage to gardens, crops and orchards and one of them, the rat, is unquestionably the most dangerous animal to human health and prosperity in the whole animal kingdom. The fecundity of the smaller, murine species, is great, and from time to time they increase inordinately in favorable places, and swarm abroad in vast and destructive migrations. Were it not for the fact that the rodents furnish the principal part of the food of predatory mammals, reptiles, and birds, and are thus kept down, the globe would soon become so populous with this tribe that hardly anything else could maintain existence.

The distinguishing anatomical characteristic of the rodents is the dentition. The canines, so essential to carnivorous, predatory animals, are here completely absent, and a long empty space intervenes between the incisors and the molars, or cheek teeth, which vary greatly in number and form among the different families. The incisors consist of a single pair in each jaw, very large and strong, and composed of vasodentine, faced only  with hard enamel, often yellow or red. As the softer substance behind the facing wears away more easily, the incisor takes a chisel shape, leaving the hard enamel in front projecting slightly as a cutting edge; thus these teeth always remain sharp. The rodents are traced back in their lineage to the order Tillodontia of Eocene time. The oldest family of modern type in the order is that of the squirrels.

The world-wide tribe of rats and mice is formed by a group of eight families, of which the typical one (Muridæ) alone contains a third of all Rodentia, and the other seven creatures differing greatly from these familiar models. Many are small, such as the house mouse (originally a native of southeastern Asia, as also were the rats that commerce has carried all over the civilized globe), and the even tinier harvest mice, gray or brown in plain color, and with long, slender and nearly hairless tails and legs fairly equal in size. Thence in size they grade up to the stature of the rat, and from that on to the South African "springhaas" which is as big as a rabbit, and to our muskrat, two feet long, counting in its tail. Although essentially alike in structure some have varied widely from the ordinary type. Thus the jerboas, several species of which inhabit the plains of Asia and Africa, have the hind legs so long that their bones are considerably longer than the distance from the root of the tail to the nose; and they progress in long rapid leaps, balancing themselves by long tails, often tufted at the end. The big "jumping hare" of South Africa has much the appearance of a kangaroo with a squirrellike tail; and a genus of exquisitely dressed mice in our sandy Southwest are called "kangaroo" mice. In fact one of our commonest reddish field mice, found all over the country, has similar proportions, and is remarkable for its long leaps when hurried.

A shortening of the tail is seen in the voles, to which the common meadow mice of various species belong, and still more in the lemmings, in the Old World mole rats, and in our pouched gophers. All these are not only ground-keeping kinds, but burrowers, and have no use for a long tail, save in the case of the muskrat, which is really a big vole that has taken to an aquatic life, and needs an oar to scull himself through the water; for muskrats swim more by means of their tails than by their feet. The foremost burrowers are the pouched gophers, whose long tunnels, and food-getting, do so much damage to crops in the central plains region of this country. They must be distinguished from the ground squirrels, also called "gophers."

An interesting diversity of habits may be met with here. Some rodents live in deeply excavated burrows, others in shallow diggings or holes in stumps and rock crevices; some, like the water voles, reside in holes in the banks of streams, or, like the muskrat, heap up "houses" in a marsh in which to pass the winter in security; while still others construct ball-like nests among the herbage, or in bushes and trees. Some truly hibernate in cold countries, like the famous dormice of Europe, and our equally sound sleeper, the American jumping mouse; but mostly they stay in snug habitations and live through the winter on collections of food, or, like field mice, gather seeds abroad even in the coldest weather, or poke about under the snow for food, as do the lemmings. From time to time certain species, especially of the short-tailed field mice and the lemmings, multiply excessively in some district, and then are forced to spread away from their birthplace in those migrations of myriads which form the "plagues" that devastate large tracts of country. They march on until an accumulation of enemies and an epidemic of illness combine to kill them off.