Order 13.-RODENTIA.

Gnawing animals.

Squirrels, marmots, rats, rabbits, hares, hamsters, the beaver, capybara, porcupine, and several other animals similarly organized, belong to the order of gnawers or rodents.

Instead of canine teeth, they have strong chiselshaped teeth in front and molar teeth behind. These teeth are adapted for gnawing. The brain of rodents is very like that of the insectivores. Rats and mice are well-known examples of this order, and have been transported by man to all parts of the world. They also appear, like man, to possess constitutions adapted to any climate, living as well in the torrid zones as in the far north, or the temperate parts of the world.

In "the struggle for existence," so well delineated by Mr. Darwin, in his " Origin of Species," the rat is mentioned as an example of which one species frequently takes the place of another species under the most different climates.

Mr. F. Buckland, a good authority, believes that the so-called old English rat-the black rat-is not the aboriginal rat of the British soil, but was introduced from France. At all events, the commor brown rats have very nearly extirpated the black rats, once so very abundant, by the simple process of devouring their sable brethren.* The cave rat

of America is undoubtedly a striking example of the effect of the use and disuse of bodily organs, as insisted on by Mr. Darwin in the modification of species.

The cave rat is blind, although the eyes are of

* Buckland's "Curiosities of Natural History."