Another group of familiar animals materially reinforces the results already established. After what has been said, it will not be difficult to perceive the meaning of the resemblances among mice of the house and field, and of rats and rabbits and squirrels. All of them possess heavy curved gnawing teeth, or incisors, and lack the flesh-tearing or canine teeth. They agree in many other respects which distinguish them as a separate natural order of the mammals called the rodentia. Again we find a highly aberrant form in the flying squirrel, which leads toward an order with another plan of body. This animal is a true rodent, which lengthens its leap from branch to branch by means of a fold of skin stretching between its fore and its hind limbs. It is an animated aeroplane, and it shows in part how bats have originated. The wing of a bat is an elastic membrane stretching not only between the two legs of one side, but also between the greatly lengthened "fingers" of the fore limb. But the bones of arm, wrist, and fingers are almost precisely the same in number and relation as in walking forms. The fact that this peculiar wing adheres to a plan belonging to the anterior legs of walking or climbing types has no reasonable explanation save that of evolution. The well-known group of hoofed animals, including horses and cattle, is also valuable for our present purposes, as well as in a later connection when the evidence of fossils is described. The elephant possesses five toes armed with well-developed nails or hoofs. A tapir has four or three toes, and it would seem that its ancestor had had five toes, of which one or two had been lost. A rhinoceros possesses three toes, and its foot is constructed internally like the elephant's with the outer elements absent. The horse comes last with one large toe and hoof, but on either side of the main bones of this digit are vestiges of what must have been toes in its ancestors. Among the even-toed forms the hippopotamus has four which reach the ground, with a vestige of a fifth, so this animal has apparently descended from a typical mammal with the full number along a different line from that taken by the odd-toed forms. A pig has a cloven hoof, made up of what we may call the third and fourth members of a series of five digits, but the second and fifth fingers and toes are present, though they are withdrawn from the ground so as to be no longer functional; this animal seems to have proceeded further along the same line taken by the hippopotamus. A deer, with still smaller rudiments at the sides of its double foot, leads in the comparative series to the camel with a cloven hoof devoid of any such relics.