this pressure, together with the concussion with the ground, appears to have permitted the protrusion of the middle line in the form of a keel, while the lateral parts have been supported and even compressed. The reptilian ancestors of the mammals do not possess these keels.
| Fig. 2. | Fig. 3. | Fig. 4. |
| Fig. 2.—Hind-foot of Primitive Cameloid Poëbrotherium labiatum, showing grooved astragalus and first toe-bones without keel in front at lower end. (From Colorado.) | ||
| Fig. 3.—Hind-foot of Three-toed Horse (Protohippus sejunctus) (from Colorado), showing grooved astragalus, and trace of keel on front of lower end of first bone of middle toe. | ||
| Fig. 4.—United First Bones of Two Middle Toes of Deer-Antelope (Cosoryx furcatus), showing extrusion of keel on front of lower end. (From Miocene of Nebraska.) | ||
Now, the lines of mammalian descent displayed by paleontology are characterized, among other things, in most instances, by the gradual elevation of the heel above the ground, so that the animal walks on its toes. It is evident that in this case the concussion of running is applied more directly on the ends of the bones of the foot than is the case where the foot is horizontal. As a consequence we find the keel is developed farther forward in such animals. But in many of these, as the carnivora, the hippopotamus, and the camels, there is developed under the toes a soft cushion, which greatly reduces this concussion. In these species the keel makes no further progress. In other lines,
