THE ENERGY OF LIFE EVOLUTION. 



793 



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 op Primitive Cameloid Poebrotherium labiattim, 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 sejvncfvt) (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-Antklope (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, 



