KINETOGENESIS. 293 



lateral parts have been supported and even com- 

 pressed. The reptilian ancestors of the mammals do 

 not possess- these keels. 



Now, I have shown that 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 ani- 

 mals. But in many of these, as the Carnivora, hip- 

 popotamus, and the camels, there is developed under 

 the toes a soft cushion, which greatly reduces this con- 

 cussion. In these species the keel makes no further 

 progress. In other lines, as those of the horse, the 

 pig, and of the ruminants, the ends of the toes are ap- 

 plied to the ground, and are covered with larger hoofs, 

 which surround the toe, and the cushion is nearly or 

 quite dispensed with. These animals are especially 

 distinguished by the fact that their metapodial keels 

 extend entirely round the end of the bone, dividing 

 the front, as well as the end and back (Fig. 71) ; since 

 the front of the metapodial is out of the reach of the 

 sesamoid bones, its keel would seem to be a mould- 

 ing to the groove of the first phalange, which is itself 

 moulded by the middle and posterior part of the meta- 

 podial keel (Wortman.) 



A third and similar example is furnished by the 

 elbow- joiijt of the Quadrumana and Diplarthra. In 

 the lower 'Mammalia, including the Carnivora (Fig. 

 73), the distal end of the humerus presents a subme- 

 dian groove, which receives the ulna, and on the inner 



