24 EVOLUTION OF THE HORSE 



Hippidium. Pleistocene. South America. The feet are 

 like those of Equns, except that they were short and stout. The 

 teeth are like those of Pliohippus, from which it is supposed to be 

 descended. The skull is large and long with very long slender 

 nasal bones. Casts of the skull and limbs presented by the 

 Museo Nacional of Buenos Ayres, Argentine Republic, are ex- 

 hibited here. 



Meaning of the Change in Feet and Teeth. 



Along with the disappearance of the side toes in the evolution 

 of the Horse there is a considerable increase in the proportionate 

 length of the limbs, and especially of the lower part of the leg and 

 foot. The surfaces of the joints, at first more or less of the ball- 

 and-socket kind, which allows free motion of the limb in all direc- 

 tions, become keeled and grooved like a pulley-wheel, permitting 

 free motion forward and backward, but limiting the motion in 

 all other directions and increasing considerably the strength of 

 the joint. By this means the foot is made more efficient for 

 locomotion over a smooth regular surface, but less so for travel- 

 ing over very rough ground, and it becomes of little use for 

 striking or grasping or the varied purposes for which the feet of 

 poly dactyl animals are used. 



The increased length in the lower leg and foot increases the 

 length of the stride without decreasing its quickness. The 

 heavy muscles of the leg are chiefly in the upper part, and to in- 

 crease the length of the lower part changes the centre of gravity 

 of the limb very little. Consequently the leg swings to and fro 

 from the socket nearly as fast as before, since in an ordinary 

 step the action of the leg is like that of a pendulum and the 

 speed of the swing is regulated by the distance of the centre of 

 gravity from the point of attachment, as that of a pendulum is 

 by the height of the bob. To increase the length of lower leg 

 and foot therefore gives the animal greater speed ; but it puts 

 an increased strain on the ankles and toe- joints, and these must 

 be strengthened correspondingly by converting them from ball- 

 and-socket joints to "ginglymoid" or pulley joints. Additional 

 strength, likewise at the expense of flexibility, is obtained by 

 the consolidation of the two bones of the fore-arm {ulna and 



