24 EVOLUTION OF THE HORSE 
Hippidium. PLEISTOCENE. SoutTH AmERIcA. The feet are 
like those of Equus, except that they were short and stout. The 
teeth are like those of Pliohtppus, 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 
polydactyl 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 (ulua and 
