700 EVOLUTION OF THE HORSE. 



state ; because the increase of the area of support thus 

 offered by the feet, was a direct help in preventing the 

 animal from sinking too deeply in the mud over which he 

 travelled. It is evident that this lateral play of the digits 

 entailed loss of speed in progression on hard ground, on 

 account of expenditure of muscular power required to 

 restore them to their normal position, and from increased 

 friction. The less lateral play the digits have, the faster 

 would the animal be able to travel over hard ground. We 

 may therefore infer that the decrease in the number of the 

 digits of the horse's foot, has been due to residence on 

 increasingly dry soil during many thousands of genera- 

 tions. With the loss of lateral play in the foot, there has 

 been a consequent increase of speed, which was necessary 

 for protection against the attacks of carnivorous animals, 

 like the puma, wolf, and lion, whose conformation was 

 unsuited for predatory operations in the morasses which 

 afforded an asylum for ancestors of the horse. 



Evolutional specialization in one structure is not neces- 

 sarily accompanied by specialization in other structures. 

 Although, as regards the number of toes, the horse's foot 

 is better suited for high speed than that of any other 

 animal ; the fact remains that (for instance) the cheetah 

 (Fig. 3), hare, wolf, and wild dog, which have four working 

 toes on each foot, are comparatively much faster than 

 any wild horse or ass, because their muscles are relatively 

 a great deal longer. The extreme length of the limbs of 

 the black buck (Fig. i) compensates him to a large extent 

 for the disadvantage of having two working toes on each 

 foot. In his ancestors, the tendency to place weight both 

 on the third and fourth toe, was so evenly distributed on 

 these two digits, that the balance between them has re- 

 mained practically undisturbed for ages, and under modi- 

 fying conditions which have nearly doubled the compara- 

 tive length of his limbs. 



