336 EVOLUTION OF THE HORSE. 



horse, also seems to prove that his type is the more ancient 

 one of the two. Judging by the general conformation, 

 especially as regards the comparative size of head, Burchell's 

 zebra appears to be the nearest akin, among asses, to the 

 horse ; and the mountain zebra, the furthest removed. The 

 recent extinct quagga more closely resembled the horse, than 

 does Burchell's zebra. 



Conditions which Modified the Form of the Horse. 



— The conditions which have produced an animal (the horse) 

 with a single toe to each of its feet, from ancestors with five 

 toes, have been, apparently, those of soil, combined with a 

 tendency to place the weight of the limb more on one 

 particular digit than on any other digit. The fact that the 

 feet of the tapir have undergone, practically, no modification 

 for several millions of years, proves that his conditions of life 

 at the present day are nearly the same as they were when 

 the ancestors of the horse, like those of the tapir, had four 

 hoofs on each front foot, and three on each hind one. Feet 

 like these, which, under the influence of pressure, had the 

 faculty of spreading out, were admirably suited for going 

 through soft ground similar to that over which the tapir still 

 roams in a wild state ; for 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 for progression on hard ground, on 

 account of expenditure of muscular power required to restore 

 them to their normal state, and from increased friction. The 

 less lateral play the digits would have, the faster would the 

 animal be able to travel over hard ground. For instance, the 



