HORSES. 7 



upon their three toes, which correspond to the second, third, 

 and fourth rows in the pentadactyle, or five fingered hand, 

 as seen at Fig. 1 a. But these Paleotheres eventually faded 

 away (at the close of the Eocene period), and were succeeded 

 (in the Miocene) by another tri-dactylous quadruped, which, 

 on account of its general resemblance to the horse, as shown 

 by its fossil bones, has been called the liipparion. And, 

 although this horse-like animal had three toes upon each foot, 

 the same as its predecessors, yet upon the hard ground it 

 really walked upon one, the same as the horse does, inasmuch 

 as the outer toes had become considerably shortened, and re- 

 duced in size, as is seen at Fig. 1 b. 



The dentition, as well as the close resemblance of the feet, 

 point to a transitional position, which the hipparion must have 

 occupied in the " survival of the fittest " between the paleo- 

 theres and the modern horse. And, believing that this same 

 process of " derivative" m6dification may have been carried 

 still further, we should ultimately expect to find a creature in 

 which those outer toes were wholly wanting ; and this is 

 really the case in all the equine species. Fig. 1 c. " Thus 

 the succession in time," says Professor Owen,* " accords 

 with the gradational modifications by which paleotherium is 

 linked on to equus." 



Hence we see that the so-called " splint-bones," in the 

 limbs of the horse are simply the rudiments of those lateral 

 toes in the tridactyle feet of their ancient predecessors. 

 But occasionally we find a horse with these supplementary 

 ancestral hoofs. " In one of the latest examples," says Pro- 

 fessor Owen,f " the inner splint-bone, answering to the 

 second metacarpal of the pentadactyle foot, supported pha- 

 langers and a terminal hoof, in position and proportion to the 

 middle hoof, resembling the corresponding one in hipparionJ^ 



Such examples of tridactyle feet in the horse, though 

 usually regarded as " monsters," clearly illustrate the natu- 

 ral law of hereditary descent. 



There is a great variety in the form of the limbs, or ambu- 



* See his closing chapter on the Anatomy of Vertebrates, vol. iii. 



tin his Comparative Anatomy and Physiology of Vertebrate Animals,l vo. iii, 



