302 EVOLUTION OF THE HORSE. 



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

 wild pig, whose feet (Fig. 375) spread out a good deal 

 more than those of the steinbok (Fig. 395), is much better 

 through ''dirt" (to use a racing and hunting expression) 

 than this speedy South African antelope, but is far inferior 

 to him in a gallop 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 during thousands, if 

 not millions, of generations on dry soil 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 cheetah (Fig. 3) and 

 wolf, whose conformation was unsuited for predatory operations 

 in the morasses which had afforded an asylum for ancestors 

 of the horse. 



Points of Speed and Strength in the Fossil Horse. 



— At the outset of this investigation, I am met with the perti- 

 nent question . what is a horse ? My natural reply to this 

 will be that the horse is an animal which has only one toe to 

 each of its legs. I might also add, for the benefit of those 

 of my readers who are interested in comparative anatomy, 

 that his teeth are plentifully supplied with crtista petrosa 

 (p. 295), and that the pulley-like processes on his astragalus 

 are directed forward and outward (p. 58). The fact of unity 

 of digit^ — a peculiarity which distinguishes the horse from all 

 other mammals — will probably suffice for ordinary inquirers. 

 Taking this test, we cannot with propriety apply the term 

 horse to animals further back in equine descent than the 

 Anchltherium. The Phenacodus resembles a carnivorous 

 animal (a Dandy Dinmont or otter hound) more than he 

 does one of the horse tribe. His great length of humerus 

 must have given him considerable power in raising the 

 fore hand by the play of the shoulder and elbow joints, 

 the diminution of which play is compensated for in the 

 horse by the action of the fetlock joint His hocks and 

 knees, like those of the dog and cat, were '' well let down." 



