THE ANCESTRY OF THE HORSE 139 



because they are midway in time and structure between 

 the horse of the past and present, tell us that then all 

 horses were small and that all had three toes on a foot, 

 while the fore feet bore even the suggestion of a fourth 

 toe. From this to our Eocene Eohippus with four 

 toes is only another long-time step. We may go even 

 beyond this in time and structure, and carry back the 

 line of the horse to animals which only remotely re- 

 sembled him and had five good toes to a foot; but 

 while these contained the possibility of a horse, they 

 made no show of it. 



Increase in size and decrease in number of the toes 

 were not the only changes that were required to trans- 

 form the progeny of the Eohippus into a horse. 

 These are the most evident; but the increased com- 

 plexity in the structure of the teeth was quite as 

 important. The teeth of gnawing animals have often 

 been compared to a chisel which is made of a steel plate 

 with soft iron backing, and the teeth of a horse, or of 

 other grass-eating animals, are simply an elaboration of 

 this idea. The hard enamel, which represents the steel, 

 is set in soft dentine, which represents the iron, and in 

 use the dentine wears away the faster of the two, so that 

 the enamel stands up in ridges, each tooth becoming, 

 as it is correctly termed, "a grinder." In a horse the 

 plates of enamel form curved, complex, irregular pat- 

 terns; but as we go back in time, the patterns become 

 less and less elaborate, until in Eohippus, standing at 

 the foot of the family tree, the teeth are very simple in 

 structure. Moreover, his teeth were of limited growth, 

 while those of the horse grow for a considerable time, 

 thus compensating for the wear to which they are sub- 

 jected. 



We have, then, this direct evidence as to the 

 genealogy of the horse, that between the little Eocene 



