170 ANIMALS OF THE PAST 



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 patterns ; but 

 as we go back in time, the patterns become 

 less and less elaborate, until in the Hyraco- 

 there, standing at the foot of the family tree, 

 the teeth are very simple in structure. More- 

 over, his teeth were of limited growth, Avhile 

 those of the horse grow for a considerable 

 time, thus compensating for the wear to which 

 they are subjected. 



We have, then, this direct evidence as to 

 the genealogy of the horse, that between the 

 little Eocene Ilyracothcre and the modern 

 horse we can place a series of animals by 

 whicli we can pass by gradual stages from one 

 to the other, and that as we come upward 

 there is an increase in stature, in the com- 

 plexity of the teeth, and in the size of the 

 brain. At the same time, the number of toes 

 decreases, which tells that the animals were 



