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.” Ina 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, while — 
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 Hyracothere and the modern — 
horse we can place a series of animals by 
which 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 
