EQUIDA 379 
islands of the two softer substances, dentine and cement, separated 
by variously reduplicated and contorted lines of intensely hard 
enamel, resulted (Fig. 157, ¢). The crown continued lengthening 
until in the modern Horses it has assumed the form called “ hyps- 
odont” (Fig. 158, 3). Instead of contracting into a neck, and 
forming roots, its sides continue parallel for a considerable depth in 
the socket, and as the surface wears away, the whole 
tooth slowly pushes up, and maintains the grinding 
edge constantly at the same level above the alveolus, 
much as in the perpetually growing Rodent’s teeth. 
But in existing Horses there is still a limit to the 
growth of the molar. After a length is attained 
which in normal conditions supplies sufficient grind- 
ing surface for the lifetime of the animal, 
a neck and roots are formed, and the 
tooth is reduced to the condition of that 
of the brachydont ancestor. It is per- 
fectly clear that this lengthening of the 
crown adds greatly to the power of the 
teeth as organs of mastication, and en- sD 
ables the animals in which it has taken Fic. 158.—a, Outer view of second 
place to find their sustenance among the Upper molar tooth of Anchitheriwm, 
. (brachydont form); b, corresponding 
comparatively dry and harsh herbage tooth of Horse (hypsodont form). 
of the open plains, instead of being 
limited to the more succulent vegetable productions of the marshes 
and forests in which their predecessors probably dwelt. 
The modifications of the limbs which took place pari passu with 
those of the teeth must have been associated with increased speed, 
especially over firm and unyielding ground. Short, stout legs, and 
broad feet, with numerous toes, spreading apart from each other 
when the weight of the creature is borne on them, are sufficiently 
well adapted for plodding deliberately over marshy and yielding 
surfaces, and the Tapirs and the Rhinoceroses, which in the 
structure of the limbs have altered but little from the primitive 
Eocene forms, still haunt the borders of streams and lakes and 
the shady depths of the forests, as was probably the habit of 
their ancient representatives, while the Horses are all inhabitants of 
the open plains, for life in which their whole organisation is in 
the most eminent degree adapted. The length and mobility of 
the neck, position of the eye and ear, and great development of the 
organ of smell, give them ample means of becoming aware of the 
approach of enemies, while the length of their limbs, the angles 
the different segments form with each other, and especially the 
combination of firmness, stability, and lightness in the reduction of 
all the toes to a single one, upon which the whole weight of the 
body and all the muscular power are concentrated, give them speed 
a 
