The Evolution of the Horse. 
125 
of that of the brachydont ancestor. It is perfectly clear that 
this lengthening of the crown adds greatly to the power of the 
teeth as organs of mastication, and enables the animals in which 
it has taken place to find their sustenance among compara- 
tively dry and harsh herbage, the stalks of which often contain 
much hard mineral matter, instead of being limited to the more 
soft and succulent vegetable productions of the marshes and 
forests, in which the primitive brachydont forms of Ungulates 
mostly dwelt. 
The incisors, or front teeth of the horse, have a peculiarity 
not found in those of any other mammal, and seen only in the 
EquidcB of comparatively recent geological formations. In the 
most primitive species these teeth were simple, and chisel or 
awl shaped ; when their crowns became worn in consequence of 
long-continued use, they presented an external ring of enamel, 
surrounding a core of the dentine, or ivory, of which the bulk of 
the tooth is composed. The next modification consisted in the 
development of a ridge along the hinder border of the base of 
the crown. By the continuous increase of this ridge, and its 
union with the edges of the main part of the crown, a deep hole is 
produced, the orifice of which is transversely elongated, and placed 
rather behind the cutting-edge of the tooth. This is the condition 
seen in a colt s incisor which has just cut the gum. As wear 
takes place, the surface, besides the external enamel layer, as in 
an ordinary simple tooth, shows, in addition, a second inner ring 
of the same substance surrounding the pit, which, of course, 
adds greatly to the efficiency of the tooth as an organ for biting 
tough, fibrous substances. This pit, generally filled in the living 
animal with particles of food, is conspicuous from its dark 
colour, and constitutes the £; mark" (Fig. 3 a), by which the 
age of the horse is judged ; and in consequence of its only 
extending to a certain depth in the crown, it becomes obliterated 
as the tooth wears away, which then assumes the character of 
that of an ordinary incisor, consisting of only a core of dentine, 
surrounded by an external enamel layer. 
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 quietly walking over marshy and 
yielding surfaces, and the tapirs and rhinoceroses — which, as 
mentioned before, have altered comparatively little from the 
primitive Eocene forms — still haunt the borders of streams and 
the shady depths of the forests, as was probably the habit of their 
