ere parallel, but that they frequently cross; and that thus, in the
act of mastication, a hard surface in the one is constantly applied to a
soft surface in the other, and _vice versa_. They thus constitute a
grinding apparatus of great efficiency, and one which is repaired as
fast as it wears, owing to the long-continued growth of the teeth.

Some other peculiarities of the dentition of the horse must be noticed,
as they bear upon what I shall have to say by and by. Thus the crowns of
the cutting teeth have a peculiar deep pit, which gives rise to the
well-known "mark" of the horse. There is a large space between the outer
incisors and the front grinder. In this space the adult male horse
presents, near the incisors on each side, above and below, a canine or
"tush," which is commonly absent in mares. In a young horse, moreover,
there is not unfrequently to be seen in front of the first grinder, a
very small tooth, which soon falls out. If this small tooth be counted
as one, it will be found that there are seven teeth behind the canine on
each side; namely, the small tooth in question, and the six great
grinders, among which, by an unusual peculiarity, the foremost tooth is
rather larger than those which follow it.

I have now enumerated those characteristic structures of the horse which
are of most importance for the purpose we have in view.

To any one who is acquainted with the morphology of vertebrated animals,
they show that the horse deviates widely from the general structure of
mammals; and that the horse type is, in many respects, an extreme
modification of the general mammalian plan. The least modified mammals,
in fact, have the radius and ulna, the tibia and fibula, distinct and
separate. They have five distinct and complete digits on each foot, and
no one of these digits is very much larger than the rest. Moreover, in
the least modified mammals, the total number of the teeth is very
generally forty-four, while in horses, the usual number is forty, and in
the absence of the canines, it may be reduced to thirty-six; the incisor
teeth are devoid of the fold seen in those of the horse: the grinders
regularly diminish in size from the middle of the series to its front
end; while their crowns are short, early attain their full length, and
exhibit simple ridges or tubercles, in place of the complex foldings of
the horse's grinders.

Hence the general principles of the hypothesis of evolution lead to the
conclusio