paratus 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 grinders. 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 [comparative forms] 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
conclusion that the horse must have been derived from some quadruped
which possessed five complete digits on each foot; which had the bones
of the fore-arm and of the leg complete and separate; and which
posses