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 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 possessed forty-four teeth, among which the crowns of the incisors and grinders had a simple structure; while the latter gradually increased in size from before backwards, at any rate in the anterior part of the series, and had short crowns.

And if the horse has been thus evolved, and the remains of the different stages of its evolution have been preserved, they ought to present us with a series of forms in which the number of the digits becomes reduced; the bones of the fore-arm and leg gradually take on the equine condition; and the form and arrangement of the teeth successively approximate to those which obtain in existing horses.