Page:Popular Science Monthly Volume 10.djvu/306

they resemble the horse in the broad features of their organization. They differ in the characters of their fore and hind limbs, and present important features of difference in the teeth. The forms to which I now refer are what constitute the genus Anchitherium. We have three complete toes; the middle toe is smaller in proportion, the inner and outer toes are larger, and in fact large enough to rest upon the ground, and to have functional importance—not an animal with two dew-claws, but an animal with three functional toes. And in the forearm you find the ulna a very distinct bone, quite readily distinguishable in its whole length from the radius, but still pretty closely united with it. In the hind-limb you also meet with three functional toes. The structure of the hind-foot corresponds with that of the fore-foot; but in the hind-leg the fibula is better developed. In some cases I have reason to think that it is complete; at any rate this lower end of it is quite distinctly recognizable. But the most curious change is that which is to be found in the character of the teeth. The teeth of the Anchitherium have, in the first place, so far as the incisors are concerned, a rudimentary pit. The canine teeth are present in both sexes. The molars are short; there is no cement, and the pattern is somewhat like this (drawing on the blackboard). In the upper jaw, there are two crescents and two oblique ridges, while in the lower jaw you have the double crescent. It is quite obvious that this (illustrating from drawing) is a simpler form than that. By increasing the complexity of those teeth there, we have the horse's teeth. These are all the forms with which we are acquainted respecting the past history of the horse in Europe. When I happened to occupy myself with this subject some years ago, notwithstanding certain difficulties, the facts left no doubt whatever in my mind that we had here a general record of the history of the evolution of the horse. You must understand that every one of these forms has undoubtedly become modified into various species, and we cannot be absolutely certain that we have found those species which constitute the exact line of mollification, but it was perfectly obvious that we had here in succession, in time, three forms of the horse-type, of which the oldest came nearest to the general mammal. We saw that the forms which had existed afterward had undergone a reduction of the number of their toes, a reduction of the fibula, a more complete coalescence of the ulna with the radius. The pattern of the molar teeth had become more complicated and the interspaces of their ridges had become filled with cement. In this succession of forms you have exactly that which the hypothesis of evolution demands. The history corresponds exactly with that which you would construct a priori from the principles of evolution. An alternative hypothesis is hardly conceivable, but the only one that could be framed would be this, that the Anchitherium, the Hipparion, and the horse, had been created separately and at separate epochs of time. For that hypothesis there could be