The body of the lecture consists of an attempt to show, from the osteology of the genus Equus, that our modern horse proceeds from the Orohippus. The lecturer first describes the characteristics of the horse, using the term “horse” in a general sense as equivalent to the technical term Equus, and meaning not only what we now call the horse, but also asses and their modifications—zebras, etc. He invites us to pay a special attention to the foot and the teeth of the horse; and then he reasons as follows:
“If the hypothesis of evolution is true, what ought to happen when we investigate the history of this animal? We know that the mammalian type, as a whole, that mammalian animals are characterized by the possession of a perfectly distinct radius and ulna-two separate and distinct movable bones, We know, further, that mammals in general possess five toes, often unequal, but still as completely developed as the five digits of my hand. We know, further, that the general type of mammals possesses in the leg not only a complete tibia, but a complete fibula. The small bone of the leg is, as a general rule, a perfectly complete, distinct, movable bone. Moreover, in the hind-foot we find in animals in general five distinct toes, just as we do in the fore-foot. Hence it follows that we have a differentiated animal like the horse, which has proceeded by way of evolution or gradual modification from a similar form possessing all the characteristics we find in mammals in general. If that be true, it follows that, if there be anywhere preserved in the series of rocks
a complete history of the horse—that is to say, of the various stages through which he has passed—those stages ought gradually to lead us back to some sort of animal which possessed a radius, and an ulna, and distinct complete tibia and fibula, and in which there were five toes upon the fore limb no less than upon the hind limb. Moreover, in the average general mammalian type, the higher mammalian, we find as a constant rule an approximation to the number of forty-four complete teeth, of which six are cutting teeth, two are canine, and the others of which are grinders. In unmodified mammals we find the incisors have no pit, and that the grinding teeth as a rule increase in size from that which lies in front towards those which lie in the middle or at the hinder part of the series. Consequently, if the theory of evolution be correct, if that hypothesis of the origin of living things have a foundation, we ought to find in the series the forms which have preceded the horse, animals in which the mark upon the incisor gradually more and more disappears, animals in which the canine teeth are present in both sexes, and animals in which the teeth gradually lose the complication of their crowns, and have a simpler and shorter crown, while at the same time they gradually increase in size from the anterior end of the series towards the posterior.”
The professor then proceeds to show that all these conditions are fulfilled:
“In the middle and earlier parts of the pliocene epoch, in deposits which belong to that age, and which occur in Germany and in Greece, to some extent in Britain and in France, there we find animals which are like horses in all the essential particulars which I have just described, … but they differ in some important particulars. There is a difference in the structure of the fore and hind limb, … but nevertheless we have here a horse in which the lateral toes, almost abortive in the existing horse, are fully developed.”
This horse is the Hipparion.
In the miocene formations “you find equine animals which differ essentially from the modern horse … in the character of their fore
and hind limbs, and present important features of difference in the teeth. The forms to which I now refer are what are known to constitute the genus Anchitherium. We have here three toes, and the middle toe is smaller in proportion, the lower toes are larger … and in the fore arm you find the ulna, a very distinct bone,” etc., etc.
Lastly, in the oldest part of the eocene formation we find the Orohippus, which is the oldest specimen of equine animals:
“Here we have the four toes on the front limb complete, three toes on the hind limb complete, a well-developed ulna, a well-developed fibula, and the teeth of simple pattern. So you are able, thanks to these great researches, to show that, so far as present knowledge extends, the history of the horse type is exactly and precisely that which could have been predicted from a knowledge of the principles of evolution. And the knowledge we now possess justifies us completely in the anticipation that when the still lower eocene deposits and those which belong to the cretaceous epoch have yielded up their remains of equine animals, we shall find first an equine creature with four toes in front and a rudiment of the thumb. Then probably a rudiment of the fifth toe will be gradually supplied, until we come to the five-toed animals, in which most assuredly the whole series took its origin.”