The Pamphlet Collection of Sir Robert Stout: Volume 31
The Teeth of the Horse
The Teeth of the Horse.
As might be expected, and as I have already said, the apparatus for providing this page 31 machine with the fuel which it requires is also of a very highly differentiated character. A horse has, or rather may have, 44 teeth, but it rarely happens that in our existing horses you find more than 40—for a reason which I will communicate directly—and in a mare it commonly happens that you find no more than 36, because the "tushes," or canine teeth of the mare, are rarely developed. Then there are some curious peculiarities about these teeth. As everyone who has had to do with horses know, the cutting teeth—the incisors—are six above and six below, and those incisors present what is called a "mark;" at least, that mark is usually present in horses up to a certain age. It is a sort of dark patch across the middle of the tooth. The presence of that dark patch arises from a great peculiarity in the structure of the horse's incisor tooth. It is in fact in sections shaped in this fashion (illustrating), considerably curved, and with a deep pit in the middle, and then a long fang. In the young foal this pit is very deep. As the animal feeds, this space becomes filled up with its fodder, that fodder becomes more or less carbonized, and then you have the dark mark, and the reason the dark mark serves as an indication of age is, that as the horse feeds this is more and more worn down, until at last, in an aged horse, the tooth is worn beyond the bottom of the pit, and the mark disappears. Then, as I said, the male horse generally has canine teeth. We need not notice their structure particularly. In the female, these are rarely present. Following that, you may notice a very small and rudimentary tooth, but that is very often absent. It really represents the first tooth of the grinding series. Then there are usually to be found six great teeth, with exceedingly long crowns. The crowns, in fact, are so long that the teeth take a very long time to wear down, whence arises the possibility of the great age to which horses sometimes attain. This is shown in the side diagram. Then the pattern and structure of a horse's tooth are very curious. The crown of the horse's tooth presents a very complicated pattern; that is to say, supposing this to be one of the grinders of the left side (illustrating) above, there is a kind of wall like a double crescent. Then there are two other crescents, which fall in that direction, and these are complicated by folds, and all the spaces between these crescentic ridges are filled up by a kind of bony matter which is called cement. Consequently, the surface of the tooth is composed of very uneven materials—of the hard mass of the tooth, which is called dentine, then a very much harder enamel, and a softer cement between, the practical effect of which is the same as the lamination of a millstone. In consequence of the lamination of the millstone the ridges wear less swiftly than the intermediate substance, and consequently the surface always keeps rough and exerts a crushing effect upon the grain. The same is true of the horse's tooth, and consequently the grinding of the teeth one against the other, instead of flattening the surface of the teeth tends to keep them always irregular, and that has a very great influence upon the rapid mastication of the hard grain or the hay upon which the horse subsists.
I think that will suffice as a brief indication of some of the most important peculiarities and characteristics of the horse. 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 moveable 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 mammal possesses in the leg not only a complete tibia, but a complete fibula. The small bone of the leg is almost always smaller than the tibia. The small bone of the leg is as a general rule a perfectly complete, distinct, moveable 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 page 32 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 44 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 toward 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 complications 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. Let us turn to the facts and see how they bear upon the requirements of this doctrine of Evolution.
In what is called here the pliocene formation, that which constitutes almost the uppermost division of the tertiary series, we find the remains of horses. We also find in Europe abundant remains of horses in the most superficial of all these formations—that is, the post-tertiary, which immediately lies above the pliocene. But these horses, which are abundant in the cave deposits and in the gravels of England and Europe—these horses, of which we know the anatomical structure to perfection, are in all essential respects like existing horses. And that is true of all the horses of the latter part of the pliocene epoch. But 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, and the general character of which is so entirely like that of the horse that you may follow descriptions given in works upon the anatomy of the horse upon the skeletons of these animals. But they differ in some important particulars. There is a difference in the structure of the fore and hind-limb, and that difference consists in page 33 this, that the bones which are here represented by two splints, imperfect below, are as long as the middle metacarpal bone, and that attached to the extremity of each is a small toe with its three joints of the same general character as the middle toe, only very much smaller, and so disposed that they could have had so very little importance that they must have been rather of the nature of the dew claws which are in the ruminant annuals. This Hipparion, or European horse, in fact presents a foot similar to that which you see here represented, except that in the European Hipparion these smaller fingers are further back, and these lateral toes are of smaller proportion ional size.
But nevertheless we have here a horse in which the lateral toes, almost abortive in the existing horse, are fully developed. On careful investigation you find in these animals that also in the fore-limb the ulna is very thin, yet is traceable down to the extremity. In the hind-limb you find that the fibula is pretty much as in the horse itself. That is the kind of equine animal which you meet with in these older Pliocene formations, in which the modern horse is already or becomes entirely absent. So you see that the Hipparion is the form that immediately preceded the horse. Now let its go a step further back (illustrating) to these which are called the Miocene formations, and which constitute the middle part of the deposits of the tertiary epoch. There you find in some parts of Europe—in Germany, Central Germany, in France, and in Greece—there you find equine animals which differ essentially from the modern horse all that they resemble the horse is in the broad feat tires of their organization. They differ still further 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 are known to constitute the genus Anchitherium (illustrating). We have these three toes, and the middle toe is smaller in proportion, the lower 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 fore-arm you find the ulna a very distinct bone, quite readily distinguishable in its whole length from the pradius, but still pretty closely united with it. In the hind-limb you also meet with three functional toes. There is the same structure in the Hipparion's hind limb that there was in the ease of the Anchitherium, and in the hind-leg the fibula is longer. In some cases I have reason to think that it is complete; at any rate this lower end of it (illustrating) is quite distinctly recognizable as a separable though not exactly separated piece of bone. 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 more rudimentary pit—the pit is vastly smaller than in the horse. 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). There are two crescents and two oblique ridges; while in the lower jaw you have the double crescent and a very slight complication at the extremity. 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 there was some difficulty in tracing them, but they left no doubt whatever in my mind that we had here a genuine record of the history of the evolution of the horse. You must understand that every one of these forms in time has undoubtedly become modified into various species and the like, and we cannot be absolutely certain that we have the exact line of modification, but it was perfectly obvious that we had here in succession, in time, three forms, fundamentally modified, in the horse type, of which the oldest came nearer to the general mammal—was far less modified than the Hipparion and what has taken place afterward. We saw that the animals which had existed after and had undergone a reduction of their limbs and toes, a reduction of the lower bones of the hind-leg, a more complete coalescence of the fibula with the tibia. The pattern of the molar teeth has become more complicated, and the entire space has become filled with cement.
Consider what other alternative hypothesis lies open to you unless you admit this. In this succession of forms you have exactly that which the hypothesis of evolution demands. The history corresponds exactly with that you would construct a priori from the principles of Evolution. An alternative hypothesis is hardly conceivable, page 34 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, and for that there could be no scientific evidence. And in the first place it is not pretended that there is the slightest evidence of any other kind that such successive creation has ever taken place. When I was investigating this subject, all the collections in Europe were accessible to me, and they had led myself, and I may say, as I happen to know by correspondence with him, had also led that very eminent anatomist Professor Lartet, of Paris, to the same conclusion. Indeed the story is so plain that no one deserves any particular credit for drawing so obvious a conclusion. And since then, palaeontological inquiry has not only given us greater and greater knowledge of the series of horse like forms, but by and by enabled us to fill up the gaps in the series, and to extend that series further back in time.