Animals of the Past

by Frederic A. Lucas

Still we have not the least little bit of positive proof that such was the case, and although the site of many an ancient Indian village has been carefully explored, no bones of the horse have come to light, or if they have been found, bones of the ox or sheep were also present to tell that the village was occupied long after the advent of the whites. It is also a curious fact that within historic times there have been no wild horses, in the true sense of the word, unless indeed those found on the steppes north of the Sea of Azof be wild, and this is very doubtful. But long before the dawn of history the horse was domesticated in Europe, and Cæsar found the Germans, and even the old Britons, using war chariots drawn by horses—for the first use man seems to have made of the horse was to aid him in killing off his fellow-man, and not until comparatively modern times was the animal employed in the peaceful arts of agriculture. The immediate predecessors of these horses were considerably smaller, being about the size and build of a pony, but they were very much like a horse in structure, save that the teeth were shorter. As they lived during Pliocene times, they have been named "Pliohippus."

Going back into the past a step farther, though a pretty long step if we reckon by years, we come upon a number of animals very much like horses, save for certain cranial peculiarities and the fact that they had three toes on each foot, while the horse, as every one knows, has but one toe. Now, if we glance at the skeleton of a horse, we will see on either side of the canon-bone, in the same situation as the upper part of the little toes of the Hippotherium, as these three-toed horses are called, a long slender bone, termed by veterinarians the splint bone; and it requires no anatomical training to see that the bones in the two animals are the same. The horse lacks the lower part of his side toes, that is all, just as man will very probably some day lack the last bones of his little toe. We find an approach to this condition in some of the Hippotheres even, known as Protohippus, in which the side toes are quite small, foreshadowing the time when they shall have disappeared entirely. It may also be noted here that the splint bones of the horses of the bronze age are a little longer than those of existing horses, and that they are never united with the large central toe, while nowadays there is something of a tendency for the three bones to fuse into one, although part of this tendency the writer believes to be due to inflammation set up by the strain of the pulling and hauling the animal is now called upon to do. Some of these three-toed Hippotheres are not in the direct line of ancestry of the horse, but are side branches on the family tree, having become so highly specialized in certain directions that no further progress horseward was possible.

Backward still, and the bones we find in the Miocene strata of the West, belonging to those ancestors of the horse to which the name of Mesohippus has been given because they are midway in time and structure between the horse of the past and present, tell us that then all horses were small and that all had three toes on a foot, while the fore feet bore even the suggestion of a fourth toe. From this to our Eocene Hyracothere with four toes is only another long-time step. We may go even beyond this in time and structure, and carry back the line of the horse to animals which only remotely resembled him and had five good toes to a foot; but while these contained the possibility of a horse, they made no show of it.

Fig. 34.—The Development of the Horse.

Increase in size and decrease in number of the toes were not the only changes that were required to transform the progeny of the Hyracothere into a horse. These are the most evident; but the increased complexity in the structure of the teeth was quite as important. The teeth of gnawing animals have often been compared to a chisel which is made of a steel plate with soft iron backing, and the teeth of a horse, or of other grass-eating animals, are simply an elaboration of this idea. The hard enamel, which represents the steel, is set in soft dentine, which represents the iron, and in use the dentine wears away the faster of the two, so that the enamel stands up in ridges, each tooth becoming, as it is correctly termed, "a grinder." In a horse the plates of enamel form curved, complex, irregular patterns; but as we go back in time, the patterns become less and less elaborate, until in the Hyracothere, standing at the foot of the family tree, the teeth are very simple in structure. Moreover, his teeth were of limited growth, while those of the horse grow for a considerable time, thus compensating for the wear to which they are subjected.

We have, then, this direct evidence as to the genealogy of the horse, that between the little Eocene Hyracothere and the modern horse we can place a series of animals by which we can pass by gradual stages from one to the other, and that as we come upward there is an increase in stature, in the complexity of the teeth, and in the size of the brain. At the same time, the number of toes decreases, which tells that the animals were developing more and more speed; for it is a rule that the fewer the toes the faster the animal: the fastest of birds, the ostrich, has but two toes, and one of these is mostly ornamental; and the fastest of mammals, the horse, has but one.

All breeders of fancy stock, particularly of pigeons and poultry, recognize the tendency of animals to revert to the forms whence they were derived and reproduce some character of a distant ancestor; to "throw back," as the breeders term it. If now, instead of reproducing a trait or feature possessed by some ancestor a score, a hundred, or perhaps a thousand years ago, there should reappear a characteristic of some ancestor that flourished 100,000 years back, we should have a seeming abnormality, but really a case of reversion; and the more we become acquainted with the structure of extinct animals and the development of those now living, the better able are we to explain these apparent abnormalities.

Bearing in mind that the two splint bones of the horse correspond to the upper portions of the side toes of the Hippotherium and Mesohippus, it is easy to see that if for any reason these should develop into toes, they would make the foot of a modern horse appear like that of his distant ancestor. While such a thing rarely happens, yet now and then nature apparently does attempt to reproduce a horse's foot after the ancient pattern, for occasionally we meet with a horse having, instead of the single toe with which the average horse is satisfied, one or possibly two extra toes. Sometimes the toe is extra in every sense of the word, being a mere duplication of the central toe; but sometimes it is an actual development of one of the splint bones. No less a personage than Julius Cæsar possessed one of these polydactyl horses, and the reporters of the Daily Roman and the Tiberian Gazette doubtless wrote it up in good journalistic Latin, for we find the horse described as having feet that were almost human, and as being looked upon with great awe. While this is the most celebrated of extra-toed horses, other and more plebeian individuals have been much more widely known through having been exhibited throughout the country under such titles as "Clique, the horse with six feet," "the eight-footed Cuban horse," and so on; and possibly some of these are familiar to readers of this page.

So the collateral evidence, though scanty, bears out the circumstantial proof, derived from fossil bones, that the horse has developed from a many-toed ancestor; and the evidence points toward the little Hyracothere as being that ancestor. It remains only to show some good reason why this development should have taken place, or to indicate the forces by which it was brought about. We have heard much about "the survival of the fittest," a phrase which simply means that those animals best adapted to their surroundings will survive, while those ill adapted will perish. But it should be added that it means also that the animals must be able to adapt themselves to changes in their environment, or to change with it. Living beings cannot stand still indefinitely; they must progress or perish. And this seems to have been the cause for the extinction of the huge quadrupeds that flourished at the time of the three-toed Miocene horse. They were adapted to their environment as it was; but when the western mountains were thrust upward, cutting off the moist winds from the Pacific, making great changes in the rainfall and climate to the eastward of the Rocky Mountains, these big beasts, slow of foot and dull of brain, could not keep pace with the change, and their race vanished from the face of the earth. The day of the little Hyracothere was at the beginning of the great series of changes by which the lake country of the West, with its marshy flats and rank vegetation, became transformed into dry uplands sparsely clad with fine grasses. On these dry plains the more nimble-footed animals would have the advantage in the struggle for existence; and while the four-toed foot would keep its owner from sinking in soft ground, he was handicapped when it became a question of speed, for not only is a fleet animal better able to flee from danger than his slower fellows, but in time of drouth he can cover the greater extent of territory in search of food or water. So, too, as the rank rushes gave place to fine grasses, often browned and withered beneath the summer's sun, the complex tooth had an advantage over that of simpler structure, while the cutting-teeth, so completely developed in the horse family, enabled their possessors to crop the grass as closely as one could do it with scissors. Likewise, up to a certain point, the largest, most powerful animal will not only conquer, or escape from, his enemies, but prevail over rivals of his own kind as well, and thus it came to pass that those early members of the horse family who were preëminent in speed and stature, and harmonized best with their surroundings, outstripped their fellows and transmitted these qualities to their progeny, until, as a result of long ages of natural selection, there was developed the modern horse. The rest man has done: the heavy, slow-paced dray horse, the fleet trotter, the huge Percheron, and the diminutive pony are one and all the recent products of artificial selection.