The Meaning of Evolution

by Samuel Christian Schmucker

We can only surmise why this creature should have undergone such a change, but the presence of flesh-eating animals having the size of a fox, and presumably of the fox's swiftness, probably tells the story. The little bands of early horses, pursued by their carnivorous foes, were slowly modified into swifter creatures. It is not so much that running made them fast, as that the slow ones were continually being caught. If this process of constant elimination of the slow members of any herd is kept up long enough, the group will necessarily develop speed. As time goes on, of these early horses those which happened to have longer legs and stood higher upon their toes won in the race, and handed on their qualities to their long-legged descendants. As the animal rose upon his toes, the inner toe, corresponding to our thumb, was first raised off the ground and rendered useless, while a similar change came over the corresponding toe on the hind foot. The hard work of running being done on the latter, this superfluous toe was more detrimental there than on the front foot, and disappeared, consequently, more rapidly. In time, however, it also disappeared from the front foot. Gradually the further elevation of the foot lifted the toe, which corresponds to our little finger, off the ground, and this now disappears also.

With increasing toughness of the grasses, as the climate becomes drier and the region more elevated, the teeth of the horse are given harder work. The points begin to spread into ridges and to unite with each other in such way as to form the crescents, which are later to be so characteristic of the teeth of the modern horse.

By the middle of the Tertiary this ancestral horse has risen in height until he is taller and heavier than a setter dog. Three toes are found on each front foot. The middle toe is getting constantly more developed, though the smaller toes are evidently still of use. The ridges of the teeth are quite crescentic now on the outer side, and becoming better adapted to the evidently firmer food which the creature is obliged to eat.

As we come toward the end of the Tertiary, the development which had been all pointing in one direction has advanced very much further. The creature now would be undoubtedly recognized by anyone as a horse. The legs are longer and straighter; the middle toe has become the only useful toe, though on each foot a smaller toe, slender and probably useless, still hangs on either side. Two similar useless toes to-day hang at the back of the foot of the cow, which is now walking upon her two toes, which give her the appearance of carrying a cloven hoof. That is to say, the first toe on the foot of the cow has disappeared, the second and fifth hang useless and much diminished at the back of the foot, while the third and fourth are both well developed and serviceable in walking.

The late Tertiary horse has grown to be the size of a burro of to-day, though probably it was a little more slender. The teeth are quite horselike, both in shape of the crescentic ridges on their surface, in the length of the teeth in the jaw bone, and in the fact that the crinkled edges of enamel on the upper surface are protected on either side by dentine or by cement. These surfaces, being softer than the enamel, wore away somewhat more rapidly and allowed the sharp edges of enamel to stand up in ridges. This plan increases the grinding power of the teeth.

With the oncoming of the Era of Man the horse reaches his modern splendid development. During the early Quaternary the horse was perhaps in some of his representatives a larger creature than he is to-day. Each foot now has but a single toe. The nail has spread around firmly and heavily, until it has become a splendidly developed hoof, permitting the animal to travel with speed over firm and often stony ground. The side toes have disappeared completely from the outside of the horse's leg, although upon removing the skin it is easy to find the long splints, which are the remnants of toes, which have not yet quite disappeared. His heel has been lifted in the air until it is eighteen inches off the ground, and he is standing like an expert dancer upon the tip of his toe. The body of the horse thus being lifted far off the ground, a new development becomes necessary. All through the growth of the creature the neck and head have been obliged to lengthen correspondingly. Every animal must be able to bring its head down to the level of its feet in order that it may drink. Various animals use different methods to accomplish this result. The giraffe, with his enormously long legs, has a correspondingly long neck, which lowers his mouth to the ground. Even with this extended neck the giraffe's legs are so exceedingly long that he is obliged to spread his front feet when he wishes to reach the ground with his head. The elephant has pursued exactly the reverse plan. Using his tremendous head as a battering ram in fighting, and using his enormous tusks both in battle and in uprooting young trees, a lengthened neck is absolutely out of the question. Furthermore his front teeth have grown so prodigiously that they would interfere with his getting his mouth to water. Accordingly, his nose has lengthened its tip until it reaches the level of his feet, and this nose becomes to him the main organ of grasp and of touch. To drink, its end is inserted in the pool and water is drawn up the nostril. If the animal were to attempt to draw it all the way back into his throat, it would inevitably strangle him by getting into his windpipe. Accordingly, when the nose is well filled with water, the tip of it is inserted in his mouth, and the water discharged by a quick puff. The horse has taken a method intermediate between these. It had moderately lengthened both neck and head in order to get to the ground with its nipping teeth, and thus to gather the grasses which serve as its principal food.

The mammalian teeth, while of four kinds, really in most animals serve but two purposes. The front teeth consist of the incisors and canines, and are used for biting. The hind teeth, consisting of premolars and molars, are used for grinding. In the horse, the jaw has lengthened between these two sets, carrying the biting teeth far forward of the molars. It is this gap in the row of the horse's teeth which makes it possible for us to insert the bit into his mouth.

Now comes a strange accident into the life of our American horse. Creatures of the same kin had been evolving in Europe and Africa, but the developments are more distinctly horselike, it would seem, in our own country. Then for some reason the horse disappeared completely from American soil. Doubtless two things happened. First of all, some of them migrated across a stretch of open country which then connected America with Asia in the neighborhood of Bering Strait. These creatures spread first over Asia and then over Africa and Europe, leaving their skeletons scattered over this enormous stretch of country. Asses and zebras are still found abundantly and widely scattered, but the wild horse of to-day is seen only in western Asia. What happened to those who remained in America we shall possibly never know. Some surmise that a fly not unlike the tsetse-fly of Africa killed them out. Perhaps the members of the cat family, which are steadily growing larger and fiercer, fed on their young if not upon the older ones, and exterminated them. Perhaps the Glacial period which followed was too cold for them. But, whatever may have been the cause these horses died out not only in North but also in South America, to which country they had spread.

The old world horse was the companion of man. The skeletons of those found with early man in the caves of Europe look as if the horse had been a creature to draw man's burdens and to serve him for food, rather than to bear him upon its back. Its roasted bones are often found about the old tribal fires. Upon the discovery of the new world the Spaniards brought with them to Mexico and to the Mississippi Valley the horses which carried them in their battles against the Indians. In the course of these frays many riders were killed and their horses roamed wild. Slowly they made their way to the western plains; gradually they became tougher and more wiry; their diminished hoofs learned to catch more carefully in the rocks of their mountain home; and the mustang and bronco of more recent years are the descendants of the little dawn horse, whose dainty skeleton is found in the rocks over which his later descendants, after a long stretch of perhaps four million years, are now running.