CHAPTER V

HISTORY OF THE HORSE

The early history of the horse is both interesting and instructive. It is interesting because of the marked changes that have taken place in the size and conformation. It is instructive because it affords the best-known illustration in existence, of the adaptation of a race of animals to its environment, and shows clearly the influence of selection-whether natural or artificial.

The history of the horse, as now worked out, extends farther back into the past than that of any other farm animal. He was one of the first animals to receive the attention of progressive breeders, his improvement antedating that of cattle, sheep or swine. Nor do we wonder at this when we consider the intimacy of horse and master, their constant companionship, and the dependence of man upon his horse in the chase, in the pursuit of his foes, or in the escape from his enemies.

Antiquity of the horse. The ancestry of the horse family has been traced back, without a single important break, to the Eocene epoch in the Tertiary period.* During this long period, estimated at three millions of years, the animals of the horse family have passed through important changes in all parts of the body, but especially in the feet and teeth, adapting them more perfectly to their environment. Thus the earliest known ancestors of the horse family differed widely from the horse of the present time. These early ancestors were very small animals, not larger than the domestic cat. They possessed four complete toes on each forefoot and three on each hind

*The geologist divides time into eras, ages, periods, epochs and the like. Fossiliferous remains of the horse are found in the Tertiary period, which is divided into four epochs, Eocene, Oligocene, Miocene and Pliocene; as well as in both the Quarternary and Recent periods.

foot. Some scientists believe that the still more ancient ancestors possessed five toes on each foot. The teeth of these very early animals were short-crowned and covered with low, rounded cusps of enamel somewhat similar to those of swine, and differing widely from the longcrowned, rather complicated, molars of the horse.

Distribution of prehistoric horse. In the latter part of the Tertiary and in the early Quarternary periods, wild species of the horse were to be found on every continent except Australia. Remains of the horse have been found in all parts of the United States, in Alaska, in Mexico, in Central and South America, as well as in Europe, Asia and Africa. The first discovery of these fossil horses in the United States was made by Mitchell, in 1826, near the Navesink Highlands in New Jersey. About the middle of the century Leidy made similar discoveries in Nebraska. Following these came other discoveries, until the wide distribution of the horse in America became well established. Specially rich localities are on the Niobrara River in Nebraska, in central Oregon, in the phosphate mines near Charleston, South Carolina, in central Florida, in southern Texas, Arizona, Kansas, Louisiana and many other states. In fact, the fossil remains of the horse are so abundant in deposits of rivers and lakes of the Pleistocene epoch that the formation in the western United States has received the name of Equus Beds.

Evolution of the horse. The prehistoric development of the horse has been thoroughly investigated by the American Museum of Natural History. Twelve stages have been recognized in the evolution of the horse family from the early Tertiary period to the present; each stage being characteristic of its peculiar geological time, with the horse becoming more and more developed toward the present-day type, as the periods become more and more.

A few of these stages have been found in various parts of the world, but by far the most complete and bestknown series comes from the Tertiary Badlands of our

western states. Of some stages all parts of the skeleton have been found, while of others only the jaws, or jaws and feet, have been discovered. The stages properly grouped are as follows:*

1. Hyracotherium.-From the Lower Eocene. This is the most primitive stage known, and only the skull has been found, so that it has not been determined what the feet were like. The teeth display six rounded cusps on the upper molars and four on the lower. The premolar teeth have only one main cusp, except the third and fourth premolars, in each jaw, which have two and three respectively. The animals were no larger than the domestic cat. Found only in London clay, England.

2. Eohippus. From the Lower Eocene. Teeth vary somewhat from above, the molars having the cusps more clearly fused into cross-crests, and the last premolar is beginning to look like one of the true molars. The forefoot has four complete toes and the splint of a fifth. The hindfoot has three complete toes and splint. The animals were about the size of a fox terrier dog and known as "Dawn Horses." Found in New Mexico and Wyoming.

3 and 4. Protorohippus and Orohippus.-From the Middle Eocene. In these animals the splints have disappeared, leaving complete toes, as in the Eohippus. The crests in the molars are more apparent, and the last premolar has become almost like the molars, while the next to the last premolar is beginning to assume a similar form. This type was about the size of a small dog, perhaps about 14 inches high. The Protorohippus was found in 1880 in the Wind River Badlands of Wyoming and was described under the name of the "Four-Toed Horse."

5. Epihippus. From the Upper Eocene. Only incomplete specimens have been found of this stage of the evolution of the horse. The molar teeth have the round

*Made up from Guide Leaflet, No. 9. American Museum of Natural History, W. D. Matthew.

cusps almost completely converted into crescents and crests, with another premolar tooth becoming like the molars. There are four toes in front and three behind, but the central toe in each foot is larger, stronger and more important than the toes on either side.

6 and 7. Mesohippus.-From the Middle and Upper Oligocene. There are three toes on each foot and a splint representing a fifth toe on the forefoot. The middle toe

FIG. 48.-CHANGES IN THE SKULL, TEETH AND FEET SHOWING EVOLUTION OF THE HORSE

has become much larger than the side toes, which bear very little of the weight of the body. Three of the premolars have become like full molars, with crests on the crown completely formed. The outside crests in the upper molars have taken the shape of two crescents. The animals found in the Middle Oligocene were about the size of a coyote, while those of the Upper Oligocene were as large as a sheep. Of these species all parts of the skeleton are known.

8. Anchitherium.-From the Lower Miocene. This stage is much like the Mesohippus, but is larger and has

the crests of the teeth higher and more complete. Some persons think that this form is not in direct line of descent, and consider it a side branch. Found in both America and Europe.

9. Parahippus and Hypohippus. From the Middle Miocene. In Parahippus the crests of the teeth are much higher and the upper molars are changing in form, at second pair of crescents appearing inside the outer pair. Like Anchitherium, Hypohippus is off the direct line of descent. The teeth of the two forms are much alike, and they are often confused, though Hypohippus is much larger, equaling a Shetland pony in size, a specimen of this size being found near Pawnee Buttes, Colorado, in 1901. The forefoot has small rudiments of the first and fifth toes, but there is no splint of the fifth, as in Mesohippus. The second and fourth toes still touch the ground, though lightly. These animals have been called the "Forest Horses," and are supposed to have lived in forests and on lowlands. The feet of Parahippus were much like those of Hypohippus, with side toes smaller.

10 and 11. Protohippus and Pliohippus.—From the Middle and Upper Miocene. The crowns of the upper molars have become much longer, the two pairs of crescents on the upper molars are complete, with two half-separated cusps within the inner pair. Cement fills the valleys between the crests, so that with the wear of the teeth the edges of hard enamel are backed inside the dentine and outside the cement. Thus the surface of the tooth has a series of enamel ridgęs projecting somewhat above the grinding surface, due to the softer material on each side wearing down into hollows, yet never breaking off, because they are braced so thoroughly on each side. This provides an efficient instrument for grinding hard grasses. In these two forms, especially Protohippus, the crowns of the teeth are not so long as in the modern horse.

The feet in these two forms have but one toe touching the ground. The side toes are complete, but are more