After Osborn FIG. 21. EVOLUTION OF THE HIND FOOT OF THE HORSE Side views of six stages, Hohippus, Mesohippus, Miohippus, Merychippus, Hipparion, Equus 31
CAUSE OF THE EVOLUTION HE evolution of the horse, adapting it to live on the dry plains, probably went hand in hand with the evolution of the plains themselves. At the commencement of the Age of Mammals the western part of the North American continent was by no means as high above sea level as it now is. Great parts of it had but recently emerged, and the Gulf of Mexico still stretched far up the valley of the Mississippi. The climate at that time was probably very moist, warm and tropical, as is shown by the tropical forest trees, found fossil even as far as Greenland. Sucha climate, with the low elevation of the land, would favor the growth of dense forests all over the country, and to such conditions of life the animals of the beginning of the mammalian period must have been adapted. During the Tertiary the continent was steadily rising above the ocean level, and at the same time other influences were at work to make the climate continually colder and drier. The coming on of a cold, dry climate restricted and thinned the forests and caused the appearance and extension of open, grassy plains. The ancient forest inhabitants were forced either to retreat and disappear with the forests, or to adapt themselves to the new conditions of life. The ancestors of the horse, following the latter course, changed with the changing conditions, and the race became finally as we see it to-day, one of the most highly specialized of animals in its adaptation to its peculiar environment. At the end of the Age of Mammals the continents stood at a higher elevation than at present, and there was a broad land connection between Asia and North America, as well as those now existing. At this time the horse became cosmopolitan, and inhabited the plains of all the great continents, excepting Australia.
It isa question whether the direct ancestry of the modern horse is to be searched for in western America or in the little known interior plains of eastern Asia. It is also unknown why the various species which inhabited North and South America and Europe during the early part of the Age of Man should have become extinct, while those of Asia (horse and wild ass) and of Africa (wild ass and zebra) still survive. Man, since his appearance, has played an important part in the extermination of the larger animals; but there is nothing to show how far he is responsible for the disappearance of the native American species of horse. 32
PARALLEL EVOLUTION IN OTHER RACES T is interesting to observe that while the evolution of thé horse was progressing during the Tertiary period in North America another group of hoofed animals, the Litopterna, now extinct, in South America evolved a race adapted to the broad plains of Argentina and FIG. 22. THREE-TOED PSEUDO-HORSE Fore and hind feet, one-half natural size. Diadiaphorus of the Miocene of South America, although so closely like the three-toed horses in the feet, has a wholly different skull and teeth, and belongs to a different order, the Litopterna, peculiar to South America and now extinct. From specimens in the American Museum 33
34 AMERICAN MUSEUM GUIDE LEAFLETS Patagonia and singularly like the horse in many ways. These animals likewise lost the lateral toes one after another, and concentrated the step on the central toe; they also changed the form of the joint-surfaces from ball-and-socket to pulley-wheel joints; they also lengthened the limbs and the neck; and they also lengthened the teeth, and complicated their pattern. Unlike the true horse, they did not form cement on the tooth, so that it was by no means so efficient a grinder. This group of animals native to South America became totally extinct, and were succeeded by the horses, immigrants from North America, which in their turn became extinct before the appearance of civilized man. Many of the contemporaries of the horse in the northern hemisphere were likewise lengthening the limbs, lightening and strengthening FIG. 23. ONE-TOED PSEUDO-HORSE _ Fore and hind feet, one-half natural size. The Thoatherium has gone even further than the modern horse in reducing the side toes to tiny rudiments instead of splints. The skull and teeth show the animal to be closely related to Diadiaphorus (Fig. 22) and widely different from true horses. Thoatherium was a little larger ‘than a fox terrier and lived in South America during the Miocene Epoch. From specimens in the Princeton Museum and the American Museum of Natural History
EVOLUTION OF THE HORSE IN NATURE 3D the feet, elongating the tooth-crowns to adapt themselves to the changing conditions around them, although none paralleled the horse evolution quite so closely as did the pseudo-horses of South America. But the camels in America, the deer, antelope, sheep and cattle in the Old World, progressed on much the same lines of evolution, although their adaptation was not to just the same conditions of life. PRINCIPAL STAGES IN THE EVOLUTION OF THE HORSE.
. SYNOPTIC PANEL On the right hand at the entrance to the Horse Alcove is a double panel (pages 36 and 37) showing the chief stages in its evolution. On the right-hand panel is the series of skulls, fore feet and hind feet, each stage set opposite the geological epoch during which it lived. Thus all horses found as fossils in the rocks of the Eocene Epoch are Four-toed Horses; all from the Oligocene are small Three-toed Horses with shortcrowned teeth, and so on.
On the left-hand panel is a corresponding series of palates to show the gradual change in pattern of the upper teeth, and conversion of the premolars into ‘molariform’ grinders; and a series of single teeth to show the progressive lengthening of the crown. The change in the brain, the fore leg and hind leg are also shown in this panel, and side views of the fore and hind foot show more clearly the staves in reduction of the side tees, 6
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