CAUSE OF THE EVOLUTION. The evolution of the horse, adapting it to live on the dry plains, probably went band 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 so high above sea-level as now, great parts of it had but recently emerged, and the Gulf of Mexico still stretched far up the valley of the Missis sippi. The climate at that time was probabil very moist, warm, and tropical, as is shown by the tropical forest trees found fossil even as far north as Greenland. Such a 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. These conditions restricted and thinned the forests, and caused the appearance and extension of open grassy plains. The ancient forest inhabitants must then either retreat and disappear with the forests, or adapt themselves to the new conditions of life. The ancestors of the horse, adopting the latter course, changed with the changing condi tions, and the race became finally—as we see it to-day—one of the most highly specialized of ani mals in its adaptation to its peculiar environ ment. 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 horses be came cosmopolitan, and inhabited the plains of all the great continents, excepting Australia.
It is a question whether the direct. ancestry of the modern horse is to he searched for in Western America or in the little known interior plains of Eastern Asia. It is also unknown why the vari MIS 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) 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 was responsible for the disappearance of the native American species of (horse.