EVOLUTION OF THE HORSE 29 
mals the western part of the North American continent was by 
no means as 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 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. Such 
a climate, with the low elevation of the land, would favor the 
growth of dense forests all over the country, and to such condi- 
tions of life the animals of the beginning of the mammalian 
period must have been adapted. During the Tertiary the con- 
tinent was steadily rising above the ocean-level, and at the same 
time other influences were at work to make the climate continu- 
ally colder and drier. The coming on of a cold, dry climate re- 
stricted 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 is a 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 appear- 
ance, has played an important part in the extermination of the 
larger animals; but there is nothing to show how far he is re- 
sponsible for the disappearance of the native American species 
of horse. 
