HISTORY OF THE HORSE 63 



the animal being compelled to devote all its time to 

 getting enough to eat. 



Cause of the evolution. — ^The evolution of the horse, 

 adapting him to live on the dry plains, probably went 

 hand in hand with the evolution of the plains. At the 

 beginning of the Age of Mammals the western part of 

 the North American continent was not as high above the 

 sea level as now. Much of the country had but recently 

 emerged, and the Gulf of Mexico stretched far up the 

 Mississippi valley. The climate was probably very moist, 

 warm and tropical, as is emphasized 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, and to such conditions of life 

 the animals of the beginning of the mammalian period 

 must have been adapted. 



During the Tertiary period the continent was steadily rising 

 above sea level. At the same time other influences were 

 at work rendering the climate continually colder and 

 drier. The coming of a cold, dry climate thinned and 

 restricted the forests and in their place appeared the open 

 grassy plains. The early forest inhabitants were forced 

 either to retreat and disappear with the forests or adapt 

 themselves to a life on the plains. The ancestors of the 

 horse, following the latter course, changed with the 

 changed conditions, and the race became, as it is today, 

 perhaps the most specialized of animals in its adaptation 

 to its environment. 



First uses of the horse. — The first association of man 

 with the horse, so far as discovered, existed in the Early 

 Paleolithic or Chipped Stone Implement Age. The 

 earliest evidence of the existence of primitive man is his 

 crude implements chipped from stone, and since these are 

 found in caves, river gravels and rocks associated with 

 the. fossil remains of the prehistoric horse, we have no 

 history of man which is not associated with that of the 

 horse. It is assumed that man first hunted horses for 



