Evolution and History of the Hokse 13 



natural causes of evolution 



It is interesting to note the probably natural cause of the evolu- 

 tion of the horse. During the early Tertiary period much of the 

 western part of the Xorth American continent was not as high 

 above the sea as now. Indeed, much of the country had but re- 

 cently emerged from the Gulf of Mexico which stretched far up 

 the Mississippi Valley. The climate was probably very moist and 

 warm and tropical, as is emphasized by tropical forest trees found 

 fossil often as far as Greenland. Such a climate, with a low eleva- 

 tion of land, would favor the gTowth 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 the 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 to either retreat and disappear 

 with the forests or adapt themselves to life on the plains. Most 

 of the horse's early relatives followed the former course and dis- 

 appeared, while the horse himself followed the latter course, 

 changed with the changing conditions, and the race became as it 

 is today, perhaps the most specialized of animals in its adaptation 

 to its environment. 



