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 

 pecuHar 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 modem 

 Horse is to be searched for in Western America or in the httle 

 known interior plains of Eastern Asia. It is also unknown why 

 the various species which inhabited Nortli and vSoulh .\morica 

 and Europe during the early part of the Age of Man shoukl 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. 



