106 THE THEORY OF EVOLUTION 



accessible by a junction with northeastern Asia, 

 where now are Bering Strait and Sea. 



Though the equine family did not originate in 

 North America, its principal development took place 

 here and went on unbrokenly through the long ages of 

 the Tertiary period. After they had arrived at a 

 high stage of advance and differentiation, the horses 

 spread to other continents and eventually reached all 

 of them except Australia. Invading South America 

 in the late Miocene or early Pliocene, after the two 

 continents had been joined by the upheaval of the 

 Isthmus of Panama, they there gave rise to a number 

 of peculiar and characteristic forms. In the Pleis- 

 tocene epoch great herds of horses, belonging to no 

 less than ten distinct species, ranged through the 

 forests and over the plains of North America, extend- 

 ing from the Atlantic to the Pacific and from Alaska 

 through Mexico. In size, they varied from a Shet- 

 land pony to species exceeding the largest modern 

 draught-horses in stature. Then, for some unknown 

 reason, they died out completely all over the western 

 hemisphere, a fate which was by no means confined 

 to the horses or to the Americas; the close of the 

 Pleistocene witnessed over three-fifths of the land- 

 surface of the globe, the extinction of a multitude of 

 the largest and most conspicuous mammals, but the 

 causes of such a tremendous mortality can only be 

 conjectured. 



In addition to the main line of equine descent, 

 leading up to the modern horses, there were several 



