THE AGE OF MAMMALS 



neck were short, and the limbs of moderate length, show- 

 ing no remarkable adaptation for speed. This genus 

 had a remarkable range, having apparently originated in 

 England (then a part of Western Europe), and migrated 

 by way of Europe and Asia, and what is now Behring 

 Strait, to America, where it got as far east as New 

 Mexico. This migration of Eohippus shifted the scene 

 of evolution to the western hemisphere, for while 

 examples of it are continually and continuously found 

 there in succeeding strata it only appears occasionally 

 in Europe, as if the remains there had been those of mere 

 emigrants. 



Later on the horse developed in America, growing 

 larger till it was first as big as a collie-dog, with signs 

 of being more adapted for speed. It then had four 

 toes on its foot. It continued, though very gradually, 

 to grow larger, and even more gradually its unneces- 

 sary toes grew fewer and fewer till at last they disap- 

 peared. 



At length appeared the horse which had only one toe. 

 This type, that of the modern horse, first becomes 

 known in the Upper Pliocene beds of Europe, and repre- 

 sents the culmination of the race. The completeness 

 of the record of the evolution of the horse tells us some- 

 thing of the enormous numbers of ancestral forms which 

 must have existed in the more than two million years 

 that have elapsed since the first diminutive horse appeared 

 in North America. While not strongly given to migra- 

 tion, in the course of time these animals wandered over 

 the entire world, with the exception of such inaccessible 



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