72 Proceedings of the Royal Physical Society. 



ing to Sir Charles Lyell ("Principles of Geology," llth ed., 

 vol. ii. p. 340), remains of no less than twelve species, referred 

 to seven genera, have been discovered in the Pliocene and 

 Pleistocene formations of that country ; but to what extent 

 they resembled or differed from those of the Old World I 

 am unable to say. This is a most interesting point in the 

 evolutionary history of the horse, and one which I should 

 like to see handled by some competent palaeontologist. To 

 find, however, an exact parallelism in the development of 

 these animals on both sides of the Atlantic would not at all 

 surprise me, because, since the days of the Hipparion, there 

 was only one outlet by following which higher efficiency 

 could be attained on the natural lines of horse develop- 

 ment. These lines have been followed, and, for a time, the 

 results were successful. As to the ultimate fate of these 

 American horses, the following remarks by Flower and 

 Lyddeker ("Mammals Living and Extinct," 1891, p. 381) 

 may be quoted with advantage : — " Fossil remains of horses 

 are found abundantly in deposits of the most recent geo- 

 logical age in almost every part in America, from Eschscholtz 

 Bay in the north to Patagonia in the south. In that con- 

 tinent, however, they became quite extinct, and no horses, 

 either wild or domesticated, existed there at the time of the 

 Spanish conquest, which is the more remarkable, as, when 

 introduced from Europe, the horses that ran v^ild proved by 

 their rapid multiplication on the plains of South America 

 and Texas that the climate, food, and other circumstances 

 were highly favourable for their existence. The former 

 great abundance of Equidae in America, their complete 

 extinction, and their perfect acclimatisation when reintro- 

 duced by man, form a curious but as yet unsolved problem 

 in geographical distribution." 



As to the causes which led to the extinction of the 

 American horses, I believe a satisfactory explanation will 

 be found in the fact that, after coming to the end of their 

 evolutionary tether in the attainment of speed — the sole 

 means by which they could escape from their enemies — 

 they fell an easy prey to one or more of these enemies, who, 

 meanwhile, had succeeded in improving their methods of 



