146 ORIGIN OF LIFE IK AMERICA 



America from Asia in pre-Glacial times. Long before the 

 advent of the European conquerors in the New World, herds 

 of another large ungulate, the horse, roamed about these same 

 prairies and no doubt shared the abundant fodder with the 

 bison. When the Spaniards landed in America in 1521 it was 

 already extinct, and the natives had not any knowledge even of 

 the former existence of the horse in their continent. Yet even 

 in Pleistocene times several different kinds of wild horses 

 still lived in North America and were probably contem- 

 poraneous with early man. One of these (Equus giganteus) 

 seems to have exceeded in size any known race of horse either 

 living or extinct.* What caused the sudden extinction of the 

 wild horse all over America we do not know. Professor 

 Osbornf suggests that a disease of the nature of the African 

 " rinderpest " might have done it. The " tse-tse fly " renders 

 thousands of square miles of Africa uninhabitable for horses, 

 and the invasion of a similar pest into America might pos- 

 sibly have swept away the whole of the equine stock in a short 

 time. But the interest aroused among zoologists by the dis- 

 covery of fossil horses in America was not only connected with 

 their unexplained disappearance in modern times, it yielded 

 what was thought to be absolutely demonstrative evidence of 

 the theory of evolution. Fossil forms no doubt had already 

 been discovered in Europe which seemed to indicate that 

 the remote ancestors of the existing horse had five digits on 

 every foot while intermediate stages with three fully deve- 

 loped toes were known. In America, horses, or at any rate 

 animals possessing all the essential characters of a horse, 

 have been brought to light from very early Tertiary deposits, 

 possessing four toes and a rudimentary fifth on the hind foot 

 and short-crowned teeth. These are succeeded in Oligocene 

 and Miocene strata by others with three toes and short- 

 crowned teeth. In still more recent deposits, horses occur 

 with three toes and long-crowned teeth which are finally 

 followed by horses of the modern type with one toe and long- 

 crowned teeth. 



* Gidley, J. W., " Revision of North American Species of Equus," 

 p. 137. 



t Osborn, H. F., "Causes of Extinction of Mammalia," p. 835. 



