862 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



northern limit of trees further to the south. The large rumi- 

 nants were thus forced southeast, southwest, and west, since 

 the desert plateaus of central Asia shut off a retreat to the 

 south. 



By far the most impressive of all the examples of 

 extinction laiown to us is this continental extermina- 

 tion of nunierous forms of mammalian life, both 

 carnivores and herbivores, which was contemporaneous 

 with this great secular change of climate. It is no 

 exaggeration to say that the faunal aspect of Australia, 

 South America, North America, Asia, and Europe 

 was profoundly altered as it had never been before. 



This alteration entirely changed the zoologic aspect 

 of four-fifths of the earth's surface, and it is unavoid- 

 able that we should attribute it to the long series of 

 direct and indirect changes connected with the four 

 or five waves of advancing and retreating low tem- 

 perature and moisture. 



The end of the Pliocene and beginning of the 

 Pleistocene found North America populated with the 

 kinds of great herbivorous quadrupeds tabulated 

 below, all of which disappeared in North America 

 during the Ice Age, although some of them survived 

 in other forms in South America, Europe, and Asia. 



Herhivorous quadrupeds of North America that became extinct during the Ice Age 



It would be natural to assume that extinction was 

 directly brought about by these successive changes of 

 temperature and moisture and the changes in the 

 fauna and flora consequent upon the great physio- 

 graphic changes; but this simple explanation is beset 

 with many difficulties and contradictions, and the 

 results must be analyzed with some care. In Europe 

 the Mediterranean Sea presented a barrier to escape 

 or migration southward, but in North America there 

 were broad continental areas and high plateaus that 

 afforded easy routes of migration southward and every 

 means of escape. It is therefore more in accord with 

 the facts to say that contemporaneous with the 

 glacial epoch in North America there were developed 

 certain new conditions of life that directly or indirectly 

 resulted in extermination. 



Numerical diminution in hard winters. — These 

 phenomena have been discussed by Nehring (1890.1) 

 in connection with the Ice Age in noting the effects 

 of severe storms on the steppes of Russia, where 

 animals are not protected by stretches of forest. 

 Garman (1883.1) maintains that snow was the chief 

 cause of the extermination of the Pleistocene species 

 of bison and horses in North America. We owe to 

 Seton (1909.1) a review of the observations of Mun- 



son, Spears, Bunn, Henry, and others of exceptional 

 periods of numerical diminution on the Great Plains. 

 Henry in 1799 (1897.1, vol. 1, p. 174, and footnote) 

 counted "herds" of drowned and mired buffalo along 

 the banks of Qu'Appelle River. Treachei'ous ice in 

 the spring of 1801 destroyed great numbers of buffalo 

 along the shores of Red River (Alberta) ; for two daj^s 

 and nights a continuous line of carcasses floated by. 

 The blizzard of 1871-72 diminished the last great 

 buffalo herd of South Dakota. In 1880 large numbers 

 of buffalo perished in a blizzard in the same region, 

 50 skeletons being found together in one ravine. 

 The hard winter of 1893 (thermometer -61° F.) 

 killed off four-fifths of the antelope near Fort Assinni- 

 boine, Mont. Carcasses of about 900 animals, which 

 had starved to death on account of the deep snow, 

 were found in one ravine. 



Dangers of numerical diminution. — While a sharp 

 distinction must be drawn between actual extermi- 

 nation on a continent and a temporary diminution 

 caused, for example, by cold waves, floods, or other 

 unfavorable conditions of life, it is very important to 

 observe, as first suggested by Darwin, that extreme 

 diminution in numbers has as great dangers as- 

 extreme multiplication in numbers and may lead to- 



