THE PLIOCENE OF EUROPE, ASIA, AND NORTH AMERICA 369 



with the anmials which it contains, subsequently confused by both Hayden 

 and Leidy themselves ^ with the very much older horizon of true Upper 

 Miocene age as part of their Horizon F. 'Loup River' thus became iden- 

 tical in its loose significance with 'Loup Fork,' and the error spread into 

 all subsequent literature. 



It appears, therefore, that ' Loup River ' was the original name applied 

 to a fluviatilc formation containing mammals solely of Upper Pliocene or 

 Lower Pleistocene age, namely, Equus and Elephas. 



Conclusions as to American Pliocene. — Few generalizations can be 

 made from this scanty fauna and unsettled stratigraphy of the Pliocene. 

 We are confronted with more gaps in our knowledge and with more un- 

 solved problems than in any other epoch. Among the unsolved problems 

 is the direct ancestry of the South American cameloids {Auchenia), the 

 llamas, and guanacos, as well as of the true Old World camels (Camelus). 

 We should discover here deer leading to the American type of deer (Odo- 

 coileus); we should also discover horses intermediate between the most 

 progressive stages with tridactyl feet (Protohippus, Pliohippus) and the 

 monodactyl Equus. It appears finally that while the Pliocene plains 

 fauna is sparsely known, the Pliocene forest fauna is wholly unknown. 



Causes of Pliocene Extinction 



The disappearance of the browsing Herbivora and evolution of the graz- 

 ing Herbivora, in the Great Plains region, we have observed as cumulative 

 processes, beginning in Oligocene times (p. 240) and reaching a climax in 

 the Pliocene. 



Increasing aridity and prevailing summer droughts were characteristic 

 of late Miocene and early Pliocene times in Europe, and evidence is accu- 

 mulating that the same conditions prevailed in western North America. 



Secular desiccation has been the fate of portions of three great con- 

 tinents in Pliocene and Pleistocene times, in each of which we observe a 

 gradual modification and extinction of certain kinds of quadrupeds. These 

 regions of increasing aridity are: (1) the Western plains region of North 

 America, including the arid plateau and mountain region, beginning in 

 Miocene tunes; (2) South America, Patagonia, and the Pampean region, 

 beginning in late Pliocene times; (3) north central Africa, beginning in 

 late Pleistocene times; (4) central Australia, beginning in Pleistocene 

 times.- 



Combining the facts brought together by many observers. Cope, Ko- 

 walevsky, Scott, Matthew, Hatcher, Stirling, we observe that the diminu- 

 tion of the softer kinds of vegetable food and increase of the harder kinds, 



^ See Leidy, op. cit., 1869, pp. 15-21. 



^ Osborn, H. F., The Causes of Extinction of Mammalia. Amcr. Natural., Vol. XL, 

 nos. 479 and 480, Nov. and Dec, 1906, pp. 769-795, 829-859. (Sec especially, pp. 783-787). 



