OSBORN, REVIEW OF THE PLEISTOCENE 235 



mals is consistent with the climatic theory of subtropical temperature 

 and alternate dry and rainy seasons. 



Various phenomena point to increasingly long periods of drought and 

 progressive secular desiccation of this great region as the Pleistocene 

 advanced, resulting in the partial extinction and partial migration of the 

 great equatorial life into central and southern Africa. 



Eurasiatic Invasion. — At the close of the Quaternary the bear (Ursus), 

 as a characteristic forest-dweller, requiring a moist climate, became ex- 

 tinct, while the Eurasiatic deer, wild sheep, wild boar, smaller mammals 

 of European type, survived and established for this region its present 

 affinity with Europe and its Palsearctic fauna. We must account for 

 this northerly, or Eurasiatic, fauna of north Africa as having entered the 

 continent during the latter part of the Pleistocene Epoch and as sur- 

 viving in the forested regions of present and prehistoric times so as to 

 unite northern Africa closely with modern and prehistoric Europe. 

 Xorth Africa thus becomes a part of the Palaearctic Eegion. 



Thus in no region of the world have more profound changes occurred 

 during and since Pleistocene times than in Africa north of the Sahara 

 Desert. 



Sources of the Pleistocene African Life. — -It is premature to attempt 

 to ascertain the original sources of all the various members of this im- 

 posing assemblage of mammals. There remains always a great element 

 of doubt which can be eliminated only b3 r the discovery of the complete 

 Csenozoie history of Asia and Africa. It would appear probable from 

 our previous studies that the several continents contributed to the remote 

 original ancestry of the African fauna somewhat as follows : 



Africa or Asia, elephants and mastodons. 



Northern Eurasia, deer and bear, wild sheep, wild boar. 



Southern Eurasia, wild cattle and buffalo. 



North America or northern Eurasia, rhinoceroses, various Equidce, camels. 



The most comprehensive comparison of the fauna of Africa and Europe 

 is that of Stromer, 8 in which the entire fauna of Europe and north 

 Africa, including the Beptilia and Mammalia, is compared from Lower 

 Eocene to Pleistocene times. This author observes 9 that during the 

 middle period of the Tertiary the mammal fauna of southwestern Eu- 

 rope, western Asia, India to China, partook of the tropical or subtropical 



8 Stromer, Ernst : "tiber die Bedeutung der fossilen Wirbeltiere Afrikas fiir die 

 Tiergeograpbie." Verhandl. d. Deutsch. Zool. Gesellschaft, pp. 204-218. 1906. 



9 Stromer, Erxst : "Die einstige Verbreitung afrikaniscber Saugetiere." Naturwis- 

 senscbaftlicbe Wocbenscbrift, N. P., X Band ; der ganzen Reibe, XXVI Band, No. 51, 

 pp. 814-816. Dec. 17, 1911. 



