PLEISTOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 501 



changes in the fauna and flora consequent upon the great g(>ologic and 

 I^hysiographic changes of Glacial times; but this simple explanation is beset 

 with many difficulties and contradictions, and the results must be analyzed 

 with some care. The extinction of the horse in North America, for example, 

 does not admit of such a simple explanation. 



While in Europe the Mediterranean Sea presented a barrier to escape 

 or migration to the south, in North America there were broad continental 



R E C ENT 



Fig. 219. — Holocene or Recent times. A period of continental depression. Asia and 

 North America coniplotdy separated, preventing further intermigrations of mammals. The 

 island systems of the New and Old Worlds and of Australia mostly separated. Africa united 

 with Europe by a narrow desert strip, a barrier to further migration. 



areas and high plateaux affording easy migration routes southward, and 

 every means of escape. 



It is, therefore, more in accord with the facts to say that the Glacial 

 Period in North America originated certain new conditions of 

 life which directly or indirectly resulted in extinction. 



These conditions include diminished herds, enforced migrations, the pos- 

 sible overcrowding of certain southerly areas, changed conditions of feeding, 

 disturbances in the period of mating and reproduction, new relations with 

 various enemies, aridity, and deforestation; in short, a host of indirect causes. 



Protective adaptation to secular cold. — The resistance of mammals to 

 cold depends upon (a) the internal heat-producing power, which is a 

 progres.sive adaptation of the higher Mammalia, correlated with (6) the 

 accession of a warm external covering in the form of hair, wool, or blubber 



