240 ANNALS NEM^ YORK ACADEMY OF SCIENCES 



Final Elevation. — This extreme subsidence was followed in late Post- 

 glacial times both in Europe and America' by reelevation which gave the 

 continents their present contours and climates. 



ALTERNATIONS OF CLIMATE AND FLORA 



Fluctuations of temperature and of moisture in Pleistocene times are 

 indicated first by the advances and retreats of the ice caps, second by the 

 presence of arctic or temperate Mollusca in the coast waters, third by the 

 variations in the flora in glacial and interglacial times, fourth by the 

 alternate appearance of the northerly or southerly types of mammals and 

 birds, fifth by the nature of the geologic depositions, sixth by the nature 

 of the land Mollusca in the loess. Combining the evidence derived from 

 these various sources the theoretic broad divisions of the climatic se- 

 quence are as follows : ( 1 ) the cold and moist phases connected with the 

 successive glacial advances and retreats; (2) the warm to temperate 

 climates of the First and Second Interglacial Stages and first half of the 

 Third Interglacial; (3) the dry and cold climate of the second half of 

 the Third Interglacial Stage and early Postglacial times; (4) the damp 

 and cool climate of late Postglacial times favorable to forests. 



The theoretic alternating conditions of each complete glacial cycle are 

 as follows :^* 



Subsidence : Glacial Maximum : Tundra flora and fauna 



Glacial Retreat : Cool and moist forest flora and fauna 



Elevation : Interglacial : Dry conditions, flora and fauna. Steppe 



Glacial Advance: Cool forest fauna and flora 



Subsidence : Glacial Maximum : Cold tundra flora and fauna 



Low Glacial Stage Temperatures. — Low temperatures during the pe- 

 riods of glacial advance are attested both by the advent of northern Mol- 

 lusca, marine and terrestrial, northern flora, and the repeated arrival in. 

 Europe of members of the cold fauna of the arctic tundras, including both 

 the smaller and the larger mammals and the birds, as well as the cold 

 fauna of the high, arid steppes of western Asia. Low temperatures are 

 attested also in early Postglacial times during the great Aurignacian- 

 Magdalenian art period by the heavy covering of hair indicated on all 

 the animals depicted by the Upper Palaeolithic artists. This hairy cov- 

 ering coincides exactly with that of the extreme northern tundra types of 

 reindeer, woolly rhinoceros (Z). antiquitatis) and woolly mammoth (E.. 

 primigenkis) found imbedded in the ice or frozen soil. 



^ Compare Wiist. 



