242 ANNALS NEW YORK ACADEMY OF SCIENCES 



have long since been extirpated, the northern limit of similar living trees 

 now lying far to the south. 



■Alternations of Flora. — It is clear from these great successive fluctua- 

 tions of temperature, moisture and aridity during Pleistocene times that 

 the flora cannot be treated as a unit nor as progressing in a single direc- 

 tion like the flora of preceding epochs; the flora as well as the fauna 

 presented alternations of arctic, boreal and temperate species which 

 migrated southward and northward following the advances or retreats of 

 the glacial cap. Thus we may observe evidences of changes of climate 

 and flora from forested conditions to steppe conditions and back to for- 

 ested conditions. From the beginning of the Fourth Interglacial in- 

 terval to the present time, the Alps region (Penck, Bruckner, 1909) has 

 apparently gone through a cycle of changes such as the following: 



Vegetation Climate Epoch 



Fourth, forest conditions Western European, oceanic Modern 



Third, steppe conditions Western Asiatic, continental Fourth Glacial and 



Postglacial 



Second, tundra conditions Northeastern-European. Fourth Glacial 



subarctic 



First, forest conditions Western-European, oceanic Third Interglacial 



The elephants (Hilzheimer, 1913) in the structure of their grinding 

 teeth afford clear indications of the plant life, whether consisting mainly 

 of grasses or forests, but not of climate except in so far as vegetation is 

 dependent upon moisture. The advance and retreat of the ice is de- 

 pendent both upon moisture and extreme cold and involves the frozen 

 subsoil conditions of the tundras which are fatal to forests. Cases of 

 alternation of conditions favorable to Elephas trogontherii, which is be- 

 lieved to be a grass-eating form, and of Elephas antiquum, which is be- 

 lieved to be a forest-living form, are observed in Taubach by Wiist. 25 

 This author observes in the lower layers Elephas antiquus succeeded in 

 the middle layers by E. trogontherii and then in the upper layers again 

 by E. antiquus, and deduces from this succession a change of conditions 

 from forest, to steppe, to forest. 



FAUNAL LIFE ZONES OF EUROPE 



Tn the whole history of the Mammalia in various parts of the world we 

 know of no conditions so unusual and complex as those which prevailed 

 in Europe in Pleistocene times. These conditions were the product of 



25 Wust, Ewaid : "Die plistozanen Ablagerungen des Travertingebietes der Gegend von 

 Weimar und ihre Fossilienbestande in ihrer Bedeutung fur die Beurteilung der Klimasch- 

 wankungen des Eiszeitalters." Zeitschr. Naturw., lid. S2, pp. 161-252. Leipzig, 1911. 



