PLEISTOCENE OF EUROPE, NORTH AFRICA, AND NORTm AMERICA 387 



epochs. It presents alternations of arctic, boreal, and temperate species, 

 which migrated southward and northward following the advances or re- 

 treats of the glacial cap. The broad divisions of the climate are as follows : 



1. First cold phase, connected with the first glacial period. 



2. The fairly warm climates of the interglacial periods. 



3. The damp and cold climates of the last glacial advances. 



4. The dry and cold climate of the period of the last glacial retreat 



in the age of the reindeer. 



There is evidence both in Europe and North America that especially in 

 certain of the long, warm interglacial intervals the climate in the northern 

 hemisphere was somewhat more equable and milder than at present, with a 

 higher mean temperature and at certain intervals a greater precipitation of 

 moisture} There was perhaps more sunshine than now. A^ a result of 

 such favorable conditions arboreal vegetation flourished to the far north. 

 The present tundras of Siberia and British America then supported forests 

 which have long since been extirpated, the northern limit of sunilar living 

 trees now lying far to the south. ^ 



A picture of the flora of the very long and warm Mindel-Riss' interval 

 of the Second Interglacial epoch, the Chellean Stage of human culture 

 according to Penck, in which the hippopotamus appears for the last time in 

 northern Europe, is preserved in the tnf de la Celle, which contains remains 

 of the sycamore maple (Acer pseudoplatanus) , willows (Salix), the Austrian 

 pine (Pinus laricia). Higher up in the same deposits we find the box tree 

 (Buxus), not uncommonly the fig (Ficus); the sweet bay (Laurus nobilis) 

 appears less frequently. In the upper part of the tuf de la Celle where Chel- 

 lean palaeoliths have been found, the fig and sweet bay are absent.^ The cli- 

 mate was more damp and certainly milder than that of the present time in 

 this region, the mean annual temperature being eight to nine degrees higher. 

 In Lorraine below the level of the third Pleistocene fauna there occurs a 

 flora in which the most northerly varieties of the larch (Larix) and the 

 mountain pine (Pinus lambertiana) predominate. In still higher plant beds, 

 the tufs de Pont-d-Mousson in eastern France, there are remains of forests 

 composed of deciduous trees some of which have since migrated farther 

 south. These are a few of the many instances showing the southward and 

 northward migration of the flora in Pleistocene times, similar to those 

 to be mentioned as occurring in the Toronto Formation of Canada 

 (p. 448). There is strong ground for the beliefs that there were cycles of 

 climatic change beginning in earlier interglacial and succeeding glacial 



1 Croll, J., On Arctic Interglacial Periods. Philos. Mag., Ser. 5, Vol. XIX, 1885. p. 36. 



2 Nathorst. Engler's Bot. Jahrb., 1881, p. 431; also Schroter, C, Die Flora der Eiszeit, 

 Ziirich, 1883. 



3 De Lapparent, A., Traite de Geologie. Paris, 1906, p. 1703. 



^ Penck, A., Die alpinon Eiszcitbildungen und der prahistorische Mensch. Archiv. An- 

 thropoL, U.S., Vol. I, no. 8, 1904. 



