E. V. Wulff — 154— Historical Plant Geography 



that from Africa was also closed, since the land-bridges that had 

 formerly connected Europe and Africa were no longer in existence. 



The Ice Age and Its Effect on Vegetation: — The climatic changes 

 that commenced about the middle of the Tertiary period led to the 

 Quaternary glaciation of the northern hemisphere. Having first started, 

 presumably, in North America, it spread over Europe and, to some 

 extent, over northern Asia. Considerable expanses of territory in the 

 latter continent, due to the more continental character of the climate, 

 remained uncovered by the glaciers. In all probability the present 

 climatic conditions of Greenland closely approximate those prevailing 

 over a considerable part of the northern hemisphere many thousands of 

 years ago. Glaciers covered all of western Europe, extending as far 

 south as the Thames, on the west, and the Carpathians, on the east. 

 All of northeastern Europe was completely covered by the great ice 

 sheet, which extended along the valleys of the Dnieper and the Don as 

 far south as 49°-5o° N. In Siberia, on the other hand, it did not cross 

 the 6oth parallel, and a number of regions, particularly in the north- 

 eastern part, remained uncovered by the ice-sheet. The extension of 

 the ice-sheet was not uninterrupted: now it advanced, now it re- 

 treated. Glacial epochs were followed by interglacial epochs, charac- 

 terized by comparatively mild climatic conditions. The vegetation 

 destroyed by the glaciers was gradually restored as a result of the re- 

 turn of some of the species that had been forced to migrate to the 

 south. As has been shown by various collections of fossil plants, in- 

 cluding those made at Hotting (near Innsbruck) in the Tyrolian Alps 

 (Wettstein, 1892), there grew in these regions during the last inter- 

 glacial epoch many plants not found there at the present time. One of 

 the chief stations of the chestnut was located at an altitude of 1,080 

 m. above sea level; grapes grew in the same places as now; Buxus 

 and Rhododendron ponticum were widely distributed, whereas now in the 

 Mediterranean Basin there have survived only a few relic specimens in 

 isolated localities; Laurus canariensis, Ficus carica, and Cercis Sili- 

 quastrum grew near Paris; fossil remains of Euryale ferox, an aquatic 

 plant very closely related to Victoria regia, now distributed in India 

 and eastern Asia, were found in central Russia near the Oica River 



(SUKATSCHEFF, 1908). 



The glaciation of enormous expanses of territory in the northern 

 hemisphere could not but be reflected in a lowering of temperature in 

 other parts of the globe, even including the tropics, which led to the 

 formation there of a snow cover— or, if such already existed on moun- 

 tain tops, to a lowering of the snow-line— and to an increase in precip- 

 itation throughout the southern hemisphere, i.e., to so-called pluvial 

 periods. 



A study of glacial phenomena has shown that the glacial epochs 

 did not constitute catastrophes that suddenly descended upon the plant 

 and animal world of the continents. The advance and retreat of the 

 ice occupied a period of thousands of years, during which, as a result of 

 changes in climatic conditions, plants and animals migrated, as the ice 

 advanced, from the north to the south and from the upper altitudinal 

 zones to the plains and valleys, and, as the ice retreated, returned, in 

 part, to the north and to the higher altitudes. 



