Chapter IX — 157 — On Migration 



glaciated territories in North America than in those of similar terri- 

 tories in Europe. 



But this return of species to their old habitats does not by any 

 means account for all the migrations of species that resulted from the 

 Ice Age. The territory freed from the ice offered favorable conditions, 

 due to the absence of competition and of a closed vegetation, for the 

 invasion of plants from lands but sHghtly affected by the Ice Age. 

 Moreover, the alternation, in post-glacial times, of dry and humid cli- 

 matic conditions led to corresponding migrations of the hydrophytic 

 and xerophytic elements of the various floras. Asia constituted the 

 chief center from which there flooded into Europe a new vegetation. 

 South Siberian and Central Asiatic species, which penetrated into Eu- 

 rope, presumably, south of the Urals, species of the Aral-Caspian desert 

 flora, mountain species of the Caucasus and Western Asia — these were 

 the elements which enriched the flora of Europe and the advance of 

 which to the west is, in all probability, still in progress. 



Climatic Changes and Successions of Vegetation; The Post-Glacial 

 Period: — The retreat of the ice-sheets was not achieved at one stroke. 

 They alternately advanced and retreated several times, each succeeding 

 advance being less than the preceding. Similarly, the snow-line receded 

 to higher altitudes, at each stage lying 300-400 m. higher than before. 

 The present line lies at approximately this distance above the ridge of 

 moraine left by the last advance of the glaciers. After the final retreat 

 of the glaciers and ice-sheets the territory previously occupied by them, 

 including all the northern part of eastern Europe, was covered by 

 numerous lakes and the entire Baltic depression was filled with water. 

 These lakes have only been preserved in small part to the present day; 

 most of them, thanks to a gradual deepening of the river beds, have 

 either dried up or become converted into peat-bogs. 



Climatic conditions in the post-glacial period, according to Blytt 

 (1876, 1882), were characterized by age-long oscillations, warm, humid 

 periods alternating with dry, colder periods. Assuming such climatic 

 oscillations, it is possible to understand the present-day distribution 

 and grouping of species in the flora, let us say, of the Scandinavian 

 peninsula. In particular, as regards the flora of Norway, Blytt es- 

 tablished six groups of species, each having a definite geographical 

 distribution : 



The first group is confined in southern Norway to the mountains 

 and in northern Norway to the plains and bears a purely arctic charac- 

 ter. Typical species of this group are Dryas octopetala and a dwarf 

 willow, Salix reticulata. This type of vegetation is likewise character- 

 istic of Greenland, Spitzbergen, and other arctic regions. 



The second group is composed of species which, like the preceding, 

 avoid humid habitats, growing far from the shore and at altitudes up to 

 2,000 ft. above sea level. However, this group is considerably richer in 

 species than the preceding group. Here we already find deciduous 

 shrubs and trees, such as the hazelnut, elm, linden, ash, maple, oak 

 (Quercus pedunculata) , Sorbus aria, etc., and also a considerable di- 

 versity of herbaceous species. This type of flora Blytt designated as 

 boreal. 



