Chapter IX — 161 — On Migration 



tributed throughout the entire region. The spruce at first is limited 

 to mountain habitats in the Alps and Carpathians, but it is later found 

 also in the more northern parts of the mountain chains. Only in the 

 western mountainous districts is the fir beginning to become distrib- 

 uted. The birch and pine are becoming less widely distributed. 



3. Mixed Oak Forest Stage {Atlantic Period). Mixed oak forests 

 occupy a dominant position or, in any case, are characteristic of the 

 forests of central Europe of this period. At first, besides the oak, the 

 elm {Ulmus) and the lime or linden {Tilia) dominate in the forests; 

 later only the oak. In districts where the spruce and fir are distrib- 

 uted, these trees, which find themselves at a stage of expanding 

 distribution, compete with the oak. Finally, the fir in the central alti- 

 tudinal belts of the mountains in the west and the spruce in the east 

 and in the higher altitudinal belts of the west predominate, while the 

 oak retains its dominant position in the deciduous forest districts. The 

 hazel has by this time become only a shrub. Toward the end of the 

 Atlantic period the forests of central Europe are enriched by the beech, 

 which becomes widely distributed, together with the hornbeam {Car- 

 pinus). 



4. Beech Stage (Sub-Boreal-Sub-Atlantic Period). The beech (Fagus) 

 occupies a dominant position in the deciduous forests of central Europe. 

 It is widely distributed in coniferous forests, but here it finds itself in 

 competition with the conifers, particularly the fir. 



5. Stage of Man's Influence on Forests. The forests are destroyed 

 over a considerable part of the territory of central Europe. Reforesta- 

 tion destroys the virgin state of the forest. In coniferous forest dis- 

 tricts deciduous species are crowded out by the spruce and pine, while 

 in deciduous forest districts the distribution of the pine is artificially 

 fostered. 



From the hazel stage to the beech stage the upper limit of the 

 forests, as well as the altitudinal boundary of a number of forest 

 species in the mountains of central Europe, was several hundred 

 meters higher than now, corresponding to the more northern distri- 

 bution of a number of species in Scandinavia during the warm, post- 

 glacial period. 



This complex picture of the succession of vegetation cannot but be 

 the result of a number of different causes, just as at present the areas of 

 species are the result not of any one factor but of a combination of 

 factors. Moreover, there is no doubt that the chief of these factors is 

 cUmate, and also that the described succession of plant formations is 

 due primarily to changes in climatic conditions. There no longer exist 

 any disagreements on this point, but the precise determination of the 

 climatic periods, as based on changes in the composition of the vege- 

 tation, and their chronological sequence are problems that are far from 

 being solved and that are still the subject of heated dispute. 



All the foregoing leaves no doubt as to the significance of historical 

 causes — of the changes in climate and in the distribution of land and 

 sea that have taken place on the globe in different geological periods — 

 for an understanding of the present distribution of species and floras 

 and their groupings. We shall now examine various current theories 

 regarding the historical changes that have occurred on the face of the 



