THE DISTRIBUTION OF COMMUNITIES 351 



of the community. Entire associations or fragmentary associations 

 may be carried from one place to another on the feet of water birds or in 

 the wool of sheep. Kreh (1929) studied the establishment of a Bidente- 

 tum tripartiti after the artificial drainage of a little lake near Stuttgart. 

 In 1927 the lake bed was, for the first time, dry during the entire sum- 

 mer. In 1928 this very peculiar association was well developed; and in 

 1929 it was at its optimum. 



The changes of climate which began at the close of the glacial 

 period resulted in great migrations of vegetation in Central and north- 

 ern Europe and, indeed, in all the northern hemisphere. Almost all of 

 the plant communities in these regions are either of recent origin 

 geologically speaking or have immigrated in very recent geological 

 time. 



Origin of Communities. — We cannot hope to know with certainty 

 the origin and history of more than a very few plant communities. 

 Paleontologic records and pollen analyses yield quite inadequate data. 

 In most cases we must be content with analogies and with searching for 

 the existing centers of distribution and lines of travel. 



On general principles, the center of distribution of an association is 

 to be sought where the association attains its best development, its 

 greatest extent, and its richest floristic composition, where it varies 

 into numerous facies, and where characteristic species are present in 

 largest numbers. 



On this basis one can explain the poverty of species and the lack 

 of characteristic species of many water and moor communities found in 

 the inner alpine valleys, while the same associations are richly devel- 

 oped in the alpine foothills and are well supplied with characteristic 

 species. The Molinietum so widely distributed all over northern 

 Switzerland and southern Germany dies out at Zizers (540 m.) in the 

 Rhine valley. At this point only Iris sihirica, Gentiana pneumonanthe, 

 and Pulicaria dysenterica remain as characteristic species. 



The center of distribution of the familiar Carex firma association 

 should be sought in the southeastern Alps. There it is most richly 

 and luxuriantly developed and teems with characteristic species. The 

 farther we follow it toward the north and west the fewer are the char- 

 acteristic species, until in the Bernese Oberland it is all but gone 

 (Braun-Blanquet, 1926, p. 235). 



There is a community of western origin in northern Switzerland on 

 leached high-terrace gravels. It is mostly very fragmentary, contain- 

 ing Quercus pedunculata, Q. sessiliflora, Calluna, Genista germanica, 

 Deschampsia flexuosa, Carex pilulifera, Hypericum pulchrum, Centaurea 

 nigra, etc. Even at the edge of the Black Forest, which bounds the 



