SYNTHETIC CHARACTERS OF THE COMMUNITY 63 



(especially geographically isolated types) have lost their capacity for 

 variation and dissemination — Berardia subacaulis, Saussurea depressa, 

 Xatartia scabra, Borderea p^rena/ca,— characteristic species of certain 

 gravel-slide associations of the southern Alps and the Pyrenees are 

 very good examples. 



2. Besides these characteristic relict species there are others whose 

 fidelity is due to a narrowly specialized adaptation to definite physico- 

 chemical relations of the habitat, such as associations of rock crevices, 

 dunes, and epiphytes; e.g., the association Violetum calaminariae on 

 zinc soils investigated by Schwickerath (1931) contains several char- 

 acteristic species narrowly limited to this special substratum. 



3. Certain combinations of genes of polymorphic groups are more 

 vigorous and of greater efficiency in certain communities than in others 

 and increase more rapidly than related types. This gene combination 

 is thus enabled gradually to crowd out the others. Thus there results 

 a break in the series of polymorphic forms, and the new gene combina- 

 tion which prospers so vigorously in a certain community (association, 

 alliance) may attain the rank of an elementary species. This segrega- 

 tion i.? helped by geographic isolation of certain gene combinations 

 (e.g., Chrysanthemum vidgare var. delarhrei of the Festucetum spadiceae 

 of Auvorgne and many "races" of mountain species of Androsace 

 canica, Saxifraga moschata, etc.) whereas it would encounter great 

 difficulties where the presence of a polymorphic population constantly 

 made possible an exchange of genes. A. Kozlowska (1925) concludes 

 from her studies of variability in Fesiuca ovina that the association is an 

 important factor in the segregation of elementary species. 



Apogamy Hkewise favors the differentiation of sociologically more or 

 less specialized forms such as the characteristic species of the Festucetum 

 halleri from the Pilosellina group and those of the Seslerieto-Semper- 

 viretum from the Villosa and Vulgata groups of Hieracium. 



4. IVIany species are less selective in a region {e.g., Central or 

 northern Europe) with optimum conditions of life, and flourish in 

 several plant communities, but in other regions {e.g., southern Europe) 

 they become exclusively characteristic species of certain associations, 

 because there they find their life requirements fulfilled in only one 

 definite community, e.g., Neottia in the Fagetum, Centaurea jacea var. 

 typica in the Arrheyiatherum-Narcissus tazetta association. 



5. Characteristic species may be considered such ecologically 

 specialized forms as stand in direct dependence upon certain organisms 

 or groups of organisms or upon the life conditions brought about by 

 these. H(u-e belong man}^ mycotroj)hic species, saprophytes, nitro- 

 philes, and many acidophilous and humicolous plants. Parasites are 



