62 PLANT SOCIOLOGY 



in other disturbed societies or in artificial habitats which have no social 

 characteristics. 



2. Only the tangible sociological observed facts are of value. 

 Occasional human intervention (clearing, burning, or flooding) 

 causes sudden changes of vegetation, often giving a sociologically- 

 unstable but temporary transition phase, with the occurrence of relicts 

 of a former vegetation (forest or swamp relicts) or of isolated pioneers of 

 the succeeding association. Consequently the developmental stage of 

 the vegetation must be considered. 



Regional and General Fidelity. — The vegetation of a region with 

 uniform climate and uniform geological history shows usually great 

 uniformity in the species and in their grouping. But since general 

 climatic conditions are never alike over extensive areas and the 

 historical relations of the flora differ from region to region, it is plain 

 that both the grouping (Du Rietz, 1923, gives numerous examples) and 

 the fidelity of species will show regional changes. We have there- 

 fore to distinguish between regional and general fidelity, according to 

 whether a species is characteristic of an association throughout the 

 entire range of the association or only in a part of its range. Silene 

 otites is an exclusive species of the Xerobrometum in all the alpine 

 foothills and in a part of southern Germany. But in the dry spots of 

 the inner alpine valleys it thrives also in the Festucetum vallesiacaeand 

 in other associations closely related ecologically to the Xerobrometum. 

 Silene otites is therefore a regional characteristic species of the Xero- 

 brometum. Fidelity always involves the question of how a species is 

 related to a certain community. Of secondary importance is the 

 question of the social relations which this species has in all parts of its 

 range. Thus, one and the same species may be exclusive in different 

 regions to two or more distinct and different communities. Crepis 

 biennis in the region of the Arrhenatheretum is a characteristic species 

 of this association, but in another region it is characteristic of the 

 Trisetetum flavescentis. These two tall grass associations replace one 

 another in climatically different regions. 



Obviously determinations of fidelity must not be taken on too small 

 an area. But in the present stage of research these limitations need 

 hardly be mentioned. 



Causes of Fidelity. — The narrow social confinement of highly 

 characteristic species is the result of various causes. 



1. Many paleoendemics of geologically ancient regions are sociolog- 

 ically narrowly localized, e.g., Tertiary species of Mediterranean asso- 

 ciations. Long-continued rigorous selection seems to have confined 

 these species to narrowly circumscribed habitat relations, so that many 



