E. V. Wulff — 66 — Historical Plant Geography 



oj development of the plant. To this latter type belong species charac- 

 terized by seasonal dimorphism, as established by Wettstein. By 

 this we understand, for example, the finding on one and the same 

 meadow two closely related forms differing in time of development, one 

 flowering in the spring, the other in the autumn. Such seasonal 

 dimorphism is occasioned by a change in the initial form, resulting in 

 its conversion into two local varieties under the effect of the economic 

 use of the meadow by man. 



All forms of intraregional vicarism, including seasonal dimorphism, 

 not having geographical significance, are only of minor interest for 

 historical plant geography. 



In addition to the various types of true vicarism, there is also a 

 vicarism that is only seeming, called by Vierhapper pseudo-vicarism. 

 While in the case of true vicarism the vicarious species arise from a 

 common initial form, becoming differentiated either within the limits of 

 the latter's area or upon penetration into new habitat conditions fol- 

 lowed by isolation, in the case of pseudo-vicarism the seemingly vi- 

 carious species, although related to one another, have arisen from 

 different initial forms, and their apparent vicarism is occasioned by the 

 secondary penetration of a second species into the area of a first and 

 by its occupation of those portions of this area not occupied by the 

 first species due to their not being suited to its biological peculiarities. 

 Usually such pseudo-vicarism may occur only in the case of the ab- 

 sence of truly vicarious species. Hence, in the case of true vicarism 

 the penetration of the initial form into a given region is the first step, 

 followed by its breaking up into vicarious races adapted to different 

 habitat conditions. In the case of pseudo-vicarism, on the other hand, 

 the origin of the different races is the first step, after which comes the 

 occupation of separate territories. 



Pseudo-vicarism may be of various types, viz.: (i) the two forms 

 penetrating a given region may be fully established species or races; 

 (2) one of the species or races has its origin in the given region, while 

 the other does not penetrate this region until later. As an illustration, 

 we may cite an example given by Vierhapper. The genus Erigeron 

 has a number of vicarious species, geographically markedly distinct. 

 Among them there are two species in the Alps: Erigeron polymorphus, 

 growing on calcareous soils and descending far down toward the foot 

 of the mountains, and E. uniflorus, in contrast to the other, avoiding 

 calcareous soils and closely confined to the higher altitudinal zones. 

 The nature of these two species gives apparent grounds for believing 

 that they were derived from a common initial form and became 

 differentiated as a result of adaptation to different, mutually exclusive 

 habitat conditions. However, a study of the entire cycle of forms to 

 which these two species belong has shown that we have to do here with 

 species which, although related, are of different origin. E. uniflorus 

 arose, presumably, not in the Alps but in the mountains of Asia or in 

 the Arctics, whence it penetrated during the Ice Age into the Alps 

 within the limits of the area of E. polymorphus, already established 

 there at the time, and occupied places not already occupied by the 

 latter because of edaphic conditions. 



Pseudo-vicarism, consequently, despite its being seemingly identical 



