E. V. Wulff —54— Historical Plant Geography 



Just as we can imagine the origin of identical forms and varieties at 

 different points of the area of a species, we can also imagine the simul- 

 taneous origin of identical species at different — often entirely separate 

 and very distant from one another — points of the area of a genus, 

 provided that: (/) these points are within the area of the maternal 

 form; (2) habitat conditions at these points are similar but not 

 necessarily identical (Briquet, 1901). 



Before proceeding to a discussion of these theories it is necessary to 

 point out that the terms monotopic and polytopic should not be 

 confused with the terms monophyletic and polyphyletic. The former 

 refer to the geographical location of the center of origin of a genus or 

 species and the origin of its area; the latter to the origin of a given 

 taxonomic unit from one or several initial roots. In the latter case, 

 due to adaptation to similar habitat conditions or similar biological 

 factors, separate organs (or the entire organism) may acquire such a 

 similar structure that there are apparent grounds for referring such 

 similar species to the same taxonomic unit. But, naturally, once its 

 polyphyletic origin has been established, such an artificial unit must be 

 divided into as many separate units as initial roots participated in the 

 development of the externally similar organisms. For instance, the 

 subclass Sympetalae is of polyphyletic origin, and it should, therefore, 

 be abolished, and the genera comprised within it should be referred to 

 corresponding families of apopetalous Angiospermae. The smaller a 

 taxonomic unit, the easier it is to trace its origin. Thus, polyphylesis 

 of genera occurs very rarely, and, as for species, their polyphyletic 

 origin is exceedingly unlikely. Even species of hybrid origin cannot be 

 regarded as of polyphyletic origin, as is clear from the universally 

 accepted definition of this term as given by us above. Such species 

 may be called polymorphic (Rozanova, 1938) but by no means poly- 

 phyletic. Hence, in discussing the monotopic or polytopic origin of 

 species, we have in mind only monophyletic species. 



In order to make clearer the concept of the monotopic or polytopic 

 origin of species, we shall cite a number of examples. Let us imagine 

 a species distributed over a considerable territory at the foot of a 

 mountain chain having a number of separate peaks. If this species 

 at a certain time in its dispersal begins to ascend the slopes of this 

 chain and, after attaining its summits, for some reason dies away on 

 the slopes and at the foot of the chain, the habitats on the summits 

 of the chain become absolutely isolated. Naturally, knowing the his- 

 tory of the dispersal of the species, we cannot regard these stations on 

 separate peaks as having arisen polytopically. 



Now let us imagine another case: A lowland species, upon reaching 

 these same summits and being subjected to the ecological conditions of 

 the high-mountain belt, forms a tetraploid race, which can be ranked 

 as a separate species. All of the peaks of our mountain chain will be 

 inhabited by this same tetraploid species, whose origin will again be 

 monotopic, not polytopic. It arose from a diploid species having a 

 continuous area and, if its habitats are isolated on separate mountain 

 peaks, the reason for this lies not in the origin of the species but in the 

 fact that the peaks are isolated from one another. If the mountain 

 chain had been topped not by separate peaks but by a continuous 

 plateau, the tetraploid species would have had a continuous area. 



