E. V. Wulfif — 30 — Historical Plant Geography 



From the foregoing it is clear that the farther from the center of an 

 area the more rarely do conditions suitable for the growth of a given 

 species occur, which results in the peripheral regions of an area being 

 more sparsely inhabited by the species than the center. Moreover, 

 plants growing under conditions unsuitable for them will quite natur- 

 ally find themselves subjected to competition and crowding out by 

 closely related species for which these same conditions are more suit- 

 able. 



This, however, can by no means be taken as an unconditional and 

 universal proposition. We can assume also the occurrence of such 

 cases — and they actually do occur — where a species, spreading in the 

 direction of the periphery of its area, encounters, often far from the 

 place of its origin, favorable habitat conditions, perhaps even more favor- 

 able than existed in the center of the area, which give an impetus to new 

 form-genesis. But such form-genesis, leading even to the origin of new 

 species on the periphery of the area of the initial species, may also be 

 due to unfavorable conditions. We shall discuss this in more detail 

 later. We should, therefore, distinguish between the center of origin 

 of an area and the center of its development, the latter in such cases 

 being necessarily regarded as a secondary mass center (or centers, since 

 there may be several of them) of the area. 



Hence, when a species arises at any point of its future area there is 

 created, first of all, a center of propagation of this species, the center of 

 frequency of its area, and then there develops its differentiation into a 

 number of forms of different taxonomic rank, the creation of a center of 

 maximum variation, or a mass center of the area. In young species 

 the latter center may not exist at all, or it may coincide with the center 

 of frequency. Later on, such coincidence will be broken, since the 

 primary mass center will remain in its original place, while the initial 

 center of frequency may disintegrate and arise anew at one or another 

 point in the migration of forms issuing from the primary center of 

 formation of a species. A species during the course of its dispersal 

 may, under especially favorable conditions, enter into a phase of new 

 form-genesis, as a result of which there will arise a secondary rnass 

 center of the area, which in contrast to the primary center of its origin 

 constitutes a center of the subsequent development of the area and 

 wiU be characterized by the presence of younger forms. The secondary 

 nature of such a center may be established by a combination of various 

 methods of botanical study, by which it would be shown that in the 

 center of origin there is a concentration of more ancient and primitive 

 forms, as compared with those concentrated in the secondary center of 

 development. 



From the foregoing we may conclude that the region of frequency of 

 stations may ordinarily be expected to coincide with the region of 

 maximum variation, i.e., with the center of origin of the area, in those 

 cases where the distribution of the given taxonomic unit has not yet 

 been subjected to any later influences inducing alterations in the 

 character of the area. 



As regards the area of a species, correspondence of the center of 

 frequency with the center of the area is characteristic, as we noted 

 above, for areas of young species. To illustrate this point we may 



