E. V. Wulff —56 — Historical Plant Geography 



Primula but for the entire family of Primulaceae, the development of 

 which proceeded in different directions, leading in the end to the poly- 

 phyletic origin, in two hemispheres of the globe, of entirely identical 

 species, a conclusion scarcely acceptable to anyone. 



This problem may be solved considerably more simply by assuming 

 the very probable migration of P. farinosa from North America along 

 the Cordilleras and Andes to the Far South. This is confirmed by the 

 fact that this primula is not the only plant in South 'America 

 with such a range. Here are found species of a number of genera, 

 such as Draha, Saxifraga, Gentiana, Alopecurus, Carex, Phleum, etc., 

 identical or very close to arctic-alpine species of these genera. 

 According to Pax (1889) in his monograph on the genus Primula, 

 P. farinosa first occurred in the southern hemisphere only after it had 

 become widely distributed in the northern hemisphere. Its migration 

 to the south may have taken place during the Ice Age, when the 

 cUmatic conditions in the Andes were more humid than at present, 

 which favored the growth of arctic-alpine species. In the post-glacial 

 period these species were preserved in the mountains of North America 

 and the antarctic region of South America but died out in the inter- 

 mediate habitats, a bipolar area thus being formed. 



Similarly, as regards the high-mountain vegetation of the islands of 

 the Tyrrhenian Sea, which served as the basis for the conclusions 

 drawn by Briquet, we find in the work of Braun-Blanquet (1923) 

 on the origin of the flora of the central massif of France the following: 

 "The hypothesis ... of a 'polytopic' origin . . . gradually vanishes 

 in thin air and has to yield its place to a better grounded explanation 

 that is also in accord with morphogenetic data. The latter show that 

 the definitive separation between the Betic Cordillera and the Moroccan 

 Rif took place at the beginning of the Pleiocene period; the islands of 

 the Tyrrhenian Sea were separated from the mainland at the end of 

 the Tertiary period, at which time they already possessed a diverse, 

 orophytic flora. On the summits ... of the Central Massif there 

 must also have existed alpine species" (pp. 206-7). 



The advocates of the polytopic theory consider possible the inde- 

 pendent origin by mutation of absolutely identical species in two, 

 widely separated points of the initial area. But up to the present 

 time we do not know of a single case of the origin by mutation of one 

 and the same species, with an absolutely identical complex of charac- 

 ters, at widely separated points. Likewise there cannot arise one and 

 the same hybrid species at two independent and very distant points of 

 the areas of the parental species. Species having extensive areas are 

 not fully homogeneous throughout the entire extent of their areas, but 

 constitute series of vicarious races. Consequently, when these species 

 are crossed at widely separated points of their areas, the differences 

 between the geographical races to which the parental forms belong 

 exclude the possibility of the occurrence of completely identical 

 hybrid species. 



All this leads us to the conclusion that similar habitat conditions 

 may induce the polytopic origin of similar forms, distinguished from 

 one another by only a few characters, e.g., glabrous or pubescent forms, 

 forms with glandular hairs, with liguleless leaves (in cereals), with or 



