Chapter IV —55— Origin of Areas 



Thus, in such cases as the above, either the discontinuity of the area 

 is only apparent, each hnk being actually connected with the area of 

 the initial species, or— in case the latter has become extinct or there 

 have occurred some great changes in the earth's surface, e.g., the break- 

 ing away of part of the mainland in the form of an island — the species, 

 though at present isolated, had their origin in the continuous area of an 

 initial form. Hence, in order to estabhsh the history of the flora of a 

 given locaUty having in its composition isolated elements, it is neces- 

 sary to penetrate deeply into the history of their initial forms and seek 

 to ascertain whether or not their area might possibly have been con- 

 tinuous in the past. In such a case the work should be conducted 

 along the Hnes customary for investigations in the field of historical 

 plant geography. 



Quite different will be our approach in cases where we assume the 

 possibility of the origin of identical races in two distant and isolated 

 points, without their occurring in intermediate localities characterized 

 by analogous habitat conditions. For instance, Wetterhan (1872), 

 one of the first to advance this theory, considered it possible to assume 

 that, as a result of slow changes in the structure of a plant and the 

 preservation of the most favorable of these changes, there might arise 

 identical species in entirely different and far distant localities, as, for 

 example, in arctic and alpine regions. This same viewpoint is held by 

 a number of other authors (see Briquet, 1891, 1901, and the appended 

 bibliographies). 



For example. Briquet (1901), in his investigation of the origin of 

 the Alpine flora of Corsica, points out that the mountains of this 

 island are considerably more ancient than the Alps and never were in 

 contact with the latter. Nevertheless, on the mountains of Corsica 

 and on the Alps there are found exactly identical species. At the same 

 time, the species most common in the Alps and having special dis- 

 persal mechanisms for the transport of their seeds are not found in 

 Corsica, and, conversely, the species most common on this island are 

 not found in the Alps. On the other hand, rare Alpine species, whose 

 transport there are no grounds at all to suspect, are found on the 

 mountains of Corsica. Briquet considers that only one supposition is 

 possible, viz., that these identical species arose in such widely separated 

 stations polytopically, i.e., independently of each other. 



Another example : Primula farinosa, which is distributed through- 

 out the entire northern hemisphere, has an isolated portion of its area 

 in the southern hemisphere, in South America on the shores of the 

 Straits of Magellan, separated from the North American part of its 

 area by a distance of ninety degrees of latitude. This antarctic prim- 

 ula, although it is ranked as a special variety, var. magellanica, is 

 almost indistinguishable from typical P. farinosa. According to the 

 adherents of the polytopic theory, the finding of this primrose along the 

 shores of the Straits of Magellan is ascribed to its entirely independent 

 origin there. But the question immediately comes to the fore: From 

 what initial forms did this species with its double origin arise? Since 

 there was not a common initial form with a continuous area of distri- 

 bution, it would be necessary to assume that the forms which gave 

 origin to this species are the initial forms not only for the genus 



