E. V. Wulfif —138— Historical Plant Geography 



result of the spread of individual plants during the process of their 

 multiplication and continues until the climatic boundaries are reached, 

 provided that no other obstacle has previously barred their advance, we 

 must arrive at the only conclusion possible, viz., that successions of 

 floras result primarily from changes in the location of the climatic 

 zones. It may thus be considered as definitely established that cli- 

 matic zones have always existed on the earth but that the location of 

 these zones has changed many times. Furthermore, the upheaval of 

 whole mountain systems and the drying up of seas have created new 

 territories, free from competition, whither species have drifted in their 

 migrations. Spreading from slopes to mountain peaks and from forest 

 communities to the arid beds of former seas, these migrants underwent 

 processes of form-genesis, whole cycles of polymorphic forms and ge- 

 ographical series of species often being created. 



Hence, summarizing what has been said, we may presume that the 

 present distribution of plants is the result not only of the autochtho- 

 nous development of floras but also of the migrations of species and 

 floras (due to climatic and physiographic changes), migrations made 

 possible by the dissemination of seeds and modified by the character of 

 the topography and the ecological conditions of the given territory. As 

 Blytt (1882) so aptly stated, present-day vegetation reflects, as in a 

 mirror, the geological history of a country, different groupings of species 

 being the expression of different stages in that history. 



In this concept we fail to find, however, an explanation of why and 

 how, as a result of climatic changes, the areas of species are altered. 

 "We cannot say," wrote Darwin ("Origin of Species," 6th ed., 1911, 

 P- 533)1 "why certain species and not others have migrated; why 

 certain species have been modified and have given rise to new forms, 

 whilst others have remained unaltered. . . . why one species ranges 

 twice or thrice as far, and is twice or thrice as common, as another 

 species within their own homes". 



It is, nevertheless, possible to elaborate a working hypothesis ca- 

 pable of explaining how successions of floras may have taken place. 

 Such a hypothesis has been postulated by Good (1931) under the name 

 of the "theory of tolerance". This theory implies "that a species is 

 able to occupy only those parts of the world where the external con- 

 ditions are within those of its range of tolerance. This total area which 

 a species can occupy in virtue of its tolerance is conveniently termed 

 its 'potential area'. The size of the potential area will tend to vary 

 with change in external conditions" (p. 155). 



If species possessed unlimited tolerance, unlimited capacity for 

 adaptation and for the formation of new races adapted to any changes 

 that might occur in habitat conditions, and if these adaptations 

 occurred with the same rapidity as the climatic changes, then species 

 would be able to remain within the limits of their original areas. In a 

 few cases this has actually taken place; relic species and areas testify 

 to this. In various floras we may find species that have, by adapting 

 themselves to new habitat conditions, undoubtedly survived from 

 former geological periods characterized by different climatic condi- 

 tions. This holds particularly true in those regions where a cold 

 climate has been replaced by a warmer one, as, for instance, in South 

 Africa. 



