E. V. Wulff — 142— Historical Plant Geography 



the habitat or habitats) of a species. This was pointed out by Kash- 

 KAROV and Korovin (1931), who showed, on the basis of the migration 

 of different groups of vegetation in Soviet Central Asia, that there 

 exists a correlation between plant migration and changes in habitat 

 conditions. Thus, an alteration in ecological conditions, e.g., the ex- 

 pansion of desert areas, creates new routes of dispersal {viae oecologicae, 

 using these writers' term) for sand vegetation. These new routes may 

 embrace extensive territories, if habitat conditions are homogeneous, as 

 was the case, for example, in the Tertiary period in Eurasia. A change 

 in ecological conditions, accompanied by an advance of these routes of 

 dispersal into new territories, constitutes a factor in the further modi- 

 fication and dispersal of both plants and animals. 



As a result of the migrations of species, both the size and shape of 

 their areas are altered, since it is impossible to imagine that such mi- 

 grations could occur in so regular a fashion that the areas could retain 

 their original contour. The diverse topography of our globe, the dis- 

 tribution of land and sea, excludes the possibility of an area retaining 

 its former shape. Consequently, as a result of their migrations, the 

 areas of some species increase in size, as others decrease. A continuous 

 area may, due to topographical conditions, break up into two or more 

 parts, becoming a discontinuous or even insular area. Furthermore, 

 the potential area of a species may increase in size at a considerably 

 more rapid rate than the actual dispersal of the species, the result being 

 that the species, temporarily or permanently, ceases to occupy the 

 whole of its potential area. 



The processes involved in the geographical distribution of plants 

 require for their realization exceptionally long periods of time, far 

 exceeding the period of man's life upon this planet. Hence, there can 

 be no thought of direct observation or experimental proofs of the fore- 

 going postulates; only by indirect proofs can their soundness be es- 

 tablished. As one of such indirect proofs we may cite the fact that the 

 individuals of any one species react in a more or less similar manner to 

 external conditions, resembling one another in tolerance as well as in 

 morphology. This is particularly clear as regards climatic factors in the 

 case of cultivated plants, where competition has been eliminated. It is 

 true that even in this case there may be found some individuals or 

 strains that are characterized by greater tolerance to external condi- 

 tions than others, but they undoubtedly represent separate races or 

 incipient species possessing their own specific tolerance. In confir- 

 mation of this we may cite the opinion of White (1926, 1928), who 

 considers that a species whose area embraces different climatic zones 

 without any doubt consists of several races differing in their tolerance 

 to temperature conditions. This, in his opinion, accounts for the fact, 

 known to horticulturists, that plants brought from the northern part 

 of the area of a species are better able to withstand cold than those 

 brought from the southern part of the area. The differences between 

 these races are not manifested in the morphological structure of the 

 plants, or, if they are so manifested, the differences are so slight that 

 they are not detectable by the naked eye. White concludes that the 

 ability to withstand low temperatures, in the case of species not 

 accustomed to such temperatures, may be ascribed exclusively to the 

 genesis, by mutation, of cold-resistant races. 



