E. V. Wulff —70— Historical Plant Geography 



experiments have been repeated by many investigators, not only with 

 animals but also with plants, and it has been definitely established 

 that mutations may arise as a result of external factors not only under 

 artificial conditions but also in nature. 



The origin of new characters by mutation, Hnked with changes in 

 the germ-cells, may be accompanied either by changes in separate 

 hereditary factors, gene mutations, or by a change in chromosome 

 number, a chromosome change. As regards the occurrence of gene 

 mutations as a result of external influences, we know as yet only cases 

 where they were artificially induced under experimental conditions. 

 In contrast to this, the ever-increasing number of established cases of 

 chromosome changes include some where they undoubtedly occurred 

 in nature as a result of environmental conditions. 



From an evolutionary point of view of particular interest is the 

 fact that an increase in chromosome number may be accompanied by 

 heritable morphological changes, thus providing a basis for giving such 

 polyploid forms specific rank. 



Polyploidy may arise as a result of an increase in the chromosome 

 number following hybridization, allopolyploidy (which for problems of 

 plant geography are of less interest), or as a result of a simple doubling 

 of the plant's own chromosome set, autopolyploidy. The occurrence of 

 autopolyploidy as a result of the effect of external factors is now es- 

 tablished beyond any doubt. It has been induced experimentally by 

 treating the vegetative tissues or germ-cells with various agencies 

 (temperature. X-rays, colchicine, etc.). It may also be considered 

 proved that the phenomenon of polyploidy, including autopolyploidy, 

 is of common occurrence in nature, constituting an important factor in 

 species-formation. 



A study of autopolyploid species and forms has shown that in 

 many cases they have very definite geographical areas differing from 

 the areas of the initial diploid species. Moreover, these areas are con- 

 centrated, for the most part, in mountainous regions in the higher 

 altitudinal zones or in far northern or equatorial latitudes, or in other 

 localities where conditions are unusual for the given plant. This cir- 

 cumstance indicates that extreme climatic or other habitat conditions 

 induce doubling of the chromosome number presumably as a result of 

 irregularities at meiosis. Chromosome doubling is linked with the 

 origin of a new mutant form differing from the initial species in a 

 number of characters, which accounts for its adaptability to Hfe under 

 the indicated extreme habitat conditions. These conclusions are of 

 prime importance for plant geography, since they reveal one mode of 

 origin of vicarious species. 



Chromosome doubling within the limits of a species usually occurs 

 not throughout the whole extent of the latter's area but only in a 

 portion of it. The new polyploid forms differ not only cytologically but 

 always, to a greater or less degree, in their morphological structure. 

 This morphological difference may be expressed both in qualitative and 

 quantitative or only in quantitative changes. The taxonomic signifi- 

 cance of these morphological races is rated variously by different 

 investigators. Some consider that these changes are only of an intra- 

 specific nature; others regard them as adequate basis for ranking the 



