Chapter V — 69 — Types of Areas 



species. Hence, there arise species extending farther north or higher 

 up in the mountains than the initial species. 



The above-described phenomena do not take place in the case of 

 relic species in which the capacity for variation is greatly lowered. 

 Finding themselves in unfavorable environmental conditions, they cease 

 to multiply and gradually die out, their area of distribution becoming 

 more and more contracted, being preserved only in occasional, isolated 

 habitats. 



Even earlier than Kerner, Moritz Wagner (1868) advanced his 

 law of plant migration (which we have already mentioned), based on 

 similar observations. In this work, as well as in his later work, "tJber 

 den Einfluss der geographischen Isolierung und Kolonienbildung auf die 

 morphologischen Veranderungen der Organismen" (1870), Wagner 

 developed the idea that hybridization, inevitable among individuals 

 growing together in a habitat, results in the smoothing over of any 

 deviations arising as a result of the variability of organisms. An indi- 

 vidual plant with hermaphrodite flowers (or a pair of plants having 

 diclinous fiowers), happening to penetrate into a locahty isolated from 

 the region of mass distribution of representatives of the same species, 

 undergoes, as a result of external factors, changes that are preserved 

 due to isolation. This isolation may be expressed not only in great 

 distances or the transfer of occasional individuals over obstacles of 

 various kinds — mountains, seas, rivers — hindering the distribution of 

 the initial species; it may be any spatial isolation, regardless of extent, 

 provided it makes possible the preservation of changes induced by 

 habitat conditions — a circumstance that Kerner did not take into 

 account. 



Since Kerner's and Wagner's time, particularly during the past 

 forty years, the external morphology of plants has ceased to be the 

 only criterion for determining their taxonomic position. Geographical 

 distribution has acquired significance as one of the chief criteria for 

 establishing the existence of a taxonomic unit of the rank of a species 

 or subspecies. Hence, it must be recognized that geographical factors 

 exert an influence not only on the distribution but also on the origin 

 of a species. 



As a result of the adoption of the geographico-morphological method 

 in studies of genera and species, a method that was given a firm basis 

 by the works of Wettstein (1898) and Komarov (1901), many data 

 have been accumulated and critically evaluated which leave no doubt 

 that the origin of vicarious species is linked with the effect of environ- 

 mental factors encountered during the course of the dispersal of these 

 species. This has been confirmed by the mutation theory, according to 

 which mutations manifest themselves not only in changes in major 

 characters but also in numerous minor changes. 



In 1927 MuLLER induced mutations in Drosophila by X-ray treat- 

 ment and thus demonstrated that such changes may arise not only as a 

 result of internal causes but also as a result of external conditions. 

 This controverted the view, formerly held to be incontrovertible, that 

 the role of the environment in the formation of new hereditary charac- 

 ters is not of a creative nature, that the environment cannot directly 

 induce the appearance of new characters in plants and animals. These 



