Chapter III 



—29- 



On Areas 



finds itself in favorable conditions, since otherwise there would be no 

 occasion for its arising there. If a species should accidentally arise in 

 unsuitable conditions, it would be immediately destroyed as a result of 

 natural selection. A new species is highly variable, reacting to all the 

 micro- and macro-conditions of its habitat, and, hence, gives rise to a 

 large number of forms. 



Pachosky (1921) considers that the above-noted decrease in number 

 of representatives of a species or variety toward the periphery of its 

 area is closely connected with adaptability to definite habitat condi- 

 tions, particularly to a definite type of soil. In the center of an area, 

 where, as a rule, the habitat conditions of a given species most nearly 

 approximate the optimum, it can grow under fairly diverse conditions, 

 even on different soils. On the other hand, farther from this region] 

 i.e., nearer to the periphery of the area, not only is an optimum com- 

 bination of factors of more rare occurrence but often there is lacking 



Fig. 3. Centers of frequency in areas of species of the genus Hieracium on the Scandina- 

 vian peninsula (Swedish West Coast) : left, Hieracium meticeps; right, B. Moroleucum (After 

 Samuelsson). 



even that minimum of conditions required for the normal existence of a 

 species. For example, the beech tree, ordinarily capable of growing on 

 a variety of soils, on the periphery of its area is confined solely to lime 

 soils. Relic species and species becoming extinct likewise prefer 

 localities with lime or chalk soils for their habitats. Apparently the 

 physical conditions of the substrata of these soils provide for 'such 

 species more favorable conditions as regards competition with other 

 species, thus allowing them to maintain themselves, despite the fact 

 that the habitat conditions as a whole deviate considerably from those 

 normal for them. 



As regards the effect of climatic factors on the distribution of a 

 species within the hmits of an area, Graebner (1910) distinguishes 

 between the "region of compact distribution", within which asp ecies 

 finds Itself in optimum conditions as regards climatic factors and the 

 'absolute limit of distribution", where the stations of a species are 

 confined to certain localities having specific habitat conditions. 



