E. V. Wulff —68— Historical Plant Geography 



in optimum conditions, and the entire cycle of its development is each 

 year realized without hindrance. In the other part of the area, comprising 

 chiefly the periphery, the habitat conditions are only to a minimum 

 degree suited to its biology. Here the species may mature its seeds 

 only in favorable years, while in unfavorable years, if it finds this at all 

 possible, then only in especially sheltered spots. This leads to a de- 

 crease in the number of progeny. Hence, nearer the periphery of its 

 area a species is represented by an ever smaller number of individuals 

 growing in ever more rare and isolated spots, while nearer the center 

 of its area it is represented by an ever larger number of individuals 

 occupying an ever wider range of habitats. 



In the center of its area, where habitat conditions are most favor- 

 able for the existence of a species, there is, in Kerner's opinion, very 

 little probabihty of the origin of new species, since, if such should 

 arise there, they would be ehminated as a result either of competition 

 with the numerous individuals of the initial species or of inevitable 

 hybridization with the latter. On the periphery of the area, on the 

 other hand, these obstacles to the origin of new species disappear. The 

 initial species no longer covers the territory so completely, some habi- 

 tats being unsuitable or inaccessible to it; hybridization with it, due 

 to the isolation of its habitats and to the decrease in number of indi- 

 viduals, becomes ever more rare or does not occur at all. At the same 

 time, the different chmatic conditions at the boundary of an area fur- 

 ther the origin of aberrant forms even in species that are not ordinarily 

 subject to great variation. These circumstances make clear the rela- 

 tive abundance of young species near the periphery of areas of old, 

 initial species. 



The periphery of the area of a species should be understood to refer 

 not only to its horizontal distribution. Altitudinally there arise analo- 

 gous (to those just described) changes in climate and soil substrata and, 

 consequently, analogous changes in habitat conditions. Hence, even in 

 that portion of the area where, due to favorable conditions, the species 

 is represented by the greatest number of individuals, there may exist 

 analogous possibilities for the origin of new species. In consequence 

 of this, the fact that on maps there are often shown within the areas of 

 initial species tiny areas denoting young species does not indicate 

 intermingling, -since this is only seemingly so, as they are located at 

 different elevations above sea level. 



Changes in edaphic conditions presumably led to the origin of such 

 specialized species as Androsace Hausmanni and Asplenium Seelosii, 

 which grow in southern Tyrol, where the soil is predominantly of 

 dolomite formation, and which were apparently derived from Androsace 

 glacialis and Asplenium septentrionale, which are linked in their habitats 

 with slate soils. Similarly, the unique species Asplenium serpentini, 

 which grows on serpentine soils in Austria, Moravia, and Bohemia, 

 apparently arose from Asplenium adiantum nigrum, which grows on 

 non-serpentine soils. The existence of such species, now known as 

 vicarious but which Kerner called "parallel forms", testifies to the 

 influence of external factors on the origin of species. 



Such new species are able to extend their areas beyond the limits, 

 set by climatic and soil conditions, to the distribution of the initial 



