Chapter IV —49— Origin of Areas 



quently there does not take place a contraction of the area previously 

 occupied; and, second, that a discontinuous area is to be explained bv 

 the breaking up of a once-continuous area into separate sections either 

 as a result of the dying out of the species in the intervening places, due 

 to changes in climatic conditions, or as a result of changes in the 

 location of seas and continents causing the rupture of such a continu- 

 ous area. 



Another important question needs to be clarified: Should we con- 

 sider that a species arises from a single bisexual individual, two dicli- 

 nous individuals, or a few individuals and that, consequently, the center 

 of origin will occupy a territory limited by the number of progeny of 

 this one individual or few individuals that take root, or should we 

 consider that in the process of species-formation many individuals may 

 participate over a considerable extent of territory, which in such case 

 constitutes as a whole the center of origin of the area? In our opinion, 

 both modes of origin of an area are possible, depending on the way in 

 which a species arises. 



If a species arises, for example, as a result of hybridization, the 

 initial number of individuals of this species is determined by the 

 number of seeds in the fruits of the given plant which succeed in 

 finding favorable conditions for germination and growth and in main- 

 taining themselves in competition with vegetation already established. 

 Since plants spread by means of their dispersal mechanisms only very 

 gradually, it is clear that the initial center of such a hybrid species 

 will occupy a very small territory. If, on the other hand, a species 

 arises, let us say, as a result of an autopolyploid change in the bio- 

 logical and morphological characteristics of an initial species caused 

 by an increase in chromosome number induced by climatic or other 

 conditions, the changes in such a case may affect a large number of 

 individuals over an extensive territory, which will thus constitute the 

 center of the area. The latter in such a case might rather be called 

 the region of origin, but the sense remains precisely the same, since 

 this is the initial starting point of the dispersal of a new species and the 

 formation of its area. 



Let us take, for example, a species extending its area of distribution 

 toward the north. There comes a time when the vanguard plants at- 

 tain the climatic barrier beyond which they cannot pass. If under the 

 influence of the new climatic conditions there should arise by mutation 

 an autopolyploid species (and that new species do so arise has now been 

 proved for an ever larger and larger number), such a process of species- 

 formation might embrace not a single individual plant but all the 

 plants that had attained the indicated barrier over quite an extensive 

 territory. Likewise, at a time of great climatic changes on the earth's 

 surface, as, for instance, during the glacial and interglacial periods, 

 when seas advanced or receded and great mountain masses were up- 

 lifted, the effect of such climatic revolutions also must have extended 

 over vast areas, inducing processes of species-formation in a large 

 number of individuals of one and the same species. 



If one studies the areas of genera both as at present constituted and 

 particularly if one adds to the present territory the regions embraced 

 by these areas in former geological periods, he cannot but marvel at 



