352 THE EVOLUTION THEORY 



remain a region of 2 x 5 = i o stretches upon which the influence of the 

 non-varying regions would be without effect. There Avould therefore 

 arise a new race in relation to the duration of the winter dress, and 

 this would not cease abruptly, but would gradually pass over into the 

 neighbouring regions, which however would be pure at their centre, 

 just as is probably the case in reality, if we regard B as any point in 

 the line of distribution from south to north. 



The harmony of the individuals within a species will therefore 

 depend in part upon the mingling of hereditary primary constituents 

 associated with reproduction, but in greater part upon adaptation to 

 the same conditions ; it is a similarity of adaptation, and the strongest 

 influence which sexual reproduction exerts lies not in the mingling 

 of these hereditary constituents alone, but above all in the reduction 

 in the germ-plasm of the two parental hereditary contributions — 

 a reduction which results from and through the sexual intermingling. 

 It is only this that prevents these primary constituents from varying 

 'at too unequal a rate in the transformations of species, and causes 

 them ultimately to resemble each other closely again. 



But while mutual sterility is not an absolutely necessary con- 

 dition in the separation of species, it would be going too far in the 

 opposite direction to regard mutual fertility as something general, 

 or to attribute to it a rule in the origination of new species. 



Certain botanists, like Kerner von Marilaun, regard the mingling 

 of species as a means of forming new species with better adaptations ; 

 they suppose that fertile hybrids may, in certain circumstances, crowd 

 out the parent species, and themselves become new species. It will be 

 admitted that such cases do occur, that, for instance, in the north of 

 Europe the hybrid between the large and the small Avater-lily, 

 Nu]yha.r luteum and Nuphar puinihim^ to which the name Niipliar 

 intermedium has been given, has driven both the parent species from 

 the field, because its seeds mature earlier, and it is therefore better 

 adapted to the short vegetative period of the north, but nevertheless 

 we must maintain that the evolution of species on the whole does not 

 take place through hybridization. Such cases are probably nothing 

 more than rare exceptions. This is corroborated by the entire insig- 

 nificance of hybridization in animals, among which species appear in 

 the same way as they do in plants, and where the mingling of two 

 species occurs only sporadically and in a few species, never to any 

 very great extent. 



If species are complexes of adaptations, based in each case on 

 the given physical constitution of the parent species, then we can 

 readily understand the fact that they are in our experience not fixed 



