OTHER THEORIES OF SPECIES-FORMING. 349 



In closing this confessedly inadequate consideration of 

 the important work and theorising of de Vries we should 



not fail to note that the mutations theory is 

 Mutations . ,1 c 



theory con- m stron g contrast to any theory of species- 



trastedwith forming based on Lamarckian principles in that 



Lamarckism. . . 



the newly appearing differences in organisms 

 leading to the establishment of new species are purely con- 

 genital : that is, the mutations arise in one or both of the 

 sex cells and only later appear in the adult organism. There 

 is no question of the transference to the germ-cells of 

 changes induced in the soma by use or disuse or functional 

 stimulus in such a way as to result in the photographic 

 reappearance of these changes in the offspring. Mutations 

 are true congenital or blastogenic variations. 'The muta- 

 tion theory," well says Conklin, 27 "is a theory of the evolu- 

 tion of organisms through the evolution of their germ-cells." 

 The mutations theory is also in sharp contrast to the 

 theory of species-forming by geographical isolation (see 

 chapter ix). According to de Vries many dis- 

 theoryTnTrasted tinct species (de Vriesian elementary species) 

 with the isola- can anc j d o exist side by side in the same range. 



tion factori 



In fact they "are found to be heaped up in the 

 centre of their area of distribution, but are more scattered 

 at the periphery." Now according to Wagner, Gulick, 

 and Jordan two closely allied species, i. e., stock and off- 

 shoot, are found practically never to inhabit the same range, 

 except in those cases where a migration of one type into 

 the territory of the other has taken place after the differ- 

 entiation has been effected (by previous segregation). 



It would carry us into too extended a discussion to at- 

 tempt to sum up here the pertinent criticism that has been 

 directed against the mutations theory. As already indicated, 

 there is plenty of it and of distinctly non-negligible char- 

 acter. But just now it seems to me sufficient simply to call 

 attention to the extreme meagreness in quantity of the real 



