VARIATION AND HEREDITY 125 



pletely absent between these units as they 

 are between the molecules of the chemist." 



"The adoption of this principle influences 

 our attitude towards the theory of descent 

 by suggesting to us that species have arisen 

 from one another by a discontinuous, as 

 opposed to a continuous, process. Each new 

 unit, forming a fresh step in this process, 

 sharply and completely separates the new 

 form as an independent species from that 

 from which it sprang. The new species ap- 

 pears all at once; it originates from the 

 parent species without any visible prepara- 

 tion, and without any obvious series of 

 transitional forms." 



"The mutation theory is opposed to that 

 conception of the theory of selection which is 

 now prevalent. According to the latter view 

 the material for the origin of new species is 

 afforded by ordinary or so-called individual 

 variation. According to the mutation theory 

 individual variation has nothing to do with 

 the origin of species. This form of variation 

 . . . cannot even by the most rigid and sus- 

 tained selection lead to a genuine overstep- 

 ping of the limits of the species and still less 

 to the origin of new and constant characters." 

 "Of course every peculiarity of an organism 

 arises from a previously existing one; not, 

 however, by ordinary variation, but by a 



