4 Introduction. 



The Mutation theoiy affects not only our views on 

 the origin of species but in my opinion bears strongly 

 on the whole question of hybridization. For it shows 

 us that the units with which we deal in hybridization are 

 not the species themselves but the single characters which 

 compose them — the so-called elements of the species. 

 This principle leads to an entirely new method of hand- 

 ling the subject, by enabling us to proceed gradually from 

 the simpler to the more complicated phenomena instead 

 of following the present custom which consists in dealing 

 with the complex cases first. 



This work therefore falls into two main parts of 

 which the first treats of the origin of species and varie- 

 ties by Mutation, and the second of the principles of 

 hybridization. 



The Mutation theory is opposed to that conception 

 of the theory of selection which is now prevalent. Ac- 

 cording to the latter view the material for the origin 

 of new species is afforded by ordinary or so-called in- 

 dividual variation. According to the Mutation theory 

 individual variation has nothing to do with the origin 

 of species. This form of variation, as I hope to show, 

 cannot even by the most rigid and sustained selection lead 

 to a genuine overstepping of the limits of the species 

 and still less to the origin of new and constant char- 

 acters. 



Of course every peculiarity of an organism arises 

 from a previously existing one ; not however by ordinary 

 variation, but by a sudden though minute change. It 

 is perhaps appropriate to compare such a change with 

 a chemical substitution. 



The name I propose to give to this "species-forming" 

 variability is Mutability — a term in general use before 



