210 The Origin of Species by Mutation. 



form ixHtJwut any advantage whatsoez'er in the struggle 

 for existence zvill maintain itself provided (1) that it 

 is sufficiently vigorous and fertile and (2) that it does 

 not arise merely once but repeatedly during a long period 

 of time.^ 



Delboeuf^s generalization has received little atten- 

 tion. Nevertheless it seems to me, in principle and in the 

 light of the facts of mutation, to be sound. It explains 

 in a very simple way the existence of the vast number 

 of specific characters which are quite useless or at any 

 rate as to the use of which we have no idea at all — as 

 for example the differences between the oft-cited species 

 of Draba verna. 



According to the commonly accepted theory of selec- 

 tion onl}^ characters advantageous to their possessors 

 should arise; according to the theory of mutation on the 

 other hand useless and even disadvantageous ones may 

 also appear. And according to Delboeuf^s view, the 

 latter may also persist through long intervals of time 

 side by side with the useful variations. The premises 

 from which he starts are at any rate warranted by actual 

 experience. 



^ With regard to the probability of this last condition I refer the 

 reader to the instances in § 25, pp. 193-196, and to the repeated ap- 

 pearance of sterile maize (in my experiments) both of which support 

 this view. 



