480 Mutations 



tendency to mutation must be universally 

 present in the whole species. Another observa- 

 tion, although it is of a negative character, 

 gains in importance from this point of view. 

 I refer to the total lack of intermediate steps 

 between normal and peloric individuals. If 

 such links had ordinarily been produced pre- 

 vious to the purely peloric state they would no 

 doubt have been observed from time to time. 

 This is so much the more probable as Linaria 

 is a perennial herb, and the ancestors of a 

 mutation might still be in a flowering condi- 

 tion together with their divergent offspring. 

 But no such intermediates are on record. The 

 peloric toad-flaxes are, as a rule, found sur- 

 rounded by the normal type, but without inter- 

 grading forms. This discontinuity has already 

 been insisted upon by Hofmeister and others, 

 even at the time when the theory of descent was 

 most under discussion, and any link would 

 surely have been produced as a proof of a slow 

 and continuous change. But no such proof has 

 been found, and the conclusion seems admissible 

 that the mutation of toad-flaxes ordinarily, if 

 not universally, takes place by a sudden step. 

 Our experiment may simply be considered as a 

 thoroughly controlled instance of an often re- 

 curring phenomenon. It teaches us how, in the 



