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 otfspring. 

 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 theorj^ 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 



