504 Mutations 



barren, and incapable of the reproduction of tlie 

 race. 



This last is therefore, of necessity, always 

 continued by means of individuals whose devia- 

 tion from the mean average is the least. But 

 in many cases the varieties are so highly dif- 

 ferentiated that selection has become quite su- 

 perfluous for practical purposes. I have al- 

 ready discussed the question as to the actual 

 moment, in which the change of the grandiflor- 

 um variety into the new plenum form must be 

 assumed to have taken place. In this respect 

 some stress is to be laid on the fact that the im- 

 provement through selection has been gradual 

 and continuous, though very rapid from the first 

 moment. But with the appearance of the first 

 stray rays within the disk, this continuity sud- 

 denly changed. All the children of this original 

 mutated plant showed the new character, the 

 rays within the disk, without exception. Not 

 on all the heads, nor even on the majority of the 

 heads on some individuals, but on some heads all 

 gave clear proof of the possession of the new 

 attribute. This was present in all the repre- 

 sentatives of the new race, and had never been 

 seen in any of their parents and grandparents. 

 Here there was evidently a sudden leap, at 

 least in the external form of the plants. And it 

 seems to me to be the most simple conception, 



