570 Mutations 



Fluctuating variability, as a rule, is subject to 

 reversion. The seeds of the extremes do not 

 produce an offspring which fluctuates around 

 their parents as a center, but around some point 

 on the line which combines their attributes with 

 the corresponding characteristic of their ances- 

 tors, as Vilmorin has put it. No reversion ac- 

 companies mutation, and this fact is perhaps the 

 completest contrast in which these two great 

 types of variability are opposed to each other. 



The offspring of my mutants are, of course, 

 subject to the general laws of fluctuating varia- 

 bility. They vary, however, around their own 

 mean, and this mean is simply the type of the 

 new elementary species. 



VII. The mutations taJce place in nearly all 

 directions. 



Many authors assume that the origin of spe- 

 cies is directed by unknown causes. These 

 causes are assumed to work in each single case 

 for the improvement of the animals and plants, 

 changing them in a manner corresponding 

 in a useful way to the changes that take 

 place in their environment. It is not easy to 

 imagine the nature of these influences nor how 

 they would bring about the desired effect. 



This difficulty was strongly felt by Darwin, 

 and one of the chief purposes of his selection- 

 theory may be said to have been the at- 



