313 THE THEORIES OF EVOLUTION 



and at other times leaving the plants unchanged dur- 

 ing long successions of generations. All lines of the 

 genealogic tree show alternatively mutating and con- 

 stant species. Some lines may be mutating at the 

 present moment ; others may momentarily be constant. 

 The mutating lines will probably sooner or later re- 

 vert to the inactive state, while the powers of devel- 

 opment now dormant maj T then become awakened on 

 other branches. In a complete and systematic enu- 

 meration of the real units of nature, the elementary 

 species and varieties are thus observed to be discon- 

 tinuous and separated by definite gaps. There is no 

 reason to suppose that the world is reaching the end 

 of its development, and so we are to infer that the 

 production of new species and varieties is still going 

 on. In reality, new forms are observed to originate 

 from time to time, both wild and in cultivation, and 

 such facts do not leave any doubt as to their origin 

 from other allied types, and according to natural and 

 general laws." 1 



De Vries decided to watch out for the actual ap- 

 pearance of the phenomenon and after experiments 

 of eight years' duration he finally succeeded in ob- 

 serving in Lin aria vulgaris a mutation which resulted 

 in a new variety. It was another plant, however, of 

 the genus CEnoihera, which enabled him to make de- 



1 Species and Varieties. Their Origin by Mutation, by H. De Vries, 

 1906, passim. 



