iQis] DE VRIES—OENOTHERA BIENNIS 195 



0. biennis mut. sulfurea easily yields constant races of a uniform 

 sulphur color. 



3. The question whether there is any causal relation between 

 partial sterility of the sexual cells, hybridism, and mutability has 

 to be studied in all those instances in which mutations are known 

 to occur or to have occurred. In some of these cases, at least, the 

 conditions are far more simple than for the evening primroses, as 

 for example in Capsella Bursa-pastoris. 



4. The mutative condition of 0. biennis may be ascribed to some 

 "germinal disturbance" of its hereditary quaUties. Or, if we 

 replace this vague and meaningless expression by a sharp hypothesis, 

 we may assunle as its cause the presence of one or more pangens 

 in a labile position. The transition from biennis to Lamarckiana 

 would then require the addition of one or more pangens in the 

 same state, in order to explain the higher percentage of mutants 

 and the larger number of their different forms. The presence of 

 such labile pangens seems well proven by the results of numerous 

 crosses. 



The contention, however, that the transition of "undisturbed 

 germinal material into a state of disturbance," or of one or more 

 pangens from the stabile into the labile condition, may be induced 

 by external influences in pure species, has not as yet found general 

 acceptance. Some authors believe that crosses between different 

 types are required to secure this effect. At this moment, it seems 

 difficult to give experimental evidence for or against this view. 

 Until this is reached, we must rely upon comparative studies in 

 order to answer the main question whether or not the observed 

 mutations in the evening primroses are analogous to those by 

 which the mutation theory explains the evolution of the animal 

 and vegetable kingdorns. 



5. The mutants of 0. Lamarckiana all agree with that species 

 in certain characters, and not one of them shows any indication 

 of a reversion toward any of the allied wild types. If the muta- 

 bility was an effect of crossing, some marks, at least, of the other 

 parent would be expected to reappear. 



Besides this consideration, the available evidence lies in the 

 fact that the derivatives of O. Lamarckiana, originated in my 

 garden, differ from one another in marks, which are, although not 



