1915] DE VRIES—OENOTHERA BIENNIS 195 
O. 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 O. biennis may be ascribed to some 
“germinal disturbance” of its hereditary qualities. Or, if w 
replace this vague and meaningless expression by a sharp oie. 
Wwe May assume 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 kingdoms. 
5. The mutants of O. 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 
