706 Mutations 



great numbers in which our dwarf variety of the 

 Oenothera are yearly produced are suggestive 

 of such a condition. On the otlier hand, the 

 laevifolia- and b revistt/li s-mutatiojis have not 

 been repeated, at least not in a visible way. 



From this discussion we may infer that it is 

 quite possible that a large part of the progres- 

 sive changes, and a lesser part of the retrograde 

 mutations, are combined into gi^ups, owing 

 their origin to common external agencies. The 

 periods in which such groups occur would 

 constitute the mutative periods. Besides them 

 the majority of the retrograde changes and 

 some progressive steps might occur separately, 

 each being due to some special cause. De- 

 gressive mutations, or those which arise by 

 the return of latent qualities to activity, would 

 of course belong with the latter group. ^ 



This assumption of a stray and isolated pro- 

 duction of varieties is to a large degree suj)- 

 ported by experience in horticulture. Here 

 there are no real swarms of mutations. Sud- 

 den leaps in variability are not rare, but then 

 they are due to hybridization. Apart from this 

 mixture of characters, varieties as a rule appear 

 separately, often with intervals of dozens of 

 years, and without the least suggestion of a com- 

 mon cause. It is quite superfluous to go into 

 details, as we have dealt with the horticultural 



