236 MASS MUTATIONS AND TWIN HYBRIDS OF 



not for the mutability itself, since this is not a special trait of the 

 Lamarckiana, but for the exceptionally high degree of develop- 

 ment of the quality in that species. 



Later investigations of different authors, and especially those of 

 Renner, have confirmed this conception, since they do not offer 

 an explanation of the problem involved on the basis of other excep- 

 tional qualities of my plant. The experiments described for 0. 

 grandiflora, however, prove that there is still another relation, since 

 the twins may be considered as the result of the fecundation of 

 sexual cells which are, for a large part, in the condition of mutated 

 gametes. It is evident that in crosses these latter may give different 

 hybrids from those of the normal gametes of the same parent. I 

 shall now try to show that the results of my crosses confirm this 

 view in almost all their details. 



We have to start from the assumption that the mass mutations 

 take place in the same numerical proportions as those required by 

 the formula of Mendel for monohybrids; in other words, that the 

 two kinds of gametes are produced in equal numbers and among 

 the pollen as well as among the egg cells. Fecundation with a different 

 species must then produce two kinds of hybrids, each of them in 

 about 50 per cent of the offspring. Our table for the production 

 of laeta and velutina in such crosses gave on the average 52 per 

 cent for the first and 46 per cent for the latter, and thus fully con- 

 firms our conception. When the crosses are repeated with mut. 

 ochracea instead of the type of the species itself, no twins must be 

 the result, but only uniform hybrids of the type corresponding with 

 the laeta. Until now 1 have tried only one instance, ochraceax 

 0. Cockerelli and its reciprocal. They produced only one of the 

 twins, namely, laeta, and thus confirm our view. Other combina- 

 tions should be studied for the same purpose. 



If Mendel's law were applicable to the production of the twins, 

 these must split after self-fertilization into three or more types. 

 Our table shows that this is not, or at least not always, the case. 

 The velutina never split, nor do the laeta of 0. biennis, 0. syrticola, 

 and 0. biennis Chicago produce a splitting. Only those of 0. Cockerelli 

 show this phenomenon, but here it is limited to the repetition of the 

 mass mutation into velutina. From these facts we must conclude 

 that the hereditary factors involved are not in the condition required 

 by Mendel's laws. In Gruppenweise Artbildung I have called this 

 deviating condition labile, leaving the question open whether it 

 may be determined by means of linkage or otherwise. 



