364 OENOTHERA RUBRINERV1S, A HALF MUTANT. 



responsible for the dominance of the brittleness. This seems to be 

 the case at least in 0. nanella, but the number of crosses in each of 

 my different families of dwarfs is too small to decide whether this 

 is the real cause. Crosses of 0. rubrinervis with other mutants than the 

 dwarfs have also given sometimes the brittle form and sometimes 

 the subrobusta for the second hybrid. 



If we keep in mind that the hybrid rubrinervis is only a brittle 

 form of the hybrid subrobusta, and that the one may be substituted 

 for the other for unknown reasons, the following descriptions will 

 easily be understood. I might add, however, that from a single cross 

 between two individual parents both types never arise simultane- 

 ously in the first generation. In the succeeding generations the 

 rubrinervis as a rule are constant, whereas the subrobusta may split 

 off the brittle form. 



If we assume the gametes of 0. Lamarckiana to consist of equal 

 parts of typical ones and of velutina, and those of 0. rubrinervis to 

 consist of deserens and velutina, 0. LamarckianaxO. rubrinervis must 

 yield 25 per cent typicax deserens, 25 per cent typicaxvelutina, 25 

 per cent velutinax deserens, and 25 per cent velutinaxvelutina. The 

 last combination will produce empty grains, since the same lethal 

 factor comes in from both sides; on the other hand, the three first 

 named combinations must give viable seeds. Typicaxvelutina is the 

 formula for 0. Lamarckiana, and velutinax deserens that for 0. 

 rubrinervis and subrobusta, and so the occurrence of these hybrid 

 types is easily explained. The remaining combination typicax deserens 

 must then be assumed to give the new hybrid lucida, and this can 

 be verified by crossing 0. deserens with 0. Lamarckiana. All these 

 deductions are, of course, the same for the reciprocal crosses. If these 

 deductions are reliable, they show that the polymorphy of the 

 first generation of hybrids between the two older forms is due to 

 the combination of their capacities to produce twins in other crosses. 

 In other words, it is a natural sequence of their secondary mutability. 

 I shall now describe the experiments which seem to me to justify 

 these deductions. 



0. LamarckianaxO. rubrinervis. — According to the deductions 

 just given the expectation for this cross is 



O. LamarckianaxO. rubrinervis— 



(typica+ velutina) x (deserens -f-velutina)= 



typ. xdes.+vel. xdes.-f-typ.x vel. + vel.x vel.= 



lucida + subrobusta + Lamarckiana + empty grains 



or 

 lucida + rubrinervis + Lamarckiana + empty grains. 



