iQig] DeVRIES— OENOTHERA RUBRINERVIS 13 



Strong fibers, and as a rule it is so, but not always. The two con- 

 trasting cases have occurred mainly in strains derived from different 

 initial plants, and some hidden mutation might be responsible for 

 the dominance of the brittleness. This seems to be the case at least 

 in O. 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 O. 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 O. Lamarckiana to consist of equal 

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

 consist of deserens and velutina, O. LamarckianaXO. rubrinervis 

 must yield 25 per cent typicaXdeserens, 25 per cent typica Xvelutina, 

 25 per cent velutinaXdeserens, and 25 per cent velutina Xvelutina. 

 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. Typica Xvelutina 

 is the formula for O. Lamarckiana, and velutinaXdeserens that for 

 O. 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 O. deserens with Q. Lamarckiana. 

 All these deductions are, of course, the same for the reciprocal 

 crosses. If these deductions are reliable, they show that the poly- 

 morphy 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. 



