1919] DeVRIES— OENOTHERA RUBRINERVIS 9 



Summing up the results of all the tables, we may conclude that 

 all specimens of 0. rubrinervis, derived from various sources, and 

 the mutants as well as their offspring, show mass mutation into 

 O. deserens, besides a considerable number of empty seeds. Taking 

 into consideration the unavoidable losses in the numerical estima- 

 tions, we may further conclude that O. rubrinervis produces about 

 one-fourth empty seeds, and among the living offspring about one- 

 third 0. deserens, which are constant in their progeny and have no 

 empty grains, or almost none. This points to a relation of i : 2 : i for 

 the whole harvest. The phenomenon is thus shown to be parallel 

 to the sphtting of the hybrid mutant of O. gigas nanella and to the 

 mass mutability of O. grandiflora and of 0. Lamarckiana itself. 



Crosses between O. rubrinervis and O. deserens. — If this 

 explanation is true, it may be confirmed by means of crossing O. 

 rubrinerois with its mass mutant. The sexual cells of the first are 

 about one-half deserens without a lethal factor, and the rest velutina 

 provided with such a factor; those of 0. deserens, however, are 

 uniformly so. We must expect, therefore, a splitting into almost 

 equal parts of deserens Xdeserens = 0. deserens, and of velutina X 

 deserens = 0. rubrinervis. I made both the reciprocal crosses in 

 191 5, cultivated 58 and 50 specimens of their offspring in 1916, and 

 counted them at the beginning of the flowering period in July, 

 finding as follows: 



O. rubrinervis XO. deserens. . 

 O. deserens XO. rubriner\'is. . 



Percentage of j Percentage of 

 rubrinervis deserens 



48 

 78 



52 

 22 



The two types of hybrids resembled their parents exactly, and the 

 figures point to numerical equality of the two groups, although the 

 cultures were only small. Thus we see that the expectation from 

 our formula is confirmed by the experiment. 



Twin hybrids of O. rubrinervis. — The twin hybrids of O. 

 Lamarckiana and 0. grandiflora are now explained as the result of 

 the mass mutation of these species, but the experimental proof is 

 not complete as yet because neither of these species is known to 

 occur without that form of mutation. In this respect the case of 



