No. 565] CHANGES PRODUCED BY SELECTION 31 



distribution was 19.8 ± .28 leaves. A 10-leaved plant of 

 this lot was selfed and gave a progeny with a mean of 

 17.9 ± .08 leaves and a variability of 11.24 per cent. ± .33 

 per cent. What interpretation can be given these facts? 



"We believe a distinct mutation occurred, a mutation 

 different from those of DeVries. At least DeVries be- 

 lieves that the mutations that he has observed always 

 breed true. If the following hypothesis as to the origin 

 of the 12-leaved plant be true, it is unnecessary to sup- 

 pose with DeVries that mutations always breed true or 

 even that they often breed true. Of course DeVries be- 

 lieves that his Oenothera mutations obey laws different 

 from those of whose mechanism we know a little. He be- 

 lieves that species crosses always breed true; that they 

 do not Mendelize. This belief we hold to be unfounded. 

 Species crosses have never been shown to breed true. 

 There have been statements to the effect that crosses be- 

 tween Rubus species breed true, but no good evidence has 

 been submitted in their support ; while the data of Tam- 

 mes ( :11) on Linum species crosses, Davis ( :21) on (Eno- 

 thera species crosses, and of East ( :13) on Nicotima 

 species crosses, concur in showing that species as well as 

 varieties obey Mendel's Law of segregation and recom- 

 bination. Furthermore, we think that Heribert-Nilsson 's 

 ( :12) beautiful experiments on DeVries 's own material 

 show that the latter did not collect sufficiently exact data 

 on his own crosses to find out whether they bred true or 

 not. 



If one is to believe that a mutation in a hermaphroditic 

 plant breeds true he must suppose that constitutional 

 changes occur both in the male and the female gam- 

 etes, or that the change occurs after fertilization. But it 

 seems more probable that such a change will take place 

 either in the one or the other gamete and not in both. This 

 we believe to be the explanation of the appearance of the 

 12-leaved tobacco plant. A mutation occurred in either 

 an egg cell or a pollen cell. It does not matter in which 

 one it is assumed because there is no evidence favoring 

 either case to the exclusion of the other. This cell with 



