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THE AMEBIC AN NATURALIST [Vol. XLIX 



dd, and the double Dd, cross-pollination giving ldd :lDd. 

 There is usually (Saunders, 1910) an excess of singles; 

 here, as in Matthiola, the heterozygous form is the one 

 deficient in numbers, and it is also the one which appears 

 inferior in vegetative vigor. 9 Probably the deviation 

 from the 1 : 1 ratio is due in Petunia to selective elimina- 

 tion of doubleness. 



We have, then, in Matthiola and Petunia, hybrids 

 evidently due, not to the crossing of widely different 

 forms, but to mutation within the race, 10 and yet they 

 are partially sterile, and perhaps even lacking in vegeta- 

 tive vigor because of their hybridity ! In connection with 

 the vigorous discussion of mutation now going on, it 

 seems worth while to ask whether, in a case like that of 

 (Enothera, hybridization is the cause of mutation or 

 mutation one great cause of hybridity; apparently both 

 views may be in part correct. 



Miss Saunders favors a dihybrid scheme for Petunia, 

 evidently supposing the difference here also to depend on 

 two complementary factors, both necessary for single- 

 ness. Her assumption that singleness is dominant, as in 

 Matthiola, seems absolutely untenable. In considering 

 the last point, we may ignore the dihybrid feature, since 

 this evidently concerns only the deviation of the ratio 

 from 50 per cent. 



Her formulation, as thus simplified, makes the singles 

 Ss and the doubles necessarily ss; the data then indicate 

 that the functional single pollen is all S-carrying (the 

 reverse of the case in Matthiola), since self-pollinated 

 singles produce no doubles. Then, either the single eggs 

 are S + s, and the double pollen s + s, or the single eggs 

 are s + s, and the double pollen S + s. The latter 

 assumption is obviously impossible, since it not only con- 

 tradicts the assumption that singleness is dominant, but 



