SHORTER ARTICLES AND DISCUSSION 



MR. MULLER ON THE CONSTANCY OF MENDELIAN 

 FACTORS 



In discussing the selection experiments of Phillips and myself 

 with hooded rats, 1 Mr. Muller 2 accepts the explanation of " modi- 

 fying factors " which we offered to account for certain peculiar 

 results obtained, but rejects the idea which we also suggested, 

 that the chief genetic factor concerned may be undergoing quan- 

 titative variation. He rejects it on the ground that this ex- 

 planation is not ' ' in harmony with the results of Johannsen and 

 other investigators." The work of Johannsen with seed-size in 

 beans and the work of others with Drosophila is cited in support 

 of this statement. 



It is difficult to understand how the experiments of Johannsen 

 have any direct bearing on the case since no single Mendelizing 

 unit-factor was demonstrated in that connection; but in the 

 hooded pattern of rats a Mendelizing unit-factor is unmistakably 

 present and it is the quantitative variation of this which is under 

 discussion, not the presence of many or few additional factors, 

 concerning which Muller adopts our explanation. Appeal to the 

 work of Johannsen with bean-size to show that our conclusions 

 concerning color pattern in rats are incorrect is illogical because 

 the cases are not parallel. The citation by Muller of the work 

 on rabbit-size by MacDowell and myself 3 is equally non-germane, 

 because no demonstrable Mendelizing unit-factor is involved in 

 that case either. He might with propriety cite the bean work as 

 bearing on the interpretation of the inheritance of body size in 

 animals, or vice versa, since both involve blending inheritance. 

 But neither of these cases has any direct bearing on the question 

 of unit-character constancy, since in neither case has a unit- 

 character, either constant or inconstant, been shown to exist. 



The citation of work with Drosophila is more to the point, since 

 the " mutations " of Drosophila Mendelize. But is it certain 

 that they do not vary? Muller admits that they do occasionally 

 vary, stating that " in one case (possibly in two or three cases) 



1 Castle and Phillips, "Piebald Eats and Selection," Publ. No. 195, Car- 

 negie Institution of Washington. 



2 Ameb. Nat., Vol. 48, p. 567. 



8 Publ. No. 196, Carnegie Institution of Washington. 



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