No. 583] ROLE OF ENVIRONMENT IN DBOSOPHILA 419 



and absence" hypothesis, even although I myself prefer 

 a more non-committal form of factorial interpretation 

 than that offered by the " presence and absence" theory. 



The abnormal male (Ab) has one dose of abnormality 

 and the degree of his abnormality is the same as that of 

 the female (Ab, Ab) with two doses. But the hetero- 

 zygous female, AbN, has only one dose (or factor) for 

 abnormality. The degree of abnormality that she shows 

 is very variable ; she is less abnormal on an average, than 

 the abnormal male. 



Which condition is to be interpreted as absence — the 

 real absence of one Ab in the male, or the absence of one 

 Ab in the other (normal) chromosome of the female? A 

 moment's thought will show, however, that nothing of any 

 value can come from a discussion of this question, be- 

 cause the heterozygous female (AbN) differs from the 

 male not simply by the factor N, but by a whole chromo- 

 some including amongst other factors a factor which in 

 duplex produces a female. Moreover, an advocate of 

 presence and absence might maintain that the relation 

 of a dominant to the normal allelomorph is not the same 

 as the relation of a normal allelomorph to a recessive for 

 it is the latter that is 1 ' absent. ' ' In other words, he might 

 conceivably accept the hypothesis of absence for a reces- 

 sive, but reject it for a dominant mutation. 



I have pointed out elsewhere that it seems to me un- 

 warrantable to interpret the absence of a character to 

 mean necessarily an absence of a factor in the germ 

 plasm. 5 Yet this literal interpretation of the presence 

 and absence hypothesis has often been made. If the 

 linear arrangement of factors in the chromosomes be ad- 

 mitted as a plausible hypothesis the absence of a factor in 

 this literal sense would mean a hole in the chromosome, 

 and a corresponding displacement of the linear sequence 

 of factors. The evidence does not support this hypothe- 

 sis. On the other hand, if the locus of a factor be con- 



5 Although of course a changed factor might cause the failure of some 



