128 PHYSIOLOGIC \l. GENETICS 



of a sexual difference in regard to the action of autosomal genes 

 depends upon the type of reaction involved and whether the 

 quantity of its products, or threshold values for its action, arc 

 interfered with by the different conditions of the developmental 

 system in both sexes. 



A number of facts of the same general type as the previous 

 example are available, all of which tend to corroborate the 

 foregoing interpretation. There is a first set of facts not relating 

 to specific genes but to general sexual differences. We have 

 already mentioned the fact that it is possible to reproduce 

 certain different ial features of one sex as a phenocopy in the other 

 sex by action of temperature shocks in the critical period. As a 

 matter of fact, some of the earliest phenocopies produced by 

 Standfuss, and also later by Federley and Kosminsky, in Lepi- 

 doptera were of tins type (see page 4). A second series of facts 

 relates to the action of temperature upon the phenotype of 

 mutants. We reported earlier (see page 16) Driver's analysis 

 of the Bar eye in Drosophila and Harnly's work on the vestigial 

 wing. In both cases, the response was somewhat different in 

 both sexes. In addition, it may be stated that the sensitive 

 period for the action of temperature may be situated at different 

 times of development. This is to be expected on the basis of 

 such differences in the time relations of processes of differentiation 

 as are known to obtain for the two sexes of many animals and 

 which have been registered in all the work on details of growth. 

 A third group of facts is of special interest because it contains a 

 certain quantitative element. There are known mutational 

 changes which are caused by the simultaneous action of a 

 number of mutant genes with additive effects, and in some such 

 instances these effects differ quantitatively in both sexes. 

 According to Goldschmidt (1921b), melanism in the nun moth 

 (Lymantria monacha) is caused by two autosomal and one sex- 

 linked gene, which have an additive effect: if all three mutant 

 genes are present in homozygous state, the wings are black; in 

 their absence, they are white; and the different homozygous and 

 heterozygous combinations produce all intermediate conditions 

 according to definite rules. If we picture side by side a series of 

 males and females of the same genetic constitution with increasing 

 number of the three melanistic genes, we find the males always 

 about one class of darkening ahead of the females; i.e., a male Aa 



