228 THE VARIATION OF ANIMALS IN NATURE 



(a) Observations indicating that dominance is not a fixed 



property of the gene, but depends on the genetic 

 environment in which it is placed. We shall not 

 deal with this, since we consider that, as far as it 

 goes, the evidence is satisfactory. 



(b) Observations indicating that Drosophila mutants are 



recessive in their external effects but neutral in 

 certain slight internal ones. 



(c) Observations on certain cases of polymorphism, in 



which the phenomenon of dominance presents 

 unusual features. 



(b) Ford (1931, p. 37) and Fisher (1931, p. 353) have 

 pointed out that certain Drosophila mutants produce a visible 

 effect (e.g. white eye) and an internal effect (e.g. change in 

 proportions of the spermatheca). In all the examples investi- 

 gated the external effect is recessive and the internal one is 

 neutral, i.e. the heterozygotes are intermediate. It is argued 

 from this that selection has acted only on those effects of the 

 gene which are harmful, visible changes such as those in eye 

 colour being more likely to affect the life of an animal than 

 minute changes in internal structures. This argument appears 

 to us to fail in two directions. First, the small internal effects 

 are just the sorts of variants which, in the case of specific differ- 

 ences, are assumed to be selected. Secondly, many specific 

 characters are admitted to be probably of no survival value 

 to their possessors, but are supposed to be correlated with 

 more important, possibly physiological, adaptations. If the 

 dominance of the wild type has been evolved by selection, we 

 can see why the adaptive characters would have been made 

 dominant, but the useless specific characters should have 

 remained neutral. So far as the conception of the wild type 

 has any meaning at all, this is not the case. As a rule we do 

 not know why the mutant forms of Drosophila are less viable 

 than the wild type. Sometimes, as in serious malformations, 

 the character by which the mutant is recognised might be 

 expected to have a direct effect, but in most mutants this is 

 not the case. We might therefore have expected the unknown 

 harmful effects to have become recessive, while the small 

 visible effect would have remained neutral. Possibly it is 

 wrong to assume that selection can alter one part of the effects 



