D.— ZOOLOGY 83 



making for the manifestation of reduced eyes were eliminated. In 

 competition with the wild-type allelomorph, eyeless would be eliminated ; 

 but in stocks pure for eyeless, the genes to be eliminated will be the plus 

 modifiers of the mutation. 



Selection of this type, it now appears, is a constant and indeed normal 

 process. It has become almost a commonplace in animals used for genetic 

 analysis to find that mutant types which at first are extremely difficult 

 to keep going, after a few generations become quite viable. This has 

 repeatedly occurred in Gammarus, for instance, as well as in Drosophila, 

 and is also known in mice and nasturtiums. The explanation is 

 essentially similar to that for the case of eyeless. 



R. A. Fisher has extended this concept to explain dominance and 

 recessiveness in general. Mutation is always throwing up new genes ; 

 the majority of these will inevitably be deleterious, and will also be re- 

 peatedly produced. Obviously the great majority will be carried in 

 single dose, so that it will be an advantage to minimise any activity shown 

 by them while in this heterozygous state. Thus a harmful mutation will 

 inevitably be forced into recessivity by selection acting on the rest of the 

 gene-complex. Haldane has given a somewhat different explanation of 

 the origin of the recessive character of most mutations, based upon multiple 

 allelomorphism ; but this too involves selection acting upon other genes 

 than the mutant. On either hypothesis, dominance and recessiveness 

 are to be regarded as modifiable characters, not as unalterable inherent 

 properties. Dominant genes, or most of them, are not born dominant : 

 they have dominance thrust upon them. Mutations become dominant 

 or recessive, through the action of other genes in the gene-complex. 



There remains the difficulty that most mutations so far investigated 

 are deleterious. If mutations are the raw material of evolution, some of 

 them in some cases must be, or must become, advantageous. However, 

 this also is not so serious as at first sight appears. Since the gene- 

 complex is an elaborately co-ordinated system, any changes in it are much 

 more likely to act as defects rather than as improvements. Further, 

 the larger the change the less likely is it to be an improvement ; and in- 

 evitably the geneticist will detect large changes more readily than small. 

 Recent analysis, however, has revealed numbers of gene-differences 

 with extremely small effects, down almost to the limit of detectability. 

 It is not only possible but probable that among these are to be sought 

 the chief building blocks of evolutionary change, and that it is by means 

 of a series of small multiple-allelomorphic steps, each adjusted for via- 

 bility and efficiency by changes in the genie background, that an organism 

 usually achieves gradual but well-defined alteration. 



But in addition to the initial or intrinsic usefulness of certain small 

 mutations, we have also the fact that mutations which are deleterious 

 in what may be described as normal conditions may become advantageous 

 either in an altered environment or in an altered genie background, 

 and the further fact that many mutations or Mendelizing variations 

 cannot be described as intrinsically useful or harmful, but vary in their 

 selective effects with variation in environmental conditions. 



Let us take a few illustrative cases. In conditions near the optimum, the 

 vestigial-winged mutant of Drosophila is much shorter-lived than the wild 



