RADIATION AND THE STUDY OF MUTATION IN ANIMALS 1251 



150, 151, 83, 154). These workers studied the bristle pattern in a series 

 of multiple allelomorphs at the scute locus in Drosophila melanogaster . 

 It seemed at first as if the bristles could be arranged in a specific linear 

 order and that the different allelomorphs affected the bristles of this 

 order in blocks. Serebrovsky and Dubinin then assumed that the 

 arrangement of the subgenes in the chromosome corresponded to that 

 of the bristles in the linear series. In this way each of the allelomorphs 

 was due to the simultaneous change (loss at first) of neighboring sub- 

 genes. Later work of Dubinin (39) and of Sturtevant (170o) has shown 

 that the original linear seriation does not hold and (Sturtevant) that 

 different seriations are required at different temperatures. In the 

 meantime, Sturtevant and Schultz (171) were concerned to show that the 

 premise of the hypothesis — the identification of a developmental pattern 

 with gene structure — is untenable. Both they and Goldschmidt (50) 

 suggested developmental interpretations, which have been rejected by 

 Dubinin and Friesen (40). It may well be that the specific develop- 

 mental interpretations are faulty; bristle pattern in Drosophila is little 

 understood. But this in itself emphasizes the danger of projecting 

 developmental data on to the gene, without a clear knowledge of what is 

 under discussion. 



One may perhaps note a significant contrast in the primary interests 

 of the participants in the presence or absence controversy. The 

 modern proponents of the theory are chiefly interested in simplicity as far 

 as concerns the theories of what genes do. Their adversaries, on the other 

 hand, are most engaged with the evolutionary process, which they cannot 

 imagine without qualitative changes in genes. Indeed, it has been stated 

 that the evolution of genes "implies the possibility of qualitative change 

 ... as a necessary condition." The periodic table of chemical elements 

 may, however, be recalled. Here is an example of the results of 

 quantitative changes which may then, a priori, suffice to give an extensive 

 enough array of units for any evolution. Moreover, the analogy is 

 useful in another sense, the distinction between qualitative and quantita- 

 tive changes in genes is a relatively useless one unless it is sufficiently 

 specified so that predictions can be made. Now the specifications which 

 are required to make an adequate definition we are not at all prepared 

 to give. The methods used all involve inferences from phenotypic 

 changes to the structure of the gene, and the arguments used against 

 Serebrovsky and Dubinin's subgenes apply equally well to the more 

 recent attempts to test the "molecular" structure of the gene (Demerec, 

 21a). This is particularly the case when the introduction of the position- 

 effect hypothesis into the field is considered. Mechanical displacements 

 and molecular changes become effectively one, as far as the experiments 

 to date are concerned. It is patent that new methods must be devised, 

 which will deal with the genes by more specific methods than those now 

 available before it is profitable to discuss gene structure. 



