Foundations of the Mutation Concept. 21 



In the single species Drosophila melanogaster, the "vinegar 

 fly," over 200 mutations have been observed. These include more 

 than 25 factors for eye colour, and many more for body colour and 

 form of wings, while many structural differences are also involved 

 in other mutations. The majority of these have appeared but 

 once. Some mutations are, however, more frequent. White eyes, 

 a mutant from the wild red-eyed fly, are known to have occurred 

 three times in the New York experiments (Morgan, 1919, p. 248) 

 and several times in the cultures of other observers. Vermilion 

 eye-colour has appeared at least six times, " rudimentary " wings 

 five times, cut wing four times, truncate wing frequently but 

 probably owing to different changes. As with the (Enothera 

 experiments then, certain mutations repeat themselves, but 

 apparently with much less frequency. This suggests that in 

 (Enothera the mutation often appears much later than the germinal 

 change (pre-mutation) which gave rise to it, being suppressed in 

 many cases by lethal factors causing sterility. 



A number of cases of probable reversion from the mutant 

 character to the original wild type have occurred, but most of these 

 are uncertain on account of the possibility of contamination. In 

 the experiments of May (1917a) with bar-eye, however, this 

 possibility is excluded. This character is dominant to normal 

 (round) eye, and the reversal takes place with sufficient frequency 

 to eliminate the possibility of error. Reverted individuals give 

 only normal offspring. Bar-eye differs from the normal in having 

 fewer facets, and the stock showed considerable variability. In an 

 experiment in selecting for more or fewer facets, May obtained 

 six full-eyed males and five females heterozygous for full-eye. They 

 were indistinguishable from normal and were explained as simple 

 reverse mutations. In a later paper (May 1917b), however, a more 

 complex explanation of this reversal is favoured. It is assumed 

 that the normal wild fly carries a limiting factor with respect to 

 facet number, and that by " partial non-disjunction," the factor 

 passes from one chromosome to another (presumably during 

 meiosis) so that one chromosome is without a limiting factor while 

 the other member of the pair has two. The egg retaining the 

 latter chromosome would produce a bar-eyed individual. If in such 

 a race a second non-disjunction occurs, separating the two factors, 

 one chromosome would result having triple factors and one with 

 a single factor. The latter would give a reversion to a full-eyed 

 male or a heterozygous female. Hut it is doubtful ,ii such a 



