No. 576] MUTATIONS IN DROSOPHILA 



711 



It should be added that only one of Ghryenot's two 

 lines gave dark flies after treatment with ultra-violet light. 

 This might seem to indicate that the first result was acci- 

 dental, or due to the presence of a recessive mutation in 

 the stock prior to treatment were it not that a careful 

 control is recorded. Guyenot himself speaks with much 

 caution concerning the interpretation of his results. De- 

 cision as to their value may be reserved until repetition of 

 the experiment gives confirmation. Our own experience 

 with DrosopMla shows that mutations appear under con- 

 ditions where all the other flies in the same culture are 

 normal and we have become unduly sceptical perhaps 

 towards evidence which refers a particular mutant to some 

 unusual treatment to which the flies have been subjected. 

 Until we can get definite information as to how mutants 

 arise, whether through external influences, through acci- 

 dents of mitosis, through hybridizing, or through changes 

 in the chromosomes with its consequent dislocations of 

 the machinery of crossing over, or in some other way, it 

 seems futile to discuss the question. 



