No. 583] BOLE OF ENVIRONMENT IN DROSOPH1LA 429 



ratios were the same in the late as in the earlier gen- 

 erations. 



Lastly the tests made of individuals that were pheno- 

 typically normal, but genetically ;il>n<>nual. -bowed in all 

 cases the validity of the genetic evidence, which would 

 not have been the case if the apparent exceptions had been 

 due to contamination of the genes. I may also cite the 

 two peculiar matings, B and C, recorded on page 402. 

 where an expected class did not appear. It might have 

 appeared that here actual contamination had occurred. 

 In reality, the result turned out to be due to a lethal 

 factor. Our study of these lethals, that give verifiable 

 results, fully under control, made it possible to interpret 

 this case that otherwise would have been inexplicable, and 

 might have been cited in favor of the view of contamina- 

 tion of genes. Taken all together the results obtained 

 with this mutant type make out a strong case against the 

 supposition that genes become contaminated tlinnmh jux- 

 taposition. I shall not discuss here, therefore, the un- 

 pragmatic character of such a supposition, but rest the 

 case on the evidence from the experiments. 



