MUTATION 271 



There is evidence in Drosophila melanogaster show- 

 ing that the same mutation to white eyes has occurred sev- 

 eral times, and the additional and all-important proof 

 has been obtained that it is the same locus that has pro- 

 duced the white-eyed mutant. This may appear to give 

 some slight support to the view that albino mutants 

 appearing in other related species may be due to the same 

 mutative changes, but without additional evidence this 

 conclusion is problematical. 



In the mammals melanic individuals have been fre- 

 quently described, but there is no direct evidence to show 

 that they are due all to the same change. In the roof rat 

 there is a black type that is dominant to the gray of this 

 race, while the black type of the Norway rat is recessive 

 to the gray of that race. It seems probable that they are 

 different mutations, but not necessarily so. 



Yellow in the mouse is dominant and lethal ; two races 

 of yellow rats are known, both recessive forms. The rela- 

 tion of yellow to black in mice is different from the rela- 

 tion of either of the yellows to black in the Norway rat. 

 If the blacks are the same mutant the yellows are differ- 

 ent ; if either yellow of the rat is the same as the yellow of 

 the mouse, the blacks must be different, etc. 



The uncertainty of reaching any conclusion in regard 

 to the nature of the mutation from the appearance of the 

 character of the mutant is' excellently illustrated in such 

 a group of mutants as that of the fruit fly, where a con- 

 siderable number of cases are known in which mutants 

 that are almost indistinguishable externally have been 

 shown to be due to mutations in different parts of the 

 germ-plasm. There are five kinds of black mutants, three 

 or more yellows and several eye colors that are practically 

 indistinguishable. The evidence showing their difference 

 is obtained from the results of crossing, where, as a rule 

 (except, for example, cases of complete or incomplete 

 dominants), reversion to the wild type occurs. In addi- 



