94 THE PROBLEMS OF EVOLUTION 



Drosophila which, while evidently a mutation and 

 a Mendelian character, was governed in its appear- 

 ance by the amount of moisture contained in the 

 food of the insects bearing the mutant gene. The 

 character, abnormal abdomen, was dominant over 

 normal and was sex linked. Heterozygous females 

 were produced at will by mating abnormal males 

 with wild females, and the offspring reared in 

 moist bottles disclosed the Mendelian behavior of 

 the character. If reared in dry bottles, even the 

 individuals bearing the mutation were normal, 

 although they could produce offspring showing the 

 abnormal character. In drawing conclusions from 

 his experiments, Morgan writes: "The evidence 

 shows in the clearest manner that the condition of 

 the parent, whether normal or abnormal in type, 

 has no effect on the character of the offspring," a 

 statement whose truth must be freely admitted. 



While this case stands out as a striking example 

 among the experimental records of genetics, it is 

 by no means unique. Compare it with any of the 

 individual responses previously considered and a 

 marked similarity is evident. The human body 

 inherits the ability to deposit pigment, otherwise 

 we would be albinos, but indoor workers of the 

 white race remain pale while individuals constantly 

 exposed to sunlight become deeply tanned. The 

 Mendelian basis for pigmentation is not fully un- 

 derstood, but it is obviously an inherited character, 



