36 PHYSICAL BASIS OP HEEEDITY 



gous condition, since a fly homozygous for notch dies. The 

 race is therefore necessarily maintained in a hetero- 

 zygous state. In each generation females that were 

 genetically notch, but had normal wings, were selected 

 and bred to normal males. The selection was away from 

 notch {i.e., toward normal) . After a time more than half 

 of the notch flies had normal wings. The effect produced 

 proved to be due not to a change in the notch gene through 

 contamination, but to modifying genes ; for at the end of 

 the selection the original notch could be recovered at any 

 time by removing the influence of the modifying factor. 

 It has been sometimes stated, usually by the opponents 

 of Mendel's theory, or by advocates of doctrines of evolu- 

 tion that appeared to be compromised by the Mendelian 

 conception of "unit factors," that Mendelism deals only 

 with such superficial characters as the color of flowers 

 or the hair color of mammals. This statement contains 

 an element of truth in so far as it covers most of the 

 kinds of characters that students of heredity flnd most 

 convenient to study; but it contains an entirely false 

 inference as to the limitations of Mendelism. The issue 

 involved is this : changes in superficial characters are not 

 so likely to affect the ability of the organism to survive 

 as are changes in essential organs ; hence they are the best 

 kind of hereditary characters for study. But there is no 

 evidence that such superficial characters are inherited in 

 a different way from "fundamental" characters, and 

 there is evidence to the contrary. A common class of 

 characters showing perfect Mendelian behavior are 

 so-called lethals that destroy the individual when in homo- 

 zygous condition. There can be no question as to the 

 fundamental importance of such factors. Between these 

 extreme cases and the superficial shades of eye color, 

 for example, all possible gradations of structure, physio- 

 logical and pathological, are known. The only possible 

 question that might be seriously raised is whether these 

 characters are all losses or deficiencies, while progres- 



