46 AN ANALYSIS OF THE EFFECT OF SELECTION. 



Hayes has, by selection from a mixed population, established four 

 different grades of variegation (including self-colored and colorless) 

 that breed true and that represent four allelomorphs. The two in- 

 termediate types, " mosaic" and " pattern," are the ones of special 

 interest in the present connection. When these two types were 

 crossed, the mosaic type was dominant, but there was an increase in 

 variability in FI and some individuals with more pigment than either 

 parent were obtained. The parent races had been selfed and selected 

 for about six generations before the cross was made. In view of the 

 great amount of heterozygosis that seems to be normally present in 

 maize, and the large number of chromosome pairs (20?), this seems to 

 be hardly sufficient to make certain that both races were pure for their 

 modifiers. The increased variability of FI is therefore not surprising; 

 and that phenomenon would of course be expected to be followed by 

 a still greater increase in variability in F 2 . Such an increase was, in 

 fact, observed, and is the chief basis for Hayes's conclusion that con- 

 tamination may occur. The data are not sufficient to demonstrate 

 that new allelomorphs arise more often in heterozygotes than in homo- 

 zygotes; and even if it be shown that they do so, it does not follow that 

 there has been contamination of allelomorphs. There are too many 

 unknown factors involved in the production of these new allelomorphs 

 for such a conclusion to be valid without very careful controls. 



It appears from the foregoing review that the cases cited as illustra- 

 tions of contamination of allelomorphs or imperfect segregation are 

 all explicable on the multiple-factor view, or rest on extremely indefinite 

 data. 



One series of data bearing on the question has been presented in 

 this paper (p. 32), and has been interpreted as giving evidence against 

 contamination. Three other cases have been worked out by Muller 

 (1916) and Marshall and Muller (1917). Muller kept three mutant 

 characters of Drosophila in heterozygous condition for about 75 

 generations. The factors were kept constantly in flies heterozygous 

 for their normal allelomorphs, so that the characters remained unseen 

 for a long time. 



Muller extracted one of these characters (dachs) from this stock, 

 and measured the tarsi, using the length of thorax as a standard of 

 comparison. Dachs flies are characterized by shortened tarsi; and 

 the flies from the heterozygous stock were found to have tarsi actually 

 a trifle shorter than those found in a stock that had been kept pure for 

 dachs. This result was not very conclusive, chiefly because it was 

 based on a very few flies. 



Marshall and Muller made much more extensive studies with the 

 wing characters, curved and balloon, derived from the same heterozy- 

 gous stock. They obtained a similar result; the wings were no nearer 



