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 F, 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 F, is therefore not surprising; 
and that phenomenon would of course be expected to be followed by 
a still greater increase in variability in F). 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 
