AN ANALYSIS OF THE EFFECT OF SELECTION. 51 
arising in the hooded factor, then the “mutant” variation is evidently 
the only case of that sort that Castle has reported. 
GENERAL CONCLUSIONS. 
That many characters may be influenced by more than one pair of 
genes has long been recognized, and this is the essence of the multiple- 
factor view. That genes exist which require the action of other genes 
before they produce visible effects has also been long known. Further- 
more, that there are genes which produce very slight visible effects 
is now another commonplace. Given these three facts, and the 
hypothesis (which is supported by much specific evidence) that most 
races are heterozygous for a number of such genes is all that is re- 
quired to complete the conception that is held by most adherents of 
the view that multiple factors or modifying genes are responsible for 
the results of selection. 
In specific cases, the existence of definite modifying genes has been 
demonstrated by Dexter, Bridges, Muller and Altenburg, and the 
author. All other data in question fit in with the view that selection 
ordinarily acts only by isolating modifiers. 
Modification of factors by selection, crossing, fractionation, or 
similar means is undemonstrated in any given case, and has been 
shown not to occur in other cases that are typical of the results usually 
obtained. Factors do change, and more than two forms are possible 
for certain loci; but there is no known method of inducing such changes, 
and they are ordinarily quite rare and definite. 
SUMMARY. 
(1) Dichet is a dominant character, the gene being lethal when 
homozygous (yellow-mouse case). The gene is in the third chromo- 
some. 
(2) Dichet flies are more variable in bristle number than are not- 
Dichets. This variability is partly environmental, partly genetic. 
(3) Selection was effective in isolating both plus and minus Dichet 
lines. 
(4) A cross between two separate inbred plus lines gave no increase 
in variability and no increase in parent-offspring correlation. There- 
fore the two lines were presumably of very similar constitution, though 
independent in origin. 
(5) A cross between an inbred plus line and an inbred minus line 
gave the results characteristic of such crosses—increased variability 
in F, and increased parent-offspring correlation. 
(6) Linkage tests demonstrated that modifying genes exist in the 
selected lines. Several lines were shown to differ in one or more sec- 
ond-chromosome modifiers, and at least one of these modifiers was 
shown to cross over from the speck gene. 
