EFFECT OF SELECTION ON CROSSOVER VALUES 335 
a change in genes between two or more fixed points may be 
accompanied by a slight disturbance of the crossing-over ratios 
between these fixed points. Sex, to be sure, has a striking effect 
on crossing over, for the male Drosophila does not show this 
phenomenon even in the autosomes. 
Whenever one observes a large number of Drosophila females 
A B 
of the generalized zygotic formula r, it is common to 
a D 
find much variability with respect to the amount of crossing 
over, even though the cultures are kept at the usual normal 
temperature and no striking genetic modifiers of crossing over 
are known to exist. Just what this variability is due to is not 
known. Some of it may represent fluctuations of sampling and 
some of it may be due to age, but very frequently the devia- 
tions are so wide as to arouse a suspicion that hitherto unknown 
causes may be effective. If this variability is due, at least in 
part, to genetic causes, then selection should have an effect, 
particularly if environmental fluctuations do not mask or obliter- 
ate the effect of genetic modifiers. It was with this thought in 
mind that the senior writer began to select for high and low 
crossover values. 
MATERIALS AND METHODS 
The selection experiments consisted of four series: 
Series A, low selection; 
Series A', derived from series A in F7; 
Series B, low selection, a second experiment duplicating series A; 
Series C, high selection. 
Each series began with a single white-eyed miniature-winged 
female mated to a wild red long male. These strains were 
chosen because the characters are easily recognized, show little 
or no variability, and have at least fair viability. To classify 
any female with respect to her 'crossover capacity' requires the 
classification of all the progeny which we can obtain from her. 
Thus in Fg, series A (table 1), we classified 86'60 offspring to 
obtain the necessary data on fifty-six F5 females for the purpose 
of selection. In the usual selection experiment, individuals are 
