24 AN ANALYSIS OF THE EFFECT OF SELECTION. 
Dexter (1914) showed by similar methods that the chief gene for 
the variable character beaded is in the third chromosome, and that 
there is a modifier in the second chromosome. These results have 
been verified by Muller (1917). 
In making such tests for modifying factors it is very desirable that 
the test stocks, as well as the selected stocks that are to be tested, 
should be homogeneous for modifying genes. This is desirable in 
order that the tests may be repeated, and in order that results ob- 
tained with different stocks may be compared. It was for this pur- 
pose that the speck minus, or 1331, line of this paper was obtained. 
(See above for history of this line.) This line, in the later generations, 
was homozygous for the recessive mutants speck (chromosome II) 
and sepia and rough (chromosome III). Since it had been inbred 
and minus selected for several generations, it was probably uniform 
in constitution with respect to modifiers. Since the other selected 
lines also became homozygous for rough in later generations, it was 
desirable to have a uniform not-rough line. For this purpose a pair 
was taken from the speck stock, wild type in other characters. From 
this pair a line was established, and continued by strict brother-sister 
pair matings, in order to obtain a stock nearly or quite homozygous 
for all its genes. This material is designated “‘speck stock.” All 
individuals from it that were used for test purposes came from 8 or 
more successive brother-sister matings.? 
Sex-linked modifiers would become apparent in F; when two races 
were crossed, since the males from reciprocal crosses would differ and 
each type would resemble the maternal race. There is no clear evi- 
dence of the existence of such modifiers in this experiment, so the sex 
chromosome will be ignored in the discussion that follows. 
The method used for detecting second-chromosome modifiers is 
as follows: Two lines are crossed, one of which contains speck, the 
other not; one or both have Dichet (fig. 13). The F, Dichets are 
then heterozygous for speck, and for any second-chromosome 
modifiers in which the two lines were different. If an F, male is now 
mated to a speck not-Dichzt female, there will be no crossing over 
between speck and the modifiers. Therefore the not-speck Dichets 
produced will receive second chromosomes from their father which 
will be identical with those present in the P; not-speck race (in the 
diagram the Dichet race), while the speck-Dichet back-cross 
individuals will receive the second chromosome that came from the 
other P, race. Since the two types are alike in their third chromosome 
constitution, and since they have been reared in the same culture bot- 
tles, so that environmental influences were the same, any differences 
1 With the exception of culture 1737 (see Appendix), all were from 10 or more successive brother- 
sister matings. All these specks were from the same F; pair. All those before culture 2430 were 
from the same Fs pair. All those after 2430 were from a different Fs pair, but were themselves 
from the same Fi7 pair. Fis and Fis were mass cultures instead of pairs. 
