4 AN ANALYSIS OF THE EFFECT OF SELECTION. 
The averages, roughly weighted according to number of individuals, 
are: sepia Dichet, 13; Dichet spineless, 12. This agrees with the 
data of Bridges on the position of Dichet with reference to pink, since 
that locus is about 8 to the left of spineless. 
Bridges also found that homozygous Dichets are not produced. 
The gene, like that of the yellow mouse, acts as a lethal when homozy- 
gous. The result is that when Dichets are mated together they 
produce two heterozygous Dichets to one not-Dichet. This dis- 
covery has been verified by the experiments described in this paper, 
and by other experiments carried out by Muller and by the author. 
Tas_e 1. 
No. of bristles. 
vue Total. 
- 13)4]5] 6 
) 881 a 9 | 20 | 27 56 
882 1 | 23 | 29 | 30 83 
883 dy 9] 11] 11 31 
900 a; | B2 | 22) 18 67 
2715 ae 7115 3 25 
1 | 80 | 97 | 84 262 
Fras. 1 and 2.—Two types of bristle distribution 
in Dichwts—a “3” and a ‘'7.” Small post-alars are 2 and 7 bristles have also been ob- 
present in fig.2. These are never counted in the totals. served in unselected stocks. 
As shown in plate 1, fig. 1, the wings of Dichet flies are held out from 
the body and are bent back near the base. The number of dorso- 
central bristles (on the dorsum of the thorax) on the original female 
was 2 instead of 4, as is usually the case in the normal fly (plate 1, 
figs. 1 and 3). This has since been found to be a variable character. 
The number of dorso-centrals varies from 0 to 4, and sometimes 
one or more of the scutellars may be missing. In addition, the an- 
terior post-alars above and just behind the wing-base are reduced or 
absent. Plate 1, figure 1, and text-figures 1 and 2 show some common 
types. The work reported in this paper has consisted in selecting for a 
high and for a low total of scutellar and dorso-central bristles. Counts 
from five unselected cultures gave the results as shown in table 1. 
The normal flies occasionally show variations in bristle number, 
but these are much rarer than in the case of Dichet. MacDowell 
(1915) has given some data on the frequency of these variations, and 
has also reported on very extensive selection experiments with them 
(1915, 1917). These experiments will be referred to below. 
I have made bristle counts on a few unselected not-Dichet stocks, 
with the results shown in table 2. 
The normal flies have 8 dorso-central and scutellar bristles in most 
cases, while the Dichezts range from 1 to 8. But the 8-bristled Dichsets 
are still distinguishable from normals, even when their wings are not 
