4 AN ANALYSIS OF THE EFFECT OF SELECTION. 



The averages, roughly weighted according to number of individuals, 

 are: sepia Dichset, 13; Dichaet spineless, 12. This agrees with the 

 data of Bridges on the position of Dichaet with reference to pink, since 

 that locus is about 8 to the left of spineless. 



Bridges also found that homozygous Dichaets are not produced. 

 The gene, like that of the yellow mouse, acts as a lethal when homozy- 

 gous. The result is that when Dichaets are mated together they 

 produce two heterozygous Dichaets to one not-Dichaet. 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. 



TABLE 1. 



FIGS. 1 and 2. Two types of bristle distribution 

 in Dichffita a "3" and a "7." Small post-alars are 

 present in fig. 2. These are never counted in the totals. 



2 and 7 bristles have also been ob- 

 served in unselected stocks. 



As shown in plate 1, fig. 1, the wings of Dichaet 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 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 hi the case of Dichaet. 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-Dichaet stocks, 

 with the results shown in table 2. 



The normal flies have 8 dorso-central and scutellar bristles in most 

 cases, while the Dichaets range from 1 to 8. But the 8-bristled Dichaets 

 are still distinguishable from normals, even when their wings are not 



