. An analysis of the effects of selection. Natural selection. Figs. 1 and 2.â^Two tjrpea of bristle distribution in Dichcetaâa "3" and a "; Small post-alars are present in fig. 2. These are never counted in the totals. No. of bristles. Culture No. Total. 3 4 9 6 6 881 20 27 56 882 1 23 29 30 83 883 9 11 11 31 900 32 22 13 67 2715 â¢â 7 15 3 25 1 80 97 84 262 2 and 7 bristles have also been ob- served in unseleoted stocks. As shown in plate 1, fig. 1, the wings of Dichset flies are held out from the body and are bent back near the base. The number of dorso- central bristle
. An analysis of the effects of selection. Natural selection. Figs. 1 and 2.â^Two tjrpea of bristle distribution in Dichcetaâa "3" and a "; Small post-alars are present in fig. 2. These are never counted in the totals. No. of bristles. Culture No. Total. 3 4 9 6 6 881 20 27 56 882 1 23 29 30 83 883 9 11 11 31 900 32 22 13 67 2715 â¢â 7 15 3 25 1 80 97 84 262 2 and 7 bristles have also been ob- served in unseleoted stocks. As shown in plate 1, fig. 1, the wings of Dichset 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 Dichset. MacDowell (1916) 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-Dichset stocks with the results shown in table 2. The normal flies have 8 dorso-central and scutellar bristles in most cases, while the Dichsets range from 1 to 8. But the 8-bristled Dichsets are still distinguishable from normals, even when their wings are not. Please note that these images are extracted from scanned page images that may h
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Keywords: ., boo, bookcentury1900, bookdecade1910, booksubjectnaturalselection
