AN ANALYSIS OF THE EFFECT OF SELECTION. 5 
unfolded enough so that they can be separated on that basis. This 
is because the anterior pair of dorso-centrals never, so far as I have 
observed, becomes as large as the corresponding pair in normal flies. 
The anterior post-alars are also reduced in 8-bristled Dichets. This 
TABLE 2, 
6 7 8 9 10 
Stock. Total. 
Pl[ar| Qtr] Fe ose oo oe OO ok 
Wild: 
Falmouth, Massachusetts.| 0} 0] O| 1] 186] 118/11] 2] O| O 318 
Berkeley, California...... 0; 0 0} O} 95] 104 0 0 Oo; 0 199 
Mitchell, South Dakota....| 0} 0| O| O| 226] 213/ 4] 1] 0} O 444 
Amity, Oregon........... 0 0 0 o| 59 51 1 1 0 0 112 
Sydney, Australia......... Oo} oO}; O|} O| 16] 21 o| 1 0; oO 38 
Pink band wsé6566 6 saeseue es 0 0 1 5 | 103 99 1 0 0 0 209 
BIGGS 2.2. s654 cpamaaceameacatumauees 0 0 0 0 26 38 0 0 0 0 64 
BbODY ss. s eitvaae ecw ness 0 0 0 0 80 92 0 0 0 0 172 
Blistered..............25205 0 0 0 O| 114 67 0 0 0 0 181 
WhitOncissacwoam esas waneees 0 0 0 0 74 77 2 0 0 0 153 
separability is a matter of some importance, since, because of the 
lethal effect of Dichet, any Dichet culture may produce normal 
flies. However, the spread wings can be and are used for the separa- 
tion in all but the rather rare instances of failure to expand properly. 
SEXUAL DIMORPHISM. 
Calculations show that there is a slight but significant sexual di- 
morphism in bristle number in the Dichet races. Random selection 
of plus and of minus selected cultures gave the totals shown in table 3. 
TABLE 3. 
Bristle number. 
Total. 
1/2 3 4 5 6 718 
Plus Q isc cela oe} anes 4 490 | 668 | 1,702 | 81 | 6] 2,951' 
Plus o’....}...]| 3 | 25 436 | 684 | 1,527 | 53 | 8 | 2,736 
Minus 9..|...| 5 | 17 | 1,517 | 712 424] 7]...| 2,682 
Minus o”..| 1 | 39 | 177 | 1,190 | 615 332 | 2]...| 2,356 
These distributions give the statistical constants shown in table 4. 
The first three columns show that there is a slight difference in the 
means, the females being higher in both cases. In the case of the plus 
series the difference is doubtfully significant; in the minus series it is 
larger and certainly significant. The last column gives the chance 
