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. a Total. 
OM alee lo, ie} fou | Htott| [takedan bilve? 
Wild: 
Falmouth, Massachusetts.| 0/| 0]} O 1 | 186 | 118 | 11 2 Ovo 318 
Berkeley, California. ..... 0 0 0 0 95 | 104 0 0 0 0 199 
Mitchell, South Dakota...| 0 0 0 OF |) 226.213) 4 1 0 0 444 
Amity. \Oregonit 1. cie a0 0 0 0 0 59 51 1 1 0 0 112 
Sydney, Australia......... 0 0 0 0 16 21 0 1 0 0 38 
Pinksbandle nics istic tssataee 0';) 0 1 5 | 103 | 99 1 03), Oy, 0 209 
IBlackacs teeter nce OFF 30" | OTP Oee 26738) sO" rye t0y' 0 64 
IMDONY,.:a- dawellact noekiten ee OUP OL: ON On eRSOne 92) BON On eOut) O 172 
Blistered 5.5 Mc.c scr aysteaceua ies Oc On LON On 4 67 | OMe On Oot eO 181 
NWihhitelcrestors stetuselemre eters tier OP 0! | POO). | ee: ee 2a TOP I One 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 of 8 
Tei Bois ollGanilodas 4 490 | 668 | 1,702 | 81 | 6] 2,951 
IAT Wropee Elia ole ei|| er say 436 | 684 | 1,527 | 53 | 8 | 2,736 
Minus 9..|...| 5 TL OLT | 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 
