6 AN ANALYSIS OF THE EFFECT OF SELECTION. 
that differences as great as those observed between the two distribu- 
tions are due to random sampling. These values were obtained by 
Pearson’s x? method (Pearson, 1911). This column makes it quite 
certain that there is a significant sexual dimorphism in both series, 
and also brings out again the fact that the dimorphism is greater in 
the minus series. 
TaBLe 4. 
Q Mean. o' Mean. Difference. P 
Plus..... 5.468+0.010 | 5.428+0.010 | 0.041+0.014 0.0001 
Minus...| 4.583 .010 | 4.436 .012 .147+ .016 . 0000000 + 
Because of the information given by this table it has seemed de- 
sirable to present the data for males and females separately. This 
has been done in the Appendix; but since the dimorphism is slight, 
the data have been lumped in the statistical treatment given in the 
body of the paper. The data in the Appendix make it possible to re- 
calculate the constants separately if it should seem desirable to do so. 
EFFECTS OF ENVIRONMENT. 
In any selection experiment it is obviously very important to have 
some information regarding the influence of environmental conditions 
on the variable character used. If the observed variations in the 
character are largely due to environmental causes, it should be very 
difficult to accomplish much by selection; but if the environment 
plays little part in causing variability, selection should be very effective 
in isolating different types, and on the multiple-factor view variability 
should show a marked decrease after a few generations of inbreeding. 
In the case of Dichet, it has been observed that as cultures grow 
older the flies frequently have fewer bristles. In such cultures it is 
usually observed that the later flies are also smaller and that the food 
conditions in the bottle have become unfavorable. It is, therefore, 
essential in such experiments that conditions be made as nearly uni- 
form as practicable. 
The data in table 5 show that under ordinary conditions there is 
considerable environmental effect. Eight pairs from the regular series 
were transferred to second bottles, after staying the usual period in 
the first one. Offspring were thus obtained with identical pedigrees 
and differing only in that they were reared in separate bottles. No 
attempt has been made to make conditions different in the two bottles, 
which constitute a random sample of the conditions under which the 
experiments were carried out. Table 5 shows the results obtained. 
(The actual data are in the Appendix; the first three columns of the 
