AN ANALYSIS OF THE EFFECT OF SELECTION. 7 
table will enable the reader to find them.) The last three columns 
give the results of an application of the x? test to the data. The last 
column, headed P, gives the chance (1.0 representing certainty) that 
deviations from identity as great as those observed could have re- 
sulted from random sampling. It follows that in at least three cases 
(the fifth, sixth, and seventh) the results given by the two broods were 
significantly different. 
TaBLE 5.—First and Second Broods from Same Parents. 
Culture Nos. Genes 
ations a ; 
mother P 
inbred. 
Series. 
First | Second 
brood. | brood. 
1,907 Noe) Tesi spadonasaddes 4 3.74 | 3 | 0.16 
1,908 15997 |) L002 rev iea2e =f 6 5.60 | 5 -23 
1,912 Ute || LE UPRE on oc began 7 2.10 
7 
9 
4 55 
POZE | L909" | LOO rer. -tsrcciereietse 6.05 | 5 19 
ZAOTAA S251 400 | OOO Tawa siteektss sate 22.09 | 4 -0001 
2,078 | 2,141 | Test of crossbr. plus.| 11 16.81 | 4 -001 
2hOST |" 2h LAT SG4 ee Mek a cterciers 6 11 19.80 | 5 0005 
2,475 | 2,518 | Test of 1002........ 118 5.22 | 3 075 
1F\, and F\7 were mass cultures in this case. 
There is one possible source of error in these data: It has been 
shown by Bridges (1915) that the amount of crossing over in the sec- 
ond chromosome of Drosophila varies with the age of the female. 
My own unpublished data show that this is also true for the third 
chromosome. In the present case, if the female parents of the flies 
observed were heterozygous for many modifying factors, such a 
change in linkage might result in the production of genetically differ- 
ent first and second broods. However, the female parents in these 
cultures were in every case from at least four generations of brother- 
sister inbreeding (see table 5, column 4)! and in the significant cases 
for 9 and 11 generations. It is therefore very unlikely that they were 
heterozygous for many modifying factors. The two broods from 
these females must, then, be of the same genetic constitution, and the 
differences between them can only be due to environmental causes. 
It follows that in the experiments recorded below a significant part 
of the variability is not genetic, but environmental. 
METHODS. 
With very few exceptions, the flies recorded in this paper were bred 
from pairs, and in pint milk bottles. The food used was ripe un- 
cooked banana, fermented in a stock yeast-culture for from 12 to 48 
1Three cases in which the female parents were hybrids have been discarded (see 2091-2143, 
3064-3116, 3066-3118 pairs in Appendix). 
