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 2 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. 



^u and Fa 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 My own unpublished data show that this is also true for the third 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) 1 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 



1 Three cases in which the female parents were hybrids have been discarded (see 2091-2143, 

 3064-3116, 3066-3118 pairs in Appendix). 



