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" niethod (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. 



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 inform.ation 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 accompHsh 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 Dichset, 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 



