THE 'bar eye' mutant OF DROSOPHILA 517 



number of facets, the average of 250 individuals at the beginning 

 of the experiment being 98.04 and of the same number at the 

 end of the experiment 98.03. There is thus no change in facet 

 number in the absence of selection. 



In making the first selection, food containing eggs and larvae 

 was removed from the culture of the 'bar eye' stock and placed 

 in glass vials. Every twelve hours the individuals which had 

 emerged from the pupal cases were slightly etherized and an 

 estimate was made of the number of facets under the low power 

 of the microscope. Males and females with high or low numbers 

 were selected out, high being mated with high and low with low. 

 Each pair was placed in a small bottle with sufficient food to 

 last until all the offspring of the first brood had reached the 

 adult stage. When larvae began to appear in the bottle the 

 parents were killed by etherization and an exact count was made 

 of the facets. As the offspring emerged in the adult form esti- 

 mates of facet number were again made and high selections 

 were made in the 'high' lines and low selections in the 'low' lines. 

 The final exact facet counts were in all cases made in killed 

 individuals. 



The experiment has proceeded far enough so that data are 

 complete for three successive selections in each of three 'high' 

 lines, called A, B, and C, and in each of three 'low' lines, called 

 D, E and F. Fifty individuals, 25 males and 25 females from 

 a single pair, were measured for each generation in each of the 

 lines, with the exception of the third generation in line B where 

 only forty-six were available. 



The data for the individual lines are given in tables 1 to 6 

 and a summary of the six lines is given in table 7. Figures 3, 

 4 and 5 give in graphic form the course of the selections, each 

 figure combining a 'high' with a 'low' selection line. The mean 

 values, the extreme variates and the mid-parental values are 

 here represented in diagrammatic form for each of the selections. 



In all cases there is a significant shifting of the mean as a 

 result of selection. The general population mean of 98.0 is 

 changed in plus line A to 108.7 by the first selection, to 127.5 

 by the second and to 135.5 by the third. In plus line B the 



