526 CHARLES ZELENY AND E. W. MATTOON 



84.8 '41.9 



Fig. 4 The effect of selection in Plus Line B and in Minus Line E. See the 

 description of figure 3 for further details. The full data are given in tables 2, 

 5 and 7. 



selection, 0.72 for the second and 0.85 for the third. In the 

 minus hnes the corresponding figures are 0.77, 0.81 and 0.83. 

 The decrease with respect to the mean of the general unselected 

 population indicates that there is real progress during the suc- 

 cessive selections. The increase with respect to the parental 

 generation indicates that the effectiveness of the selection de- 

 creases with successive selections and that there is probably a 

 limit to the number of effective selections. It seems probable 

 that continued selection would not be able to change the number 

 of facets in the mutant stock to that of the original stock from 

 which it was derived. 



The data presented thus show that selection in the 'bar eye' 

 race of Drosophila is effective both in increasing and in decreas- 

 ing the number of eye facets. As a result there can be no doubt 

 of the existence of differences in the germinal constitution as 



