394 H - G - MAY. 



stock for seven generations. The facets in the eyes of 9,000 flies 

 were counted. 



Selection in the vestigial-winged stock had to be discontinued 

 on account of sterility and low production in the single lines. 



The sterility and low production were not due to inbreeding. 

 Sterility was not clue to inability on the part of the females to 

 lay eggs, nor does it appear to have been the final stage in the 

 reduction of the number of offspring. 



Selection in the vestigial-winged stock was effective for one 

 generation but failed to produce further effects in the second and 

 third generations. Return selections from the low lines were not 

 effective. 



A mutation appeared in a male in line 16. It was a reducing 

 factor and was sex-linked. The complete sterility of the off- 

 spring of this male prevented further study of the character. 



Selection in the long-winged stock was effective for six genera- 

 tions. In the downward selected lines most of the high flies 

 were eliminated in the first three generations and no further effect 

 of selection was noted. Return selections from the sixth gen- 

 eration, however, were still effective. The mean of the upward 

 selected lines continued to increase at approximately uniform 

 rate for six generations. The seventh generation must be dis- 

 regarded because it was not an upward selection. The range 

 in the facet number was not increased by selection. The results 

 of these experiments indicate that the hereditary differences in 

 this race of Drosophila are due to a large number of small factors. 



Crosses made in the sixth generation between low and high 

 lines indicate that the hereditary factors are not sex-linked. 



In both the long-winged and the vestigial-winged lines the 

 mean facet number was highly variable. This variability ap- 

 peared to be due chiefly to changes in the temperature of the 

 room, but may have been due also to other causes. The mean 

 facet number of flies reared in vials as single lines was slightly 

 higher than that of stock flies reared in bottles. 



The mean facet number of the vestigial-winged flies was some- 

 what lower than that of the long-winged flies. 



Six full-eyed males and five heterozygous females appeared 

 in the stock bottles and the selected lines during these experi- 

 ments. They must be regarded as reverse mutations. 



