SELECTION IN BAR-EYED RACE OF DROSOPHILA. 393 



heterozygous females should have appeared more frequently 

 than full-eyed males and full-eyed females might in rare instances 

 have appeared. If the change occurred indiscriminately in male 

 or female then the proportion of heterozygous females to full- 

 eyed males should be 3 to i, but the ratio obtained is 5 to 6. 

 The numbers obtained, however, are not large enough to be 

 conclusive. 



The fact of reverse mutation is very difficult to explain. It is 

 hard to see how the germplasm can lose a factor and still poten- 

 tially retain it and have it reappear later. So far as the present 

 data go it is possible to explain the case under consideration in 

 two different ways. 



If the normal wild fly carries a limiting factor with respect to 

 the facet number then it is possible by partial non-disjunction 

 for the factor to pass from one chromosome of a pair to the other, 

 giving one chromosome without a limiting factor and the other 

 with a double limiting factor. The bar-eyed race of Drosophila 

 may be derived from such a chromosome with two limiting 

 factors or factor groups, the mate of the chromosome having 

 been lost in the maturation of the egg. If then in the bar-eyed 

 race a second non-disjunction again separates the two factors 

 the result should be one chromosome with triple factors and one 

 with a single factor, the latter giving rise to a full-eyed male or a 

 heterozygous female. If the former passes into the egg it should 

 give rise to a further reduction in the facet number, but it is 

 possible that a fly with such a chromosome does not live. It is 

 possible also that the male with 34 facets contained a chromosome 

 with a triple reducing factor. 



A simpler explanation is that of a reversible chemical change 

 between two compounds one of which is more stable than the 

 other. If the compound that forms the basis for the bar eye 

 is the less stable then reversions are to be expected under certain 

 conditions. But the fact that the change takes place only in 

 the female is a strong argument in favor of the theory of partial 



non-disjunction. 



SUMMARY. 



Selection was carried on in the vestigial-winged, bar-eyed 

 stock for three generations and in the long-winged, bar-eyed 



