THE 'bar eye' mutant OF DROSOPHILA 527 



84.7 I4I.0 



Fig. 5 The eff6ct of selection in Plus Line C and in Minus Line F. See the 

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

 6 and 7. 



regards this characteristic among the individuals of a generation. 

 This fact is of special interest because the origin of the race by 

 sudden appearance in a single individual is known and is of 

 recent occurrence. Furthermore the behavior in crosses with 

 the normal wild race shows that the mutant differs from the 

 normal wild race in but a single Mendelian factor. If this were 

 the only germinal factor involved in facet number we would be 

 compelled to conclude that we have a case of variability in a 

 unit factor. As far as the present selection data go such a 

 provisional hypothesis would not be contrary to the facts. It 

 seems more probable however that facet number in the normal 

 wild race is represented in the germinal constitution by more 

 than one factor and that the modification occurring in the pro- 

 duction of the 'bar eye' race involved only one of these factors. 

 That this factor is a most important one is of course indicated 



