No. 583] BOLE OF ENVIRONMENT IN DROSOPTIILA 421 



suppose that a factor in the wild or agouti mouse mu- 

 tated so that the recessive black was produced as a result 

 of the activity of the new gene. Then bl = black, and Bl 

 = gray with respect to black. Likewise cinnamon agouti 

 may be represented by ci, and gray, with respect to cin- 

 namon, by Ci. Chocolate is then the double recessive 

 blci and the symbol Ch for " chocolate" becomes super- 

 fluous. All the experimental results may be explained 

 on this basis. 



It is not necessary to try to state what kind of a change 

 in the germ-plasm led to these two mutations. The fac- 

 torial hypothesis should be entire])- non-committal as to 

 the kind of change that took place, for we can know noth- 

 ing about the nature of the change, yet the results are 

 predictable as well on one view as on the other. 



There is another way to interpret a dominant factor 

 like this one that gives abnormality, namely, that there is 

 present in the normal fly a factor that restricts the yellow 

 of the abdomen to the bands. When this restrictor, ab, 

 changes (Ab) the yellow is dispersed over the abdomen 

 and the black bands fail in part or entirely to appear. 

 The new factor, acting with the rest of the cell, gives ab- 

 normality, just as the normal restrictor or inhibitor (ab) 

 acting with the rest of the cell gives normality or band- 

 ing. The interpretation is non-committal in regard to 

 the nature of the change, which is an advantage in the 

 direction of simplification. In contrast to this view, a 

 different interpretation of the meaning of a restrictor 

 might be entertained on the presence and absence view. 

 It might be said that a restrictor factor has been "lost" 

 from the normal fly, which failing to restrict the color 

 has given rise to abnormality. The first objection to this 

 hypothesis is that it postulates (as above) the nature of 

 the change in the germ plasm, because it says something 

 has been lost. The second objection is that the facts 

 show that a restrictor has not been lost sensu strictu 

 because there is a wide range of variation in regard to 



