28 T. H. MORGAN 



but were at first supposed to be a particular kind of yellow fly 

 which was recognized, however, as difTerent from the ordinary 

 yellow and had been maintained in a separate culture. When, 

 however, in the second generation from black and yellow these 

 same flies appeared, their relation to the other colors was appar- 

 ent. Forms like these, that represent a type due to absences, 

 derivable through combination of primary mutations may be 

 said to arise by permutation. 



In making the counts I have been assisted by Miss E. M. 

 Wallace, Miss Eleth Cattell and Mr. C. B. Bridges. In practi- 

 cally all cases I have checked up each count after the separation 

 had been made, so that the results stand for the agreement of 

 two observers. Only in the case of separation of yellows and 

 browns could any disagreement arise, and while I cannot claim 

 for this case that the separation has been exact, I think that it 

 is very nearly so. 



Description of the mutants 



The color differences between the normal, wild, or gray fly 

 and the mutants is shown by the accompanying plate, figures 

 1 to 4. The detailed comparisons are as follows: 



The wild fly. The upper surface of the thorax is olive yellow, 

 the olive shade being very faint. As the flies get older the- color 

 deepens. Some of the wild flies have a black trident and two 

 lateral black streaks on the upper surface of the thorax (not 

 shown in the present case), but many flies do not show this 

 marking. I have made a long series of selection experiments 

 with this marking and have produced one race that never shows 

 the trident, and another race that has a dark well-developed 

 trident. How far the character is a fluctuating one and how 

 far due to genotypic difference need not be discussed here. 



The abdomen of the female is banded with lemon yellow and 

 black. In the male there are only two black bands as a rule; 

 the end of the abdomen is black. The legs are colored like the 

 thorax but somewhat lighter. The hairs on the body are black. 



The wings are very transparent, blue gray or smoky. The 

 veins appear dark, but under the microscope are seen to be 



