468 WALTER W. MARSHALL AND HERMANN J.. MULLER 



linkage groups in Drosophila is adequate for this purpose. It is 

 important to know whether this difference is due to multiple 

 allelomorphs or multiple factors since if it were found to be 

 due to multiple allelomorphs the. further question would be 

 raised as to whether it was due to one or two mutations in the 

 factor for balloon or whether it was due to natural selection work- 

 ing on continual fluctuations in this factor. There is no evidence 

 at present to show whether the latter is true, but this question 

 too could be determined by certain breeding tests. 



Another experiment was made with the curved wing character 

 in Drosophila, which was treated in the same general way. The 

 factor curved (c), like balloon, was in the chromosome S dj cba 

 that had been subjected to the long-continued outcrossings. 

 After it was purified by a series of crosses in a manner similar 

 to those made in 'freeing' balloon, it was compared to stock which 

 had been kept homozygous for an extended period. Owing to 

 the infertility of the cm'ved winged flies the number of resulting 

 offspring was small. Nevertheless the results corroborate those 

 already given for the balloon character. The character curved 

 appears as a convexity throughout the general surface of the 

 wing and the grades are based on the degree of variation found. 

 After careful observation 4 grades of intensity were established, 

 which may be called: (1) 'Slight curvature,' (2) 'moderate curva- 

 ture,' (3) 'fair curvature', and (4) 'marked curvature,' and in 

 addition there were also found a number of cases which had 

 to be grouped in a fifth grade, (5) 'spongy wing.' In this experi- 

 ment there were only two strictly comparable lots of stock and 

 outcrossed flies (made up and counted on the same day). Owing 

 to the small numbers of stock offspring, an extra lot of stock 

 flies started on a different day was counted. 



Table 3 shows the correlation between the right and left 

 wings in the case of curved, as table 1 did for balloon. 



Table 4 gives the values obtained for the different lots of 

 flies examined. In these examinations, as in the case of the 

 balloon, the flies of one kind were marked and then mixed with 

 those of the other kind. The grades of the wings were then 

 determined before they were identified as stock or outcrossed. 



