DIRECTION AND FREQUENCY OF MUTATION 221 



than in the other dh'ection. The relative frequency of reverse 

 mutations as compared with direct ones shows that the stabiUty 

 of the material composing the gene is much greater at the full- 

 eye position than at any other. At the bar-eye position, where 

 mutation occurs in both directions, the return to full occurred 

 fifty-two times as opposed to three changes in the other direction 

 to ultra-bar, though the latter number probably is incomplete 

 because some heterozygotes may have been overlooked in that 

 part of the observations in which facet counts were not made. 



Jj.. Is recent origin an explanation of the frequency of reverse 

 mutations? It might be supposed that the frequency of muta- 

 tions in the direction ultra-bar to bar to full is due to the fact that 

 bar and ultra-bar have originated recently and are therefore 

 relatively unstable. If this were a true explanation it would 

 apply equally well to full-eyed stocks recently derived by reverse 

 mutation from bar or ultra-bar. As shown in table 1 , there is 

 not a single case of return to bar or ultra-bar in such stocks. 

 The same consideration applies to bar derived from ultra-bar. 

 It does not furnish a single case of return to ultra-bar (tables 2 

 and 3). 



5. Are shorter jumps more frequent than longer ones? In the 

 direct mutations the original ultra-bar came by two steps from 

 full. The number of observed mutants, however, is too small 

 for any conclusion as to relative frequency of short as compared 

 with long jumps. 



Among the reverse mutations there are five from ultra-bar to 

 full in 8681 individuals, or a coefficient of 0.00058. Bar to full 

 appears in fifty-two cases among 85008, or a coefficient of 0.00061, 

 and ultra-bar to bar in three out of 8681, or a coefficient of 

 0.00035. It is clear that these data furnish no support for the 

 view that shorter jumps are more frequent than the longer ones. 

 Until more adequate data are obtained the conclusion may be 

 drawn that there is no essential difference between the two. 



6, Does direction of selection have an effect upon direction or 

 frequency of mutation? It has been claimed that the direction 

 of selection has an influence upon the direction of mutation. 

 According to this view, there should be a preponderance of 



