MULTIPLE FACTORS 241 



tiple cumulative factors are conspicuously present — 

 namely, the great variability of the F* (or the back- 

 cross), as compared with either of the Pi or the Fi 

 groups, and the greater smoothness of its curve, 

 as compared with that which would result by 

 combining the Pi and Fi curves in 1:2:1 (or 

 1:1) ratio. 



In certain cases showing characteristics of multi- 

 ple factor inheritance, the interpretation has not 

 at first been so clear as in the cases given above, 

 owing to the existence of certain peculiar features 

 which seemed rather to call for the assumption 

 of genetic phenomena of totally different and 

 hitherto undemonstrated sort, such as a continual 

 fluctuation within the factor itself. 



A case in point is that of truncate wing in Dro- 

 sophila (Fig. 18, h), investigated by E. Altenburg 

 and H. J. Aluller, which was the first of these 

 refractory cases to be solved. The race of truncate 

 flies is never uniform: it usually throws flics with 

 wings of various grades ranging all the way from 

 short truncate to normal. It was attempted, 

 through over 100 generations of selection, to obtain 

 a pure stock, but altiiough the proportion of trun- 

 cates was raised to about 90 per cent., the normals 

 were never eliminated completely, and the gi-ade of 

 truncation was still subject to mucli fluctuation. 

 Crosses of truncates to wild type flies produce 

 varying results: in F, to S per cent, show some 

 truncation; in F.. the proportion with any trace of 

 truncation is highly inconstant, being sometimes as 



