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 F, 
groups, and the greater smoothness of its curve, 
as compared with that which would result by 
combining the P; and F, 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, 6), investigated by E. Altenburg 
and H. J. Muller, which was the first of these 
refractory cases to be solved. The race of truncate 
flies is never uniform: it usually throws flies 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 although the proportion of trun- 
cates was raised to about 90 per cent., the normals 
were never eliminated completely, and the grade of 
truncation was still subject to much fluctuation. 
Crosses of truncates to wild type flies produce 
varying results: in F, 0 to 8 per cent. show some 
truncation; in F, the proportion with any trace of 
truncation is highly inconstant, being sometimes as 
