242 MULTIPLE FACTORS 
low as 0 and rarely higher than 20 per cent.; in 
later generations, however, the proportion and the 
intensity of the extracted truncates may again be 
raised, by selection, to about 90 per cent. That the 
variation in truncation within the original highly 
selected stock, as well as within the extracted stock, 
is not merely a somatic effect due to uncontrollable 
environmental influences was shown by the fact 
that the variations in both lines were to a large 
extent heritable: selection of normals resulted in a 
much lower percentage of truncate offspring than 
were thrown by their truncate brothers and sisters, 
and selection of intermediates gave, on the average, 
intermediate results. The fact that such genetic 
differences are still constantly occurring in the 
original stock, in spite of the long-continued se- 
lection, seemed to indicate that here at least there 
was a case of instability of factors or contamination 
of allelomorphs. 
An analysis of the factorial composition of the 
truncate flies was then made by crossing them to 
flies containing in each of their chromosomes other 
mutant factors whose hereditary behavior was 
known. In the second generation of the cross 
(back-cross) these other factors served as identifying 
marks which disclosed just which chromosomes of 
the P, truncate fly each F, individual had or had 
not received. By observing the amount of trun- 
cation which accompanied each ascertained combi- 
nation of chromosomes it could thus be determined 
just what role each of the chromosomes played in 
