MULTIPLE FACTORS 245 
combination of chromosomes as before will once 
more have been produced (among other offspring); 
such males may therefore be obtained and back- 
crossed generation after generation, to black pink 
females. In this way, although the flies are con- 
tinually crossed and kept heterozygous, a “pure 
line’? may be maintained in the sense that the same 
chromosome combination is continually transmitted 
without recombination. Two such heterozygous 
lines were thus propagated for twelve generations, 
and selected for truncate and for normal wing 
respectively. At the end of that time they were 
found still to be unchanged and like each other in 
respect to the amount of their truncation. This 
proved that there could not have been any con- 
tamination or miscibility of the truncate factors 
with their allelomorphs, or any instability of these 
factors; the genetic variability of the original 
truncate stock must therefore have been due en- 
tirely to the recombination of multiple factors, 
which was prevented in the present selection experi- 
ment. It also follows that all of these differential 
factors for truncate must be located in the chro- 
mosomes (in the three large chromosomes), since 
only the recombination of such factors had been 
prevented by the experimental technique. 
It remained to be determined why, in the original 
truncate stock, recombinativn of factors could still 
be going on, since the prolonged selection should 
in any ordinary case have long since resulted in a 
homozygous condition. Special tests were there- 
