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, recombination 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- 



