MULTIPLE FACTORS 191 



ences in size that appear might therefore be selected 

 in order to produce new and larger strains. 



A more difficult case than those given above is that 

 of truncate (Fig. 18, b) in Drosophila, worked out by 

 E. R. Altenburg and H. J. Muller. The F 2 resulting 

 from a cross of a truncate fly to a normal long-winged 

 fly consists of 85-92 per cent, of long-winged and the 

 rest truncates and flies with wings of various inter- 

 mediate grades. The extracted truncates do not 

 breed true; by selection it is possible gradually to 

 reduce the longs to about 5 per cent., but even after 

 about 100 generations of selection the proportion of 

 longs could not be reduced any further. These longs 

 produce some truncates, but do not, on the whole, 

 produce nearly as high a percentage of them as do 

 their truncate brothers and sisters. The longs, 

 therefore, differ genetically from the truncates, and 

 the fact that these genetic differences are constantly 

 occurring in this stock, in spite of the long-continued 

 selection, seemed to indicate that here at least there 

 was a case of instability of factors or contamination 

 of allelomorphs. 



By means of linkage experiments it was shown that 

 in the production of this character there are involved 

 at least three factors (Ti,T 2 ,T 3 ), one in the first, one 

 in the second, and one in the third chromosome. 

 The character cannot make its appearance without 

 the factor in the second chromosome (T 2 ), but it 

 may appear without either of the other two factors, 

 which are, therefore, in the nature of intensifiers. 

 Moreover, truncate is influenced by still other fac- 



