274 HEREDITY AND ENVIRONMENT 



as is shown in the right half both of Fig, 59 

 and of Fig. 60, and this suggests that the 

 differential factors for these characters are car- 

 ried in these chromosomes. 



By a series of ingenious experiments Foot 

 and Strobell have shown recently that the dif- 

 ferential factors for certain sex-limited char- 

 acters in insects, that is characters which are 

 limited to one sex, are not contained in the 

 "sex chromosomes," and they argue that the 

 differential factors for secv and for sex-linked 

 characters cannot be located in these chrom- 

 osomes. Morgan does not admit the validity 

 of their conclusions, but it must be admitted 

 that the evidence that particular determiners 

 can be located in particular chromosomes is 

 not entirely conclusive, and this question may 

 for the present be regarded as an open one. 

 However there is good evidence that the fac- 

 tors for the determination of sex and of sex- 

 linked characters are distributed in the same 

 way as the "sex chromosomes" are, and it 

 would be a surprising thing if these two phe- 

 nomena should be found not to be related 

 causally. 



