280 HEREDITY AND ENVIRONMENT 



and if these are interbred there are produced 

 in the F 2 generation equal numbers of red- 

 eyed and white-eyed males and females 

 (Fig. 60). 



The distribution of the maternal and pa- 

 ternal sex chromosomes ( A") * exactly parallels 

 this distribution of this sex-linked character, 

 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 sex 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 



* Morgan has found that there are two different sex chromo- 

 somes (XY) in the male of Drosophila, instead of one (XO) as 

 Stevens supposed; the Y is found only in males. 



