SEX INHERITANCE 91 
the female. It does not follow from this that 
there is only one sex factor involved that is car- 
ried by the sex chromosomes. The symbols used 
here, viz., XX-XY and WZ-ZZ, are intended 
primarily for the chromosomes, but also for the 
sex factors. 
These formule for the Drosophila type and for 
the Abraxas type raise the question as to whether 
the postulated sex factors are identical in the two 
cases. The employment of different letters for the 
two types suggests, of course, that the sex factors 
may be different. And it is true that the two sets 
of letters are used to avoid an apparent paradox 
that appears if we use only X and Y in both cases. 
If this is done, XY on one scheme represents the 
male and on the other scheme the female. Never- 
theless, for the present the employment of different 
letters need not necessarily mean that different 
factors for sex are present in the two great classes, 
for these reverse results may be due to the action of 
the same factor-difference. in a different setting. 
For example, acid may be the color differentiator in 
a setting of a certain solution containing it and litmus 
(with one drop of acid the color being blue, with 
two drops red), but in a setting containing Congo 
red the same differentiator may produce just the 
opposite effects (one drop red, two drops blue). On 
the other hand, it is conceivable that the setting (lit- 
mus and acid) may remain the same and yet a reverse 
result be produced by having a different differentiator 
—alkali instead of acid. 
