Sex Determination in Phylloxerans and A phids 339 
Abraxas case along these lines shows that Wilson’s assumption, 
based on the chromosomal evidence, that two X’s produce the con- 
ditions leading to femaleness, one X produces the conditions lead- 
ing to maleness will not explain the results, while the assumption 
of one X for femaleness and no X for maleness fits in with the 
observed relations. ‘This interpretation has the advantage of 
giving a simple explanation for Abraxas. In fact it is little more 
than the hypothesis of Bateson and Punnett made more general in 
the sense that it assumes a unit character that determines female- 
ness, but for this very reason it introduces an interpretation of 
sex that is extremely hypothetical. For, the unit character is no 
longer simply a quantitative factor but a special element that has 
the power of turning maleness into femaleness. It is an entirely 
imaginary factor and lacks observational evidence in its sup- 
port. It leads to further assumptions in regard to the secondary 
sexual characters in the male, supposed to be absent in the 
female. ‘These are referred to another unit character and it 
is suggested that this is the Y-element of Wilson. If so, such 
characters should be absent when Y is absent, which appears not 
to be the case. In so far as the view rests on a quantitative con- 
ception of sex determination it contains elements that are in har- 
mony with the views here discussed, but in so far as it calls for a 
Mendelian element whose presence turns maleness into femaleness 
it meets with serious difficulties and must make further hypotheti- 
cal assumptions for its support. One of its more serious draw- 
backs we have already referred to, namely that postulating male 
and female gametes it assumes that they conjugate without regard 
to their condition as sex-bearers, male fertilizing male at times, etc. 
This difficulty might be met by assuming that the unit character 
for sex (sporophyte) has nothing to do with the conjugation prop- 
erties of the gametes and the signs male and female are used only 
as symbols for the presence and absence of this differential charac- 
ter. But such a view ignores the conjugating element itself whose 
signs cannot be explained on the assumption, here adopted, of 
their determination by their association with the sex element. 
We owe to Wilson the most complete analysis of the present 
evidence relating to sex determination. His conclusion is based 
