External Factors of Sex Determination 385 
are 114 male births to 100 female births. The statistics for the 
nobility of Sweden show 98 male births to 100 females, while for 
the clergy of the same country 108.6 male to 100 female births. 
Diising, in particular, has developed this point of view. It 
is not evident, however, that because more food is supplied, the 
individual is necessarily better nourished, for this will depend 
on other conditions as well. The amount of food taken, or 
accessible, does not necessarily mean that more is digested. 
It has also been pointed out in man that when twins develop, 
each gets less nourishment than when only a single embryo is 
present. The smaller size of twins shows that less food is 
available, yet the proportion of male and female twins is the 
same as that for single births. 
Punnett has examined some statistics based on the census 
of London for 1901. He finds more females born in the poor- 
est classes, and more males in the highest classes. Intermediate 
classes give intermediate results, 7.e. more nearly an equal pro- 
portion. Even after taking into account the differential birth 
rate, the results are still the same; for this and other factors 
diminish the proportion of males in the poorer classes and make 
even more probable the conclusion that more males are born 
under favorable conditions. Punnett draws the conclusion 
that the determination of sex is independent of parental nutri- 
tion. 
If nutrition were in reality a factor of any importance in sex 
determination, it is surprising to find so little difference under 
apparently very favorable and unfavorable conditions. It seems 
much more probable that if the nutrition affects in any way the 
proportion of the sexes, it does so indirectly by elimination, and 
not by determining either the sex of the embryo or of the egg. 
This conclusion is borne out by the results of some experi- 
ments on other mammals recently carried out by Cuénot and 
by Schultze. 
Cuénot reared two sets of albino rats: one'set was well fed 
with a variety of food; the other set was poorly nourished, being 
fed mainly on bread. The well-nourished individuals produced 
2C 
