External Factors of Sex Determination 385 



are 114 male births to 100 female births. The statistics for the 

 nobihty of Sweden show 98 male births to 100 females, while for 

 the clergy of the same country 108.6 male to 100 female births. 



Dusing, 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, i.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 Cuenot and 

 by Schultze. 



Cuenot 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 



