26 ORIGIN OF SEX. 
a lessened or increased vigor may determine this increased ten- 
dency toward the production of females ; in fact, anything which 
operates upon the animal economy by distracting it from the busi- 
ness of reproduction, such as rapid and vigorous growth, develop- 
mental processes, mental anxiety, or incipient disease, will cause 
a lessened fecundity and an increase in the proportion of female 
offspring. | 
Mrs. Mary Treat* of Vineland, N. J., has, by repeated experi- » 
ments on butterflies, found that by overfeeding a certain number, 
a large proportion of female eggs was produced, and that by 
underfeeding, or partly starving them, the poporo of male 
eggs was increased. : 
~ The writer has shown in his paper on “The Nationality of — 
Parents as Affecting the Fecundity and the Proportion of Sexes — 
in Births” + that foreign mothers (who are unquestionably more — 
“vigorous” than native mothers) have a much larger proportion 
of boys among their children. 
Mr. H. H. Howorth, in his paper entitled *Strictures on Dare a 
winism,”{ says that he “cannot but conclude that sterility is — 
induced by vigorous health and by a plentiful supply of the neces- 
saries of life, while fertility is induced by want and debility, and 
that this law acts directly against Mr. Darwin’s theory, in that it 
is constantly recruiting the weak and the decrepit at the expense . 
of the hearty and vigorous, and is constantly working against the 4 
favorite scheme of Mr. Darwin, that in the struggle for existence 
the weak are always being eliminated by the strong.” 3 
It will be seen by the above that the views of Mr. Darwin and — 
_ Mr. Howorth are both extreme; the former believing that the 
_ greatest fecundity and best products belong to the most vigorous, — 
while the latter believes that the most feeble are most prolific, and 
have the most vigorous offspring. 
The writer is of the opinion, from a careful study of the wubjett 
that a medium condition between these two extremes is most 
favorable to fecundity and the production of healthy, vigorous 
offspring, namely, developmental maturity of the parents, and . 
moderate supply of food in connection with a life most in accord- 
ance with nature. o‘ 
* AMERICAN a 1873, Laws Controlling the, Sexes in Butterflies. — 
oS 
Philadelphia 
eee ee Anthropological Institute. ‘London, April, 1872, pp. 21-40. P- or 
t 1, Fertility and Sterility, 
