424 Evolution and Adaptation 
earlier experiments in this direction are those of Born and of 
Yung. By feeding one set of tadpoles with beef, Yung found 
the percentage of females that developed to be greatly in- 
creased, and a similar increase was observed when the tad- 
poles were fed on the flesh of fish. An even greater effect 
was produced by using the flesh of frogs, the percentage 
rising to 92 females in every hundred. These results have 
been given a different interpretation by Pfliiger and by 
others, and, as will be pointed out later, there is a possible 
source of error that may invalidate them. 
Somewhat similar results have been obtained by Nussbaum 
for one of the rotifers. He found that if the rotifer is abun- 
dantly fed in early life, it produces female eggs, that is, larger 
eggs that become females; while if sparingly fed, it produces 
only small eggs, from which males develop. It has been 
claimed also in mammals, and even in man, that sex is to 
some extent determined by the nourishment of the individual. 
Some experiments made by Mrs. Treat with caterpillars 
seemed to show that if the caterpillars were well nourished 
more female moths were produced, and if starved before 
pupation more males emerged. But Riley has pointed out 
that since the larger female caterpillars require more food 
they will starve sooner than the males, and, in consequence, 
it may appear that proportionately more male butterflies are 
born when the caterpillars are subjected to a starvation diet. 
This point of view is important in putting us on our guard 
against hastily supposing that food may directly determine 
sex. Unless the entire number of individuals present at the 
beginning of the experiment is taken into account, the results 
may be misleading, because the conditions may be more fatal 
to one sex than to the other. 
In some of the hymenopterous insects, the bees for example, 
it has been discovered that the sex of the embryo is deter- 
mined by the entrance, or lack of entrance, of the spermato- 
zoon. In the honey-bee all the fertilized eggs produce 
