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 Pfluger 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 



