PROVOCATIVE BIOLOGICAL THEORIES 83 



"metabolic theory of sex." In this theory, sex is deter- 

 mined by the rate or level at which metabolism takes 

 place in the embryo. The theory takes no account of 

 metabolism as such, but simply of the rate of metabolism. 

 One may probably best suggest the causes which brought 

 this theory into existence under the following four heads : 

 (1) Dr. Riddle, of Cold Spring Harbor, performed a 

 number of experiments with pigeon eggs some years ago 

 which well exemplified this metabolic theory. The pigeon 

 lays two eggs, the first one laid always developing into a 

 male, while the second develops into a female. Riddle 

 carefully analyzed the eggs and found that the male- 

 producing egg not only contained less stored food than 

 the female-producing egg, but that it also contained more 

 water and, as a consequence, such eggs had a higher rate 

 of metabolism and a higher oxydizing capacity than the 

 female-producing egg. (2) Dr. Whitney performed exper- 

 iments with a rotifer, Hydratina senta, which animal lays 

 two kinds of eggs, large ones, which develop partheno- 

 genetically into females, and small eggs, which develop 

 parthenogenetically into males. It was observed that 

 when this rotifer was fed green flagellates, it laid small 

 eggs, whereas when it was fed colorless flagellates it laid 

 large eggs. (3) Dr. R. Hertwig had kept eggs of frogs for 

 quite a time before fertilizing them. They thus became 

 overfilled with water before fertilization. A predominance 

 of males resulted. Miss Helen King, of the Wistar Insti- 

 tute, performed a somewhat similar experiment on the 



