External Factors of Sex Determination 387 



the sex of the egg or embryo of mammals is not determined by 

 nutrition. 



Geddes's and Thomson's Theory of Sex 



The theory of sex that Geddes and Thomson elaborated in 

 their book on "The Evolution of Sex" is largely based on evi- 

 dence of the sort just examined. We have seen that all of the 

 earlier experiments that seemed to show that the sex of the em- 

 bryo is determined by the amount of food supplied are open 

 to grave suspicion. Hence the strongest evidence brought for- 

 ward by Geddes and Thomson in favor of their theory is no 

 longer valid ; but since the theory is also based on certain sup- 

 posed analogies between the male and female condition in 

 general, we may briefly consider it here. 



Geddes and Thomson think that the greater activity or 

 "vitality" of the male is due to a greater breaking down or 

 katabolism of the tissues ; while the less active female condition 

 is more one of building up, or of anabolism. "Deficient or 

 abnormal food, high temperature, deficient light or moisture, 

 and the like are such as tend to induce a preponderance of 

 waste over repair a relatively katabolic habit of the body 

 and these conditions tend to result in the production of males. 

 Similarly the opposite set of factors, such as abundant and rich 

 nutrition, abundant light and moisture, favor constructive pro- 

 cesses, i.e. make for a relative anabolic habit, and these conditions 

 tend to result in the production of females." It is pointed out 

 that the males themselves are often smaller, more active, have 

 a higher temperature and a shorter life ; while the females are 

 larger, more passive, vegetative, and conservative forms. The 

 authors handle the entire problem of sex under the quasi-sym- 

 bolic terms of anabolism and katabolism without stating definitely 

 how such a condition determines sex. In fact, so vague and gen- 

 eral are most of the statements regarding sex determination that 

 their interpretation belongs rather to that class of hypothesis, 

 so common in much of our biological speculation, in which the 

 issue is obscured by the appeal to phenomena as uncertain 





