REPRODUCTION AND SEX 525 



other such circumstances. Let us consider various forms of this 

 second theory. 



Hof acker (1821) and Sadler (1830) independently published 

 statistics in support of the theory that when the male parent is 

 the older, there are more males among the offspring, and that when 

 the female parent is the older, there are more females. In short, the 

 sex of the offspring depends on the relative ages of the parents. 

 Statistical evidence has been found both supporting and con- 

 tradicting this theory. Schultze's experiments on mice tell strongly 

 against it. 



It seems fair to notice, that if the germ-cells remain for some time 

 undetermined in regard to the sex which they will express — if, in 

 other words, they retain for some time the potentiality of either — 

 there is no a priori reason against the hypothesis that the absolute 

 and relative ages of the parents may have influence. 



Or again, even if there are two kinds of egg-cells and two kinds 

 of sperm-cells, which are from the first determined towards develop- 

 ing into females or towards developing into males, the age of the 

 parent may favour the production of one kind rather than of the 

 other, or may favour the survival of one kind rather than of 

 the other. 



There seems some evidence in mankind of a correlation between 

 the age of the mother and the sex of the child. The younger mothers 

 tend to have more female children; the older mothers tend to have 

 more male children. On this the self -regulating numerical balance 

 of sex in a nation is said to depend. When females are scarce — for 

 instance, in a colony — they mate early, and supply the demand 

 for girls. When men are scarce — for instance, after war — there are 

 more late marriages, and therefore more boys. 



By many authors, e.g. Girou, and at various dates, the theory 

 has been propounded that the sex of the offspring tends to be that 

 of the "more vigorous" parent. This is a favourite opinion among 

 breeders and the fathers of many boys, but it lacks substantiation, 

 and the concept of comparative vigour is too vague to be useful. 

 So far as parental vigour may depend on strained reproduction, 

 or on deterioration supposed to result from close inbreeding, 

 Schultze's experiments on mice do not in the least confirm the view 

 that it has any effect on the proportions of the sexes. 



Starkweather was responsible for the theory that the sex of the 

 offspring tends to be the opposite of that of the "superior" parent; 

 but "superiority" and "comparative vigour" are far too vague to 

 be scientifically discussible. We have not been able to discover data 

 warranting the conclusion that a prepotent sire gives his offspring 

 a bias either towards his own sex or towards the opposite. 



According to Van Lint, the offspring has the sex of the parent 

 whose sex-cells are the weaker at the time of fertilisation. The sure 



