THE FACTORS WHICH DETERMINE SEX 647 



trolling sex in Man lends no support to it. Moreover, Schultze's 

 experimental investigation i on the sexes produced by mice of 

 different ages has led hkewise to a negative result.^ 



Influence of Parental Vigour or SwperiorUy. — Considerable 

 importance has been attached by breeders and others, and 

 notably by Starkweather,^ to the comparative vigour or con- 

 dition of the parents as a factor in sex determination. Ac- 

 cording to Starkweather, the superior parent tends to produce 

 the opposite sex. This theory has been accepted by Alhson,* 

 who beheves it to be applicable to thoroughbred horses. It 

 is obvious, however, that in attempting to apply Starkweather's 

 hypothesis much depends on the signification to be attached 

 to the term " superiority," and for this, if for no other reason, 

 the theory is unsatisfactory. Furthermore, Schultze ^ has 

 shown that long-continued or strained reproduction in female 

 mice has no effect on the proportion of the sexes produced. 

 The results of experiments on the effects of inbreeding were also 

 indefinite or contradictory. 



Influence of Nourishment. — Of the various external factors 

 which have been supposed to have direct influence in determin- 

 ing sex, nourishment seems to have found more favour than 

 any other. In some cases this factor is supposed to act upon 

 the developing embryo or larva (see p. 624), and so to determine 

 its sex, while in other cases it is concluded that sex is established 

 at an earher period. 



Geddes and Thomson have elaborated the idea that favour- 

 able nutritive conditions tend towards the production of females, 

 and unfavourable ones towards the development of males, and 

 certain of the evidence referred to above (p. 624) has been cited 



Washington, 1904. Newcomb states that the first-born child of any mother 

 is more likely to be a boy in the proportion of about eight to seven. 



^ Sohultze, "Zur Frage von den geschleohts-bildendeu Ursachen," Arch, 

 f. Mikr. Anat., vol. Ixiii., 1903. 



^ This theory, and that which follows, shoald possibly be included among 

 those which assume that sex is settled at fertilisation ; for if sex is determined 

 by the age of the parents, it seems to follow that no event occurring during 

 embryonic life can alter it. This point, however, does not appear to have 

 been raised by the authors of the theory. 



^ Starkweather, The Law of Sex, London, 1883. 



^ Allison, The British Thoroughbred Horse, London, 1901. 



^ Sohultze, loc. cit. 



