684 THE PHYSIOLOGY OF REPRODUCTION 



investigation l on the causes controlling sex in man lends no support 

 to it. Moreover, Schultze's experimental investigation 2 on the sexes 

 produced by mice of different ages has led likewise to a negative 

 result. 3 



Copeman and Parsons, 4 however, found that does aged over six 

 months produced more males than does under that age, but in 

 view of the evidence in favour of male heterozygosis as regards sex 

 among Mammals recorded above, little stress can be laid upon such 

 observations. 



Influence of Parental Vigour or Superiority. Considerable 

 importance has been attached by breeders and others, and notably 

 by Starkweather, 5 to the comparative vigour or condition of the 

 parents as a factor in sex-determination. According to Starkweather, 

 the superior parent tends to produce the opposite sex. This theory 

 has been accepted by Allison, who believes 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 7 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 in-breeding were also indefinite or 

 contradictory. 



Influence of Nourishment. Of the various external factors which 

 have been supposed to have direct influence in determining 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. 662), and so to determine its sex, while in other cases 

 it is concluded that sex is established at an earlier period. 



Geddes and Thomson have elaborated the idea that favourable 

 nutritive conditions tend towards the production of females, and 



1 Newcomb, "A Statistical Inquiry into the Probability of Causes of the 

 Production of Sex in Human Offspring," Carnegie Institution (Washington) 

 Publications, 1904. Newcoinb states that the first-born child of any mother 

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



2 Schultze, "Zur Frage von den geschlechts-bildenden Ursachen," Arch. f. 

 Milcr. A not., vol. Ixiii., 1903. 



3 This theory, and that which follows, should 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. 



4 Copeman and Parsons, " Observations on the Sex of Mice," Proc. Roy. Soc., 

 vol. Ixxiii., 1904. For other observations of a similar kind see Goldsc'hmidt, 

 loc. cit., where full references may be found. 



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



Allison, The British Thoroughbred Horse, London, 1901. 

 7 Schultze, loc. <-it. 



