DETERMINATION OF SEX DONCASTEPw 475 



cumstances may affect the percentage of the sexes among the off- 

 spring born ; among these we may choose a few of the more recent as 

 examples of the kind of result obtained. Pearl^ shows that of over 

 200,000 births in Buenos Aires, the proportion of males is signifi- 

 cantly greater when the parents are of different racial stocks than 

 when they are of the same. The difference ranges from about 1 per 

 cent to about 5 per cent, but is always on the same side. Punnett" 

 finds that in London the proj^ortion of males is lowest in the poorest 

 portion, highest in the wealthiest, and intermediate in the interme- 

 diate portion. The males per 100 females were, respectively, 99.5, 

 102.2, and 100.7; but he points out that these differences are probably 

 Avholly explicable on the grounds of differential infant mortality, 

 birth rate, and probably marriage rate. Heape,^ from statistics of 

 over 17,000 greyhounds, concludes that whilst males are always con- 

 siderably in excess (averaging 118.5 to 100 females), the proportion 

 is noticeably higher in the season during which fewest pups are born. 

 In a later note in the same volume (loc. cit., p. 201) Heape gives some 

 curious figures with regard to canaries, showing that in one aviary 

 (out of 200 birds hatched) the ratio of males was about 77 to 100 

 females, while in another (out of 68 birds) the males were in the ratio 

 of 353 to 100 females. Evidence is given that these differences are not 

 ascribable to mortality; Heape supposes that in both cases the propor- 

 tion of the sexes is due to a selective action of conditions on the ova 

 which are matured. He assumes that ova bear either maleness or fe- 

 maleness, and that some forms of environment favor the maturation 

 of one kind, some of the other. The same explanation is applicable to 

 other cases in which the proportion appears to be influenced by exter- 

 nal circumstances. 



It appears therefore that the idea that the proportion of the sexes 

 may be influenced by conditions acting on the parents is not incon- 

 sistent with the hypothesis that the germ cells bear only one or the 

 other sex, as long as the primary germ cells do not all come to ma- 

 turity. This is the case at least in the females of the higher verte- 

 brates ; and it is from them that the greatest amount of evidence in 

 this direction has been obtained. Russo * has recently maintained 

 that treatment with lecithin causes an increase in the number of fe- 

 male ova matured in the rabbit and believes that he can distinguish 

 the male from the female eggs in the ovary. He admits, however, that 

 the families in his tables are selected and it seems that the differences 



1 Biol. Bulletin, vol. 15, 1908, p. 194. 



sProc. Camb. Phil. Soc, vol. 12, 1903, p. 262. 



3 Ibid., vol. 14, 1907, p. 121. [See also Heape, ' The Proportion of the Sexes produced 

 b.v Whites and Colored People in Cuba.' Phil. Trans. Roy. Soc. B. 269, p. 271-330 

 ("vol. 200, 1909).] 



* Atti Acad. Lincei, vol. 16, 1907, p. 362. 



