500 HEREDITY AND SEX 



equal distribution of the sexes. Or how could it apply, for in- 

 stance, to such a clutch of eggs as Shufeldt reports in the case 

 of a sparrow-hawk ? The first became a male, the second a 

 female, the third a male, the fourth a female, and the fifth 

 a male, in regular alternation. Yet these were produced in a 

 short time from one ovary, and were probably fertilised by the 

 same set of spermatozoa. 



On the other hand, there are cases where a mother produces 

 a long succession of offspring all of one sex, or produces one son 

 and a long succession of daughters, and so on. Such cases 

 suggest that the constitution of the parent may be of some 

 importance, and we know that the constitution is modifiable 

 by nutrition and the other factors in nurture. 



When we pass from general considerations, such as the above, 

 and appeal to the facts, we find an interesting conflict of evidence. 



From human statistics some have tried to prove that 

 abundant food favours the production of female offspring, and 

 vice versa ; but others have concluded, also from statistics, that 

 the parental nutrition is of no moment, unless in bringing about 

 a differential death-rate. The fact that 30 per cent, of human 

 twins are of different sexes seems enough to show that the dieting 

 of the parent is not of great importance. Schenk's notorious 

 theory (1898), that the sex of children could be adjusted by dieting 

 the mothers, rested on entirely insufficient evidence a very 

 small number of cases. Moreover, he supposed that the sex 

 was determined after conception. 



In a statistical inquiry in London Mr. Punnett found that the 

 proportion of male to female infants is lowest in the poorest 

 quarter and highest in the wealthiest, yet the differences are 

 not great, and he concluded that they are due to differential 

 infantile mortality, birth-rate, and probably marriage-rate. He 

 was inclined to believe that " in man, at any rate, the determina- 

 tion of sex is independent of parental nutrition. In any case its 

 influence can be but small." 



