WHAT DETERMINES SEX? 99 



are always of the same sex. But ordinary 

 twins, which arise from two distinct ova 

 developing simultaneously, are often of differ- 

 ent sexes. Why this difference in the two 

 cases ? 



The eggs which the queen-bee lays without 

 fertilising them from her store of spermatozoa 

 (received from a drone in her nuptial flight) 

 develop into drones, but the likewise un- 

 fertilised eggs produced during the summer 

 months by green-flies or aphides develop 

 into females parthenogenetic viviparous 

 females until the end of the season, when 

 males are produced. Why are the cases so 

 different ? 



In one household the family consists of 

 boys and girls; in a second of boys only; in 

 a third of girls only. Why is this ? 



DIFFERENT LINES OF INQUIRY. Since 

 Darwin's day the problem has been attacked 

 scientifically, along three distinct lines 



(a) Some conclusions as to the determination 

 of the sex of the offspring have been based 

 on statistics, for instance, as to the relative 

 numbers of male and female offspring pro- 

 duced in different localities, at different 

 times, with different ages of parents, and so 

 on. It is said, for instance, that late marriages 

 tend to increase the proportion of boys, that 

 after prolonged wars when men are scarce 

 more boys are produced, that the younger the 

 mothers the more girls there are born. 



(b) Some conclusions as to the determina- 

 tion of the sex of the offspring have been 

 based on the minute study of the germ-cells 

 in particular cases. Thus it has been shown 



