478 HEREDITY AND SEX 



ness and femaleness ' ; and it is still a popular ' explanation ' 

 to invoke undefined * natural tendencies ' to account for the 

 production of males or females. Thirdly, it has been re- 

 cognised that the problem is one for scientific analysis " 

 (Geddes and Thomson, Evolution of Sex, 1889, revised edition 

 1901, p. 35). 



But even after the problem was recognised as one which 

 could be tackled scientifically, non-scientific solutions have been 

 attempted in scores of pamphlets and booklets — non-scientific 

 in the sense that they lay stress on unverifiable factors, such as 

 the desire of the parent ; that they base a generalisation on a 

 few cases, as in Schenk's theory ; that they universalise particular 

 instances ; that they allege the operation of factors which are 

 physiologically absurd, in a manner which can only be called 

 superstitious ; and that they confuse statistical generalisations 

 with physiological interpretations. 



Classification of Scientific Answers. — 



A. Statistical. Some generalisations as to the sex of the 

 offspring have been based on statistics, which show how the 

 relative numbers of male and female offspring vary in different 

 localities, at different times {e.g. in times of war or famine), 

 according to the relative ages of the parents, and so on. These 

 statistics are very valuable, and the more of them the better. 

 But, as we have already seen, great care is necessary in giving 

 a physiological interpretation of statistical results. 



B. Observational. Some generalisations as to the sex of the 

 offspring have been based on the observation of particular cases. 

 Thus, it has been shown that ova fertilised by spermatozoa 

 with " an accessory chromosome " (in the case of some insects) 

 develop into females ; that the unfertilised ova of the bee always 

 develop into drones ; that the parthenogenetic ova of aphides 

 always develop into females as long as summer conditions 

 persist ; that some animals have two sizes of eggs, the larger 

 developing into females ; and so on. 



