i;6 HEREDITY AND SEX 



was natural to refer to ' inherent properties of maleness 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 recognised that the 

 problem is one for scientific analysis " (Geddes and Thomson, 

 Evolution of Sex, 1889, revised edition 1901, p. 35). 



Even after the problem of the determination of sex was 

 recognised as one that must be tackled scientifically, or not at 

 all, the suggestions offered have varied greatly in their con- 

 sistency of adherence to scientific method. There are still 

 frequent appeals to " natural tendencies," and these must be 

 judged, not by their self-explanatory character (for biological 

 formulae will never be that), but by their correspondence with 

 the limits of available physiological analysis, and by their 

 applicability in the actual control of life. 



There is a library of books and pamphlets dealing with the 

 determination of sex, but a large number redolent as they are 

 of good intentions must be set aside at once because of obviously 

 fatal defects in their scientific procedure. Some lay stress on 

 what even the most tolerant must admit to be at least unverifiable 

 factors, such as the desire of the parents or parent to have a 

 male child. Others allege the operation of factors which are 

 physiologically absurd. Others base a generalisation on an 

 outrageously small number of cases. The reason for the un- 

 usual copiousness of speculation in regard to this difficult biological 

 question is to be found rather in its practical than in its theoretical 

 interest. 



The Problem Stated. The general problem is : What deter- 

 mines whether a fertilised egg-cell will develop into a male or a 

 female organism ? But let us look at particular forms of the 

 problem. It is generally admitted that what are called " true 

 twins " in the human race arise from the division of a single ovum 

 into two independently developed ova, and they are said to be 

 always of the same sex, identical in this as in their other features. 



