THE DETERMINATION OF SEX 103 



conjugate with eggs of one kind. But it must be admitted that 

 any suggestion of selective fertilisation interferes with the 

 extreme simplicity of the theory outlined above. 1 



The hypothesis here described not only explains the cases 

 which led up to it and such facts as the effects of castration 

 but also accounts for the phenomena of sexual dimorphism and 

 the inheritance of some structures by one sex only. But at 

 present the more complex cases of sexual polymorphism, such, 

 for example, as are known in the African butterflies of the 

 genus Papilio, still remain obscure, although it is probable that 

 when we have more extensive records of breeding experiments 

 these also will be found to fall into line. And it should be 

 explained that some forms of sex-limited inheritance are of 

 quite a different nature — e.g. colour-blindness and the disease 

 haemophilia in man. In these diseases the abnormal condition 

 is dominant in one sex (male) and recessive in the other and 

 may appear in the female if both parents are tainted. Possibly 

 a combination of some condition of this kind with sex-relations 

 such as we find in A. grossulariata and the Cinnamon Canary 

 may ultimately be found to account for the complex sexual 

 polymorphism found in the African Papilios. 



We have now sketched the principal lines of evidence which 

 have been collected in recent years pointing to the conclusion 

 that the sex-determinant is present in the germ-cell and is 

 probably comparable in nature with a Mendelian unit. In a 

 paper of this kind it is clearly out of place to attempt even to 

 mention a tithe of the numerous hypotheses concerning sex 

 which have been advanced even in very modern times. Many 

 of them do not concern the point at issue, dealing as they do 

 with possible factors which may influence the sex of a given 

 individual ; for we have seen that, whatever the true nature of 



1 Wilson has recently {Science, xxix. Jan. 1909, p. 53) put forward a fresh sugges- 

 tion, viz. that the idiochromosomes do not bear the determinants for maleness or 

 femaleness as such but that one idiochromosome in the fertilised egg causes it to 

 develop into a male, two into a female, so that the difference is rather quantitative 

 than qualitative. Castle (Science xxix. March 1909, p. 395) has taken up this idea, 

 with the further suggestion that while some species are as Wilson suggests, in 

 others the presence of one idiochromosome determines femaleness and absence 

 of any idiochromosome at all brings about maleness. If this last condition should 

 be found to exist in Abraxas grossulariata, it would then fall into agreement. In 

 this connection it is of interest that cases such as Wilson describes have been 

 observed in most of the chief orders of insects but not in the Lepidoptera. 



