INHERITANCE OF SEX 251 



m the two characters in the heterozygote, to account 

 for the approximate equality in the numbers of the 

 sexes ; and, secondly, the sterility of unions between 

 gametes bearing like characters. 



Castle's form of the theory has now been super- 

 seded by a much simpler one which is due to Mr. 

 Geoffrey Smith. It was briefly referred to in the 

 remarks on p. 181, relating to the possible kinds of 

 union between any two of the zygotic forms, DD, 

 DR and DD, where it was seen that there are only 

 two kinds of union in which the parental types are 

 reproduced in equal numbers, namely, DR x RR, and 

 DR X DD. Of these two the former is the only one 

 in which the two parental types differ from one 

 another in all cases ; that is to say, RR is always 

 different from DR in outward appearance, whilst 

 DD is indistinguishable from DR except in those 

 cases, like the Aadalusian fowl, in which the DR 

 zygote has a character peculiar to itself. Inasmuch, 

 therefore, as the male differs by external characteristics 

 from the female, and inasmuch as the result of their 

 union is usually the production of males and females 

 in approximately equal numbers, it is natural that 

 the suggestion that one sex was of constitution DR 

 and the other of constitution RR should be made. 



The theory is this : Femaleness and maleness 

 constitute the two characters of a Mendelian pair. 

 Femaleness is dominant ; that is to say, femaleness 

 is due to the presence of a factor the mere absence 

 of which constitutes maleness. But femaleness never 

 occurs in the homozygous condition ?? (DD)j but 



