SEX 191 



The differentiation of the individuals of a species into 

 males and females, that is, into individuals producing only 

 sperms or only ova, is, as has already been said, a com- 

 paratively late product of evolution, and one evident result 

 of this must be that inbreeding is rendered less close than it 

 is in those races where every individual can play the part 

 either of fertilising or being fertilised. Though the gametes 

 produced by the two sexes vary much in size, structure, and 

 appearance, it is a mistake to consider them, or speak of 

 them, as male or female gametes, in the sense that one 

 represents maleness and the other femaleness. As far as 

 sex is concerned, there is no suggestion, in the great 

 majority of cases, that a sperm represents maleness or an 

 ovum femaleness. Indeed, in some cases the ovum may 

 be destined to produce a male individual quite apart from 

 any subsequent fertilisation by a sperm. Thus the un- 

 fertilised ova of a queen-bee produce males only, while 

 the fertilised ova produce females, which may be either 

 functional females or neuters according to the way in which 

 the larvse are fed. We must generally regard both sperms 

 and ova as being potentially of both sexes, that is to say, 

 under certain conditions either may produce males or 

 females. 



Passing from those organisms in which the individual 

 is capable of performing both the paternal and maternal 

 functions in reproduction, we find that there is evidence that 

 sex, where it exists, sometimes remains undetermined much 

 longer in the lower than in the higher organisms. In the 

 hag-fish 1 (Myxine glutinosa), the sex of the individual 

 appears to change. The fish is apparently at first a male 

 and produces sperms, but later in life it produces ova and is 

 functionally a female. In mammals, on the other hand, it 

 is possible to distinguish between male and female at a very 

 early stage in development. 



Some direct experiments suggest that, in the case of 



1 Cunningham, J. T., Sexual Dimorphism in the Animal Kingdom, London, 

 1900. 



