SEX BIOLOGY AND PSYCHOLOGY 137 



conditions have come to resemble those found in 

 birds, with consequent great development of epigamic 

 characters and actions of one sort and another, both 

 physical and mental. Thus we see that sex, after invad- 

 ing and altering the conformation of the body, finally 

 invades and alters the conformation of the mind. 



As regards the other great biological question, 

 of the determination of sex, a very few words will 

 suffice. In the first place I have no time to consider 

 plants or lower animals. In almost all higher animals 

 that have been investigated, however, there has been 

 found some hereditary mechanism for ensuring a 

 rough constancy of sex-ratio. This mechanism 

 resides in the so-called chromosomes of the nucleus. 

 These exist for the most part in similar pairs in both 

 sexes : but one pair is dissimilar in one sex. In 

 mammals and man this sex is the male. Man pos- 

 sesses one chromosome less than woman. He 

 possesses only one member of this pair of special 

 sex-chromosomes, whereas she possesses two. All 

 her ova are alike in possessing one, whereas half 

 his sperms possess one, half possess none. There- 

 fore, when the former kind of sperms fertilize an 

 ovum, two sex-chromosomes are present in the 

 fertilized egg and a female results ; when the latter, 

 only one, and the offspring is male.^ 



Putting the matter in the broadest terms, we can 

 say that there is a different balance of hereditary 



^ See Goldschmidt, '23 ; Morgan, '195 Doncaster, '14. 



