SEX BIOLOGY AND SEX PSYCHOLOGY 137 



find display of some sort, either by the male alone 

 or by both sexes, present in the great majority of 

 species. It is at least partly in correlation with this 

 that beauty of voice and brilliant appearance is far 

 commoner in birds than in mammals 



The monkeys represent in some way a transitional 

 stage towards that seen in man, in whom the condi- 

 tions have come to resemble those found in birds, 

 with consequent great development of epigamic char- 

 acters and actions of one sort and another, both 

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

 vading 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 suf- 

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

 plants or lower animals. In almost all higher ani- 

 mals that have been investigated, however, there has 

 been found some hereditary mechanism for ensuring 

 a rough constancy of sex-ratio. This mechanism re- 

 sides 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 mam- 

 mals and man this sex is the male. Man possesses 

 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. Therefore, when the former kind 

 of sperms fertilize an ovum, two sex-chromosomes 



