In the birds, on the other hand, although here too the internal secretion of the gonad delimits a period in which alone congress of the sexes can occur, it does not act for such a sharply-limited time as in the mammal, nor is it so intense as completely to override other components of the mind. As a result, general emotional stimulus may play an important part in inducing readiness to pair, and we accordingly 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 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 invading 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 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 are present in the fertilized egg and a female results; when the latter, only one, and the offspring is male.[31] Putting the matter in the broadest terms, we can say that there is a different balance of hereditary factors in male and female, and that this difference of balance dates from the moment of fertilization, and normally determines sex. Various agencies may alter the balance. The chromosomes themselves may vary in what we must vaguely call their potency; or external agencies may affect it. As a result, we sometimes obtain strange abnormal individuals, in which the balance has been upset; in them development results sometimes in organisms permanently intermediate between male and female, sometimes in a change of sex at some period of development.