SEX BIOLOGY AND SEX PSYCHOLOGY 159 



mechanism, was converted, when they changed their 

 mode of life, into the thyroid gland: the parathyroids 

 develop from the remains of the gill-apparatus when 

 gills are discarded for lungs: the secondary sexual 

 differences which originate as accidental consequence 

 of the primary difference between the sexes are, over 

 and over again, elaborated into special characters 

 employed in courtship. 



So the sex-instinct and its associated emotion, at 

 first simply one among a number of separate and 

 scarcely-correlated instincts, has in man become the 

 basis for numerous new mental functions. It can 

 enter into the composition of various emotions, 

 though its character is often disguised and its pres- 

 ence often undetected. It contributes to some of the 

 most exalted states of mind which we can experience. 

 The sexual relationship, which in lower animals in- 

 volves neither contact nor even propinquity, but 

 simply simultaneous discharge of reproductive cells, 

 and in most animals is a purely temporary affair, 

 is very different in man. Even in those birds and 

 non-human mammals in which the sexes remain 

 associated for long periods or permanently, the 

 different departments of life are more in water-tight 

 compartments, the psychical activity is subordinate 

 to the physiological: in man the physiological side, 

 though of course still basic and necessary, is more — 

 and can be much more — subordinate to the psycho- 

 logical, and all parts of the mental life interpenetrate 

 to a much greater extent; so that the sex-instinct 



