SEX BIOLOGY AND PSYCHOLOGY 159 



obvious rule — is that acquisitions are not thrown 

 away when change occurs, but built upon, utilized 

 for some new function. The endostyle of the lowest 

 chordates, part of a very primitive type of feeding 

 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 char- 

 acters 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 presence 

 often undetected. It contributes to some of the 

 most exalted states of mind which we can experience. 

 The sexual relationship, which in lower animals 

 involves neither contact or 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 



