SEX BIOLOGY AND SEX PSYCHOLOGY 139 



characters; whereas it exerts a profound effect upon 

 mammals or birds. 



As a second result, we find that in vertebrates the 

 gonads form part of what has been called the chem- 

 ical directorate of the body — the interlocking system 

 of endocrine glands, each of which is exerting an 

 effect upon the rest. The importance of this is seen 

 in the experiments of Steinach, Sand, Voronoff, and 

 others, who have been able to obtain a rejuvenating 

 effect in senile mammals by increasing, by various 

 methods, the amount of secreting reproductive organ 

 in the body.^ 



To what then has our rapid survey led us? The 

 actual origin of sex is lost to us in the mists of a 

 time inconceivably remote. Its preservation once in 

 existence, and its present all-but-universal distribu- 

 tion seem to be definitely associated with the bio- 

 logical advantage of the plasticity which it confers. 

 Later, the primary difference between male and fe- 

 male — their power of producing different sorts of re- 

 productive cells — leads on to secondary diflferences. 

 These differences may be biologically speaking non- 

 significant, mere accidents of the primary difference. 

 Or they may be in the nature of a division of labour 

 between the sexes, this division ol labour usually con- 

 cerning the protection of the embryo or the care of 

 the young, or more rarely the preservation of the in- 

 dividual itself. Or, finally, they may concern the 

 more efficient union of the gametes; such differences 



6 See Steinach, '20; summary in Lipschutz, '19; Voronoff, '23. 



