SEX BIOLOGY AND PSYCHOLOGY 139 



directorate of the body — the interlocking system of 

 endocrine glands, each of which is exerting an eflFect 

 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 eflFect in 

 senile mammals by increasing, by various methods, the 

 amount of secreting reproductive organ in the body.^ 

 What then has our rapid survey led us to ? 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 

 female — their power of producing different sorts of 

 reproductive cells — leads on to secondary differences. 

 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 of labour usually 

 concerning the protection of the embryo or the 

 care of the young, or more rarely the preservation 

 of the individual itself. Or, finally, they may con- 

 cern the more efficient union of the gametes ; such 

 differences may merely affect the ducts and apertures 

 of the reproductive system, and be more or less 

 mechanical j or they may concern the use of these 



1 See Steinach, '20 j summary in Lipschiitz, '19 j 

 VoronofF, '23. 



