SEXUAL BEHAVIOUR 95 



male which is dominant and male behaviour only is seen. 



Because of the weight of this evidence Stone^^^ has 

 concluded that the 'hormones of the gonads appear to 

 be a sine qua non for the organization of sexual behaviour'. 

 However, there is still very little evidence as to what 

 parts of the central or sympathetic nervous systems are 

 affected or as to how they become eroticized to produce 

 this result. 



Also unanswered is the difficult question, which looms 

 so large in the higher mammals, of the part played in sex- 

 ual activity by non-hormonal factors. As Beach^i con- 

 cludes, the 'evidence already available shows that as we 

 consider first the lower mammals, then those of inter- 

 mediate phyletic status, and, finally, the higher primates 

 including man, we find that the relative importance of 

 hormonal effects to normal sexuality grows progressively 

 less apparent, while the magnitude of extra-physiologic 

 influence becomes increasingly evident. The description 

 and interpretation of this evolutionary shift in emphasis 

 is a basic task that will provide the only solid founda- 

 tions upon which our understanding of human sex life 

 can rest'. 



Finally, it may also be mentioned that, while attention 

 has been concentrated on the effects of the gonad hor- 

 mones, there is reason to believe that other hormones 

 may also be found to exert an influence on breeding 

 activities. In particular it appears that maternal be- 

 haviour may be induced by an anterior pituitary 

 hormone, distinguished by the name of prolactin. It 

 is this substance which is considered to be responsible 

 for inducing the secretion of 'milk' from the wall of the 

 pigeon's crop, and it has been described as causing the 

 appearance of parental behaviour in several fishes, birds 

 and mammals. A full review of these results, which do 

 not yet seem to be conclusive, is given by Beach.^i 



