90 VERTEBRATE REPRODUCTIVE CYCLES 



have also produced striking changes in sexual activity. 

 Female behaviour has been induced out of season and 

 after the removal of the ovaries in all the lower verte- 

 brates which have been examined, and the effect is 

 particularly clearly defined in mammals.^i Oestrogens 

 have been used to bring farm animals on to heat, and to 

 increase their willingness to receive the male. In this way 

 cattle which had never bred successfully before were 

 induced to become pregnant. 103 Treatment with 

 oestrogens induces the activity and excitement of oestrus 

 in immature mammals and also in mammals, such as 

 ferrets, from which the ovaries have been removed. ^^^ 

 In the rhesus monkey the results are similar^o and appar- 

 ently a reaction can even be obtained in the chimp- 

 anzee.^* However, the human subject again gives no 

 clear response, and any effect that may be induced is 

 obscured by the psychological reaction of the patient to 

 suggestion. 



It is important to add that the female hormone is 

 without any definite effect on the behaviour of the 

 normal adult male, even when it is administered in very 

 large quantities. It has, however, been reported that such 

 large quantities have a depressing effect on male behav- 

 iour, and for this reason oestrogens have been used for 

 suppressing excessive sex function in men. 



The hormone progesterone from the corpus luteum is 

 also said to exert a depressing effect on sexual activity, 

 so that in female rabbits and monkeys it may reduce the 

 willingness to mate. 



The general conclusion emerging from these data 

 must be that the state of the nervous system, and thus 

 the response which it provides to certain situations, is 

 powerfully aft'ected by the gonad hormones in the blood 

 stream. With an excess of male hormone the behaviour 

 is typically male, while with an excess of female hormone 

 the behaviour is typically female. Thus it is evident that 

 sexual activity is hormonally and not genetically deter- 

 mined. 



