THE CYCLE OF SEX 141 



a considerable disturbance of the internal 

 secretions of the body. 



It is clear that outliving the capacity of 

 independent sustenance is not feasible for 

 ordinary animals in a state of nature, and 

 it is not tolerated. It is also clear that each 

 kind of organism will reproduce best when 

 it is at its prime, and that reproduction by 

 organisms in their declining years would be 

 prejudicial to the welfare of the species. 

 Therefore we do not find wild animals long 

 outliving either their full vigour or their 

 effective reproductivity. In Man, however, 

 there has been an age-long successful attempt 

 to throw off the yoke of natural selection, 

 and the duration of life has probably been 

 lengthened out far beyond the primitive 

 span. For although it is a remarkable fact 

 that the average length of human life all 

 over the world is much the same in different 

 countries and conditions, we need not suppose 

 that this has always been the average. It is 

 more likely that the waning of reproductivity 

 about the fiftieth year indicates the end of 

 an older span. 



There seems to be no hard and fast rule 

 as to the decline of sex in males, but, as has 

 been indicated, it often wanes about the fiftieth 

 year. Veterans of seventy and eighty have 

 been known to retain all the vigour of 

 youth, but these are very exceptional cases. 

 Similarly, as regards women, although the 

 production and liberation of ova may be 

 exceptionally continued and the menopause 

 postponed for many years, the usual punctua- 

 tion is between the forty-fifth and fiftieth 



