96 SECTIONAL ADDRESSES 



following observations : ' In no case, as far as we can see, would an 

 inherited tendency to produce both sexes in equal numbers or to produce 

 one sex in excess, be a direct advantage or disadvantage to certain indi- 

 viduals more than to others . . . and therefore a tendency of this kind 

 could not be gained through natural selection. Nevertheless there are 

 certain animals in which two or more males appear to be necessary for 

 the fertilisation of the female : and the males accordingly largely pre- 

 dominate, but it is by no means obvious how this male-producing tendency 

 could have been acquired. I formerly thought that when a tendency to 

 produce the two sexes in equal numbers was an advantage to the species 

 it would follow from natural selection, but I now see that the whole 

 problem is so intricate that it is safer to leave its solution for the future.' 

 I myself belong to a generation whose thoughts and actions have been 

 largely moulded by Darwin's opinions and attitudes. It is but natural, 

 therefore, impressed as I am by his greatness, that I should hesitate to 

 assume that I might contribute towards the solution of a problem that 

 Darwin himself set aside. But recent developments in cyto-genetics 

 have removed many of the difficulties that surrounded this subject in 

 Darwin's time and it is, I think, now possible to re-examine the problem 

 more hopefully. I am encouraged in this view by the writings of Fisher 

 (1930) who in presenting his concept of reproductive value logically 

 maintains that the sex ratio adjusts itself under the influence of natural 

 selection in such a way that the total parental expenditure incurred in 

 respect of each sex is equal. Accepting this argument, I propose to 

 attempt to uncover the mechanisms that may be concerned in such 

 adjustment. 



From the pages of the Registrar-General's Report for igJS I have torn 

 all the information that deals with the sex ratio. The abundant tables so 

 completely detailed show that in that year in England and Wales for every 

 100 girl babies born alive there were no fewer than 105-6 boys. Expressed 

 diiTerently, the secondary sex ratio (the sex ratio that obtains amongst 

 newly born infants) was 105 -6 : 100. 



During the same year the sex ratio for the babies who died during the 

 yth-gth months of intra-uterine life was no: 100, being significantly 

 higher than the secondary sex ratio. 



The population, being classified by the Registrar- General into 5-year 

 age groups, and the sex ratio of each of these groups computed, it is 

 revealed in the most striking fashion that the sex ratio becomes greatly 

 altered as we pass from the younger to the older age groups. This swing 

 is indeed remarkable, for a numerical preponderance of males amongst 

 the earlier age groups gives place to a numerical equality of the sexes 

 among the 15-19-year olds, whilst amongst the 20-24-year olds the 

 females actually begin to outnumber the males ; and thereafter, as age 

 group succeeds age group, this female numerical ascendancy progressively 

 increases until, amongst the 85 's and over, there are more than twice as 

 many women as men. 



This swing in the sex ratio from high to very low, is shown to be the 

 result of a sexually selective mortality, for the mortality tables make it 

 clear that at all ages relatively more males are removed from the popula- 



