io8 SECTIONAL ADDRESSES 



As a general rule, the males of all groups, except those derived from 

 wide crosses in which genie balance is more likely to be unusual, appear 

 to show a relatively low viability, irrespective of their homo- or hetero- 

 gamety, and the difference in the death rates of the sexes appears to be 

 as much correlated with sexuality itself as with any particular type of 

 chromosome or gene equipment. That this should be so is perhaps not 

 surprising when the relative reproductive values of the sexes are com- 

 pared. The major task of one generation of gamete producers is the 

 production, economically and efficiently, of a succeeding generation that 

 numerically will be in harmony with the conditions and resources of the 

 habitat. That of the females of a generation is the production of the 

 requisite number of ova ; that of the males the production of sperma- 

 tozoa in numbers sufficient to make the fertilisation of every available egg 

 highly probable. The number of spermatozoa required will be deter- 

 mined by many factors — e.g. the variety of the fertilisation process, the 

 pre-natal and post-natal relations of mother and offspring, the relation- 

 ship of male to female in respect of parentage. If the male is merely a 

 fertilising agent, then economy and efficiency are observed if the male dies 

 in coitu or is for other reasons shorter-lived than the female so long as 

 there are more or less equal numbers of males and females of the age when 

 fertilisation occurs. If the male is concerned with the protection of and 

 food-finding for the female and her young then the length of life of the 

 male might be expected to be related to the duration of the period of 

 dependency of the young upon the parents and of the female upon the 

 male, and it might reasonably be assumed that when the male has served 

 his purpose he would be removed so that savings in food energy could be 

 effected and used for further reproduction. 



In the light of the facts and observations that have been presented, 

 it is desirable to examine Table 31, Volume 2, of the Report on the 

 Census of Scotland, 193 1. Here it is seen that in this human population, 

 though between the ages of 0-14 there were more males than females, 

 and between the ages of 20-100 there were more females than males, 

 the sex ratio of those in the population between the ages of 15 and 19 

 was equality. So, also, in the Regisrrar-General's Annual Review for 

 1935 will it be found that among those aged 15-19 is the sex ratio most 

 nearly equality. It can be stated, therefore, that in the case of these 

 human populations the sex ratio is equality only amongst those who 

 stand at the threshold of their reproductive prime. This fact is surely 

 not without significance. The age group in which the sex ratio is equality 

 consists of those who, biologically if not socially, are newly equipped for 

 ardent reproduction. Amongst them there is no surplus : there are 

 equal numbers of males and females. If, during the biological evolution 

 of man, pair-mating attached to itself a definite and positive value, it 

 would be expected that all the mechanisms concerned in the establishment 

 of a sex ratio of near equality among the 14-19 age group would, through 

 selection, come ultimately to be related harmoniously to this end. 



Implied in this suggestion is yet another : that in the case of any 

 population of living things, and under the conditions that exist in any 

 given place and at any given time, there is an optimum sex ratio amongst 



