loo SECTIONAL ADDRESSES 



bones more completely ossified, and so, as would be expected, it is the male 

 that suffers more from birth injuries and from the epilepsy that follows 

 upon such. The sex ratio of such cases is approximately i6o : loo. 

 There are other causes of death which operate almost exclusively upon 

 one sex : hernia is a cause of death of male infants, gonococcal infections 

 are restricted almost exclusively to the female infant. But the explana- 

 tion of this is to be found in a consideration of the anatomical differences 

 that distinguish the sexes at birth. Such sex-limited conditions obviously 

 cannot provide an explanation of the difference in the sex incidence of 

 death among early abortuses, and, in any case, they lose their significance 

 when the tables in the Registrar-General' s Annual Review giving the 

 causes and the sex incidence of death are examined, for it is seen that from 

 all causes there is everywhere a general proneness on the part of male 

 infants to die in greater numbers, and that in general the earlier in life 

 the onset of a disease, the higher is the sex ratio among its fatal cases. 

 This is true of infectious as well as of other diseases, and the only possible 

 explanation for this would seem to be that the differences between the 

 sexes in respect of capacity for continued life is greatest in early infancy. 



Violent deaths remove twice as many males as females from a popula- 

 tion in the course of a year, and therefore necessarily distort the sex ratio 

 among certain age groups, but murder, suicide, deaths on the road, 

 occupational accidents, warfare and migration cannot possibly explain 

 away the gentle swing in the sex ratio from conception to extreme 

 senescence. There is no doubt whatsoever that the whole course of sex 

 mortality in pre-natal life, in infancy, and in all subsequent age periods is 

 consistent with the view that the male in man is the inherently weaker 

 sex, more prone on account of his relative constitutional weakness to 

 developmental anomalies, to congenital debility and to death from 

 diseases of all kinds. 



This being so, it is necessar}^ to contrast male and female in order to 

 determine what fundamental differences distinguish them, and to decide 

 which of these may be held responsible for the observed differences 

 between the sexes in respect of viability. 



Male differs from female in genetic constitution. He is heterogametic, 

 possessing but a single X-chromosome and elaborating two kinds of 

 gametes, X-chromosome-bearing and Y-chromosome-bearing respectively. 

 The female is constitutionally homogametic, possessing two X-chromo- 

 somes and therefore elaborating ova all of which are alike in that each 

 carries one X. In respect of X-borne genes the male is equipped with 

 a single set, the female with two. It follows from this that in the female 

 a mutant recessive gene in one X can be cancelled out by its wildtype 

 dominant allele in the other, whereas the same recessive gene in the single 

 X of the heterogametic individual is unchecked and unrestrained and is 

 expressed to produce its full effect. Upon this firm basis of ascertained 

 fact, the sex-linked lethal theory, which seeks to account for the sexually 

 selective mortality and the swing in the sex ratio, has been built. It 

 has been advocated by Lenz (1923), Gunther (1923), Geiser (1924-5), 

 Huxley (1924), Schirmer (1929) among others, and had its beginning as 

 long ago as 1912 when Morgan first drew attention to the lower vitality 



