io6 SECTIONAL ADDRESSES 



96,008 live born chicks, 49-17 io-ii per cent, males — a figure still 

 significantly lower than 50 per cent., so that it would appear that in spite 

 of the fact that more females than males die before hatching there is still 

 a slight preponderance of females among the live born. 



As regards early post-natal mortality neither Jull (1931) nor Byerley 

 and Jull (193s) could find any evidence to show that this was in any way 

 differential between the sexes. There is no reason why there should be 

 if only viable chicks are hatched and if the conditions of brooding are such 

 as not to discriminate against one sex. In order to determine which sex 

 is less viable during early post-natal life, it would be necessary to keep the 

 chicks under less favourable conditions ; at the present time the techniques 

 of artificial incubation are not so highly developed as are those of artificial 

 brooding. 



These conclusions attach to themselves a particular importance for 

 the reason that in birds the heterogametic sex is the female. A com- 

 parative study of the sex ratio and sex incidence of mortality in mammals 

 and birds might be expected, therefore, to determine to what extent the 

 heterogametic constitution itself is connected with mortality. If in 

 mammals it is the male and in birds the female that is found to be the more 

 fragile, then it becomes possible to assert that it is the heterogametic sex 

 that is the less viable and to seek the cause of this relative inviability in 

 the heterogametic constitution. If, on the other hand, it should prove 

 in both mammals and birds to be the male that is removed by natural 

 death in greater numbers, maleness itself must be held to be the cause. 



The figures for the fowl seem to show that it is the heterogametic 

 sex upon which death falls more heavily pre-natally so that it becomes 

 possible at once to decide that in the heterogametic mechanism itself a 

 factor of importance is to be found. But more information concerning 

 birds generally is urgently needed. 



Geiser (1923) in reviewing the literature dealing with the sex ratio in 

 fish found that in populations taken at birth the sexes were either nearly 

 equally represented or else showed a slight excess of females, and that in 

 older populations the females were always more numerous ; in some 

 instances greatly so. But the evidence loses a great deal of its value for 

 the reason that the sex chromosome constitution of such fish as the plaice, 

 salmon, smelt, dog-fish and top minnow has not yet been determined, so 

 that it is impossible to relate the above findings to heterogamety. 



Concerning insects many records have been published showing that 

 the normal male does not live as long as the female. Arendsen Hein 

 (1920) found that among 13,754 young pupae of the meal worm beetle 

 taken at random the sex ratio was 100 males : 100-3 females. In a series 

 of 32 experiments he ascertained that in this species the male lives on the 

 average 60 days, the female iii. Pearl (1923) found that in the case of 

 Drosophila melanogaster (male heterogamety) the survival relations of the 

 two sexes were exactly like those in man ; at practically all ages in Droso- 

 phtla the number of survivals at any given age is higher amongst the 

 females than amongst the males. The mean duration of life for the male 

 is 31-3 days ; that for the female 33-0 days. 



There is a series of papers by Rau and Rau (1914) dealing with large 



