D.— ZOOLOGY I OS 



case of the laboratory mammals, therefore, it may be accepted that though 

 in certain stocks there is no evidence of a selective pre-natal mortality, 

 yet in others in which it undoubtedly occurs it is the male that is removed 

 in greater numbers. 



In the case of birds there is much that is anecdotal and a certain amount 

 of information that has been derived from experimentation. It will be 

 remembered that Darwin's correspondents assured him that male birds 

 were caught in greater numbers than were females, and that he accepted 

 this presumed preponderance of males in nature as a fact supporting his 

 theory of sexual selection. Pelseneer's (1921) summary closes with the 

 statement that in the known cases where observation had been possible 

 an early excess of males at hatching and in young birds may be preserved 

 into adult life, but becomes less marked owing to the shorter life and higher 

 mortality of the males, and is often followed by an equality of the sexes 

 or even by a numerical preponderance of females. Darwin accepted 

 the view that in the domestic pigeon there is good evidence either that 

 males are produced in excess or that they live longer, and that the hen 

 is generally the weaker of the two and more likely to perish. Cole and 

 Kirkpatrick (1915) found a secondary sex ratio of 105 : 100 in the pigeon 

 and recorded that post-natally there was an early period with an excess of 

 male deaths followed by an adolescent and reproductive phase with a 

 slight . excess of female deaths, but that, on the whole, there was no 

 significant change of the sex ratio with advancing age. Whitman's 

 figures (191 9) for the Japanese turtle dove, presented by Riddle, show that 

 the male is often the longer-lived, whilst Haig Thomas and Huxley (1927) 

 found that in pheasant species crosses there was a predominance of males 

 at hatching and also a large and distinctive excess of male deaths both 

 before and after hatching, so that a secondary sex ratio of 67 • i per cent, 

 males became reduced to 50-1 per cent, in the adult, they concluded that 

 in the case of these pheasant crosses the early post-natal mortality 

 of the male is much greater than that of the female. 



Concerning the sex ratio of the fowl there is an abundant literature, 

 much of which has been summarised in the papers to which reference is 

 now made. Landauer and Landauer (193 1) computed that amongst 

 67,993 live born chicks the percentage of males was 48-77, whereas 

 among those dying during the first eight weeks of life it was 52-7 ±0-5. 

 They therefore concluded that in the fowl, as also in man, post-natal 

 mortality is higher amongst males and that if any differential mortality 

 during embryonic development occurs it is the male that suffers more. 

 But Byerley and Jull (1935) present an even greater body of data. They 

 hold the view that data on embryonic mortality were dismissed as in- 

 adequate by Landauer and Landauer, and largely ignored by MacArthur 

 and Baillie (1932), and show that in the literature are to be found records 

 of 6,864 dead embryos of which 48-59 per cent, were males. They 

 themselves examined a further 17,989 dead in shell to find among these 

 47-56 per cent, males. Thus, among the whole 24,853 dead embryos 

 there are only 47-85 per cent, males, a highly significant deviation from 

 equality. 



Adding their own figures to those in the literature they find among 



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