D.— ZOOLOGY 103 



14 per cent., has been found in the male. Because of their relationship 

 to respiration the erythrocytes and hzemoglobin might be expected usually 

 to show a sex difference. Adequate measurements of both erythrocyte 

 number and hagmoglobin content have been made on the adult in man 

 (Haden, 1922, Rud, 1923), in the fowl (Blacher, 1926), pigeon and ring 

 dove (Riddle and Braucher, 193 1), and in every case the higher erythro- 

 cyte amount, 2-3-31 per cent., and the higher ha2moglobin value, 7-35 per 

 cent., is found in the male. 



In the light of these and similar observations it is reasonable to expect 

 that the higher metabolic rate of the male should render him less resistant 

 to unfavourable conditions and more prone to death. That this expectation 

 is justified is shown by the results of a considerable number of varied 

 experiments, the following of which are cited as examples. 



MacArthur and Baillie (1929) found that at 28° C. the mean duration 

 of life of the females of Daphnia magna exceeded that of the males by 

 33-35 per cent., by 15-82 per cent, at 18° C, and by only 0-43 per cent, 

 at 8° C, and they further demonstrated that this extreme response of the 

 males was probably a factor of their metabolic level. Riddle, Christman 

 and Benedict (1930) found that the basal metabolism of male pigeons was 

 more easily and extremely aflFected by such conditions as unusually high 

 temperature and low oxygen supply than was that of the females. Essen- 

 berg (1923) showed that the males of the sword-tailed minnow Xiphophorus 

 helleri are 2-25 per cent, more susceptible to KCN, alcohol and extremes 

 of temperature, whilst Hildebrand (1927) found by direct test that the 

 males of Gambusia are much less resistant to high temperature, altered 

 pa, KCN and oxygen deficiency. 



The metabolic theory is, at the present time, somewhat crude, lacking 

 the precision and smoothness that distinguish the alternative and attractive 

 sex-linked lethal theory, but nevertheless in it there lies truth. It is to 

 be expected that, with advances in our knowledge of endocrinology, and 

 with the increasing manufacture of synthetic equivalents of the elaborated 

 products of gonad, pituitary, thyroid and the rest, we shall soon be 

 equipped with the means of exploring completely those physiological 

 differences between the sexes which seem to be connected with differences 

 in mortality, and, moreover, it will then perhaps become possible to repair 

 by chemical means the deficiencies which maleness now confers upon its 

 exhibitors. 



It is desirable at this stage to find out if the sex ratio of man is peculiar, 

 or whether the facts and observations concerning the human sex ratio 

 apply also and equally to that of other living forms. 



A great mass of data relating to the secondary sex ratio in animals of 

 economic importance has been accumulated, and though herd and stud 

 books are, for various reasons, inclined to be somewhat inaccurate, the 

 records therein do possess a certain value. An examination of these shows 

 that in the case of the horse the secondary sex ratio has been found by all 

 who investigated it to be low, so also has been that of the sheep. That of 

 cattle has a wide range, from very low to very high, whilst that of the pig 

 and of the dog has always been found to be high. But, from the point 

 of view of the present study, the secondary sex ratio by itself possesses 



