432 PEARL— SEX RATIO IN DOMESTIC FOWL. 



some portion, at least, of the prenatal mortality in man. In still 

 births there is a greater excess of males over females than in living 

 births. The reviews which prevail among statistical writers regarding 

 this matter are well put by Nichols^* (p. 269) in the following 

 passage : 



" Obviously the main cause of the great preponderance of male stillbirths 

 resolves itself into the question of the comparative mortality or death rate of 

 the male and female sexes during the intrauterine period of existence. Vital 

 statistics have shown clearly that there are material differences in the mor- 

 tality of the two sexes, the death rates among males being, in general, higher 

 than among females throughout nearly the entire period of life, and the aver- 

 age duration of life of females being greater than of males. During the 

 adult and later periods of life this difference is largely or partly explainable 

 on the ground of the greater stress and strain and liability to injury imposed 

 by the greater responsibilities, more laborious occupations, and greater expo- 

 sure of men, and their greater indulgence in vicious and morbific habits ; these 

 factors scarcely being offset by the perils incurred by women during the child- 

 bearing period. But the same greater mortality of males occurs, and in the 

 most marked degree, even in the intrauterine period of existence and in the 

 early years of life before the factors mentioned begin to be operative; it is 

 therefore obvious that the male constitution is intrinsically weaker, less hardy, 

 and more susceptible to morbific and mortific influences, and has less vitality 

 and resisting power against disease, than the female. The cause of this innate 

 disparity of vitality between the two sexes we do not know ; but the fact it 

 exists, that the antenatal mortality and death rate of males much exceeds that 

 of female fetuses, accounts for the great excess of male over female still- 

 births." 



The demographic objects, in the study of sex ratios, are some- 

 what different than the purely biological. In the present instance, 

 and generally in purely biological studies on the proportion of the 

 sexes, what we really wish to know is the true proportions in which 

 zygotes of the two sexes are initially produced. This can not be di- 

 rectly observed in higher vertebrates, owing to the occurrence of pre- 

 natal mortality at all stages between the fertilization of the egg and 

 the birth of the young. The earliest easily observable datum plane 

 which one has upon which to base a conclusion as to the sex pro- 

 portions in the zygotes at the moment of their production, is the sex 

 ratio at birth. Obviously the prenatal mortality may have influenced 



1* Nichols, J. B., Mem. Amer. Anthropol. Assoc, Vol. I., Part 4, pp. 249- 

 300, 1907. 



