448 Thos. H. Montgomery, Jr. 



3 THE ORIGIN AND FIXATION OF SEX RATIOS 



There are obviously two sides to the question of sex determi- 

 nation: the one, the process that regulates kind and succession of 

 the sexes of one offspring-unit (totality of offspring of one parent), 

 the other, the process of differentiation of the sexes and the origin 

 of sex ratios. The second question is essentially phylogenetic, 

 but if it should be proved that there is a distinct inheritance of 

 sex then this phylogenetic aspect will come to have an important 

 bearing on the other. We are now concerned with the question 

 of the origin of sex and of sex ratios. 



The opinion is fairly general that in most animal species, those 

 exhibiting parthenogenesis excluded, there is no great disparity 

 in number between the sexes. This has followed from a consider- 

 ation of the numbers of the sexes in man, for so far in no other 

 animal except the horse, is there available any computation of the 

 sexes based upon a count of large numbers of individuals at birth. 

 Only such computations have value, it is hardly necessary to add, 

 that are founded upon the count of the sexes at the time of birth 

 or earlier because the mortahty after birth frequently varies with 

 the sex, the males in certain lower animals being more short-lived 

 and less resistant. We have found that in Latrodectus the male 

 ratio is 8.19, there being born more than eight males to every 

 female. For man, where the statistics are the only ones more 

 numerous than those of Latrodectus, the proportion of males to 

 females in 10,864,950 births is 1036 to 1000, the male ratio there- 

 fore 1.03, as taken from the compilation presented by Pike in his 

 Table III.* According to Morgan'' for "still-born infants, fully 

 formed, but not alive," Quetelet found 133.5 ^n^les to 100 females, 

 a male ratio of 1.33, and Bodio (from whose statistics I compute 

 an average) a male ratio of 1.3 1. Darwin^" tabulates the births of 

 English race horses, 25,560 in all, giving 99.7 males to 100 females. 

 For Bufo lentiginosus King" found in individuals that had com- 



' Pike: A critical and statistical study of the determination of sex, particularly in human offspring. 

 Amer. Nat. 41, 1907. 



' Experimental Zoology. New York, 1907. 



'" The descent of man, new ed., New York, 1886. 



" Food as a factor in the determination of sex in Amphibians. Biol. Bull., 1907. 



