Sex Ratio of an Aranead 449 



pleted their metamorphosis 241 males to 259 females, a male 

 ratio of slightly less than i ; and other students have constated a 

 low male ratio in Amphibia. These examples, based on cases 

 where relatively large numbers of individuals were counted before 

 the age of maturity, and all with the exception of Bufo at or before 

 the time of birth, are sufficient to indicate that different species 

 have different sex ratios, and that the sex ratio may be a quality 

 of the species. 



Now when there is a male ratio of 8.19 as in Latrodectus, such 

 a proportion of the sexes can be explained neither upon Newcomb's 

 theory of chance^^ nor yet upon Castle's idea of the Mendelian 

 inheritance of sex." Some other explanation is called for and, 

 as I shall proceed to argue, it is probably to be sought in the factor 

 of selection coupled with segregation. 



In the first place the distinction of the sexes is a difference of 

 reproductive power. The female is the reproductive individual, 

 while the male is not" reproductive but impregnatory, for females 

 can reproduce without males, as in cases of parthenogenesis, but 

 males are unable to reproduce of themselves. The male is dis- 

 tinctly the less important organism for the perpetuation of the 

 race. There is no reasonable proof for the formula of Geddes and 

 Thompson" that the male is more katabolic and the female more 

 anabolic, for that is merely an unfounded statement. The sexual 

 difference is one of degree of reproductive ability. 



Probably in the earliest racial species all individuals reproduced 

 in equal measure; this is probable simply because sexual dimor- 

 phism implies a rather advanced differentiation, therefore one 

 that should have developed more or less gradually. The origin of 

 sex is most easily conceived in the following manner. Indi- 

 vidual variation within a species affects so far as we know every 

 quality, therefore reproductive ability would be a quality subject 

 to variation. Among the fluctuants of a species in which sex had 

 not yet become pronounced there would be a series of individuals 



1- A statistical inquiry into the probability of causes of sex in human offspring. Carnegie Inst. 

 Publ. II, 1904. 



'5 Castle: The heredity of sex. Bull. Mus. Comp. Zool. Harvard, 1903. 

 " The evolution of sex. London, 1897. 



