THE DETERMINATION OF SEX 201 



Oesterleben in Europe summarized the data for 

 nearly sixty million human births and found that 

 an average of 106 males are born to every 100 fe- 

 males. 



According to various authorities, the relative num- 

 ber of males per 100 females is given for horses as 99, 

 for cattle 94, and poultry 95, while in pigs, rabbits, 

 pigeons, and greyhounds the corresponding number 

 of males is slightly over 100. 



This practical equality of the sexes in all sorts of 

 natural environments indicates the improbability of 

 the assumption that external conditions determine 

 sex. 



4. MONOCHORIAL TWINS 



There are two kinds of twins, namely, ordinary 

 twins, which come from two separately fertilized 

 eggs each inclosed in its own chorion, and "identical 

 twins, " that have their origin in one egg which is in- 

 closed in one chorion. Of the former, something like 

 30 per cent in man are reported as being of two sexes, 

 thus showing that it is neither nutrition nor envi- 

 ronment which determines sex. Usually when twins 

 are of the same sex, they exhibit as great a range of 

 difference in mental and physical traits as do ordinary 

 children of the same fraternity born at different 

 times, but occasionally "identical twins" are born, 

 and such monochorial twins are always of the same 

 sex. This is evidence that sex, like other somatic 

 characters, is determined in the germplasm at the 

 time of fertilization. 



