RELATION TO GENERAL BIOLOGICAL PROBLEMS 115 



when the ratios of normally multiparous species are 

 examined. Wentworth gives the following tables (I 

 and II) for swine and for dogs: 



TABLE I 

 Sex-Ratios in Swine Births 



No. of males per litter 

 Expectation (in no. of 



litters) 



Actual no. of litters. . . 



No. of males per litter 

 Expectation (in no. of 



litters) 



Actual no. of litters . . . 



28. s 



28 



O.II 



I 



TABLE II 

 Sex-Ratios in Dog LitteeS 



No. of male pups per litter 

 Expectation (in no. of 



litters) 



Actual no. of litters 



In the total of 173 litters of swine there is really no 

 significant departure from the normal distribution of 

 the sexes. 



Again no disturbance of the expected ratio occurs 

 in a total of 126 litters of pups. 



From these statistics it appears that the disturb- 

 ances in sex-ratios of plural births are limited to those 

 species that are normally uniparous. This may be due 

 partly to differential prenatal mortality, favoring the 

 survival of females. There is nothing about these 

 ratios at all out of accord with the idea that in mammals 

 sex is zygotically determined. 



