114 THE BIOLOGY OF TWINS 



increases the relative proportion of males to females 

 decreases. Nichols {loc. cit.) gives the following table: 



Number of Sons for 

 i,ooo Daughters 



Single births 1,057 



Twin births 1,043 



Triple births 1,007 



Quadruple births 548 



Similar ratios are found in sheep. The following 

 table is attributed by Wentworth^ to Pearl. In 115 

 multiple sets in sheep the following sex-ratios occurred: 



3 males to a birth ■ 16 



2 males and i female to a birth 39 



2 females and i male to a birth 22 



3 females to a birth 38 



A summary shows that the ratio of females to males in 

 these large plural births is 197 females to 148 males. 



The reason for the preponderance of females in 

 the plural offspring of these two species is not fully 

 known; the basis of it is probably a sex-difference in 

 prenatal mortality. It is well known that the prenatal 

 mortality of human male twins is greater than that of 

 females, an indication that males are less resistant to 

 abnormal uterine conditions than are females. In the 

 uterus of a species adapted for uniparous gestation the 

 crowding of several fetuses must inevitably introduce 

 untoward conditions; if males are more susceptible 

 to subnormal conditions than females it is highly 

 probable that they succumb in larger numbers than the 

 latter. 



That the mere fact of multiple offspring carries 

 with it no necessary disturbance of sex-ratios is seen 



^E. N. Wentworth, Science, N. S., XXXIX (1914). 



