No. 631] 



HUMAN TWINS 



123 



that the frequency of twins in the fraternities of fathers 

 of twins is almost the same as that of twins in the fra- 

 ternities of mothers of twins. Since the average propor- 

 tion of labors which are twin labors is 1.1 per cent, for 

 the population as a whole, we see that twins occur in the 

 fraternities of repeating fathers as well as repeating 

 mothers about four times as frequently as in the popula- 

 tion as a whole. 



To make use of more extended pedigrees, we may com- 

 pare the tendency to have twin children on the part of 

 sisters of the father and the mother of twin fraternities 

 and on the part of brothers of the fathers and mothers of 

 such fraternities. Then we obtain the results shown in 

 the following table : 



From this table, most of the items of which were based 

 upon ten or more twin labors, it appears that the sisters 

 of twin-producing parents are more apt to have twins 

 than the brothers of twin-producing parents; but the sis- 

 ters of twin-producing fathers are more apt to have twins 

 than the sisters of twin-producing mothers; also the 

 brothers of twin-producing fathers are more apt to have 

 twins than the brothers of twin-producing mothers. In 

 all cases the proportion of twin births is very high, rang- 

 ing from 4 to 7.5 times the average proportion of twin 

 births in the whole population. These statistics then in- 

 dicate that there is no important difference in the hered- 

 itary influence to twin production on the part of the father 

 and the mother of offspring which include two or more 



If. instead of considering the cases of twins in general, 

 we pick out tho.e of certain ( or hi-hlv probable) identical 

 twins, then we find, in 30 families with such twins, that 

 the mothers came from fraternities in which (in 77 labors) 

 there were 13 per cent, twin labors, and the fathers came 

 from fraternities in which (in 38 labors) there were 13 



