No. 631] 



HUMAN TWINS 



parent of twins has married twice. In 14 cases it was 

 the father who married twice, in 15 cases the mother, and, 

 in 1 case, both father and mother. In the 14 cases of 

 father of twins who had married twice, there were twins 

 by both marriages in 2 cases, or 14 per cent, of all such 

 cases. In the 15 cases where the mothers of twins had 

 married twice, there were twins by both marriages in 3 

 cases, or 21 per cent, of all snch marriages. The numbers 

 are small, but, so far as they go, in view of the average 

 occurrence of twins in only about 2 per cent, of all mar- 

 riages (and hence if chance only were at work in 4 per 

 10,000 of both pairs of double marriages), they indicate 

 that the tendency to twin production is hereditary and 

 also that not only the mothers but also the fathers have 

 great influence in determining the production of twins. 



All the foregoing statistics speak strongly for the view 

 that the father has about as much influence in the produc- 

 tion of twins as the mother. This result at first sight 

 seems quite inexplicable and indeed to reduce the whole 

 matter to an absurdity. If twin production is due simply 

 to double ovulation, what can the father have to do with 

 the result? 



The present paper does not attempt to give a final 

 answer to this inquiry. It attempts only to set forth a 

 hypothesis which suggests a line of experimentation to 

 answer the question more definitely. We have assumed 

 that 2-egg twins are due to the simultaneous bursting of 

 two Graafian follicles while single births result from the 

 bursting of a single follicle. There is, however, a good 

 deal of evidence that single births are not always the con- 

 sequence of the bursting of a single follicle merely. There 

 are indeed several other factors that determine a single 

 birth, such as the failure of one of two simultaneously 

 expelled eggs to be fertilized or the failure of one of 

 two simultaneouslv expelled fertilized eggs to develop 

 to maturity. That is, it may well be that two eggs are 

 simultaneously ovulated much more frequently than at 

 present recognized and that the comparative rarity of 

 twin-births is due either (1) to a failure of fertilization 

 of one egg or (2) to a failure of development of one egg. 



