STATISTICS OF MULTIPLE BIRTHS 19 



bination of probabilities would lead to the same result whether we sup- 

 posed it or not. The main point is that there is some preponderating 

 tendency of the pair of organisms towards one sex in some cases and the 

 opposite sex in the remaining cases. Stating probable results in per- 

 centages, they would be : 



In group A, probability of 2 males 0.77 = 59.3 per cent. 



In group A, probability of 2 females 0.23 = 5.3 per cent. 



Total unisexual percentage 64.6 



Bisexual 35.4 



In group B the results would be the same, interchanging male and 

 female. 



These numbers, it will be noticed, show the percentages actually given 

 by the statistics. The mathematical method developed in the Appendix 

 shows that the results may be accounted for by assuming a certain uni- 

 sexual tendency represented by a fraction a having the value 



=0.27 



This coefficient may be considered to express the efficiency of all the 

 causes tending to produce sex which are common to the two twin mem- 

 bers of the family. In other words, assuming this unisexual coefficient, 

 the result will be 77 per cent of twins of the same sex, and 23 per cent 

 of twins of different sexes, these being the actual results of observation. 



A most interesting fact is that, by the methods developed in the Ap- 

 pendix, we may apply this coefficient a to determine how the sexes in 

 families of triplets should be divided. There are two ways of proceeding. 

 We may assume the unisexual tendency to be the same in triplets as we 

 have found it to be in twins, as naturally ought to be the case. From 

 this we can determine what proportion of triplets should be unisexual, 

 and compare the result with statistics. The other method consists in 

 determining the value of the unisexual tendency from the statistics of the 

 triplets in order to see how much it differs from that determined from 

 the statistics of twins. Adopting the first method the problem is : We 

 have three organisms subject to such conditions that, in the case of each, 

 there is a probability of 0.77 that it will prove of one sex and of 0.23 

 that it will prove of the other. What are the respective probabilities 

 that the three organisms will be unisexual ; that is, all three of the same 

 sex; and that one shall be of one sex and two of the opposite? These 

 probabilities are found in the Appendix to be : 



Unisexual: percentage, 46.9 

 Bisexual: " 53.1 



