34 STATISTICS OF SEX 



The method of using the numbers is, from the statistics for each 

 value of n, to form conditional equations having k and a as unknown 

 quantities. These unknowns are to he determined by a solution of the 

 equations. It will be seen that h and a cannot he determined sepa- 

 rately, but only the combination ltd'. We may therefore suppose li = 1 

 without any loss of generality so far as these equations are concerned. 



We now make a practical application of this theory by determining 

 the numerical value of the unisexual tendency, a, in the respective cases 

 of twins and triplets, as enumerated in section 5 preceding. The sta- 

 tistics of twins there cited show that, of such pairs, 0.6-16 are unisexual 

 and 0.354 bisexual. Equating these percentages to the expressions for 

 the probability we find 



% -+- 21ia = 0.646 

 1/2 --21) a =0.354 



Subtracting these from each other we find 4/ia 2 = 0.292, and hence, 

 supposing li = 1, 



a 2 = 0.073 



a = 0.27 



We may now consider the case of triplets in two ways. Proceeding, 

 as in the case of twins, by equating each probability to the fraction indi- 

 cating the proportional number of the families to which it relates, we 

 have the equations: 



14 + 3/ta 2 = 0.499 % - - 3/; a 2 = 0.501 



Solving these we derive, after putting h = 1, 



a 2 : = 0.0831 a 0.29 



We may also proceed in another way by substituting in the expres- 

 sions for the respective probabilities of unisexual and bisexual triplets 

 the value of Jia" derived from the case of twins. This will give, as 

 has already been stated, the percentage 46.9 for unisexual triplets 

 instead of 49.9, as has been found from observation. It may he added 

 that this relation is not changed by changing the value of // ; it is there- 

 fore indifferent what value we assign to li. 



