STATISTICS OF MULTIPLE BIRTHS 17 



HYPOTHESIS II. Sex is entirely determined by the conditions to 

 which the germ is subject during the early stages of its development. 



All these conditions are the same ab initio for the two members of the 

 pair. The result of the hypothesis would therefore be that twin children 

 would always be of the same sex. 



The statistics show that neither of these hypotheses is correct taken 

 singly; the actual result being an intermediate one between those of the 

 two hypotheses. A child of one sex is more likely than not to have a 

 twin of the same sex ; but there is only a certain preponderance of proba- 

 bility for this. 



The negation of the first hypothesis leads to the conclusion that the 

 original germs supplied by the father, if not completely asexual, can at 

 most have no other sexual quality than a slightly greater tendency to 

 develop into one sex than into the other. While the existence of such a 

 tendency is not out of the question, the more likely conclusion would 

 seem to be that the part of the father is completely asexual, and that the 

 determination of sexes is entirelv the function of the mother. It is true 



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that this contravenes certain supposed conclusions from statistics which 

 will be considered in the next section. But these seem to me open to 

 misconstruction. 



We pass next to the actual numbers shown by the statistics of France 

 and Germany. In the following table the first line shows the number of 

 births giving rise to two males; the second to the number of bisexual 

 birth's; the third of births of two females. In the next two lines are 

 the total number of male and female children resulting from all the 

 births. The preponderance of males is, on the whole, the normal one, 

 showing that, in the production of twins, there are no causes affecting sex 

 which act in any way differently from those in the ordinary cases. Then 

 is given the normal number of bisexual births as it would have been were 

 the determination of sex, in the case of the two children, completely inde- 

 pendent, as required by hypothesis I. The proportional deficiency of 

 the actual number of bisexual births over this probable number may be 

 used to define the unisexual tendency in the case of twins. The observed 

 fact may be set forth thus : The probable proportions of unisexual and 

 bisexual twins, if the sex of each child were determined independently of 

 the other, and the percentage of pairs of the several classes would be: 



2 m., 0.260; m. and f., 0.500; 2 f., 0.240, 

 while the actual proportions are: 



2 m., 0.332; m. and f., 0.354; 2 f., 0.314. 



