204 RAYMOND PEARL AND MAUD DEWITT PEARL. 



lowing conclusion (p. 385) : "If nutrition were really a factor 

 of any importance in sex determination, it is surprising to find so 

 little difference under apparently very favorable and unfavorable 

 conditions. It seems much more probable that if the nutrition 

 affects in any way the proportion of the sexes, it does so indirectly 

 by elimination, and not by determining either the sex of the 

 embryo or of the egg." Further on Morgan says in discussing 

 Geddes and Thomson's theory of sex (p. 388) : " If, on the 

 other hand, the determination of sex is supposed to be due to 

 the nourishment of the embryo, the best ascertained facts, both 

 experimental and statistical, are opposed to the hypothesis." 

 Taking all these points into consideration it seems very doubtful, 

 to say the least, if the observed excess of males in the cross mat- 

 ings has its explanation either in whole or in part in differences 

 in the environmental complex implied by " social status." How- 

 ever, in the absence of more complete and definite statistical data 

 regarding the point one cannot be dogmatic in asserting such a 

 conclusion. 



If none of the suggested factors can reasonably be held to 

 afford an explanation of the facts regarding the sex-ratio shown 

 by the present statistics how are these facts to be interpreted ? 

 All that can safely be asserted is that the present statistics, with- 

 in their limits, show clearly that there is a definite relation be- 

 tween the character of the mating and the magnitude of the sex- 

 ratio. Is this a posi hoc or a propter hoc relation ? The data 

 themselves do not conclusively demonstrate which it is. Nor 

 does it seem probable that statistics of human births alone can 

 ever settle this question. It is one which demands experimental 

 analysis. The chief difficulty involved in maintaining that there 

 is a causal relation between the character of the mating and 

 the sex-ratio lies in the lack of knowledge as to what could 

 be the physiological mechanism by which the causation was 

 effected. In a way the phenomenon appears somewhat analo- 

 gous to the well-known phenomenon of xenia observed in plant 

 breeding, differing in that here the character influenced is sex 

 rather than some purely morphological feature of the organism. 



In conclusion it should be said that the data presented in this 

 paper are not put forth as in any way final or conclusive. They 



