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CARNEGIE INSTITUTION OF WASHINGTON. 



matings; and finally, (3) the great excess in pure matings of males 

 among children of the first birth as compared with the later births, 

 contrasted with the steady excess of males in hybrid matings. 



Table 5. — Sex ratio {male -^female) and proportion of stillbirths in pure and hybrid 



human matings. 



These facts are all explicable on the hypothesis that the male- 

 forming sperm are less likely to be eliminated, because they are less 

 able to carry genetic factors which might produce physiological incom- 

 patibility either during their intra-uterine journey or after fertili- 

 zation, than are the female-producing sperm with their larger amount 

 of foreign chromatin. Within ''pure" races, the uterine secretions 

 might at first eliminate sperm more easily and more specifically than 

 they could later on ; but the hybrids, in which there was a greater degree 

 of difference between male and female gametes, might retain their 

 ehminatory ability either for a longer time or even permanently. This 

 would result in a great excess of male offspring from first matings 

 within the "pure" races, and the continued excess of males among 

 the racial hybrids. 



The relatively smaller number of stillbirths in hybrids as compared 

 with "pure" races is an interesting fact. At first it appears that be- 

 cause of the large number of cases of venereal disease involved, still- 

 births are poor material for genetic analysis. But inasmuch as the 

 result of the infection and not its occurrence is the matter of biological 

 interest, it is entirely probable that truly genetic and biological factors 

 underlie and determine whether a fetus infected with venereal disease 

 shall or shall not die. The biological and genetic basis of factors con- 



