EFFECTS OF INBREEDING ON THE SEX RATIO 27 



whether the male belonged to the A or to the B series of in- 

 breds. The litters sired by males from the A series showed a 

 higher sex ratio (102.3 & : 100 9 ), however, than did the litters 

 sired by males from the B series (96.2 cf : 100 9 ) , but these ratios 

 are not significant, since they differ from each other and from the 

 norm by less than three times the probable error (table 10). 



Diising's contention that close inbreeding increases the relative 

 number of male offspring was based mainly on statistics of human 

 births collected from several isolated communities in which there 

 were many consanguineous marriages, and on the supposedly great 

 preponderance of male births among the Jews, who are a clannish 

 race and intermarry more frequently than do other civilized 

 races. The latter evidence is undoubtedly invalid, as Pearl and 

 Salaman ('13) have shown that the nomial sex ratio among the 

 Jews is the same as that in other races of man (105 cf : 100 9 ), 

 and that the anomalous sex ratio among them is due to faulty 

 registration, male births being recorded where those of females are 

 not. The high sex ratio in the other cases cited by Diising can 

 doubtless be attributed to a similar cause. The great excess of 

 males found in various strains of thoroughbred dogs Heape ('08) 

 ascribed in part to inbreeding, but in these cases also it is prob- 

 able that the statistics are not reliable, since female pups are 

 commonly discarded from large litters and males are registered 

 more often, as a rule, than females. 



The inbreeding experiments of Huth ('87) on the rabbit, of 

 Schultze ('03) and of Copeman and Parsons ('04) on mice were 

 made with relatively small numbers of animals, and the sex ratios 

 obtained showed no greater deviations from the norm than might 

 have been expected under the conditions of the experiments. 

 Shull ('13) found no change in the sex ratio of Hydatina senta as a 

 result of repeated inbreeding, the proportion of male-producers 

 and of female-producers remaining practically constant. In the 

 present series of inbreeding experiments with the albino rat, all of 

 the animals belonging to the earlier generations suffered severely 

 from malnutrition, which Diising ('84) considered as a very potent 

 factor in increasing the number of male offspring, yet among the 

 individuals in the first seven generations the sex ratio was only 



