4 ETHEL M. ELDERTON 



say while a brother and sister marriage would lead to 25 p. c. of the offspring having 

 the harmful character patent and another 50 p. c. having it latent, the intermarriage 

 of the cousins of the DR class among themselves would lead to the same baneful 

 results as this brother-sister marriage, while the intermarriage of the DR class of 

 cousin with the DD would also lead to 50 p. c. with the latent detrimental character. 

 In other words endogamy as far as brothers and sisters are concerned would lead to : 



25 p. c. hale. 50 p. c. latent evil. 25 p. c. patent evil, 



while endogamy in the cousinship would give us: 



56"25 p. c. hale. 37"5 p. c. latent evil. 6-25 p. c. patent evil. 



The explanation therefore of the wide-spread social feeling against endogamy in 

 the first degree, even between apparently hale individuals, is on the surface of it 

 explicable on the Mendelian theory ; also we see that, whether we look upon cousin 

 marriage as producing on the average more than six per cent, of patent evil, or in 

 the other aspect, that some cousin marriages are as detrimental as brother-sister 

 marriages, reasons can be found for their all being forbidden by tribal custom or 

 religious ordinance. But this is after all only to look on one side of the picture, 

 because the RR characteristic might be a patent good quality suddenly introduced 

 from outside into a population ; in such a case cousin marriage is distinctly to be 

 commended, and brother-sister marriage would be more effectual still. In this way 

 the endogamy of many early communities receives its due sanction. As long as a 

 species is likely to vary advantageously, endogamy between collaterals of the first 

 degree will produce 75 p. c. with patent or latent good quality, and between collaterals 

 of the second degree 62 5 p. c* ; even endogamy of ascendants and descendants may 

 be advantageous. It is probable that whenever selection is extremely stringent the 

 relative advantages of endogamy become apparent and are emphasised by tribal 

 custom. But the Mendelian theory cannot be considered as demonstrated, and if it 

 were, we could hardly at present apply it to man. We have no means of separating 

 the DR's from the DD's, short of that experimental breeding which the Mendelians 

 tell us is the only reliable guide to the gametic constitution. We cannot, however, 

 afford to bring defective children into the world to test where the endogamous union 

 will be an advantage, where a failure. The somatic characters of the individual and 

 of his or her ancestry are at present our sole possible guide to his or her gametic 

 constitution. From this standpoint we may ask what is the quantitative value of 

 the cousin in the problem of inheritance ? In predicting the probable offspring of an 

 individual is the cousinship of more or less importance than the parents' brothers and 

 sisters ? Is a knowledge of the grandparents' characters of greater value than that of 

 the cousin ? It will be clear that the cousinship while generally less accessible than 

 the sibship, is often far more accessible than the grandparentage, or in the case of 

 orphans than even the parentage ; and for the special purpose of medical diagnosis 



* This supposes that endogamy in the first degree is forbidden. 



