NATURAL SELECTION IN MAN 195 



In both the royal and the middle classes the percentage of 

 children dying under six years of age decreases as the age at death 

 of either the father or the mother increases. In other words, if 

 either parent dies young it greatly decreases the expectation of 

 life for the new born child. How are we to interpret this relation- 

 ship? It might be urged that the death of one parent would be 

 apt to involve lack of adequate care for the children. It was 

 pointed out that while this might partly account for the death 

 rate in children of the younger parents it would not explain the 

 fact that the child death rate continues to fall during the later 

 age periods in which the parents are so old that their death could 

 not possibly have fallen within the first five years of the life of any 

 of their children. It was also pointed out that in the royal fami- 

 lies in which the death of the parent would not leave the child 

 without adequate means of support there is much the same 

 correlation between the longevity of parent and child mortality 

 that is found in the middle class families. The relation of child 

 mortality to the death period of the father in these royal families 

 is especially noteworthy. 



The results are attributed by Ploetz to the inheritance of 

 different degrees of constitutional weakness. Natural selection, 

 therefore, acts not merely on the parents who are lacking in vigor, 

 but it picks out their young offspring, and thus tends to eliminate 

 stocks which transmit a defective vitality. 



It is probable that a considerable part of the infant death rate 

 that seems to be caused by external factors with little regard 

 to heredity is more strongly influenced by the hereditary factor 

 than is at first apparent. Much has been written on the high 

 mortality of artificially fed babies as compared with those which 

 are breast fed. We might be tempted to attribute this to the 

 great superiority of the mother's milk over the various substitutes 

 which are used to replace it. Certain investigations by Pearson 

 on the infant mortality of breast fed and artificially fed babies 

 of the towns of Preston and Blackburn, England, have shown that 

 the death rate of artificially fed babies depends largely on whether 

 the mothers do not want to nurse their children, or fail to nurse 



