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 
