NATURAL SELECTION IN MAN 185 



the paintroot (Lachnanthes) that is poisonous to white swine. 

 Black sheep, according to Heusinger, possess a similar immunity 

 to certain plants injurious to white sheep. And there are cases in 

 which infectious diseases are more fatal to light than to dark 

 colored breeds of animals. 



It is generally held that tuberculosis is more apt to attack 

 individuals with defective vitality. The tendency of tuberculosis 

 to run in famihes has long been recognized, but since it was 

 demonstrated that this disease is caused by bacterial infection, it 

 has not been regarded as truly hereditary. Direct transfer from 

 mother to embryo is exceedingly rare. It is probable, however, 

 that there are hereditary differences in the liability of individuals 

 to become infected. Pearson and his co-workers have collected 

 evidence to show that the correlation between parents and 

 children for tuberculosis (which lies between .4 and .6) is higher 

 than the correlation between the occurrence of tuberculosis and 

 unfavorable environment such as poor housing and bad ventila- 

 tion. A parent-offspring or a fraternal correlation is not neces- 

 sarily the result of heredity. It might also be brought about by 

 the transmission of an infection quite apart from heredity. It is 

 argued, however, that since the correlation for tuberculosis in 

 husband and wife where the chances for infection are presumably 

 equally great lies between o and .3, and as a part of this correla- 

 tion is probably due to assortative mating, or the tendency of like 

 or similarly situated individuals to intermarry, the parent-off- 

 spring correlation must be mainly tlie result of an hereditary 

 proclivity to infection. 



It may be questioned, however, if tuberculosis is as apt to be 

 conveyed in the marital relation as it is from parent to offspring. 

 If, as many authors now contend, tuberculosis is usually acquired 

 in childhood, often lying latent until some condition causes it to 

 flare up in adult life, the high value of the parent-offspring corre- 

 lation may be the result of early infection rather than a hereditary 

 diathesis. 



On the other hand, autopsies show that the great majority 

 of human beings are infected by tuberculosis some time during 



