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 families 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 the 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 
