tween husband and wife as regards tuberculosis is due to this fact,
and not to infection.[59]

Again, it is objected that the infection of children is not a family
matter, but due to tuberculous cows' milk: how then does it appear
equally among the Japanese, where cows are not tuberculous and cow's
milk rarely used as an infant food: or among such people as the
Esquimaux and Polynesians, who have never seen a cow?

But, it is argued, at any rate bad housing and unsanitary conditions of
life will make infection easier and lower the resistance of the
individual. Perhaps such conditions may make infection easier, but that
is of little importance considering how easy it is for all city
dwellers--for the population as a whole. The question remains, will not
bad housing cause a greater liability to fatal phthisis? Will not
destitution and its attendant conditions increase the probability that a
given individual will succumb to the white plague?

Most physicians think this to be the case, but they have not taken the
pains to measure the respective roles, by the exact methods of modern
science. S. Adolphus Knopf of New York, an authority on tuberculosis,
recognizes the importance of the heredity factor, but says that after
this, the most important predisposing conditions are of the nature of
unsanitary schools, unsanitary tenements, unsanitary factories and
workshops. This may be very true; these conditions may follow after
heredity in importance--but how near do they follow? That is a matter
capable of fairly accurate measurement, and should be discussed with
figures, not generalities.

Taking the case of destitution, which includes, necessarily, most of the
other evils specified, Professor Pearson measured the correlation with
liability to phthisis and found it to be .02. The correlation for direct
heredity--that is, the resemblance between parent and offspring--it will
be remembered, is .50. As compared with this, the environmental factor
of .02 is utterly insignificant. It seems evident that whether or not
one dies from tuberculosis, under present-day urban conditions, depends
mainly on the kind of constitution one has inherited.

There is no escape, then, from the conclusion that in any individual,
death from tuberculosis is largely a matter of natural selection. But
by taking a longer view, one can actually see the change to which
natural selection is one of the contributors. The following table shows
the deaths from