As to the inheritance of disease as such there is little room for
misunderstanding: no biologist now believes a disease is actually handed
down from parent to child in the germ-plasm. But what the doctors call a
diathesis, a predisposition to some given disease, is most certainly
heritable--a fact which Karl Pearson and others have proved by
statistics that can not be given here.[58] And any individual who has
inherited this diathesis, this lack of resistance to a given disease, is
marked as a possible victim of natural selection. The extent to which
and the manner in which it operates may be more readily understood by
the study of a concrete case. Tuberculosis is, as everyone knows, a
disease caused directly by a bacillus; and a disease to which immunity
can not be acquired by any process of vaccination or inoculation yet
known. It is a disease which is not directly inherited as such. Yet
every city-dweller in the United States is almost constantly exposed to
infection by this bacillus, and autopsies show that most persons have
actually been infected at some period of life, but have resisted
further encroachment. Perhaps a fraction of them will eventually die of
consumption; the rest will die of some other disease, and will probably
never even know that they have carried the bacilli of tuberculosis in
their lungs.
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