sease 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.

Of a group of men picked at random from the population, why will some
eventually die of tuberculosis and the others resist infection? Is it a
matter of environment?--are open-air schools, sanitary tenements, proper
hygiene, the kind of measures that will change this condition? Such is
the doctrine widely preached at the present day. It is alleged that the
white plague may be stamped out, if the open cases of tuberculosis are
isolated and the rest of the population is taught how to live properly.
The problem is almost universally declared to be a problem of infection.

Infection certainly is the immediate problem, but the biologist sees a
greater one a little farther back. It is the problem of natural
selection.

To prove this, it is necessary to prove (1) that some people are born
with less resistance to tuberculosis than others and (2) that it is
these people with weak natural resistance who die of phthisis, while
their neighbors with stronger resistance survive. The proof of these
propositions has been abundantly given by Karl Pearson, G. Archdall Reid
and others. Their main points may be indicated. In the first place it
must be