llion? To the first of these science can only return a tentative and
approximate answer. The subject is beset with difficulties, chief among
which is the fact that we are unable to produce the disease with
certainty in animals, with the single exception of the Jensen's tumors
in mice referred to, nor is it transferred from one human being to
another, so that we can make even an approximate guess at the precise
time at, or conditions under, which the process began.

Many theories have been advanced, but most investigators who have
studied the problem in a broad-minded spirit are coming gradually to
agree to this extent:--

First of all, that one of the most powerful influences conditioning this
isolation and revolt of the cells is age, both of the individual and of
the organ concerned. Not only does far the heaviest cancer mortality
fall between the ages of forty-five and sixty, but the organs most
frequently and severely attacked are those which between these years are
beginning to lose their function and waste away. First and most
striking, the mammary gland and the uterus in women, and the shriveling
lips and tongue of elderly men. To put it metaphorically, the mammary
gland and the uterus, after the change of life, the lip, after the decay
of the teeth, have done their work, outlived their usefulness, and are
being placed upon a starvation pension by a grateful country. Nineteen
out of twenty accept the situation without protest and sink slowly to a
mere vegetative state of existence, but, in the twentieth, some little
knot of cells rebel, revert to an ancestral power of breeding rapidly to
escape extinction, begin to make ravages, and cancer is born.

The age-preferences are well marked. Cancer is emphatically a disease of
senility, of age; but, as Roger Williams has pointed out in his
admirable monograph, not of "completed" senility.

To express it in percentages, barely twenty per cent of the cases occur
before forty years of age, sixty per cent between forty and sixty, and
twenty per cent between sixty and eighty. Thus the early period of
decline, the transition stage between full functional vigor and declared
atrophy (wasting) of the glands, is clearly the period of greatest
danger; precisely the period in which the gland-cells, though losing
their function,--and income,--have still the strength to inaugurate a
rebellion, and a sufficient supply of the sinews of war, either in their
own possession or within e