As to the heredity of cancer, all that can be said is that the burden of
proof rests upon those who assert it. It is really curious how
widespread the belief is that cancer "runs in families," and how
exceedingly slender is the basis of evidence for such a belief. There
are so many things that we do not know about cancer that any positive
statement of any kind would be unbecoming. It would be absurd to declare
that a disease, of which the cause is still unknown, either is or is not
inherited. And this is our position in regard to cancer. An overwhelming
majority of the evidence so far indicates that it is not a parasite; if
it were, of course, we could say positively that it is not inherited.
Although we are getting a discouraging degree of familiarity with the
process and clearly recognize that it consists chiefly in the sudden
revolt or rebellion of some group of cells, a tendency which quite
conceivably might be transmitted to future generations, yet it is highly
improbable, on both biological and pathological grounds, that such is
the case. If this rebellious tendency were transmitted we should at
least have the right to expect that it would appear in the cells of the
same organ or region of the body. It is a singular fact that in all the
hundreds of cases in which cancer has appeared in the child of a
cancerous parent it has almost invariably appeared in some different
organ from that affected in the parent.

For instance, cancer of the lip in the father may be followed by cancer
of the liver in the son or daughter, while cancer of the breast in the
mother will be followed by cancer of the lip in a son. Further than
this, the percentage of instances in which cancer appears in more than
one member of a family is decidedly small, considering the frequency of
the disease.

I took occasion to look into the matter carefully from a statistical
point of view some ten or twelve years ago, and out of a collection of
some fifty thousand cases of cancer less than six per cent were found to
give any history of cancer in the family. And this, of course, simply
means that some one of the relatives of the patient had at one time
developed the disease.

In fact, the consensus of intelligent expert opinion upon the subject of
heredity of cancer is, that though it may occur, we have comparatively
little proof of the fact; that the percentage of cases in which there is
cancer in the family is but little larger than might be expecte