SYMPOSIUM ON PUBLIC HEALTH PROBLEMS 101 
breeding out, at will, cancer in mice, and made it succeed as 
an experimental problem depending on what strain she selected 
with its known hereditary predisposition. 
Another significant fact in the work of this original investi- 
gator, at least as far as it applies to the study of the problem of 
cancer heredity in mice is that it is not the second or the 
third generation which shows up the predisposition to cancer 
development in families, and what it is to be; but the fourth, 
fifth or even sixth generation that emphasizes or controls the 
determining factors, 
So when the human family finally arrives at a knowledge of 
predisposition to cancer as a family problem they may be im- 
measurably aided in the study by knowing what diseases 
troubled not only their grand parents, but the great grand 
parents as well. 
It is a curious fact also in the enigma of cancer that a little 
injury to the body is more frequently followed by cancer than 
a severe one. Cancer rarely develops from a fracture of one 
of the larger bones of the trunk; but a comparatively slight 
bruise of one of these bones may develop the disease at seat of 
the injury and this too, months or years after its receipt. Just 
why this is so, can not in our present state of knowledge be 
explained. Why the supposed irritation of a slight bruise 
should be more potent in this regard than a more extensive and 
severe one merely illustrates the difficulties under which we 
labor when we try to explain some of the facts which now seem 
to be fundamental in the development of this real scourge of 
the race. 
In the beginning of this paper it was stated that there was 
only one thing known positively about this condition, viz., 
that in the beginning it is always local. Cancer never com- 
mences as a large mass. At first it is so small as to be micro- 
scopic in size. More than this, it is marvelously slow in the 
vast majority of cases, in its development; if on the surface, 
its removal or destruction is one of the simplest problems in all 
of medicine or surgery. But right at this stage usually com- 
mences the great tragedy of cancer as it affects the individual 
patient. It is usually not removed when it is a little insignifi- 
cant growth, and because it is not we have the real explanation 
