d on the
doctrine of probabilities from the average distribution. Though possibly
the offspring of a cancerous individual may display a slightly greater
tendency toward the development of that strange, curious process of
"autonomy" than the offspring of the average individual, this tendency
is so small and occurs so infrequently as to be a factor of small
practical importance in the propagation and spread of the disease.

In insanity and epilepsy we have probably the last refuge and almost
only valid instance of the old belief in the remorseless heredity of
disease. But even here the part played by heredity is probably only a
fraction of that which it is popularly, and even professionally,
believed to play. It is, of course, obvious that diseases which tend
quickly to destroy the life of the patient, especially those which kill
or seriously cripple him before he has reached the age of reproduction,
or prevent his long surviving that epoch, will not, for mechanical
reasons, become hereditary. The Black Death, or the cholera, for
instance, could not "run in a family." Supposing that children were born
with a special susceptibility to this disease, there would obviously
soon be no family left.

The same is true in a lesser degree of milder or more chronic diseases.
The family which was hereditarily predisposed to scarlet fever, measles,
smallpox, or tuberculosis would not last long, and in fact the whole
progress of civilization has been a continuous process of the weeding
out of those who were most susceptible and the survival of those who
were least so.

But when we come to deal with certain conditions, fortunately rare, such
as functional disturbances of the nervous system, which neither
seriously unfit their possessor for the struggle of life nor prevent
him from reproducing his kind, then it becomes possible that a tendency
to such disease may be transmitted through several successive
generations.

Such is the case with insanity, with epilepsy, with _hemophilia_, or
"bleeders," and with certain rare and curious disturbances of the
nervous system, such as the hereditary _ataxias_ and "tics" of various
sorts. However, even here the only conditions on which these diseases
can continue to run in a family for more than one or two generations is
either that they shall be mild in form or that only a comparatively
small percentage of the total family shall be affected by them. If, for
instance, two-thirds, one-half, or e