The inheritance of an acquired disease is not only inconceivable, in the
light of what is known about the germ-plasm, but there is no evidence to
support it. While there is most decidedly such a thing as the
inheritance of a tendency to or lack of resistance to a disease, it is
not the result of incidence of the disease on the parent. It is possible
to inherit a tendency to headaches or to chronic alcoholism; and it is
possible to inherit a lack of resistance to common diseases such as
malaria, small-pox or measles; but actually to inherit a zymotic disease
as an inherent genetic trait, is impossible,--is, in fact, a
contradiction of terms.
(3) When we come to the effects of use and disuse, we reach a much
debated ground, and one complicated by the injection of a great deal of
biological theorizing, as well as the presence of the usual large amount
of faulty observation and inference.
It will be admitted by every one that a part of the body which is much
used tends to increase in size, or strength, and similarly that a part
which is not used tends to atrophy. It is further found that such
changes are progressive in the race, in many cases. Man's brain has
steadily increased in size, as he used it more and more; on the other
hand, his canine teeth have grown smaller.
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