rm-plasm. They see only the
body-plasm. They are devoted to the dogma that if they can change the
body (and what is here said of the body applies equally to the mind) in
the direction they wish, this change will in some unascertainable way be
reproduced in the next generation. They rarely stop to think that man is
an animal, or that the science of biology might conceivably have
something to say about the means by which his species can be improved;
but if they do, they commonly take refuge, deliberately or
unconsciously, in the biology of half a century ago, which still
believed that these changes of the body could be so impressed on the
germ-plasm as to be continued in the following generation.

Such an assumption is made to-day by few who have thoroughly studied the
subject. Even those who still believed in what is conventionally called
"the inheritance of acquired characteristics" would be quick to
repudiate any such application of the doctrine as is commonly made by
most of the philanthropists and social workers who are proceeding
without seeking the light of biology. But the idea that these
modifications are inherited is so widespread among all who have not
studied biology, and is so much a part of the tradition of society, that
the question must be here examined, before we can proceed confidently
with our program of eugenics.

The problem is first to be defined.

It is evident that all characters which make up a man or woman, or any
other organism, must be either germinal or acquired. It is impossible to
conceive of any other category. But it is frequently hard to say in
which class a given character falls. Worse still, many persons do not
even distinguish the two categories accurately--a confusion made easier
by the quibble that _all_ characters must be acquired, since the
organism starts from a single cell, which possesses practically none of
the traits of the adult.

What we mean by an inborn character is one whose expression is due to
something which is present in the germ-plasm; one which is inherent and
due to heredity. An acquired character is simply a modification, due to
some cause external to the germ-plasm acting on an inborn character. In
looking at an individual, one can not always say with certainty which
characters are which; but with a little trouble, one can usually reach a
reliable decision. It is possible to measure the variation in a given
character in a group of parents and their children,