d helps the novelist out of difficulties, but
is itself inexplicable. In truth, however, the fact that a boy, like
his father, has a head and a heart and hands and feet, physical traits
characteristic of the human species, that he begins to walk and talk
and shave at about the same age as his father did--all this is the
fact of heredity. The fact that guinea pigs produce guinea pigs and
not rabbits is the fact of heredity. Often it is true that this
resemblance is strikingly particular. All know of family traits; we
may have our father's eyes or nose, our mother's hair or disposition,
a grandfather's determination or a grandmother's patience. But these
particular individual resemblances are no more and no less
illustrations of heredity than the fact that on the whole children are
more like their parents than like other human beings.

The subject of heredity is of supreme importance in the practice of
Eugenics. The facts of no other department of biological inquiry are
of equal value, and at the same time there is probably no biological
subject regarding which there is so much misunderstanding. Of the
many phases of this extremely fascinating subject there are chiefly
two with which we are particularly concerned as Eugenists. These are
the questions: first, how completely are all the distinguishing traits
of either or both parents represented in the offspring; and, second,
how completely is each trait inherited that is inherited at all? In
other words, what we are chiefly interested to know, as bearing upon
the subject in hand, is whether all or only some of the
characteristics of our parents are heritable, and whether the
offspring show each inherited trait with the same intensity shown in
the parent, or more, or less.

One of the leading British students of heredity has said that no one
should undertake the study of this subject unless he can instantly
detect and explain the fallacy involved in the familiar conundrum,
"Why do white sheep eat more than black ones?" It is perhaps the
elasticity of our language that makes possible the mental confusion
involved in this question, but yet it is certainly true that we do
tend to confuse individual and statistical statements. We must
remember, in connection with this subject particularly, that an
individual may belong to a group without representing it, and that
within a group there are many more individuals with average than with
exceptional characteristics. The mediocre is