yond the
field of heredity and led to a radical change in our notions of what
an organism really is. It is, of course, true in a sense that an
organism is a unit, an organism is one thing; but at the same time it
is true that an organism is fundamentally a collection of units, of
structural and functional characteristics which are really separable
things. A few of these units were mentioned in the first pages of this
chapter and others are mentioned on a later page. They serve as the
building blocks of organisms: individuals of the same species may be
made up of similar combinations or of different combinations. One unit
or a group of units may be taken out and replaced by others.

From the standpoint of heredity, and particularly from our eugenic
point of view, the most important results of the unit composition of
the organism lie in the fact that these units remain units throughout
successive generations and throughout successive and varying
combinations, whatever their associations may be from generation to
generation. It is a fact of the greatest eugenic significance that a
pure bred individual may be produced by a hybrid mated either with a
pure bred or with another hybrid; and that the pure bred resulting
will be just as pure bred as any. "Pure bred" now means pure bred with
respect to certain traits only. An individual may be pure bred in
certain of its characteristics, hybrid in others. Practically there is
no such thing as an individual which is either pure bred or hybrid in
_all_ its traits. One of the chief contributions, then, of Mendelism
to the subjects of Heredity and Eugenics is this--that a pure bred may
be derived from a hybrid in one generation: the pure bred produced by
a long series of hybrid individuals is just as pure as the pure bred
which has never had a hybrid in its ancestry. Another important
consequent is, that among the offspring of the same parents some
individuals may be pure bred and others hybrid. Community of parentage
does not necessarily denote community of characteristics among the
offspring. Yet by knowing the ancestry for one or two generations we
can know the qualities of the individual. Guesswork is eliminated and
the importance of the qualities of the individual is enormously
emphasized. It is necessary only to suggest the social and eugenic
significance of such facts relating to characteristics that are of
social or racial importance.

We shall have occasion in the next chap