stable substance, continuous from generation to generation since the
first appearance of life on the globe. It is in the body of the
parent, but scarcely of it. Its relation to the body is like that of
a plant to the soil or of a parasite to its host. It receives from
the body practically only transport and nourishment. It is like a
self-perpetuating, close corporation; and the somatoplasm has no
means of either controlling it or of gaining representation in it.

Says Weismann[A]: "The germ-cells are contained in the organism, and
the external influences which affect them are intimately connected
with the state of the organism in which they lie hid. If it be well
nourished, the germ-cells will have abundant nutriment; and,
conversely, if it be weak and sickly, the germ-cells will be
arrested in their growth. It is even possible that the effects of
these influences may be more specialized; that is to say, they may
act only upon certain parts of the germ-cells. But this is indeed
very different from believing that the changes of the organism which
result from external stimuli can be transmitted to the germ-cells
and will redevelop in the next generation at the same time as that
at which they arose in the parent, and in the same part of the
organism."

[Footnote A: Essays upon Heredity, p. 105.]

But if the germ-plasm has this constitution and relation to the rest
of the body, how is any variation possible? Different individuals of
any species have slightly different congenital tendencies. Hence in
the act of fertilization two germ-plasms of slightly different
structure and tendency are mingled. The mingling of the two produces
a germ-plasm and individual differing from both of the parents.