we may suppose that these variations
in the germ may arise. The first is by the direct influence upon the
germ plasm of certain unknown external conditions. The life substance of
organisms is always very unstable, and, as we have seen, acquired
variations are caused by external influences directly affecting it. Now,
the hereditary material is also life substance, and it is plainly a
possibility for us to imagine that this germ material is also subject to
influences from the conditions surrounding it. That such variations do
occur appears to be hardly doubtful, although we do not know what sort
of influences can produce them. If the germ plasm is wholly stored
within the reproductive gland, it is certainly in a position to be only
slightly affected by surrounding conditions which affect the animal. We
can readily understand that the use of an organ like the arm will affect
it in such a way as to produce changes in its protoplasm, but we can
hardly imagine that such use of the _arm_ would produce any change in
the hereditary substance which is stored in the reproductive organs.
External conditions may thus readily affect the body, but not so readily
the germ material. Even if such material is distributed more or less
over the body instead of being confined to the reproductive glands, as
some believe, the difficulty is hardly lessened. This difficulty of
understanding how the germ plasm can be affected by external conditions
has led one school of biologists to deny that it is subject to any
variation by external conditions, and hence that all modification of the
germ plasm must come from some other source. Probably no one, however,
holds this position to-day, and it is the general belief that the germ
plasm may be to some slight extent modified by external conditions. Of
course, if such variations do occur in the germ plasm they will become
congenital variations of the next generation, since the next generation
is the unfolding of the germ plasm.

The second method by which the variations of germ plasm may arise is
apparently of more importance. It is based upon the fact that, with all
higher animals and plants at least, each individual has two parents
instead of one. In our study of cells we have seen that the machinery
of the cell is such that it requires in the ordinary process of
reproduction the union of germinal material from two different
individuals to produce a cell which can develop into a new individual.
As we