ther,
and is stored temporarily in the nucleii of the reproductive cells.
During the life of the individual this germ plasm is capable of
increasing in amount without changing its nature, and it thus continues
to grow and is handed down from generation to generation, always endowed
with the power of developing into a new individual under proper
conditions, and of course when it does thus give rise to new individuals
they will all be alike. We can thus easily understand why a child is
like its parent. It is not because the child can inherit directly from
its parent, but rather because both child and parent have come from the
unfolding of two bits of the same germ plasm. This fact of the
transmission of the hereditary substance from generation to generation
is known as the theory of the _continuity of germ plasm_.

Such appears to be, at least in part, the machinery of heredity. This
understanding makes the germ substance perpetual and continuous, and
explains why successive generations are alike. It does not explain,
indeed, why an individual inherits from its parents, but why it is like
its parents. While biologists are still in dispute over many problems
connected with heredity, all are agreed to-day that this principle of
the continuity of the heredity substance must be the basis of all
attempts to understand the machinery of heredity. But plainly this whole
process is a function of the cell machinery. While, therefore, the idea
of the continuity of germ substance greatly simplifies our problem, we
must acknowledge that once more we are thrown back upon the mysteries of
the cell. Until we can more fully explain the cell machine we must
recognize our inability to solve the fundamental question of why an
individual is like its parents.

[Illustration: FIG. 50.--Diagram illustrating the principle of
heredity.

_A_ represents an egg of a starfish. From one half, the unshaded
portion, develops the starfish of the next generation, _B_. The other is
distributed without change in the ovaries, _ov_, of the individual, _B_.
From these ovaries arises the next egg, _A'_, with its germ plasm. This
germ plasm is evidently identical with that in _A_, since it is merely a
bit of the same handed down through the individual, _B_. In the
development of the next generation the process is repeated, and hence
_B'_ will be like _B_, and the third generation of eggs identical with
the first and second. The undifferentiated part of the ger