ells arise, as far as their essential and characteristic
substance is concerned, not at all out of the body of the individual,
but direct from the parent germ-cell_. This latter view Weismann holds
to be the correct one, and, on this theory, heredity depends on the fact
that a substance of special molecular composition passes over from one
generation to another. This is the "germ-plasm," the power of which to
develop itself into a perfect organism depends on the extraordinary
complication of its minutest structure. At every new birth a portion of
the specific germ-plasm, which the parent egg-cell contains, is not used
up in producing the offspring, but is reserved unchanged to produce the
germ-cells of the following generation. Thus the germ-cells--so far as
regards their essential part the germ-plasm--are not a product of the
body itself, but are related to one another in the same way as are a
series of generations of unicellular organisms derived from one another
by a continuous course of simple division. Thus the question of heredity
is reduced to one of growth. A minute portion of the very same
germ-plasm from which, first the germ-cell, and then the whole organism
of the parent, were developed, becomes the starting-point of the growth
of the child.

_The Cause of Variation._

But if this were all, the offspring would reproduce the parent exactly,
in every detail of form and structure; and here we see the importance of
sex, for each new germ grows out of the united germ-plasms of two
parents, whence arises a mingling of their characters in the offspring.
This occurs in each generation; hence every individual is a complex
result reproducing in ever-varying degrees the diverse characteristics
of his two parents, four grandparents, eight great-grandparents, and
other more remote ancestors; and that ever-present individual variation
arises which furnishes the material for natural selection to act upon.
Diversity of sex becomes, therefore, of primary importance as _the cause
of variation_. Where asexual generation prevails, the characteristics of
the individual alone are reproduced, and there are thus no means of
effecting the change of form or structure required by changed conditions
of existence. Under such changed conditions a complex organism, if only
asexually propagated, would become extinct. But when a complex organism
is sexually propagated, there is an ever-present cause of change which,
though slight in any