It is difficult to entirely free ourselves from the flattering and
almost universal idea that parents are true originators or creators of
copies of themselves. But the main truth, if not the whole truth, is
that they are merely the transmitters of types of which they and their
offspring are alike more or less similarly moulded resultants. A parent
is a trustee. He transmits, not himself and his own modifications, but
the stock, the type, the representative elements, of which he is a
product and a custodian in one. It seems probable that he has no more
definite or "particulate" influence over the reproductive elements
within him than a mother over the embryo or a vessel over its cargo.
Parent and offspring are like successive copies of books printed from
the same "type." A battered letter in the "type" will display its
effects in both earlier and later copies alike, but a purely extraneous
or acquired flaw in the first copy is not necessarily repeated in
subsequent copies. Unlike printer's type, however, the material source
of heredity is of a fluctuating nature, consisting of competing elements
derived from two parents and from innumerable ancestors.
Galton compares parent and child to successive pendants on the same
chain. Weismann likens them to successive offshoots thrown up by a long
underground root or sucker. Such comparisons indicate the improbability
of acquired modifications being transmitted to offspring.
That parts are developed in offspring independently of those parts in
parents is clear. Mutilated parents transmit parts which they do not
possess. The offspring of young parents cannot inherit the later stages
of life from parents who have not passed through them. Cases of remote
reversion or atavism show that ancestral peculiarities can transmit
themselves in a latent or undeveloped condition for hundreds or
thousands of generations. Many obvious facts compelled Darwin to suppose
that vast numbers of the reproductive gemmules in an individual are not
thrown off by his own cells, but are the self-multiplying progeny of
ancestral gemmules. Galton restricts the production of gemmules by the
personal structure to a few exceptional cases, and would evidently like
to dispense with pangenesis altogether, if he could only be sure that
acquired characters are never inherited. Weismann entirely rejects
pangenesis and the inheritance of acquired characters. This enables him
to explain heredity by his theory of the "Continuity of the
Germ-plasm."[67] Parent and offspring are alike successive products or
offshoots of this persistent germ-substance, which obviously would not
be correspondingly affected by modifications of parts in parents, and so
would render the transmission of acquired characters impossible.