Seeds of Hierarchy 215
different definition). Each id contains a determinant for every trait, and can
therefore build a body. Weismann identified ids with the disk-like microsomes,
recently observed as linearly ordered on chromosomes. The chromosomes
themselves, or "idants" in Weismann's system, carry ids in rows, and stand atop the
hierarchy of hereditary units.
Germinal selection rests upon the notion that determinants within germ cells
may be analogized to organisms within habitats. Just as organisms struggle for
limited resources (and not all can survive), determinants battle for the restricted
flow of nutriment available to any cell. The winners grow and proliferate; the
losers wither or disappear entirely. The strength of determinants governs the
phenotypic expression of their resulting trait. Thus, if the determinants of a
particular trait decrease or wither by germinal selection within cells, the trait will
suffer in expression by exhaustion of its molecular base.
Weismann viewed germinal selection as an analog to interspecific struggle,
rather than to competition among members of a single population—that is,
determinants for one organ battle for limited nutriment with determinants for other
parts of the body. But, unlike Roux's Kampfder Theile, Weismann's germinal
selection does operate as a theory of altered heredity, and therefore as a potential
evolutionary mechanism—for determinants weakened by germinal selection not
only build a diminished body part; they also pass fewer (and debilitated) offspring
determinants to germ cells of the next generation.
The ingenuity of this argument lies in its capacity to take the most serious
potential challenges to the Allmacht of selection—a group of overt phenomena that
seem to lie outside the possible control of organismal selection, and therefore
within the domain of Lamarckism or orthogenesis—and to reinterpret them as
consequences of selection acting at a lower level. For if organismal selection can
produce directional trends in phenotypes during geological time, then germinal
selection can forge trends in strengthening or weakening determinants (and their
phenotypic expressions) across generations. The gradual and unidirectional
shriveling to oblivion of an organ not subject to personal selection certainly
suggests Lamarckian inheritance and evolutionary loss by disuse; but if we
descend a level and peer within the germ cells, we may envision (though we cannot
directly see) a constant competition and selection among determinants, with losers
paying the usual price of gradual and inexorable elimination. The Allmacht of strict
Darwinism may be sacrificed, as organismal selection loses its exclusivity; but
selection itself remains preeminent by expansion:
Powerful determinants in the germ cell will absorb nutriment more rapidly
than weaker determinants. The latter, accordingly, will grow more slowly
and will produce weaker descendants than the former... Since every
determinant battles stoutly with its neighbors for food, that is, takes to itself
as much as it can, consonantly with its power of assimilation and
proportionately to the nutriment supply, therefore the unimpoverished
neighbors of this minus determinant will deprive it of its nutriment more
rapidly than was the case with its more robust ancestors (1896, p. 24).