182 THE STRUCTURE OF EVOLUTIONARY THEORY
Lamarck's argument shows more "leakage" than his words would suggest.
Consider the following, as stressed by Corsi and Burkhardt:
ONTOGENY. Although Lamarck presents the forces of adaptation as deviations
from, and therefore secondary to, the primary causes that build complexity, he
apparently developed his mechanism for progress from his previously formulated
ideas about adaptation (Corsi, 1988; and newly discovered evidence in Gould,
2000d). Still, the psychological source of a theory needn't map its eventual logical
structure, and this point, while interesting, scarcely compromises the distinctness
and ranking of the two sets.
CAUSATION. At several points, Lamarck breaches the boundaries between his
sets in discussing causation.
(1) Soft inheritance works in both sets. Whether an organism becomes more
complex because fluids carve channels by intrinsic chemistry, or becomes better
adapted because habits change in response to altered environments, the acquired
features must still be passed to offspring by direct inheritance. Still, a common
mechanism may work in two modes, and this linkage does not compromise
Lamarck's claim for distinctness.
(2) The style of action for soft inheritance in adaptation depends upon the
state of complexity engendered by opposing forces of progress. Lamarck divided
organisms into three ascending groups designated, in the old Aristotelian terms, as
insensitive, sensitive, and rational. The first group, too simple to mount a creative
response to external change, reacts to environment not by altered habits, but by
direct influence. The capacity for active response, Lamarck's famous sentiment
interieur, only arises in the second group and unleashes the tripartite causal
sequence of changed environment to altered habit to modified form.
(3) The real blurring occurs when we try to make sense of Lamarck's claim
that forces of progress can build the entire sequence from infusorian to complex
vertebrate without any environmental change. Lamarck surely makes this assertion
explicitly, without hesitation (see citation on p. 187), and the distinctness of his
two forces depends upon this potential independence. But Lamarck does not work
out a consistent justification, and several frustrated historians have even argued
that he could not have done so without contradiction—that his system, in other
words, suffers from a true defect in logic of argument on this point.
The simplest organisms, Lamarck states, are carved out and complexified by
"subtle" and "imponderable" fluids—caloric and electricity in his system. These
fluids work in their intrinsic way to produce increased complexity. But as animals
differentiate and harden, fluids must flow in preset channels; the weak
imponderables then lose their power to mold, and the body's own ponderable fluids
must assume this role. (Lamarck locates this transition at about the echinoderm
grade of organization.) At this level, the "power of life" should become inoperable
without an impetus from environmental change— and the two sets of forces should
therefore commingle. Protected inside a rigid body, and constrained to flow in
preexisting channels, how can the ponderable