194 THE STRUCTURE OF EVOLUTIONARY THEORY
to be overcome; they represent an essential part of his context for study, and they
played a major role in shaping the radical and different character of Darwin's
distinctive theory.
I believe that Lamarck had a far greater influence on Darwin than tradition
has allowed (a point advanced by other historians of science as well—see Corsi,
1978; Mayr, 1972, p. 90). I base this claim on Darwin's own contact with
Lamarck's works, his private reactions as recorded in letters, and the eventual
content of his theory. I don't claim that Darwin devised natural selection
and surely doing some service in making the subject of evolution discussible. The pairing of
Lamarck with Chambers does, therefore, represent the totality of highly noticed, fully evo-
lutionary systems available in England as Darwin formulated his theory.
This pairing becomes an important footnote to my argument because Chambers's theory is
also hierarchical in the same general sense as Lamarck's—that is, in advocating separate realms
of causes for progress and deviation, with progress at a higher level and difficult to discern, and
deviation as immediate and palpable, but incapable of generating the full taxonomic order of life.
Chambers's particular theory, however, differed radically from Lamarck's, and "Mr. Ves-
tiges" attacked his French predecessor and inspiration (via Lyell) as a man "whose notion is
obviously so inadequate to account for the rise of the organic kingdoms, that we only can place it
with pity among the follies of the wise" (1844, p. 231). But Chambers thoroughly misunderstood
Lamarck, accepting the usual caricature of mystical will leading to adaptation as Lamarck's
complete system. Somehow Chambers missed the hierarchical character of Lamarck's theory; he
understood that the force of adaptation could not yield progress, but, unaware that Lamarck had
ascribed life's ladder to different causes acting at another level, he concluded that Lamarck had
erred in trying to explain all evolution by adaptation alone.
Chambers remedied this misperception by devising a theory every bit as hierarchical as
Lamarck's actual proposal, but radically different in mechanism. Chambers's system works by
extended analogy to von Baer's laws of embryonic differentiation (Gould, 1977b, pp. 109-112).
Linear progress follows the embryonic sequence of the highest organism. The higher-level cause
of progress pushes creatures up the sequence. Meanwhile, the lower-level force of deviation
pulls organisms into adaptive configurations at various plateaus of design. A small impetus from
the cause of progress may bring a developing animal to the plateau of fishes; a greater push will
lead to reptilian, mammalian, or even human grade. Progress depends upon an ability "to protract
the straightforward part of the gestation over a small space" (1844, p. 213), thereby resisting the
lateral force of adaptation at lower plateaus. Chambers even introduces the cogent idea (still
arousing modern debate) that higher-level causes may be hard to ascertain because they operate
so rarely. They may work quite regularly and in an absolutely law like way, but only once every
million years or so—and our chance of observing them then becomes vanishingly small. (This
attack on a dogmatic uniformity, based only on observed modern causes, emerges in such recent
ideas as the bolide impact theory of mass extinction.) Thus, for Chambers, embryological
transcendence to the next stage of progress may occur regularly and rapidly, but very rarely—
while the lateral forces of adaptation operate around us all the time.
Darwin therefore faced only two widely discussed evolutionary systems as he formulated
his own. Both were hierarchical, and both proposed an elusive higher-level force of progress,
paired with a palpable but limited lower-level force of adaptation. Darwin must have been struck
by the enigma that what he could see didn't matter (in the long run of evolutionary advance), and
what mattered couldn't be seen. How much of his distinctive single-level theory of extrapolation
arose in reaction to this intractable dilemma of hierarchical theories posed in the old and invalid
style by Lamarck and Chambers? Hierarchy, as this chapter holds for its primary theme, has been
a crucial ingredient of evolutionary theories from the start, and may be more responsible than we
have recognized for the eventual character of Darwin's system.