188 THE STRUCTURE OF EVOLUTIONARY THEORY
As a primary sign of our estrangement from Lamarck's world, and our lack of
understanding for his system, all "standard" textbook examples of Lamarckian
evolution ignore his fundamental, higher-level principle of progress, and only cite
instances of lateral twigs built as highly specialized adaptations. We do this, I
suppose, because adaptation and specialization constitute the major theme in our
modern evolutionary vocabulary (in the altered guise of Darwinian causation),
while the bulk of Lamarck's system has passed beyond our notice into cognitive
dissonance. In any case, every classical example—from eyeless moles, to webbed
feet of water birds, to long legs of shore birds, to the blacksmith's strong right
arm—ranks as a lateral deviation, not a stage on the main sequence. As for the
greatest cliché and exemplar of all, the ubiquitous giraffe of our text-books,
happily munching leaves at the tops of acacia trees, Lamarck provides only one
paragraph of speculation—with no elaboration, no measurements, no data at all.
An example can become a knee-jerk standard for many reasons, with cogent,
complete documentation not always prominent among them (see Gould, 1991b, on
the evolution of horses and the size of Hyracotherium). Nor does simple repetition
enhance the probability of truth! Lamarck wrote this and only this about giraffes
(even repeating a common error about differential lengths of fore and hind limbs):
It is interesting to observe the result of habit in the peculiar shape and size
of the giraffe (Camelo-pardalis)': this animal, the tallest of the mammals, is
known to live in the interior of Africa in places where the soil is nearly
always arid and barren, so that it is obliged to browse on the leaves of trees
and to make constant efforts to reach them. From this habit long maintained
in all its race, it has resulted that the animal's forelegs have become longer
than its hind-legs, and that its neck is lengthened to such a degree that the
giraffe, without standing up on its hind-legs, attains a height of six meters
(1809, p. 122).The final, complex order of life arises from an interplay of the two forces in
conflict, with progress driving lineages up the ladder and adaptation forcing them
aside into channels set by peculiarities of local environments: "The state in which
we find any animal, is, on the one hand, the result of the increasing complexity of
organization tending to form a regular gradation; and, on the other hand, of the
influence of a multitude of very various conditions ever tending to destroy the
regularity in the gradation of the increasing complexity of organization" (1809, p.
107). In his strongest characterization of the two forces as conflicting, Lamarck
tells us in another passage that "nature's work [of progress] has often been
modified, thwarted and even reversed by the influence exercised by very different
and indeed conflicting conditions of life upon animals exposed to them throughout
a long succession of generations" (1809, p. 81).
Two additional statements in the Philosophie zoologique give dramatic
expression to the absolute distinction of the forces, and to their hierarchical
character, with progress as primary and regular, and diversity as secondary and
disturbing. The first provides a vivid iconography of the two-factor theory: