258 THE STRUCTURE OF EVOLUTIONARY THEORY
But many 19th century biologists, and many evolutionists in our day again,
feel that Darwin demoted constraint too far, and that the two domains—constraint
and adaptation—must again share potential partnership, as expressed by the
important relative frequency of each component. We would therefore restore the
strength of the dimension that Darwin first eliminated (when he reinterpreted Unity
of Type as a consequence of adaptation) and then reintroduced in weakened form
(when he allowed laws of growth to fill nooks and crannies in a domain ruled by
natural selection)—see Figure 4-4.
This full model of Figure 4-4 shows three dimensions of form and their
interactions: adaptation, constraint, and history. A current trait of an organism may
arise as an immediate adaptation to surrounding environments, as a constraint not
particular to the contingent history of its lineage (architectural or structural
principles, correlations to current adaptations), or by inheritance of an ancestral
form (often called historical or phylogenetic constraint, but quite different in
principle from nonhistorical styles of constraint). This distinction suggests a
recursion, because contributions from the axis of "history"
4 - 3. Darwin does allow minor influence for constraint apart from mere inheritance of past
adaptations. See text for details.4 - 4. Constraint reestablished as equal in importance to adaptation as an immediate cause of form.
See text for details.