1236 THE STRUCTURE OF EVOLUTIONARY THEORY
narrow crevices beneath bark, and also constitutes an aerofoil which enables it to
glide from tree to tree. Phylogenetic analysis shows that the flattening first developed
in the context of crevice use and was only later coopted to gliding" (p. 139).
Moreover, such primary adaptations as head flattening for penetration of
crevices usually work in synergy with other coopted features that operate as
exaptations in the complex and multifaceted "fit" of the organism to its new
environment. For example, when lizard heads become flattened, "the eyes do not
usually become correspondingly smaller and, in normal activity, bulge upwards
above the skull surface" (p. 139). Arnold then continues to describe the remarkable
exaptation of a mouthful of eye: "However, when a lizard flees into a narrow crevice
the eyes must be accommodated within the depth of the flattened head. They are most
usually pushed downwards by the ceiling of the crevice as the lizard moves deeper
into it, so their upper margins are flush with the skull roof and their lower sections
bulge through the palate into the buccal cavity (p. 139)." In scincids and lacertids, the
eye bulges vertically downward into the suborbital foramen. This aptation depends
upon the preexistence of this opening (obviously evolved for other reasons), as
indicated by its general distribution on the cladogram of lizards. Therefore, "as the
occurrence of the foramen on the phylogeny of the forms concerned precedes
occupation of crevices, its use for accommodating the eye within the reduced depth
of the skull is an exaptation" (p. 139).
A common, but unfounded, objection to exaptation enters the logical structure of
argument at this point. Several colleagues (Coddington, 1988, for example) have
claimed that since almost any exapted structure will undergo secondary modification
for its new role, and since these subsequent changes must count as adaptations, the
concept of exaptation becomes either useless or confusing because any primarily
exapted structure must then accrete secondary adaptations to be fully "fit" for its new
role. I raise this issue here because, as Arnold points out, the suborbital "foramen is
initially small and triangular, allowing only limited projection of the eye into the
buccal cavity" (p. 139). This hole then undergoes a secondary adaptive enlargement
to accommodate the eye more completely.
I am confident that this common objection cannot be sustained, because
hierarchical sequences of processes, each with a different name and status, practically
define the nature of complex historical change, and pose no conceptual problems (but
rather help us to understand and sort out these sequences), provided that we can
specify the order of temporal precedence and hierarchical nesting. Exactly the same
issue arises for homology and convergence, and for plesiomorphy and apomorphy.
The front appendages of bats and birds are homologous as forearms and convergent
as wings; live birth is plesiomorphic for the clade of marsupial and placental
mammals, and apomorphic for the same clade within the Tetrapoda. Similarly, the
suborbital foramen of lizards is exaptive as a preexisting receptacle for the pushed-
down eyes of lizards with flat heads, whereas the subsequent enlargement of the hole
may be adaptive for better accommodation of the eyes. The two aspects