1060 THE STRUCTURE OF EVOLUTIONARY THEORY
vertex becomes the locus of canonical causes, while contributions from the other two
vertices become constraints upon full determination by general laws of nature.
Under this theory (Fig. 10-llb), adaptive form arises from the operation of
general laws upon biological materials. But if, to understand any current adaptation,
we need to invoke strictures based either upon: (1) passive inheritance within a
specifically designated genealogical system (a constraint from the historical vertex,
imposed by a unique and contingent biological particular, thus detracting from a
claim for full causation by general laws); or (2) upon the immediate construction of a
particular adaptation by a biological process tied to specifics of adaptive pressures in
one environment at one time (a constraint from the functional vertex of current
natural selection)—then the full power of the purely physical model becomes
compromised. Thus, as I show in Figure 10-1 lb, a purely physicalist theory for the
direct generation of adaptive form by spatiotemporally invariant laws of nature,
places its canonical mechanism at the structural vertex, and regards inputs from both
the historical vertex (strictures from past particulars) and the functional vertex
(strictures imposed by the specifics of current biological situations) as constraints.
Finally (Fig. 10-11c), a pure (and caricatured) cladist, who believes that the
reconstruction of genealogical pattern (without reference to modes of causation)
defines the goal and purpose of evolutionary biology, would locate his canonical
mechanisms at the historical vertex, and view contributions from the other two
vertices as constraints upon his ability to detect a pure genealogical signal in the
currently adaptive traits of organisms. Influences from the structural vertex must be
counted as constraints because their timeless generality covers or distorts the desired
signal of particular history with an unwanted contribution from causes with no
specific genealogical content. And influences from the functional vertex impose a
confusing immediate particular—an autapomorphy offering no help at all in the
reconstruction of lineages, and therefore conventionally omitted in cladistic
analysis—degrading a phyletic signal that might otherwise map the organism's
position in the genealogical system of a more general lineage. (This insight about
particular and immediately adaptive features—autapomorphies in cladistic
terminology—has long been regarded as a truism in taxonomic practice. Darwin
himself frequently emphasized (1859, chapter 13) that new and unique adaptations
can only confound taxonomic relationships, and that systematists must privilege
characters with broad homological residence in the taxa of larger genealogical
groupings.)
Thus, the cardboard Darwinian functionalist, the cardboard physical
structuralist, and the cardboard genealogical cladist each chooses a different vertex
for canonical causation, and must then define influences from the other two vertices
as constraints upon the efficacy of his orthodox mechanism. My examples are
purposefully cartoonish, but the principle thus illustrated represents an important, and
insufficiently appreciated, generality in