1256 THE STRUCTURE OF EVOLUTIONARY THEORY
I chose the San Marco spandrels because, in this case, we have firm evidence in
both categories to reject this alternative possibility, to affirm the origin of spandrels
as nonadaptive side consequences of a larger architectural decision, and to understand
the mosaic designs as secondary adaptations within a space exapted for their utility.
(1) Direct historical data. We know that the spandrels were not built to house the
evangelists because San Marco stood and operated in its appointed role as a church
for at least three centuries before the mosaicists applied their astonishing work to a
series of constrained and previously unornamented spaces (see Demus's classic four
volume monograph (1984) on the history, architecture and iconography of San
Marco).
(2) Inferences from taxonomic structure. Human buildings cannot be ordered as
branching genealogical systems following both Linnaean and Darwinian logic—so
we cannot base our inferences upon true cladograms in this case. But a taxonomy of
ecclesiastical, and other large public, buildings does permit us to invoke a series of
standard arguments, long utilized in the comparative anatomy of organisms as well,
and all indicating both a nonadaptive original status for the spandrels, and a
secondarily adaptive role for the evangelists as good designs in exapted spaces:
- Ubiquity vs. occasionality as evidence for both priority and necessity.
Thousands of Western buildings feature domes atop rounded arches—and every
single one of them generates tapering triangular spaces at the intersections. These
spandrels are ornamented in a wide variety of ways, each appropriate to the local
circumstance. Many carry no ornaments at all (indicating that spandrels must be
generated but need not bear "adaptive" designs). - Constancy of form vs. variety in usage. How could such a diversity of
employment always generate the same housing? This particular distribution of
anatomical "features" indicates that a constant form preceded the various ornaments
thereof (both historically in phyletic time in some cases, and structurally in
ontogenetic and architectural sequence in all cases). I have, for example, noted
various religious foursomes in the spandrels of other churches —the four major Old
Testament prophets, Isaiah, Jeremiah, Ezekiel, and Daniel in many cases; or, in San
Ignazio in Rome (and "politically correct" by current standards of gender equality),
four Old Testament heroes and their weapons: David with his sling, Judith with her
sword (to behead Holofernes), Samson with his jawbone, and Jael with her tentpost
(to transfix Sisera through the head). I also have noted secular themes in civic or
scientific building—the four continents of Africa, Europe, Asia, and America under
the main dome of the Victor Emanuel arcade in Milan; four classical lawmakers
(Justinian, Pericles, Solon, and Cicero) under the glass dome in the Victorian
courtroom of the Landmark Center, St. Paul, Minnesota; four mainstays of
civilization (peace, justice, industry, and agriculture) in the County Arcade of Leeds,
England, built in 1900; or the four Greek elements in the pendentives under the main
dome at the headquarters of the National Academy of Sciences in Washington, DC!