Structural Constraints, Spandrels, and Exaptation 1257
- Suboptimal or ill-fitting designs as evidence of historical sequencing. As
emphasized in Chapter 2, in my analysis of Darwin's rich and subtle methodology for
historical inference, perfection covers the tracks of history, whereas oddities and
imperfections often reveal both the direction and the stages of temporal sequences.
When four evangelists establish an optimal design in four spandrels, we cannot
determine the sequence of events from the structure alone: either the spaces came
first, and the evangelists fit in later, or plans to depict the evangelists came first, and
architects then fashioned the spaces as appropriate housing. But a peculiar, ill-fitting,
or suboptimal design might suggest an order of historical precedence. If three
spandrels housed elegant mosaics of Genesis, Exodus, and Leviticus, while similar
designs for Numbers and Deuteronomy appeared all scrunched together in an ugly
and overcrowded fourth spandrel, we might assume (at least as a preferred hypothesis
for further test) that the four spandrels originated for a different and prior reason, and
that a later mosaicist miscalculated badly in formulating plans for placing symbols of
the Torah into these preexisting spaces.
Similarly, the foursomes in several sets of spandrels in European churches seem
rather forced or even ill fitting, thus indicating that a fixed number of spaces (and
their form) preceded any decision about embellishment. In the 16th century church of
San Fedele in Milan, for example, four concepts, personified as women, decorate the
spandrels under the central dome—the famous biblical trio of faith, hope, and charity
(1 Corinthians, 13), with the remaining fourth spandrel occupied by religion. Three
spandrels might have carried the intended design better, but architectural constraints
dictated a quartet, so the designers had to draft a fourth participant, however
unsanctioned by a very famous quotation. By etymology, "religion" may mean,
"tying together," but this particular woman seems more out of place than integrative
at San Fedele. Interestingly, the design of the great Romanesque pulpit in the Duomo
of Pisa (the building adjacent to the structurally inadaptive but touristically highly
exaptive Torre Pendente, or Leaning Tower) imposes no architectural necessity for a
quartet of spandrels. Its lectern rests upon a tripartite column that expands to three
ornamental spaces at the top. The three spaces carry heads representing faith, hope,
and charity—all by their proper selves this time, with no fourth interloper to complete
the occupation of a preexisting architectural constraint. - Invariable correlation of a specific form under discussion with broader
structural features of the totality. As an unprofound point in this case, the number of
spandrels always correlates perfectly with the number of arches supporting an
overlying dome—thus identifying the spandrels as automatic side consequences of a
broader architectural decision. We might question this argument if all buildings
mounted their domes on four arches, and therefore always generated pendentives in
sets of four. But, although four remains by far the most common number, the
comparative anatomy of large public buildings includes some variety. The central
dome of St. Paul's Cathedral in London rests upon eight arches, and the resulting
eight spandrels feature the