1250 THE STRUCTURE OF EVOLUTIONARY THEORY
When a hemispherical dome stands on a set of four rounded arches meeting at
right angles to form a square—a very common design in ecclesiastical architecture
and many other buildings—four tapering triangular spaces must appear directly under
the dome, each formed by the space left over between the dome itself (above) and the
pair of adjacent arches (at the sides) meeting at right angles (see Figure 11- 8 ). These
four spaces, called pendentives, must form as a structurally necessary side
consequence of the architect's basic decision to mount a dome on four rounded
arches, so arrayed. Lewontin and I never claimed that these spaces did nothing useful
(for obvious and trivial starters, their roofing keeps out the birds and the rain). But we
did argue that their ineluctable size and shape—their number and triangular form—
arose as side consequences of a previous architectural decision, and could not be
viewed as adaptations in themselves. (The four pendentives, in other words and by
analogy with my previous example, hold the same status as mold marks on an old
bottle—necessary side consequences of an architectural decision, not functional
features in themselves.)
The general architectural term for such "spaces left over" is spandrel— a lovely
name derived from the primordial human tool of measurement, the span of our own
hand (or of the corresponding feature in an anthropomorphized divine architect—as
in Isaiah's God (40:11-12) "who hath measured the waters in the hollow of his hand,
and meted out heaven with the span"). Classical spandrels are two-dimensional
spaces left over (Figure 11-9) including the vertical boards between steps of a
staircase, the triangular spaces between arches arrayed in a linear row, and the flat
horizontal stretches (called "spandrel courses") on large office buildings, located
between the tops of windows on the floor below and the bottoms of windows in the
next story just above. Apparently—see documentation in Gould, 1997e— some
architects restrict the term "spandrel" to two-dimensional spaces left over, whereas
others, particularly in European usage, extend the term to any space that arises as a
side-consequence of a prior decision, and not as an explicitly designed feature in
itself, thus including the three-dimensional pendentives of San Marco and thousands
of other buildings. In any case, I consciously decided to apply this remarkably
appropriate term to San Marco's pendentives because their shared property with
classical two-dimensional spandrels—their status as architectural byproducts (at least
for their forms, numbers, and placements)—cannot be denied. I also felt that biology
needed a term for such architectural sequelae of "adaptive" decisions, and that this
well known term from a related discipline could serve admirably.
an entire book by linguistic scholars on our (mostly unconscious) literary tactics (Selzer,
1993); a wise commentary by a noted scholar of medieval buildings (Mark, 1996), and,
wonder of wonders in our faintly philistine (and avowedly secular) professional community,
a burgeoning interest in at least two humanistic subjects generally shunned by scientists for
reasons of passive ignorance, or even active distaste: church architecture (Dennett, 1995;
Houston, 1997) and literary parody of the puerile, "ain't-I-clever," sort embodied in two
titles, "The Scandals of San Marco" and "The Spaniels of St. Marx." Ouch! (Borgia, 1994;
Queller, 1995).