Darwin's Dangerous Idea - Evolution and the Meaning of Life

272 BULLY FOR BRONTOSAURUS
sion. Does figure 10.3 display three different sorts of spandrels, or does it
display a spandrel on the left, and two ugly alternatives to spandrels? Like
other specialists, art historians often indulge in both strict and loose usages
for their terms. Strictly speaking, the tapering, roughly spherical surface
illustrated in figure 10.1, the sort of surface illustrated on the left in figure
10.3, is called a pendentive, not a spandrel. Strictly speaking, spandrels are
what remains of a wall once you punch an arch through it, as in figure 10.4.
(But even that definition leaves room for confusion. In figure 10.4, are we
shown spandrels on the left, and something else on the right, or do "pierced
spandrels" count as spandrels, strictly speaking? I don't know.)
Speaking more loosely, spandrels are places-to-be-dealt-with, and in that
looser sense, the three variations in figure 10.3 all count as spandrel varieties.
Another variety of spandrel (in that sense ) would be a squinch, shown in
figure 10.5.
But sometimes art historians speak of spandrels when they are talking
specifically about pendentives, the variety shown on the left in figure 10.3. In
that sense, squinches are not types of spandrels, but rivals to spandrels.
Now, why does all this matter? Because, when Gould and Lewontin say
that spandrels are "necessary architectural by-products," what they say is
false, if they are using "spandrel" in the narrow sense (synonymous with
"pendentive") and true only if we understand the term in the loose, allinclusive
sense. But in that sense of the term, spandrels are design problems,
not features that might either be designed (adaptations) or not. Spandrels in
the loose sense are indeed "geometrically necessary" in one regard: if you
The Spandrel's Thumb 273
Squinch. A corbelling, usually a small arch or half-comical niche, which is placed
across the corners of a square bay in order to form an octagon suitable for carrying
an octagonal cloister-vault or a dome. [Krautheimer 1981.]
FIGURE 10.5
place a dome over four arches, you have what you might call an obligatory
design opportunity-, you have to put something there to hold up the dome—
some shape or other, you decide which. But if we interpret spandrels as
obligatory places for one adaptation or another, they are hardly a challenge to
adaptationism.
But is there nevertheless some other way in which spandrels in the narrow
sense—pendentives—truly are nonoptional features of San Marco? That is
what Gould and Lewontin seem to be asserting, but if so, they are wrong. Not
only were the pendentives just one among many imaginable options; they
were just one among the readily available options. Squinches had been a
well-known solution to the problem of a dome over arches in Byzantine
architecture since about the seventh century.'
What the actual design of the San Marco spandrels—that is, pendentives—
has going for it are mainly two things. First, it is (approximately) the
"minimal-energy" surface (what you would get if you stretched a soap film in
a wire model of the corner), and hence it is close to the minimal surface area
(and hence might well be viewed as the optimal solution if, say, the number
of costly mosaic tiles was to be minimized!). Second, this smooth surface is
ideal for the mounting of mosaic images—and that is why the
FIGURE 10.4
1. "Whatever the origin of the dome on squinches, however, the importance of the
question, it seems to me, has been vastly overplayed. Squinches are an element of construction
which can be incorporated into almost any kind of architecture." (Krautheimer
1981, p. 359.)