1136 THE STRUCTURE OF EVOLUTIONARY THEORY
issue of this important discussion. We need to develop criteria for ordering and
evaluating our highly varied and ever-growing compendium of homoplastic results
generated along homologous developmental pathways—for these cases fall along a
continuum from narrow and controlling channels of constraint to insignificant sharing
of nonspecific building blocks. When Pharaoh "made the children of Israel serve with
rigor" (Exodus 1:13), they fabricated bricks to use in a full range of buildings: "And
they built for Pharaoh treasure cities, Pithom and Raamses" (Exodus 1:11). Now if
these bricks built every structure in the city, from great pyramids to public toilets, we
might identify a homologous generator of all final products (bricks of the same
composition made by the same people in the same way over a continuous stretch of
time). But we could scarcely argue that these homologous generators exerted any
important constraint over the differing forms of Pharaoh's final products—if only
because all realized architectural diversity shared the same building blocks.
But if I note a majestic portico of Corinthian columns in front of a building in
modern Manhattan, I recognize a strong internal constraint imposed by an
architectural module of very different status. The Corinthian column, last and most
ornate of the classical orders, consists of a slender fluted shaft (with 24 flutes in
"standard" examples), capped by a striking, distinctive, and elaborate capital (the
defining "species" character in a taxonomic analogy) adorned with stylized acanthus
leaves. Few Greek examples survive, but the Romans then used Corinthian columns
extensively and for several centuries. Vetruvius, who wrote the only surviving work
on classical architecture, described such columns in detail in the 1st century BC, and
later builders of the Italian Renaissance replicated the design in all aspects of form
and proportion, whence, ever since, buildings in classical style have often used
Corinthian columns on their facades, porticos and lobbies.
Like Pharaonic bricks, Corinthian columns hold clear status as homologous
underlying generators for their continuous phyletic history and stable form. But
whereas Pharaonic bricks did little to constrain a resulting building by their form or
structural character, and would not therefore sustain an interesting interpretation of
parallelism for two similar buildings that happened to employ them in construction (if
only because many other, very unsimilar, buildings in town also use the same bricks),
Corinthian columns do exert a strong structural constraint from an inherited past (a
homology) that can help us to identify and distinguish buildings, even 2500 years
after the invention of this unchanged form.
First of all, we can identify the lineage of the element just by looking (for the
acanthus leaves mark the species), so I know the source of constraint before
proceeding any further, whereas a brick may be difficult to peg as Pharaonic, or as a
product of independent invention for a simple, obvious and utilitarian form (a
Chinese version, for example). Second, tradition and intrinsic form dictate that this
large and elaborate column only be used in limited ways (whereas my Pharaonic
brick can build almost anything)—so the choice of the column constrains the form
and function of the building.