Structural Constraints, Spandrels, and Exaptation 1191
phyletically unique and distinctive forms (Figure 11-2); an explanation of honeycomb
cells as optimal balances of strength and holding capacity; and a convincing claim
that the hexagonal closest packing of corallites in Paleozoic colonial rugosans cannot
be read as distinctive phyletic adaptations of particular lineages, because corallites
assume circular cross sections when they grow in less confined spaces with no
contact between individuals (Figure 11-3).
11 - 3. Ecophenotypic shaping of corallites by physical crowding rather than genetic coding.
Cross sections are circular when the corallites do not touch, and hexagonal when they grow in
a tightly packed configuration. From D'Arcy Thompson, 1917.