1046 THE STRUCTURE OF EVOLUTIONARY THEORY
growth in gastropods. In any case, the West Indian land snail Cerion, the primary
subject of my own research, and perhaps the most phenotypically diverse genus of
land snails, grows with strong and invariant allometry in three recognizable stages
(Fig. 10-7): an early button-shaped or triangular phase with width increasing far more
rapidly than height; an intermediary "barrel" phase, where width increases slowly or
not at all, and height grows rapidly; and a final twisting of the aperture (phase 3 of
Fig. 10-7), before deposition of the definitive adult lip. In fact, Cerion owes its
name—from the Greek word for wax, in reference to the characteristic shape of
beehives—to the form produced by the first two allometric phases, particularly in
species with a relatively sharp transition between the upper button and the middle
barrel.
I have used inductive multivariate biometry to identify, and then to judge the
extent of influence for groups of ontogenetically correlated characters ("covariance
sets" in my terminology) that are both mechanically enforced by allometric growth,
and that also exert substantial, often controlling, impact upon patterns of temporal
and geographic variation in the phenotypic history of species. These covariance sets
usually dominate several major axes of orthogonal variation detected in such
techniques as factor and discriminant analysis (see Gould and Woodruff, 1986, for a
detailed application; Gould, 1989a, for a general statement; and Gould, 1992b, for an
analysis of the infamous "square snails" in the Cerion dimidiatum complex of Cuba, a
phenotype that Maynard Smith had declared "impossible" at the 1980 Chicago
macroevolution meeting in order to acknowledge that even he, as a strict Darwinian,
did not deny a role for constraint in precluding access to certain regions of
morphospace. His principle cannot be gainsaid, but his application failed because he
assumed a logarithmic spiral model, thus forbidding the square shape that can be
attained only by intense allometry; but Cerion's allometry leads precisely to such
squareness at the extreme of contrast and sudden transition between allometric phases
one and two).
To cite an example from one species (as a prototype for demonstrating the
dominating relative influence that such constraints can exert in particular situations),
one of the major covariance sets of Cerion's allometry—the "jigsaw