Historical Constraints and the Evolution of Development 1047
constraint" of my terminology (Gould, 1989a)—may seem almost trivial in its
obvious nature, but still exerts great influence in setting patterns of variation within
Cerion at all levels, from intrapopulational variation, to geographic variation within a
species, to chronoclines, to regional patterns of differentiation in species complexes
(see Gould, 1989a, for details). If different shells reach virtually the same final size—
and Cerion, as one of its major bio-metric advantages does, unlike most invertebrates,
reach a final size marked by the secretion of a thickened lip in the third allometric
phase—then shells with larger whorls must end their growth with fewer whorls. (In
two jigsaw puzzles with frames of the same size, the one with smaller pieces must
use more pieces to fill the common space—hence my name for the covariance set.)
The basic principle might be regarded as both obvious and entirely unprofound.
Its operation would also impose scant effect upon any molluscan shell that grew in
close conformity with the idealized logarithmic spiral—for two shells of the same
size, one with few and the other with many whorls, would then display the same
shape, and no substantial differences (beyond the number of whorls) would be
apparent. But Cerion's extensive and distinctive allometry triggers a large and
visually striking set of correlated changes, necessarily leading to obvious differences
in form between few and many whorled shells of the same size. (Such distinctions
can be readily characterized, and judged in relative strength, on factor or discriminant
axes of multivariate biometric studies.) For example, large-whorled specimens grow
fewer whorls and therefore undergo a later transition to the second allometric phase
(which invariably occurs between the 5th and 6th whorl), thereby yielding a more
triangular adult shell, as relatively less of the total growth occurs during the "barrel"
of the second allometric phase.
This single constraint, with its complex sequelae, explains virtually all the
interregional geographic variation in one of the most interesting, and certainly the
most intensely studied, species of Cerion—the geographic and morphological outlier
(also the holotype of the genus, and a species named by Linnaeus himself), Cerion
uva from Aruba, Bonaire and Curasao. Moreover, recognition of the jigsaw constraint
allowed me to resolve, in a manner congenial to all parties, the most substantial and
longstanding debate in the history of Cerion studies.
In a large monograph, published in 1924, H. B. Baker, a great descriptive
malacologist, claimed that he could distinguish four geographic domains of variation
by subtle but entirely characteristic differences in shell form: Aruba, Bonaire, Eastern
Curasao and Western Curasao. (The island of Curasao, shaped like a dumbbell with
eastern and western portions joined by a much narrower neck of land, may be
sensibly so divided; the two halves were probably separated by higher sea levels of
former interglacial epochs.) Baker used the classical and subjective criterion of a
taxonomist's "good eye," and could therefore not defend his impressions in the face
of extensive biometrical studies by Hummelinck (1940), then extended and
confirmed by De Vries (1974). These Dutch researchers, unable to identify
covariance sets with their univariate