1044 THE STRUCTURE OF EVOLUTIONARY THEORY
Hallam's persuasive flow tank experiments (1968) identified the morphological
changes that could provide greater shell stability in an evolutionary lineage beginning
with the problematical G. incurva: namely, larger size, broader shells, and decreased
coiling. The evident conclusion now simply follows: if natural selection favored all
these traits as conjoined enhancers of essential stability, wouldn't its action be greatly
aided by any internal mechanism that happened to bias variation in these directions,
or to forge correlations among these jointly beneficial characters?
Fortunately, all the morphological features already exist within the ontogenetic
(and strongly allometric) channel of the founding member, G. incurva—for young
shells of this ancestral species are relatively broader and less coiled than the adults
that will develop from them. If these features can be brought forward by
paedomorphosis into later ontogeny, greater stability can be achieved.
10 - 5. The life position of Gryphaea, with the implied adaptive advantage of coiling in keeping
the aperture of the shell above the muddy surface. From flow channel experiments of Hallam,
1968.10 - 6. A Gryphaea shell tipped onto its side—an inviable position—is actually more stable than
a shell in its life position of Figure 10-5. From Hallam, 1968.