1154 THE STRUCTURE OF EVOLUTIONARY THEORY
therefore appears to represent "all head"—an expanded part of the oral end only, with
the aboral end suppressed. Cartwright et al. (1999) found very weak expression of
Cnox- 2 throughout the full length of tentaculozooids "at approximately the same level
seen in the tentacles of gastrozooids" (p. 2185).
In an additional affirmation by experimental manipulation, dactylozooids can be
removed from the colony and induced thereby to transform into gastrozooids. Cnox- 2
expression initially decreased in the developing hypostome of the transforming polyp,
and then in the tentacle region, but not at the aboral end, "until ultimately,
dactylozooids that fully transformed into gastrozooids displayed aboral-oral Cnox- 2
expression patterns indistinguishable from that of normal gastrozooids" (p. 2185).
I believe that this crucial discovery about early emplacement of key
developmental patterns—at least in bilaterian ancestry (and to a lesser extent in all
animals)—combined with a central fact of timing in phylogeny, establishes a
framework for understanding the primary importance of historical constraint, and of
formalist (or internalist) perspectives in general, for explicating both the subsequent
pathways of animal evolution and the resulting, markedly inhomogeneous habitation
of potential morphospace in the history of life. Three logically connected and
sequential arguments (presented as the last three sections of this final movement)
combine to reset the balance of structure and function, or constraint and selection, in
evolutionary theory—
10 - 26. The four major polymorphic types of polyps in Hydractinia. A, gastrozooid; B,
gonozooid; C, dactylozooid; and D, tentaculozooid. From Cartwright et al., 1999.