246 HETEROCHRONY AND PAEDOMORPHOSIS
## A Clock Model of HeterochroyIt is often impossible to decide whether we deal with speeding up or slowing
down in evolution unless we have a standard criterion for measuring time [ E. Mehnert, 1897 I find it remarkable that so little attention has been directed toward a synthesis of the two great literatures on size and shape: the quantitative measurement of allometry, long treated as bivariate in Huxley's (1932) formulation, but now attaining a multivariate generalization (Teissier, 1955; Jolicoeur, 1963; Gould, 1966; Hopkins, 1966; Mosimann, 1970; Sprent, 1972), and the study of heterochrony, a subject that has doggedly maintained a purely qualitative and descriptive approach. The standard techniques of allometry do not provide an optimal metric for heterochrony because they subtly reinforce a prejudice directed against the dissociability upon which heterochrony depends. Mosimann (1970, p. 943) argues persuasively that the "functional relationships mold" of bivariate plotting places undue emphasis upon the functional association of size and shape. The form of a regression comes to be viewed as a primary feature. The abstracted straight line becomes a key character. Mosimann writes: "I do feel strongly that in many cases the use of functional relations in allometry has been a rebirth of the 'type' concepts of taxonomy" (personal communication, March 3, 1970). Any phyletic change is regarded as a "break" or "disruption" of this primary correlation. Association is primary, disassociation exceptional. The plotting of size and shape as a functional relation is inherently uncongenial to the notion of dissociability. But the functional relation is but one method among many. Mosimann (1970) prefers a nonfunctional approach that considers the
vectors of size and shape separately. In this model, isometry is no
longer a rigid correlation of two variables with a slope of The bivariate regression is not "truth"; it is a valid picture that directs thought in ways that are rarely appreciated because alternatives are not presented. A proper attention to dissociability requires a new picture in which the ordinate and abscissa of bivariate plots ac- |