weeks" (1930, p. 27). Though truncation is only facultative in this case, evolutionary fixation has arisen in others. Polystomum ocellatum is permanently paedomorphic and resembles the truncated form of Polystomum integerrimum (de Beer, 1930, p. 27).

Any major prolongation will be correlated with a retardation of maturation. This retardation is not a secondary consequence or an ancillary correlate of prolongation; on the contrary, selection for delayed maturation may be the major determinant of phyletic size increase (Chapter 9).

A small degree of prolongation or truncation may develop without any advance or delay in maturation, but major evolutionary changes in the length of ontogeny involve selection for acceleration or retardation of sexual maturity. Thus, prolongation and truncation are but another aspect of acceleration and retardation (of sexual organs rather than somatic characters). We can trace almost all parallels between ontogeny and phylogeny to acceleration and retardation.

DeBeer made a major contribution by forcefully linking acceleration and retardation in the single phenomenon of temporal displacement. But he introduced a regrettable confusion by applying Haeckel's term "heterochrony" to this phenomenon. To Haeckel, heterochrony was the displacement of one feature relative to other features of the same organism. For de Beer, it became the displacement of a feature relative to the time that this same feature appeared in an ancestral form.* This transmutation has been so widely adopted, however, that I have no choice but to use "heterochrony" in de Beer's sense throughout the rest of this book.

Heterochrony is the mechanism that produces recapitulation and paedomorphosis as its result. If parallels between ontogeny and phylogeny are of major significance in the history of life, it is because heterochrony—the temporal displacement of characters—is a pervasive phenomenon among evolutionary processes.

The Reduction of de Beer's Categories of
Heterochrony to Acceleration and Retardation

In a series of remarkable books that established the synthetic theory of evolution, Gavin de Beer's Embryology and Evolution was the first

* The odyssey of heterochrony is exceedingly curious. In its original use, it defined a class of exceptions to recapitulation since it precluded the complete transference of an entire ancestral adult to juvenile stages of descendants. In Mehnert's reinterpretation, it became consistent with recapitulation because its prevalence required that recapitulation be redefined for single characters only. In de Beer's altered reading, it becomes the mechanism of recapitulation.