drozoans), Mystacocarida (Crustacea), and Acochlidiacea (Mollusca). It is a testimony to the importance of heterochrony that all these groups have received a progenetic interpretation. The Actinulida are generally regarded as progenetic (Hadzi, 1963, p. 94). Hessler (1971, p. 87) has urged a progenetic interpretation of the Mystacocarida in opposition to previous claims for their "primitive" nature. Riedl (1969, p. 856) points to the paedomorphic features of gnathostomulids in suggesting their possible progenetic origin from much larger fossil conodonts. Odhner (1952) suggests that the family Microhedylidae of the Acochlidiacea may be progenetic.

The Role of Heterochrony in Macroevolution:
Contrasting Flexibilities for Progenesis and Neoteny

Progenesis

Progenesis has usually been dismissed as an agent of degeneration with no evolutionary importance. I have tried to rescue it from this charge by demonstrating its immediate significance for precocious maturation in r-selected regimes (and, in some cases, for small body size). But de Beer's dismissal was more an assessment of retrospective significance in macroevolution than a denial of adaptive value in local environments.

The link of progenesis to r selection might seem to affirm the usual denial of retrospective significance. "Evolutionary importance" is an ambiguous notion (to put it mildly); in conventional writing about macroevolution, a process partakes of this recondite "importance" when it contributes to the classical material of macroevolutionary success: slow, continuous, and sustained trends toward more complex morphology and greater diversity. These trends are the material of K selection; r selection, with its emphasis on production of offspring at the expense of perfection in morphology, can only serve as a brake upon such "progress." As Dobzhansky wrote in his prophetic article of 1950, long before theoretical ecology codified the concept: "Physical factors, such as excessive cold or drought, often destroy great masses of living beings, the destruction being largely fortuitous with respect to the individual traits of the victims and the survivors . . . Indiscriminate destruction is countered chiefly by development of increased fertility and acceleration of development and reproduction, and does not lead to important evolutionary advances" (p. 220; see also Murphy, 1968, p. 402). Levin has noted the restriction of recombination in r-selected plants, K-selected genomes, he argues, do not congeal because pathogens and herbivores exert intense pressure and