letic origin in a single species. Only one creative progenesis is required for the entire phylum.

I believe that evolutionary biologists shy away too readily from hypotheses that invoke improbable events and excuse them by citing the immensity of time. The criterion should be cogency in theory, not frequency of occurrence. Goldschmidt's hopeful monsters were rejected because they were untenable in theory, not because they needed to succeed but rarely. An extremely rare creative role for progenesis in the rapid origin of higher taxa is not only tenable in theory; it might even save Darwinism from an embarrassing situation usually swept under the rug of orthodoxy—the difficulty of explaining transitions between major groups if the transitions must be gradual and under the continual control of selection upon morphology.

Neoteny

Evolutionary trends toward greater size and complexity form the classical subject matter of "progessive" evolution as it is usually conceived-the slow and gradual fine tuning of morphology under the continuous control of natural selection. These trends display three common features marking them almost inevitably as primary products of, K-selective regimes:⁶

1. A primary role for morphology in adaptation-usually leading to increased complexity, improvement in biomechanical design, or at least the continual exaggeration of specialized structures with clear functions.

2. A general tendency to increasing size—Cope's rule. Hairston et al. (1970, p. 685) have linked phyletic increase in size with the density- dependent regulation of K regimes.

3. In most cases, a delay in the absolute time of maturation. This property has rarely been mentioned, but I regard it as unavoidable. Larger animals with a generally increased level of morphological differentiation almost surely mature later than their much smaller and more generalized ancestors. Bonner (1974, p. 27) has emphasized the usual association of increased size and delayed reproduction. Sexual maturation usually marks the termination (or at least the pronounced slowdown) of both size increase and differentiation.

The usual heterochronic result of such trends is probably hypermorphosis—an extension or extrapolation of ancestral allometries. With a delay of maturation, differentiation can proceed beyond its ancestral level into the larger sizes of descendants. The irony of this