process into the juvenile stages of ancestors. Thence they move slowly forward, gradually invading and displacing ancestral adult stages by a process that Schindewolf called proterogenesis.

In summary, the classical significance of paedomorphosis has been portrayed in the following way: Progenesis can be dismissed as a factor in macroevolution because it leads only to degeneration by truncation. Neoteny is an essential process in macroevolution for two reasons: First, it provides an escape from specialization in the replacement of inflexible adult structures by generalized juvenile forms (for instance, Clark, 1964, p. 247, on the "rescue" of vertebrates from a deuterostome stock whose adults had become "acutely specialized" by a sessile mode of life). Second, it supplies one of the very few Darwinian justifications for large and rapid evolutionary transitions, by permitting major changes in morphology without extensive genetic reorganization. In short, the classical arguments are entirely morphological and macroevolutionary.

Retrospective and Immediate Significance

With one exception—the denigration of progenesis—I do not disagree with the classical arguments. I do, however, regard them as woefully incomplete, for a fundamental theoretical reason: they are entirely retrospective in design. They look upon a case of neoteny after its descendants have evolved and attribute meaning in terms of the aggregate success. But what of the actual species that experienced neoteny? It did not realize that it would be a herald of future diversity because it had sloughed off some ancestral specializations. It was not impressed by the fact that it had gained so much in morphology for so little genetic effort. It became neotenic for its own immediate reasons—its own ecologic strategy in its own particular environment.

Retrospective significance is a legitimate inquiry in macroevolutionary studies. The ancestral species of successful modern groups formed a tiny fraction of their contemporary biotas; more than 99.9 percent of all species are not sources of great future diversity. It is therefore important to seek in retrospect the common features of these exceedingly rare sources of later success. Traditional inquiry in paleontology has done just this. We learn that successful ancestors have usually been smaller than their later descendants (Cope's law of phyletic size increase) and of fairly generalized structure (law of the unspecialized). But these "laws" say nothing about the immediate significance of these features for the ancestors themselves. The ancestors were not small to provide a potential for future change, but to gain some selective advantage in their own environment.