The Ecological and Evolutionary
The Argument from Frequency
The Importance of Recapitulation
How common is evolution by heterochrony? We can no more answer this question than we could resolve the issue of relative frequencies for recapitulation and paedomorphosis. We can be certain, however, that its effects have been catalogued thousands of times and that it is the dominant mode of evolution in many important lineages. If Haeckel's law of recapitulation is "a vague adumbration of the truth" (Julian Huxley, personal communication, August 1971), the truth must be that heterochrony is extremely important.
Although I put little stock in the tradition of argument by enumeration in natural history, I begin this chapter with selected examples of the dominant role of heterochrony in lineages ranging from phylum to endemic genus. Here, as in all the following sections of this book, I emphasize paedomorphosis (this chapter takes as its general theme the importance of distinguishing progenesis from neoteny as distinct phenomena both in ecological significance and evolutionary importance). In so doing, I do not wish to cast my lot with de Beer (1930, 1958) in claiming that recapitulation is rare and unimportant in evolution. I believe that it is every bit as significant as paedomorphosis. Recapitulation, like paedomorphosis, is a consequence of two processes with different evolutionary meaning (Table 3). The first process is hypermorphosis, the extension of ancestral ontogeny. I shall discuss hypermorphosis within the framework of retarded matu-