of entire lineages. In setting this ecological theater for the evolutionary play, different reasons for heterochrony become apparent. In particular, the timing of maturation (rather than the morphology obtained by speeding up or slowing down) seems to demand special emphasis.

Heterochrony, Ecology, and Life-History Strategies

In a few cases, proposals have been advanced for the immediate significance of paedomorphosis. Here, we can usually detect the influence of another bias in explanation. Adaptive significance has been sought in terms of morphological advantages alone. Thus, when Stubblefield (1936, p. 430) discussed the significance of progenesis in minute agnostic! and eodiscid trilobites, he could offer only the possibility of superior enrollment in forms with very few thoracic segments. Maybe so, though larger trilobites with many thoracic segments rolled up tolerably well. Stubblefield considered neither the direct significance of marked reduction in size nor, especially, the consequences of small size for population structure and dynamics—the probability of more individuals, reduced longevity, and earlier reproduction.

Classical evolutionary theory portrayed adaptation in terms of morphology, physiology, and, perhaps, behavior. The size, structure, and dynamics of populations were very rarely considered, though I expect that most evolutionists would have admitted their potential importance if pressed. No biases are more insidious than those leading to the neglect of things everyone knows about in principle. In this case, morphology, physiology, and behavior all reinforced a typological prejudice for considering individuals as efficient machines. The more efficient replace the less efficient and, in this sense, a population exists. But such an attitude does not invite attention to the individual advantages most readily inferred from population size, age structure, and turnover rates. As Cole wrote in his pioneering paper: "Comparative studies of life histories appear to be fully as meaningful as studies of comparative morphology, comparative psychology or comparative physiology. The former type of study has, however, been neglected from the evolutionary point of view, apparently because the adaptive values of life-history differences are almost entirely quantitative" (1954, pp. 134–135).

I regard the rise of theoretical population ecology as one of the most significant events in evolutionary theory during the past twenty years. For, in focusing on immediate significance in the short run of ecological time, it has proved that the components of life history