Editors' Summary: Emilio Moron argues that we cannot assume that observed behaviors are necessarily adaptive. His concerns are with agricultural productivity in the Amazonian rain forest. He argues, very persuasively, that shifting cultivation as practiced by Amazonian lowland natives is not an adaptive response to absolutely poor soils, as is commonly believed from environmentally deterministic approaches. Rather, nonciraular forms of shifting cultivation result from the interaction of mobility (required to maintain the productivity of hunting), of adaptation to species-specific rather than site-specific resources, and of the need to select agricultural practices that were relatively reliable. Manioc fulfills these conditions exceedingly well but its low-protein content favors reliance on animal protein provided by hunting. Moran makes the important assertion that some tropical forest soils can support a sedentary population of cultivators, and challenges the generalisation that all humid tropical soils are poor. The identification of the scarce high fertility soils presents a problem for local populations that is resolved by favoring adaptation to the more frequent nutrient-poor soils in combination with precise adaptations to the behavior of specific faunal resources. The reader should note that the emphasis on mobility is not an effort to replace one form of determinism (soil nutrient depletion) with another (animal protein depletion). Rather, Moran proposes an interactive model for explaining human adaptive strategies in the Lowlands of Amazonia. The relative influence of single factors will depend on site-specific conditions, both environmental and historical.