 Thus, there are good reasons for adopting normal stages to periodize model organism ontogeny, and these reasons help to explain why their continued use yields empirical success. However, similar to other standard (successful) practices in science, normal stages are often taken for granted, which means their biasing effects are neglected (Wimsatt 1980), some of which are relevant to evolutionary questions (e.g., systematically underestimating the extent of variation in a population). This is critical to recognize because the success of a periodization is not a function of the eventual ability to relax the idealizations; periodizations are not slowly corrected so that they become less idealized. Instead, new periodizations are constructed and used alongside the existing ones because different idealizations involve different judgments of typicality that serve diverse descriptive and explanatory aims. In addition to the systematic biases involved in developmental staging, most model organisms are poorly suited to inform us about how environmental effects modulate or combine with genetic or other factors in development—they make it difficult to discover details about mechanisms underlying reaction norms. Short generation times and rapid development are tightly correlated with insensitivity to environmental conditions through various mechanisms such as prepatterning (Bolker 1995).