Any comprehensive theory of adaptive evolution has to feature development. Development produces the phenotypic variation that is screened by selection. For a mutation to affect evolution, it must first affect development. In order to understand phenotypic change during evolution, one has to understand phenotypic change during development, as well as how to relate that change to selection and gene-frequency change (evolution). The evolution of the phenotype is synonymous with the evolution of development. The genotype-phenotype problem addressed by the metaphors of chapter 2 is fundamentally a problem of development. Genetic programming, the canalized epigenetic landscape, and the recipes and blueprints contained in the genes—all are metaphors for development. Development is the missing link between genotype and phenotype, a place too often occupied by metaphors in the past. The task of this chapter is to outline a concept of development that connects it to mechanisms, on the one hand, and natural selection and evolution on the other, without a potentially misleading metaphorical crutch. The portrait of development provided by developmental biology is not adequate to this task. Evolutionary developmental biology extensively treats the genomic correlates of gross morphological variation across phyla, with little or no discussion of behavior, physiology, life histories, and the kind of variation within populations that is required for natural selection to work. Some progress toward a population approach has been made in plant developmental biology (e.g., see Lawton-Rauh et al., 2000). But a strong emphasis on the genome means that environmental influence is systematically ignored. If you begin with DNA and view development as “hard-wired” (e.g., Davidson, 2000), you overlook the flexible phenotype and the causes of its variation that are the mainsprings of adaptive evolution. I begin instead with the observation that DNA activity—gene expression—is universally condition sensitive and dependent upon materials from the environment. This implies connections between a DNA-centered approach and conventional insights about adaptive evolution in variable environments. The genome affects development at nearly every turn, so genes obviously play an important role in any theory of development and evolution.