This chapter describes the importance of developmental systems to evolutionary variation and examines some general properties of biological systems that deconstrain phenotypic change. It argues that physiological variation, particularly the variation used in embryonic development, is an important substrate for evolutionary change. The chapter proposes that weak linkage, exploratory behavior, and compartmentation are major underlying mechanisms which facilitate phenotypic variation. It suggests that the theory of evolution is based on those of natural selection, heredity, and phenotypic variation.