Today, developmental and evolutionary biologists are focusing renewed attention on the developmental process, specifically those genetic and cellular factors which influence variation in individual body shape or metabolism. They are attempting to understand better how evolutionary trends and patterns within individuals might be limited and controlled. In this important work, the author reviews the classical literature on embryology, morphogenesis, and palaeontology, and presents recent genetic and molecular studies on development. The result is a unique perspective on a set of problems of fundamental importance to developmental and evolutionary biologists.