Today developmental and evolutionary biologists are focussing renewed attention on the developmental process—those genetic and cellular factors that influence variation in individual body shape or metabolism—in an attempt to better understand 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 paleontology, 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.