A set of papers are discussed laying the foundations for a comprehensive dynamical theory of discontinuities in urban population size. Abstracting from the main premise of the papers, which appeared in 1980, a thesis is advanced that an encompassing notion of dynamic comparative advantages underlies urban growth or decline. It is argued that such a theory is consistent with the notions found in Darwinian evolution. References to classical urban literature are made, linking these models to long-standing views of urban evolution.