This paper aims to bring to light the explanatory model that is hidden beneath the wealth of detail of Industry and Trade. Based on the pair innovation-routine, which is itself a variation of the Darwinian pair variation-selection, the model acts as a guide to Marshall’s investigation of firm boundaries and size, industrial organization and economic policy, providing a coherent, yet flexible, explanatory scheme. Since the model is never openly presented, it is too easy for readers to miss its purport. The paper begins with a brief survey of the reception of the book and the disappointment consequent upon the high expectations that its long haul had generated. Section 2 highlights the key role of Marshall’s model in his interpretation of industrial development. It also emphasizes that the model has no pretension to predetermine the outcome of the process. Section 3 is devoted to the related but separate issue of the contingent reasons for Marshall’s choice of nation-States as the main units of analysis. His policy proposals, less heterogeneous and fragmentary than appears at first sight, are the subject of section 4, which is followed by a few summarizing conclusions.