The successes in comparative physiology during the last 25 years have resulted from the study of those cellular or subcellular phenomena common to all animals, but approachable most easily in a few model species. In contrast, complex systems comprising many cooperating tissues and organs are characteristic of narrow taxa; analysis of such systems does not yield widely applicable generalities. An analogy between the distribution of commodities and products in biological systems and in marketing channels is developed, and the function of the circulation is considered in that context. The accumulation and assorting of products by the flow of blood from the tissues to the collecting vessels emerges as a particularly important role. The techniques that have been developed for coping with man's highly complex organizations and activities should be applicable to analogous biological systems.