In the majority of current text-books it is customary to restrict functional considerations of the evolution of the double circulation to questions of oxygen transport. As a result of this approach the reptilian circulatory system is presented as "inefficient," and the left systemic arch dismissed as "useless." Considerations of circulatory dynamics, however, make it clear that the reptilian left systemic is far from useless, and is an essential part of a circulation in which an evolving lung is not yet capable of dealing with the same minute-volume of blood as the systemic circuit carries. Similar considerations of the mammalian circulation point to the probability of a functional ductus arteriosus having existed in the mammal-like reptiles. The persistence of the ductus arteriosus in the embryo of mammals lends some support to this idea.