 It was following upon his proposed solution to the issue of organic generation that Buffon then addressed the issue of organic species and their permanence. In the fourth volume of the Natural History (1753), devoted to the large domestic quadrupeds, Buffon first raised the option of species transformism, only to reject it. In the article “The Ass”, devoted to the domestic donkey, Buffon drew attention to the close anatomical similarity between the horse and the ass, as revealed by his collaborator Daubenton’s anatomical descriptions. This similarity strongly suggested an underlying unity of plan of all the quadrupeds, a similarity raising the possibility that all quadrupeds might have been derived in historical time from a single stem (souche) which “in the succession of time, has produced, by perfection and degeneration, all the other animals” (Buffon 1753, [OP, 355]). In a move that has confused commentators every since, Buffon then rejected this possibility and instead argued that because of the infertility of hybrids, species must have been distinct from the beginning.