Early modern bodies, we are told, were conceived of as ‘fluid’ and ‘permeable’, or ‘mutable’ and ‘fluxable’, because their constituent humours were always and everywhere at the mercy of the immediate surroundings. This chapter challenges this notion. As much as the seventeenth century’s ambiguous understanding of human reproduction may seem ‘soft’, a common if not dominant view was that complexions were both innate, enduring, and, in a fundamental way, inherited. Published reckonings of professional astrologers such as William Lilly and John Gadbury; the carefully scripted, autobiographical calculations of William Bellgrave and Jonathan Hall, otherwise obscure artisans; little-studied case notes regarding humble individuals kept by those, like Jeffrey Le Neve or Richard Saunders, who considered themselves as much mathematicians or physicians – all these materials point to a widespread belief that humoral complexions and their associated body types were established from birth. The body’s mutability was a preoccupation, because of the difficulty of maintaining these self-same, innate constitutions. The chapter tests the persistence of what we might term an astro-humoral world-view by re-examining the controversy surrounding the most significant birthday of the late Stuart era: the arrival of a prince of Wales on 10 June 1688.