This article deals with the theory of the "strong nucleus of the Greek race" elaborated by the Greek physical anthropologist Ioannis Koumaris (1879-1970), who headed all academic anthropological institutions in Greece between 1915 and 1970. According to this theory human groups were in a state of "fluid constancy," meaning that the "proper" nucleus of the predominant race always persisted in a stable form despite miscegenation, and was hence capable of resurfacing. This theory footed, first, on racial theories challenging the existence of "pure races" in favor of evidencing "racial varieties" and "racial types" and, second, an early Greek national idea according to which Hellenism possessed the ability to acculturate and absorb foreign peoples or nations without losing its innate qualities. The Greek notion fili (meaning both nation and race), and its shifting semantics from religious to national and racial, is similarly instrumental to this analysis. By means of this theory racial purity was not so much rejected as it was relativized, essentially being replaced by the constancy of a race over time. With the shift from purity to constancy, the imperative of the homogeneity of an entity is not violated but, in contrast, supported by race anthropological arguments. Race hygienic theories, in turn, advanced the shift from racial consistency to purification.