This paper argues that our modern concept of biological heredity was first clearly introduced in a theoretical and practical setting by the generation of French physicians that were active between 1810 and 1830. It describes how from a traditional focus on hereditary transmission of disease, influential French medical men like Esquirol, Fodéré, Piorry, Lévy, moved towards considering heredity a central concept for the conception of the human bodily frame, and its set of physical and moral dispositions. The notion of heredity (...) as a natural force, with a wide ranging capabilities of transmitting differentially both fundamental and accidental characters was generalized by that generation of physicians with the help of contemporary naturalists and physiologists. By 1830 the term hérédité was widespread, and it shared the explanatory and semantic qualities of traditional medical concepts like constitution and temperament. An analysis is given of the main developments that led to the conception of biological (including human) bodies as consisting of a layered, hierarchical organization of characters, differentially affected by the laws of conservation (Heredity) and change (Inneity, Variation). The mid-century work of the French physician Prosper Lucas, Traité Philosophique et Physiologique de L'Hérédité Naturelle, is shown to be the culmination of the efforts of several generations of French physicians towards having a feasible, complexly structured notion of how heredity works. (shrink)