Literature is not stable : it changes. By taking the example of two genres experiencing different destinies, tragedy and comedy, we demonstrate that this change results from the pressure exerted by two opposing forces : that of respecting the rules of the genre on the one hand, and that of the personal expression of the author of the other. The study of these two forces is done by means of the distance of the plays with the centroids of their respective authors or genres, a new method in stylometry.