Since, in all ages, the size and shape and physical appointments of the theatre have determined for the playwright the form and structure of his plays, we may always explain the stock conventions of any period of the drama by referring to the physical aspect of the theatre in that period. Let us consider briefly, for purposes of illustration, certain obvious ways in which the art of the great Greek tragic dramatists was affected by the nature of the Attic stage. The theatre of Dionysus was an enormous edifice carved out of a hillside. It was so large that the dramatists were obliged to deal only with subjects that were traditional,--stories which had long been familiar to the entire theatre-going public, including the poorer and less educated spectators who sat farthest from the actors. Since most of the audience was grouped above the stage and at a considerable distance, the actors, in order not to appear dwarfed, were obliged to walk on stilted boots. A performer so accoutred could not move impetuously or enact a scene of violence; and this practical limitation is sufficient to account for the measured and majestic movement of Greek tragedy, and the convention that murders and other violent deeds must always be imagined off the stage and be merely recounted to the audience by messengers. Facial expression could not be seen in so large a theatre; and the actors therefore wore masks, conventionalised to represent the dominant mood of a character during a scene. This limitation forced the performer to depend for his effect mainly on his voice; and Greek tragedy was therefore necessarily more lyrical than later types of drama. The few points which we have briefly touched upon are usually explained, by academic critics, on literary grounds; but it is surely more sane to explain them on grounds of common sense, in the light of what we know of the conditions of the Attic stage. Similarly, it would be easy to show how Terence and Calderon, Shakespeare and Molière, adapted the form of their plays to the form of their theatres; but enough has already been said to indicate the principle which underlies this particular phase of the theory of the theatre. The successive changes in the physical aspect of the English theatre during the last three centuries have all tended toward greater naturalness, intimacy, and subtlety, in the drama itself and in the physical aids to its presentment. This progress, with its constant illustration of the interdependence of the drama and the stage, may most conveniently be studied in historical review; and to such a review we shall devote a special chapter, entitled _Stage Conventions in Modern Times_.