But besides this literary tradition there has been a hardly less powerful theatrical tradition. Tragedy has always owed a double allegiance, to literature and to the theatre. A tragedy is a play, not merely a dialogue in poetry or prose, but a play to be interpreted by actors before an audience in a theatre. To these three factors it has had first of all to suit itself. And these factors have constituted conditions and standards, different and not less variable and transient than those of the literary tradition. The plays of Æschylus, of Shakespeare, of Calderon, and of Racine, for example, were planned for widely different conditions, and for conditions also widely different from those now present in the theatres. Excepting Shakespeare's, no English tragedies of the sixteenth, seventeenth, eighteenth, or, one might almost say, nineteenth century, are acted in our theatres to-day. The effect of the acted drama is consequently not only different from that of the drama when read, it is also subject to other and variable artistic standards. It aims at some effects not at all literary and at some likely to be limited by its own day and theatre. A history of tragedy must take into account the differences of the theatre of one nation from that of another, and of one period from another period. It must remember that to those temporary conditions each dramatist necessarily conformed and that by them his achievement was directed. It may find some hostile to the best dramatic art, tending to promote melodramatic rather than tragic effects. It may find others that are divorced from any permanent meaning for the drama or literature. But the fact that such conditions are temporary should not breed contempt, for much great literature has been aimed not at the world or posterity but at the audience of the day. Out of temporary and varying theatrical conditions have arisen the permanent criteria for dramatic excellence.