The four plays exhibited by each poet might be composed on two different systems. They might form independent works of art, and have no inner connexion with one another; or they might deal with successive phases of the same legend, and be fused into a single artistic whole. The general name for the group of plays was ‘didascalia’, or a ‘teaching’[54], because in ancient times the author had to teach them to the actors. But when they were connected together by unity of subject, they were denoted by a special term. The four plays were called a ‘tetralogy’;[55] the three tragedies, regarded apart from the satyric drama, were called a ‘trilogy’. As applied to the drama, however, both words first occur at a comparatively late date:[56] and as, to judge from their etymology, they seem properly to denote groups of speeches rather than groups of plays, it is possible that their dramatic application is a secondary one, and that the grammarians applied to the drama the word ‘tetralogy’ which properly denoted such groups of four speeches about fictitious cases as those of Antiphon, and afterwards formed the word ‘trilogy’ by analogy to denote three plays connected in subject with each other but not with the satyric play. In earlier times such collective titles as Lycurgeia, Oresteia, and the like were used.[57] The practice of writing plays in trilogies and tetralogies is chiefly associated with the name of Aeschylus. Whether it was invented by him, or inherited from his predecessors, is uncertain. We have no information as to the manner in which the poets of the sixth century were accustomed to combine their plays together. But whatever the origin of the system may have been, it was undoubtedly Aeschylus who first perceived the various developments of which it was capable, and brought it to perfection. In his hands it became a mighty instrument for the inculcation of religious truths. The central idea in the moral system of Aeschylus was the disastrous effect of sin, not only upon the sinner himself, but also upon his remote descendants. The curse entailed in the sinful act clung to a family from one generation to another. In the trilogy, with its wide range of time and subject, he was able to trace the whole course of this hereditary evil, and to follow the crime from its original commission down to the period of its final expiation. The Orestean trilogy, which has fortunately been preserved, is a magnificent example of his method. The Agamemnon depicts the murder of the returning chieftain by his adulterous wife. In the Choephori vengeance is taken on the murderess, after years of waiting, by her own son. In the Eumenides the matricide, a prey to remorse, is hunted from place to place by the Furies of his mother, until their rage is at length appeased by divine intervention. These successive pictures of crime and vengeance form a series of unapproachable grandeur. The general effect of the whole may be appreciated even by a modern reader.