Cassandra and Ægisthus have not yet been considered, for they belong also to the next topic—the method in which the unity of the play is so handled that it does not interfere with, but helps to effect, the unity of the whole trilogy. The indescribable power and thrill of Cassandra’s scene may easily blind us to the slightness of the character-drawing. Simply as a character, the princess is no more subtly or carefully studied than the herald; the extraordinary interest which surrounds her arises not from what she is or does but from what happens to her. She is the analogue of Io in the _Prometheus_. The mere structure of both plays allots to Io and Cassandra precisely the same functions. Passive victims of misfortune, they are the symbol and articulation of the background in the particular drama; further, they are vital to the economy of the whole series, in that they sum up in themselves the future happenings which the later portions of it are to expound. So far, they are the same; but when we go beyond theoretical structure and look to the finished composition, Cassandra far outshines Io. The Argive maiden suffers, shrinks, and laments in utter perplexity. The Trojan suffers, but she does not quail; her lamentations are hardly lamentations at all, so charged are they with lofty indignation, and the sense of pathos in human things. Io is broken by her calamity; Cassandra is purified and schooled. The poet who in this very play sings that suffering is the path to wisdom has not made us wait long for an example. There is, too, a definite technical advance in this, that Io merely hears the prophecy of justification and the possibility of revenge, while Cassandra in her own person foretells the return of Orestes. Ægisthus also, but less obviously, is important to the progress of the trilogy. His appearance and his speeches are no anti-climax to the splendid scene of Clytæmnestra’s triumph. The queen and Cassandra have talked of the Pelopid curse; Ægisthus is the curse personified. It is through ancient wickedness that he has passed a half-savage life of brooding exile; the sins of his fathers have turned him into a man fit to better their instruction. Again, this last scene brings before us in full power that aspect of Clytæmnestra which has been almost ignored—her baser reason for the murder of her husband. This is done precisely at the right place. To dwell on the queen’s intrigue earlier would have deprived her of that measure of sympathy which throughout this first play she needs. Not to have depicted it at all would have left that sympathy unimpaired, and we should have entered upon the _Choephorœ_ fatally unable to side with Orestes in his horrible mission.