No less wonderful is the strictly dramatic economy of the play. The action is slight. Prometheus works no more; it is his part to endure. All the secondary characters act as a foil to bring the central figure into massive relief. Each has some touch of Prometheus: Hephæstus, pity without self-sacrifice; Cratos, strength without reflection; the Nymphs, tenderness without force; Oceanus, common-sense without dignity; Io, sensibility to suffering without the vision which learns the lesson of pain; Hermes, the power to serve without perception of the secret of sovereignty. Most essential of all these is Io. The only human participant in the action, she reminds us that the hand of Zeus has been heavy upon innocent mortals as well as rebel gods, and thus gives fresh justification to the wrath of Prometheus. Still more, she is vital to the whole trilogy. As Hephæstus links the _Fire-bringer_ to the second play, so does she join the second play to the _Prometheus Unbound_. It is her descendant Heracles who after thirteen generations will free Prometheus and reconcile him to Zeus; the hero of the last drama is brought in a sense upon the scene in the person of his ancestress. Prometheus himself suggests to us the thought of Christ; and yet (as has been said) the Satan of Milton is like him too. This double kinship is made possible by the conception of Zeus which here obtains. Under the sceptre of a god who hates mankind it is possible for the saviour of men to be a rebel and an outcast. Right or wrong, the Titan is godlike in his goodness, his wisdom, his courage. At one point only does his deity show a flaw; he endures his pangs not as a god, but as a man; he agonizes, he laments his pains, he utters exclamations of fear. Rightly, for if the actors in this world-drama are immortal, the spectators are not. To have portrayed Prometheus as facing his punishment without a quiver would have been perhaps sounder theology, but worse drama; the human audience must be made to understand something at least of these pangs, or the greatness of the sacrifice will elude them. A parallel on which we must not dilate cannot escape the reader. One strange outcome of his rebellion is generally overlooked. Zeus had wished to destroy mankind and create a new race. That is, he meant to treat men as he treated the Titans—or would have treated them had they been mortal. Prometheus thwarted this plan, so that we men are a survival of that pre-moral world which the new ruler supersedes. We are the younger brothers of the Titans and (so to put it) have all survived the Flood. Our pettiness and futility condemned us in the eyes of Zeus, who wished for progress; but Prometheus loved us in spite of our miserable failings, and so insisted on carrying us over into the new and nobler world at the cost of his own age-long agony.