Then as to the relationship between Worlds I and II. For closely related they must be--it was unthinkable that two eternally independent universes could have produced two near-identical Opal Crosses, Gray Twins, Clawlys, Thorn, and an uncounted host of other similars; if one granted that possibility, one would have to grant anything. No--Worlds I and II must be the results of a split in the time-stream, however caused, and a fairly recent split at that, for the two worlds contained duplicate individuals and it was again unthinkable that, if the split had occurred as much as a hundred years ago, the same individuals would have been born in the two worlds--the same gametes, under different circumstances, still uniting to form the same zygotes. The split must--of course!--have occurred when the nightmare-increase began in World I. About thirty years ago. But--Thorn's credulity almost rebelled--would it have been possible for two worlds to become so different in a short time? Freedom in one, tyranny in the other. Decent people in one, emotional monsters and cringing, embittered underlings in the other. It was horrible to think that human nature, especially the nature of people you loved and respected, could be so much the toy of circumstance. And yet--the modern world was keyed for change. Wars could, had, come overnight. Sweeping technological changes had been accomplished in a few months. And granting such an immense initial difference as the decision to keep subtronic power a government secret in World II, to make it public property in World I-- Moreover, there was a way of testing. Without pausing to consider, Thorn said, "Remember when we were children? We used to play together. Once we swore an oath of undying friendship." * * * * * Clawly II twisted toward him in the current, which was now taking them up past winking corridor entries. "You _are_ breaking," he remarked in surprise. "I never expected a play for sympathy. Yes, of course I remember." "And then about two years later," Thorn plunged on, "when our glider dropped in the lake and I was knocked out, you towed me ashore." Clawly II laughed, but the puzzled look around his eyes deepened. "Did you really believe I saved you? It hardly fits with your behavior toward me afterwards. No, as I think you know, I swam ashore. That was the day on which I first realized that I was I, and that everything and everybody else was circumstances." Thorn shivered, as much in horror of this changeling beside him as in satisfaction at having checked the date of the time-split. Then he felt revulsion rising in him, more from the body he occupied than from his own thoughts. "There isn't room in the world for even two people with that attitude," he heard himself challenge bitterly.