"But if we could only find out what destroyed your attempt, perhaps we could do something to prevent it!" His eyes glowed. "What a glorious thing for mankind! Already you have converted men from nomadic wanderers into a semi-cultured people. If that cause which destroyed--or is to destroy--your tutorship were to be removed--" Ramey faltered over the use of tense, feeling keenly the anomaly of their position as men of a future age, living in a past, being part of that past, yet knowing inerrably that which was to come--"Why, then, the whole history of mankind could be changed! There would be no decay in Egyptian culture, no Rome rising mightily, then toppling, no long Dark Ages. There would be only steady progress, ever forward, upward, to greater knowledge!" Syd sniffed, "You're day-dreaming, Ramey. The fact that we exist proves that the history of mankind took a certain channel. There's no way of changing that. Is there, Doctor?" "I don't know, Sydney. There is much to be said on either side. It may be that history is, as you say, unchangeable. But there is the problem of causality. Once this era _was_. We, having not been born then, were not here. Causes developed effects new causes--and a course of history was written leading to the world we know. "But a new factor enters an old equation. This era again _is_, but we who do not properly belong here have entered into the picture by way of a time-machine. It is conceivable that our very _being here_ is sufficient of a cause to change and divert the entire sequence of events which would otherwise have been the 'future.'" "Rot!" snorted Syd. "Excuse me, Doctor, but that's not logical. For if our being here were to change history in any slightest way--then we would automatically cease to exist! Because the exact and precarious chain of circumstances which brought us into being would have been altered." CHAPTER X Exposure So they dropped the matter there, completely unable to solve the problem, each convinced that his theory was correct, none without a lingering doubt but that the other's might be true. And the days sped by. They were fruitful days. Lake, who had a flair for the philological, spent much time studying the Gaanelian language. To Red and Ramey, as professional airmen, what was of particular interest was the matter of spaceflight. Gaanelian ships, Sugriva told them, called regularly at every inferior conjunction of Earth and Venus, Videlian craft less frequently. "But often enough," the blue lord admitted ruefully. "And these ships--" demanded Ramey eagerly. "Their method of propulsion? What is it?"