Men had built only three other cruisers capable of exceeding the speed of light, so far. The first had gone out in a direction opposite to that of the _Clarion_ and had returned to report a regular decline in culture as the distance of habitable worlds from Earth increased. The nearest was in a medieval state, the next an early bronze culture, then a stone-age one, and so on, down to the furthest explored, where the native race had barely discovered fire. It had been either impossible coincidence or the evidence of some law nobody has been quite ready to accept, save for the newly spreading fundamentalists, who maintained it proved that Earth was the center of the universe. The other two cruisers had not reported back when the _Clarion_ took off. And their own trip had only added to the mystery, and they had touched on four habitable systems. And on each, there had been evidence of a highly developed race and some vast struggle that had killed off that race completely. The furthest had lain fallow for an unguessable period of time, and in each succeeding one, evidence indicated the time interval since the destruction of the culture had been less. On the world they had left, the end must have come not more than a few thousand years before. "Suppose one race had gone along in a straight line, seeding the systems with life," Lenk guessed. "Remember, every race we found had similarities. And suppose another race of conquerors stumbled on that line and is mopping up? Maybe with some weapon that leaves no trace." Jeremy looked at him. "Suppose Graves is right, and his God wipes out all wicked races. He keeps planting races, hoping they'll turn out right, and wiping out the old ones?" he snorted. "Only, of course he thinks Earth is the only world that counts. We're dealing with facts, Lenk, not wild theories. And why should an alien race simply wipe out another race, wait a thousand years or so, and move on--without using the plant afterwards, even for a base for the next operation? Also, why should we find plenty of weapons, but no skeletons?" "Skeletons are pretty fragile. And if somebody had the mythical heat ray...." "Bunk! If it would vaporize calcium in the bones, it would vaporize some of the parts of the weapons we found." Jeremy moved a rook, considered it, and pointed. "Check. And there are always some parts of skeletons that will last more than a thousand years. I've got a theory, but it's...." Pale light cut through the viewing port, and a gong sounded in the room. Lenk jerked to his feet and moved to his screens. "Maybe we'll know now," he said. "We'll be landing on a planet in about an hour. And it looks pretty much like Earth, from here." He cranked up the gain on the magnifiers, and studied it again, scanning the surface of the planet below them. There were clouds in the sky, but through a clear patch he made out enough evidence. "Want me to set us down near a city?" he asked, pointing.