"At the time we left," the message continued, "we found no other intelligent race, though there were some capable of further evolution. Perhaps our scout ships long ago met your ancestors on some remote planet. We were never very numerous, and because we move and multiply so slowly, we are in danger of being swept out of existence in the foreseeable future. We prefer to leave while we can. The reason we must go developed on our own planet, deep beneath the cities, in the underworks, which we had ceased to inspect because there was no need to. This part was built to last a million generations, which is long even for us." Emmer sat upright, annoyed at himself. "Of course! There are always sewers and I didn't think of looking there!" "In the last several generations, we sent out four expeditions, leisurely trips because we then thought we had time to explore thoroughly. With this planet as base of operations, the successive expeditions fanned out in four directions, to cover the most representative territory." Kelburn stiffened, mingled pride and chagrin on his face. His math had been correct, as far as he had figured it. But had there been any reason to assume that they would confine their exploration to one direction? No, they would want to cover the whole Milky Way. Taphetta paled. Four times as many humans to contend with! He hadn't met the other three-fourths yet--and, for him, it wasn't at all a pleasant thought. "After long preparation, we sent several ships to settle one of the nearer planets that we'd selected on the first expedition. To our dismay, we found that the plague was there--though it hadn't been on our first visit!" Halden frowned. They were proving themselves less and less expert biologists. And this plague--there had to be a reason to leave, and sickness was as good as any--but unless he was mistaken, plague wasn't used in the strict semantic sense. It might be the fault of the translation. "The colonists refused to settle; they came back at once and reported. We sent out our fastest ships, heavily armed. We didn't have the time to retrace our path completely, for we'd stopped at innumerable places. What we did was to check a few planets, the outward and return parts of all four voyages. In every place, the plague was there, too, and we knew that we were responsible. "We did what we could. Exhausting our nuclear armament, we obliterated the nearest planets on each of the four spans of our journeys." "I _wondered_ why the route came to an end," crinkled Taphetta, but there was no comment, no answer. "We reconstructed what had happened. For a long time, the plague had lived in our sewers, subsisting on wastes. At night, because they are tiny and move exceedingly fast, they were able to make their way into our ships and were aboard on every journey. We knew they were there, but because they were so small, it was difficult to dislodge them from their nesting places. And so we tolerated their existence."