Howard shrugged his shoulders. "I do not know. Perhaps they discovered that human brains were too trivial and absurd to bother with. Perhaps we ceased to amuse them. Perhaps they grew tired of us. But it is conceivable that the sign destroyed them—or sent them back through space. I think they came millions of years ago, and were frightened away by the sign. When they discovered that we had not forgotten the use of the sign they may have fled in terror. Certainly there has been no manifestation for three weeks. I think that they are gone."