Only a few of the numberless cubes sent forth ever found a landing and response on an inhabited world - since there was no such thing as aiming them at goals beyond sight or knowledge. Only three, ran the story, had ever landed on peopled worlds in our own particular universe. One of these had struck a planet near the galactic rim two thousand billion years ago, while another had lodged three billion years ago on a world near the centre of the galaxy. The third -- and the only one ever known to have invaded the solar system -- had reached our own earth 150,000,000 years ago.