question:  What does the narrator say was the reason for Earth’s destruction?
context: The ancient rule was sink or swim—swim in the miasma of a planet without spaceflight, or sink to utter destruction! [Transcriber's Note: This etext was produced from Worlds of If Science Fiction, July 1963. The last contact the Ship had had with it—and we were the ones who dropped them—was almost 150 years ago. A planet is all right for a Mud-eater, but not for me. We have a place in the Ship like that—the Third Level—but it's only a thousand square miles and any time it gets on your nerves you can go up a level or down a level and be back in civilization. They don't let us grow for fourteen years and then kick us out to die. It means that everybody on the Ship is a person who can take care of himself if he has to. Daddy says that something has to be done in a closed society to keep the population from decaying mentally and physically, and this is it. The ship lifted impersonally away from Ninc and me like a rising bird, and in just a moment it was gone. When the sun disappears, somehow you wonder in your stomach if it's really going to come back. One of the smartest moves in history was to include horses when they dropped the colonies. I say "they" because, while we did the actual dropping, the idea originated with the whole evac plan back on Earth. Considering how short a time it was in which the colonies were established, there was not time to set up industry, so they had to have draft animals. One of the eight, as well as the two that were being built then, went up with everything else in the Solar System in 2041. I don't know how many of those planets had animals that could have been substituted but, even if they had, they would have had to be domesticated from scratch. There were five men on horseback herding a bunch of the ugliest creatures alive.
output:  Over population
options:  Losels  Over population  Lack of horses  Crime