sentences:  Their offer stands open for twenty years.At first only the most disadvantaged and unhappy members of human society take the Shaddill up on their offer; but when they return to visit Earth in a year or two with tales of their wonderful new lives, the exodus increases— which causes more and more problems for those who remain behind.By the time the Shaddill's 20-year offer expires, the only people left on Old Earth are those too stubborn or violent to make the leap to a new society, and who must subsist in the collapsed remains of the old.