Complex sentence: By the time of the story there is a firm class division between Crew and Colonists, held together by fear of'Implementation', the secret police who take criminals and dissidents to the organ banks.
Split into simpler sentences: By the time of the story, there is a firm class division between Crew and Colonists. The former's rule over the latter is enforced by fear of' Implementation', the secret police who take criminals and dissidents to the organ banks.