 Information given in the story indicates that Humanity only developed space travel far enough to attract the attention of Central Control in the 37th century AD.Norton explains the implied retardation of human development through references to nuclear wars, which presumably caused so much destruction that civilization took an extra sixteen or seventeen centuries to achieve a level of development suitable for resuming Humanity’s reach for the stars.Presented in the guise of a history lecture at an alien university, Norton's introduction explains that in the 40th century the people of Terra (the Latin name having replaced the Anglo-Saxon Earth) can only go to the stars as mercenaries.