A work of unprecedented scale in the genre, it describes the history of humanity from the present onwards across two billion years and eighteen distinct human species, of which our own is the first and most primitive. Stapledon's conception of history is based on the Hegelian Dialectic, following a repetitive cycle with many varied civilizations rising from and descending back into savagery over millions of years, but it is also one of progress, as the later civilizations rise to far greater heights than the first.