The far future has been used as a setting in many works of science fiction. The far future setting arose in the late 19th century, as earlier writers had little understanding of concepts such as deep time and its implications for the nature of humankind. Classic examples of this genre include works such as H.G. Wells' The Time Machine (1895) or Olaf Stapledon's Last and First Men (1930). Recurring themes include themes such as utopias, eschatology or the ultimate fate of the universe. Many works also overlap with other genres such as space opera, science fantasy or apocalyptic and post-apocalyptic fiction.