Olaf StapledonStanding outside the science fiction "field", he wrote fictional explorations of the futures of whole species and galaxies.
(HT: Flos Carmeli)
Interesting; Stapledon, of course, is the philosopher who wrote Last and First Men and Star Maker; C. S. Lewis's Space Trilogy (particularly Out of the Silent Planet) is a sharp criticism of the sort of attitude toward science and humanity taken in those works. Stapledon, whose academic work in philosophy had virtually no impact, had, through his 'philosophical romances' an immense impact on science fiction, practically inventing whole sci-fi tropes that have since become commonplace: galactic empires, genetically engineered species, and so forth.
Many of his out-of-print works are online, e.g.:
Philosophy and Living (1939)
A Modern Theory of Ethics
A Modern Magician is a short story about a psychokinetic who kills things in order to impress his girlfriend.