A provisional and, I hope, uncontroversial definition of science fiction might run as follows: sf is a distinct kind of popular literature telling stories that arise from actual or, more usually, hypothetical new discoveries in science and technology. The science and technology must be convincing enough to invite a certain suspension of the reader's disbelief: this is how sf, as a creation of the later nineteenth century, differs from earlier fiction in which themes such as space travel and encounters with extraterrestrials were presented in a merely fantastic or satirical light. The present essay will propose a broad evolutionary model for the development of science fiction, comprising a prehistorical and at least three historical stages. The points of transition are those at which the genre can be seen to shift from one kind of discourse to another. In all science-fiction stories, scientific and technological innovation has consequential...