The August 1931 edition of Hugo Gernsback’s Wonder Stories has a letter from author Clare Winger Harris where, after some comments on the science fiction films (or lack thereof) existing at the time, she identifies what she believes to be the 16 main categories of science fiction:
The list seems a little quaint today, but speaking as someone who’s been doing an issue-by-issue read-through of Amazing Stories (I’m currently up to 1928) it seems a pretty accurate summary of the main themes of SF literature as it existed at the time.
For example, she mentions giant insects as a plot category. As it happens, Amazing had run a number of stories on this theme: Curt Siodmak’s “The Eggs from Lake Tanganyika”, Murray Leinster’s “The Mad Planet” and “The Red Dust”, arguably Edgar Allan Poe’s “The Sphinx” and Harris’ own “The Miracle of the Lily”. The following category, gigantic man-eating plants, had also turned up a few times. Nowadays, though, it would make more sense to conflate the giant insects and plants into a single category — one that could also include King Kong and Godzilla, perhaps.
The fourth category, “adventures in the micro or macro-cosmos”, seems rather esoteric today; yet it had been covered by some of Gernsback’s earliest acquisitions: G. Peyton Wertenbaker’s “The Man from the Atom” and Ray Cummings’ “Around the Universe”, each of which suggested that the entire universe was no more than a single atom of an unfathomably larger universe.
Artificial intelligence and androids had not yet come into their own and so, in this list, are represented only by the broad category of “synthetic life”. Interestingly, Harris apparently did not consider contact with extraterrestrials to be a significant theme in itself: she includes “monstrous forms of unfamiliar life” but leaves no room for more anthropomorphic or otherwise sympathetic aliens. Her decision to split “interplanetary space travel” and “adventures on other worlds” into two categories seems redundant — perhaps, with the former section, she was thinking of Jules Verne’s lunar adventures, where the meat of the story takes place inside the vessel rather than on the Moon.
A number of the categories speak of the technology level of the day. The inclusion of “ray and vibration stories” harks back to an era when the unseen effects of various electro-magnetic waves had only recently been grasped by researchers, and were firing the imaginations of SF authors. Harris includes stories of world-wide destruction, noting that they can occur either on Earth or on other worlds, but specifies that they are caused by natural disasters — come the atomic age, the idea of a man-made apocalypse would become rather more topical.
All in all, the list is an interesting document. At the time it was printed, it did a pretty good job of summarising the dominant themes in SF. Looking back, it’s a real time capsule.