Most genres begin with a hypothesis, on the understanding that if the author doesn't undermine it then the reader won't ask how plausible it is. In spy thrillers, we must believe that in the shadows of the East and West are spymasters like chess-players, mirror images of each other. In English crime novels, a single sleuth will be presented with a finite range of suspects. In American ones, a serial killer can confidently be expected to leave his twisted trade-marks behind him. Most science fiction needs a non-relativistic universe, where interstellar travel is possible.

Here we have to believe in an unthinkably vast catastrophe. The genre divides roughly into two. Survivalist books take a group of disparate individuals from the world as it was, and follow them through the disaster: John Wyndham's The Kraken Wakes and John Christopher's The World In Winter are typical. The situation is not so much resolved as held to an impasse: the plot ends when the characters find some degree of stability. They have found a safe refuge (in The Day of the Triffids, for example, the heroes occupy the Isle of Wight; in Peter Dickinson's The Changes, the heroine at last reaches France), and they have gathered together the means to begin rebuilding.

In the other sub-genre, the catastrophe itself is long past and we are now following the lives of those engaged in the reverse process: the world is not to be transformed from society to ruins, but from ruins into society. John Christopher's Tripods trilogy is an example, as is Terence Dudley's BBC serial Survivors, or even Isaac Asimov's Foundation trilogy, where the world in question is an entire galactic civilisation.

In destroying the world, authors face two big problems. Firstly, how to do it? Answering this question, John Christopher has probably killed more people than any other author in history. Amongst his means of decimating mankind are: nuclear holocaust, instability in the Earth's crust, alien invasion, mutated strains of wheat leading to famine, a tilt in the Earth's axis plunging northern Europe into an Ice Age and throwing Africa into prosperity, and so on. He has also isolated and then demolished small communities: a skiing party in a Swiss lodge taken over by possessing aliens, a yachting group on a radiation-soaked island in the South Pacific, the passengers on a cargo ship wrecked in the Arctic.

Looking at this mad catalogue it might reasonably be assumed that the exact method doesn't really matter, any more than the particular choice of murder weapon affects the quality of a crime novel. This is true, but leads to the second big problem. The protagonist is required to represent Everyman, the ordinary citizen suddenly deprived of the life-support mechanism of society. This means that he can't really be involved in the disaster, or an expert on it. And yet the reader needs to be told about what's happening in some detail.

The standard technique is to give the main character a friend who is a scientist, a journalist with friends in high places or, in children's fiction, a teacher who says more than he ought to. In a book where a second ice age is approaching, an oceanographer will probably make an offhand remark at a dinner party in chapter 1 ("most people don't realise that if the oceans rose tomorrow..."). And so forth. This is by far the most common way to inform the reader, though sometimes the scientist is replaced by a series of newspaper stories. Alternatively, though stretching belief, the hero just so happens to have personal experience already. Wyndham's The Day of the Triffids contains both methods: the hero has been, in childhood, the first person ever stung by a Triffid, and ended up industrially farming them -- a coincidence, then, that it should be he who survives. And, he has a more reflective colleague, who makes prophetic remarks during tea-breaks at work. These can be conveniently remembered later, in great detail.

So much for the technique. The motivation is usually, and especially in Wells or Christopher, to examine by drastic means the workings of society: like looking for the cogs in a watch smashed open. They no longer move, but now you can see them for what they were. This is especially apparent in what now seems a very dated version of the genre: the SF novel about black immigration into England in the early 70s, or in other words, at the time of Enoch Powell, when English politics briefly seemed to license bigotry. Christopher Priest's Fugue For A Darkening Island and John Christopher's The World In Winter are powerful but, thankfully, now quite unnecessary pitches for tolerance.

A considerable attraction, to writers, is the pleasure in taking the world and rendering it romantically as a fallen Romanesque empire. The buildings are overgrown, as in the French early 19th-century vogue for paintings on subjects such as "The Long Hall of the Louvre in Ruins", with crumbling columns invaded by trees and shepherds. The secret of electricity is lost, leaving lonely and disused pylons across the landscape. The fabled "atomic power" plays a similar role in Asimov's Foundation, which is transparently a sci-fi version of Gibbon's Decline and Fall of the Roman Empire. John Christopher's The Pool of Fire ends with, among other things, an elegy for the rural peace which is lost as civilisation resumes.

Herbert George Wells, born the son of a professional cricketer and unsuccessful businessman, began as an apprentice draper, then became a teacher and -- despite poverty and ill-health -- a student of science. The Time Machine (1895) is counted by some as the first proper science-fiction novel, but he also wrote comic realism about his lower-middle-class origins, and at the end of his life became an influential speaker on social issues. He lived to see and comment on the Hiroshima bombing, a sad end for a committed humanist with a powerful imagination for the ways in which society transforms its physical rules and settings. In The War In The Air (1907), for example:

There had been talk of mono-rails for several years. But the real mischief began when Brennan sprang his gyroscopic mono-rail car upon the Royal Society. It was the leading sensation of the 1907 soirées...

In a little while no one thought anything of crossing an abyss on a wire, and the mono-rail was superseding the tram-lines, railways, and indeed every form of track for mechanical locomotion... The mono-rail cable standards became a striking fact in urban landscape, for the most part stout iron erections rather like tapering trestles, and painted a bright bluish green... All day and all night the fast cars from Brighton and Hastings went murmering by overhead -- long, broad, comfortable-looking cars, that were brightly lit after dusk. As they flew by at night, transient flares of light and a rumbling sound of passage, they kept up a perpetual summer lightning and thunder-storm in the street below.

Presently the English Channel was bridged -- a series of great iron Eiffel Tower pillars carrying mono-rail cables at a height of a hundred and fifty feet above the water, except near the middle, where they rose higher to allow the passage of the London and Antwerp shipping and the Hamburg-America liners.

Wells collapses this transformation into a page, giving plausible notes on how the changing landscape affected the Smallways family, whose young men have a bicycle repair shop but (like the Wright brothers in 1903) dream of aviation. Wells was not the first author to observe the extraordinary speed with which technology could overtake the landscape -- the arrival of the railways, with the consequent abandonment of long-distance roads to grass and sheep, is noted in many nineteenth-century novels, such as Thomas Hardy's The Return of the Native. But Wells was arguably the first to project such changes ahead, into the future.

The War of the Worlds (1898) was the first and greatest apocalypse novel, a short book of poetic force, which is in many ways still definitive. It is a prototype for almost every book of the "surviving the catastrophe" variety, and certainly for the works of Wyndham and Christopher. Wells's most important characters met along the wayside, the curate and the artilleryman, are archetypal of the wandering madmen and potential fascists who inhabit the ruined Earths of all such novels since.

Wells wrote the novel when he was living in Woking, and gave a tongue-in-cheek plan of the novel in one of his letters:

I completely wreck and sack Woking -- killing my neighbours in painful and eccentric ways -- then proceed via Kingston and Richmond to London, selecting South Kensington for feats of peculiar atrocity...

Briefly, Martian meteorites land in Horsell Common, but turn out to be artificial cylinders. Creatures struggle out, resist all attempts at communication or attack with a vicious heat ray, and build tripod carriages to stalk the earth. These sweep across to London, laying waste with a black poison gas (an eerie anticipation of the First World War). Refugees and victims are everywhere, and the Martians' triumph is complete: but they are defeated, helplessly, by a bacterial disease, and the last sight of their camp is as pathetic as that of the ruins of London surrounding it.

There have been numerous adaptations of The War of the Worlds. Most famous is still Orson Welles's radio version, set in New Jersey and broadcast on 30 October 1938. Next is probably the 1953 film, which created a look surviving in the late 1980s American TV version and in much other science fiction besides.

The same style colours V (1984), a series of TV movies in which healthy-looking aliens arrive bearing gifts but then establish a dictatorship which various even more healthy-looking Americans have to overthrow. V is on the whole astonishingly silly and is at its most hapless when trying to make a point, as in its wincingly inadequate Holocaust parable. But it's so kitsch as to be quite good fun, at least until it turned into a follow-up television series, which was execrable.

But I don't think any of the mutant forms of The War of the Worlds (most recently the movie Independence Day) do justice to the book. It's out of copyright and anyone can download it from the Project Gutenberg archives.

The debts of British sci-fi to John Wyndham are not always appreciated nowadays, his reputation having fallen since his death. He will always be known for the triffids, the word "triffid", like the word "Dalek", having passed into dictionaries. But his most important works, The Day of the Triffids (1951), The Kraken Wakes (1955) and The Midwich Cuckoos (1957) are undeservedly often out of print. A vast number of Penguin paperbacks have been printed, though, so that they will never be hard to find in Church jumble-sales and second-hand shops. The Kraken Wakes, the title being a reference to Tennyson's poem of a sea-monster rising from the deep, is one of the finest apocalyptic novels in the literature.

Born John Harris, the son of a barrister, he was -- like John Christopher -- at times a hack writer, under numerous other pseudonyms. He called his works "logical fantasy" rather than "science fiction", having been a great admirer of Wells and Jules Verne in youth, but nowadays they are considered on the cosy side of catastrophe. This is partly because it is his earlier novels which are most remembered. Even there, however, it would not be quite fair to say that Wyndham's characters have a jolly adventure of it, as they do suffer attacks of nervous tension, and worse. But the early Wyndham is ultimately positive and cheery. If one can imagine a Dornford Yates or John Buchan secret agent a little older, married and working for the BBC, that would serve as a Wyndham hero.

The critic John Griffiths observes that:

Where Christopher examined the dilemma of choice between survival and barbarism, Wyndham's is between evolution and extinction... it is interesting to follow his development from Man surviving the Monster in The Kraken Wakes... to the alternative solutions of his two late 1950s books with a pessimistic outcome in Man's destruction of an alien, superior and quasi-human intelligence in The Midwich Cuckoos and the triumphant emergence of homo superior in The Chrysalids.

Critics of Doctor Who and other television sci-fi are only now beginning to talk up Wyndham's influence, particularly the later Wyndham of subtle alien presences.

John Christopher paints an interesting portrait, in a 1999 interview:

...Nice to be mentioned in the same breath. I did indeed admire his work... [someone] described John's as cosy catastrophes. Not true, but his characters are pleasanter, and he plainly likes them more. But he himself was one of the nicest and most amiable of men. The only sour note I recall is in that short story, "Survival", where a horror of the female seems unexpectedly to emerge. He was educated at that (then) advanced public school, Bedales, which was a pioneer of co-educational boarding, and which one would expect to help its pupils overcome the normal English male inadequacies. In fact he was terribly shy with women, and didn't marry until he was over sixty. He then retired to Petersfield, within walking distance of the school. Life is strange at times.