GALAPAGOS. By Kurt Vonnegut. Delacorte/Seymour Lawrence. 312 pp. $16.95.

CONSIDER the human brain, urges Kurt Vonnegut in Galapagos, his first novel since Deadeye Dick. The size of that brain (next to the cognitive instruments of the "lower" animals, a disastrously big organ) has doomed us to evolutionary failure, suckering us down a primrose path of misconceptions, neuroses, and crackpot notions to our own inevitable extinction.

But suppose that we could sidestep extinction by jettisoning large pieces of our cerebral furniture, trading in our hands for flippers, and dieting exclusively on fish. In the long run, wouldn't these sacrifices turn out to be improvements, utterly happy evolutionary adaptations? In Galapagos, which often has the flavor of a jeremiad delivered more in sorrowful whimsy than in angry disgust, Vonnegut expertly stacks the deck to force us to conclude that forfeiting our big brains would be far better than either (1) continuing to inflict our vicious selves on this lovely planet or (2) going the way of the dodo as a viable species.

In fact, to structure this simultaneously funny and harrowing novel -- his most controlled and inventive, and so his best, since 1969's Slaughterhouse-Five -- Vonnegut has stood a hoary science fiction clich,e on its head. Almost to a person, sf writers have bought the premise that only greater brainpower will ensure humanity's survival and of course its total triumph over the physical cosmos. The superman has been a staple in speculative fiction from H.G. Wells' The Food of the Gods (1904) through Olaf Stapledon's Odd John (1935) all the way up to the 1953 category "classics" More Than Human by Theodore Sturgeon and Chilhood's End by Arthur C. Clarke, and beyond the 1950s to the present day. Vonnegut, who rigorously eschews genre classification not only to write whatever he deems important but also to keep from artificially limiting his audience, frog-marches the post-Darwinian concept of the superman right back to the sea. Galapagos, then, is both a harrowing litany of big-brain abuses against life and good order and a semihilarious paean to the joys of devolution.

The story resists easy summary. Seven people have gathered in the Ecuadoran seaport, Guayaquil, to take passage aboard the Bahia de Darwin for a trip to the historic Gal,apagos Islands. The trip's promoters have billed it as "the Nature Cruise of the Century," but a world monetary crisis and the outbreak of war between Peru and Ecuador -- among other disasters, including the sudden appearance of a bacterium that renders human women permanently infertile -- conspire to turn a couple of these would- be passengers, and six girls of the local Kanka-bono cannibal tribe who are hustled aboard under desperate circumstances, into the ancestors of the happily devolved sea-going human beings who people the Ecuadoran isles in the year 1,001,986 A.D.

Oh, yes. The novel's narrator is a ghost -- specifically, the ghost of Leon Trotsky Trout, son of the not-so-famous sci-fi writer Kilgore Trout. Leon is also a Marine deserter who helps build the Bah,ia de Darwin in Malmo, Sweden, after fleeing Vietnam. Leon's ensuing spirituality -- a crane at the dockyard decapitates him -- permits him to get inside the other characters' heads and also to provide sardonic commentary on the action -- the bulk of which takes place between 1986 and 2016 -- from a vantage one million years down our common timeline. Most of this gentle rant centers on the duplicity and the domineering arrogance of humanity's three-kilogram brains:

"Even at this late date, I am still full of rage at a natural order which would have permitted the evolution of something as distracting and irrlevant and disruptive as those great big brains of a million years ago. If they had told the truth, then I could see some point in everybody's having one. But these things lied all the time!"

The other characters include Mary Hepburn, a widow and former biology teacher from Ilium, New York; James Wait, a serial bigamist who marries widows for their money and then deserts them: Zenji Hiroguchi, the inventor of a simultaneous voice translator called Gokubi and of an advanced model called Mandarax that can diagnose a thousand human illnesses as well as disgorge malapropian quotations from world literature; and the brothers von Kleist: Sigfried, owner of a tourist hotel in Guayaquil and victim of Huntington's chorea, and Adolf, captain of the cruise ship and later the unsuspecting progenitor of all of latter-day humanity. Various celebrities, tortoises, pilots, sharks, and dancers (both human and blue-footed boobian) also make appearances, but the gutsiest and most admirable of the featherless bipeds proves to be Mary Hepburn.

No more about the story or its characters. Vonnegut has nearly always resorted to reductio ad absurdum to score points against our species' most dismaying follies, and in Galapagos he does so again, as precisely as he has ever wielded this instrument. I can imagine hostile readers indicting him for easy pessimism or simplistic misanthropy, but Vonnegut -- in keeping with his advocacy of smaller brains? -- has never pretended to be a deep thinker along the lines of Sartre or Bellow. He prefers common sense to arcane intellectualism, grotesque exaggeration to austere despair, and humor to hysteria. Therefore, his best work may sometimes seem a rare combination of the unsubtle and the subtle -- a sledgehammer blow to the forehead followed by a series of tenderly administered feather tickles. No one else writing in America today can duplicate this wily effect, and an eagerness to experience it anew has always typified the most avid Vonnegut freaks.

Gal,apagos is a book that makes me glad to declare my membership in that granfalloon ("a proud and meaningless association of human beings"). I am equally glad that my brain has sufficient size and weight to allow me a perplexed appreciation of this newest addition to the Vonnegut canon. In the long run, like William Shakespeare and Kilgore Trout, the man may be writing -- to quote the ghostly Leon -- "with air on air," but for the present, thank God, we can still find his words in our bookstores and libraries.