The Perfect Winter Book : DARWIN’S SECRET : A Novel of the Amazon <i> by Richard Hoyt (Doubleday: $18.95; 322 pp.) </i>

I have always believed that summer reading calls for challenge: books whose length or depth or complexity need the freedom of uncommitted time to bring out their pleasure. By this standard, “Darwin’s Secret” is a winter book. The deepest of winters; February, with three simultaneous working deadlines, an intractable family quarrel, and lost car keys. Its pleasure is all skin.

But who am I to quarrel with tradition? Or rather, I am someone who is paid to quarrel with it but not necessarily to win. So go ahead and take “Darwin’s Secret” to the beach, and see how you do with Proust six weeks past New Year.

You need prodigious energy, a wild streak and a madcap sense of the zany to write a comic adventure story that transcends such funereal publicizing words as prodigious, wild, madcap, and zany.

Richard Hoyt possesses such qualities. His humor is dilapidated and unexpected, and he listens to it and only forces it once in awhile. The adventures his boatload of characters encounter in Brazil make no effort to be believable, but they avoid the industrially offbeat. Hoyt has his share of contemporary and hip, and probably he’s watched too many movies. But over and over in “Darwin’s Secret,” we hear the authentic voice of the storyteller; specifically, the tall-storyteller, asking himself “What if?” and cackling.

Best of all, Hoyt has got himself a river; in this case, the Amazon. A river is a marvelous antidote to the tendency of a picaresque tale to overheat and go dead. It refreshes the dull bits and cools down the engine rattle. If Providence had seen fit to pave the Mississippi, “Huckleberry Finn” would be just another “On the Road.”

Hoyt’s story is more delta than river; there are any number of side trips and stretches of mainstream that peter out into marsh. The miscellaneous band assembled aboard the river boat Ingaranha--a jaunty, white-painted affair, ship-shape and comfortable and with two glass fish eyes stuck on the bow--is bound for a fabulous gold-rush town far into the interior. Each has an entirely different reason for going there; they never do get there, in fact, and it doesn’t much matter.

After a pseudo-learned preface referring to an apocryphal explorer’s account of Albino catfish 20 feet long, the book begins gaudily. A procession of 30 gleaming Mercedes Benzes bump through the jungle. In the lead Mercedes ride a tough American adventurer and his blonde and entirely naked companion. Suddenly a swarm of tiny Indians surround the motorcade, armed with blowpipes and rolls of barbed wire.

That’s it. Only well into the story, on one of the side-trips taken by the boat party, do we read of a curious group of what seem to be prayer-mounds. Upon investigation, they turn out to be 30 Mercedeses, each tightly cocooned in barbed wire, each occupied by skeletons (one without clothes).

The book is full of such disconnected air bubbles; disquieting, comic and buoyant; small energizing jolts. Many of the jolts come as we meet the half-dozen voyagers on the Ingaranha, supplemented eventually by four more. Most have multiple identities, successively revealed.

There is the captain, who is also a brilliant painter of river fish, and the mate, who is also a naturalist. There is a Catholic priest who is trying to recover a tiny clay fish, an ancient Indian relic stolen as it was about to be exhibited. There is a Candomble priestess, intent on getting hold of the fish herself. There is a Japanese trader with a mysterious cargo of crates, all carefully hasped and sealed. “Apples,” he replies when questioned as to their contents.

There is Nicholas, an itinerant novelist who hopes to find literary material. And there is Deedee, an activist doctor who has fled the United States after serving a platterful of foreskins disguised as calamari at a cocktail party for a surgeons’ convention. Among other things, Deedee is against circumcision except for religious reasons. She is also against rich surgeons.

Here is Hoyt describing the conventioneers milling about, complacently addressing each other as “doctor.” “They almost broke the word into two syllables, almost smelling it, swirling it like wine in a goblet--there being a hint of credit card about its bouquet, a suggestion of new-car smell, with just the tiniest soupcon of freshly trimmed putting greens.”

Excessive, no doubt, and a little awkward; but with the exhilaration of a writer going too far in a direction entirely his own. Certainly, the adventures of the Ingaranha party have that quality.

They are much too elaborate to recount; they include an expedition to a society of leech-hunters, to a ruby mine and to a ranch burned out of hundreds of square miles of jungle. The ranchers raise thin cattle, mix their meat with fat imported--aboard the Ingaranha--from Kansas City, and ship the mix back to the United States for sale by a giant hamberger chain.

The adventures include attacks by monstrous catfish capable of devouring entire river boats, and lethal and mysterious quarrels among the passengers. They include a visit by agents of Xu Xu, a Utopian community hidden for three centuries in the depths of the Amazon, and distinguished by gentleness, imagination and outrageous whimsy. Among other things, the XuXuians were the ones who gave Charles Darwin the idea for “The Origin of Species”; his work consisted simply of verifying in the field.

There is a lot more, including some not-over-obvious digs at various types of civilized greed. There are, finally, the grisly deaths of almost everyone aboard the Ingaranha. It is a disembodied kind of grisliness, though; overwhelming, but overwhelmed in turn by the far-fetched high spirits of Hoyt’s storytelling.

“Darwin’s Secret” is often ramshackle and windy, frequently careless, and with moments of foundering cuteness. It never quite sinks, though. Hoyt’s air bubbles always hold it up. An ideal book for winter.