Kevin Schafer/ Martha Hill

Tourists stick to trails to avoid disturbing wildlife.

Tui De Roy/ Minden

Park workers relocating a giant-tortoise nest.

THE POLARIS STOPS AT MANY of the larger islands: Española, Floreana, Genovesa, San Salvador, Santa Cruz, Fernandina, and Isabela. The strong ocean currents, the distances between the islands (anywhere from three to 110 miles), their raw black volcanic profiles and ragged cliff faces, create a forbidding impression of isolation. Less than four million years old, the Galápagos Islands are some of the youngest and most active volcanoes in the world (the most recent major eruption occurred on Isabela in 1998). We cruise past the rocky oceanic outcrop called Roca Redonda, the tip of an enormous underwater mountain, and spend a morning in Puerto Ayora, on Santa Cruz, at the Charles Darwin Research Station. The 41-year-old station is funded by Ecuador, UNESCO, and various scientific organizations (the San Diego Zoo foots the bill for the station's giant-tortoise projects), and is staffed by biologists who oversee its conservation and protection programs.

The animals they study are as fierce and strange as the geological features of the islands: delicate little finches called vampires survive by pecking other birds and drinking their blood; fluffy masked booby chicks routinely kill their siblings to better their own chances; male albatross, with wingspans wider than a man is tall, gang-rape females returning from sea. Because these creatures evolved without predators in the picture, they have no fear of us, and we stand inches away from basking marine iguanas and sea lion pups. As our Zodiac pulls up to the narrow concrete landing on uninhabited Fernandina, one of the most pristine oceanic islands left in the world, we're instructed to give some room to a sea lion mom who's just given birth on the jetty, the afterbirth dripping into the water. On a trail through the interior of Española, the southeasternmost island, we have to take care not to step on a Galápagos snake or to disturb the rooster-size offspring of the waved albatross standing on the trail. Here, tiptoeing through this miniaturized Jurassic Park, we're the dinosaurs, looming over the inhabitants.

Not that they can't take care of themselves. At Punta Suarez, on Española's windward side, we're surrounded by thousands of breeding pairs of blue-footed boobies, masked boobies, and most of the planet's population of waved albatross, not to mention crowds of the island's endemic subspecies of mockingbirds, as well as shearwaters, frigate birds, and Galápagos hawks, all whistling, hooting, cawing, and shrieking. The blue-footed boobies are engaged in their bizarre foot-fetish rituals, ever so slowly lifting each blue foot to show their prospective mates. Albatross pairs are furiously clicking enormous yellow beaks that look like they could take your arm off. It's no place for people who have panic attacks brought on by a certain Hitchcock film.

As tour guides go, Fowler and the naturalists she oversees on the Polaris are remarkably frank about everything Galápaganean. Paula Tagle, an Ecuadoran geologist, unblushingly tells us about the sexual proclivities of the small black males of the Genovesa iguana, who live in an environment harsh even by Galápagos standards--raked by the sun and with meager food sources--and who are less likely to get lucky than iguanas on other islands. The females, for whom size apparently does matter, find the males puny and unappealing. The smaller males have developed their own compensatory routine: They work up a load of sperm by masturbating on a rock for a few minutes, ejaculate into a pouch, and then, when an unwitting female walks by, hop on, releasing the sperm.

Our guides are also hypervigilant, knowing full well that our very presence stirs the evolutionary stew. They make certain that we stay on the trails, that we constantly rinse our feet and snorkeling gear (so as not to track sea-lion poop onto the boat or carry seeds from one island to another), and that we strictly observe the No-Giving-Water-to-the-Mockingbirds rule (although flocks follow us everywhere, aggressively begging for a sip from our bottles on islands where the only shade trees are 30-foot cacti). Everywhere we go, the guides preach the gospel of Darwin and promote the destruction of introduced and invasive species. Lest the Galápagos go the way of Hawaii, with its dozens of extinctions caused by introduced pigs, snakes, insects, and plants, the fittest cannot be allowed to survive. One afternoon, as we gather for lunch in the dining room aboard the Polaris, we're greeted by an entire roast pig, a cheery apple stuffed into its surprised-looking mouth, shot that morning on Floreana. Message received.