Over the last 20 years the number of tourists visiting the Galapagos Islands, site of some of Charles Darwin's most important insights into evolution, has increased from 20,000 a year to a 120,000. Yet the islands still feel remote and desolate as one walks the carefully marked tourist paths; and there is the comforting thought that at least most of the visitors are unlikely to be - or to remain once they have seen the place with their own eyes - creationists. True, there might be two or three other boats moored near the permitted landing places when you wake in the morning after a night of rough sailing, but if you get onto land early enough you rarely see the people they carry, and when you do they rarely succeed in being obtrusive. It is part of the character of these bleak, sparse, strange volcanic outcrops in the vast Pacific that their aloofness absorbs itinerants, making it seem that the islands ignore humans just as their resident iguanas, tortoises and flightless cormorants do.

We were a family party in a 96-foot two-masted sloop, 12 of us with five crew, and we sailed from Baltra, whose small airport is the destination for flights from the Ecuadorian mainland. In the days and nights that followed we sailed all the way round the islands from Santa Cruz via Santa Fe to Espanola, and then to Floreana where, divided into two truncated teams representing England and Ecuador, we played football at Post Office Bay (I was in goal for Ecuador, and we won two nil).

From there we crossed a plunging stretch of Pacific to Isabella, the largest island, and sailed up its west coast in a series of hops, stopping to walk the lava fields and to kayak in the mangrove lagoons.

Then to Fernandina, the youngest and most pristine of the islands, and from it north round Isabela to Santiago, another long sail in which the sloop rolled and pitched incessantly, stopping first on the latter's north-west shore and then across from the huge black lava fields of its eastern side at the little island of Bartolome, home of the famous Pinnacle Rock.

The wealth of wildlife on these at first so barren-looking islands is astonishing. What makes the Galapagos archipelago special is that it lies on the equator, yet is washed by the cold Humboldt current that sweeps up from the Antarctic. Its remoteness has kept the wildlife populations of the separate islands segregated from continental influences and from one another; hence the evidence of evolutionary differences among subspecies that hits one in the eye.

Man's depredations have damaged the tortoise and fur seal populations, and have introduced ravening species of goats and rats which seriously interfere with habitats. But the evolutionary laboratory discovered by Darwin is still there, and the work of the National Park and the Charles Darwin Research Centre is steadily repairing some of the damage.

Despite Ecuador's strained resources it is proving a good steward of the island's scientific importance, no mean feat in the face of competing pressures: from a hungry tourist industry, from angry local fishermen, from illegal immigration (mainlanders wishing to cash in on the tourist boom) stretching local resources to breaking point.

As one swims among sea lions in the sticky Pacific breakers on a white sand beach on Floreana or one of the other islands, those troubles seem very far away, and so does the rest of the world. Every day there is something new and astonishing to see. Blue-footed boobies really do have bright blue feet and legs; huge albatrosses have to hurl themselves off cliff-tops to take flight; the marine iguanas, reptilianly crawling over one another with their expressionless ancient faces held up to the sun, sneeze great sprays of salt at you as you walk past; the sea lion pups invite you to play; the turtles glide in tranquil silence under your dinghy as you approach the shore. There is not a moment when you are unconscious of being somewhere fantastic and significant.

And after a time the black volcanic fields, the bare landscapes, the scanty thorn bushes, the big broken-topped volcanoes, begin to assume an austere beauty invisible at first because of the initial rebarbative face with which the islands greet you.

Describing one characteristic encounter must serve to stand for dozens of others. While more robust members of our party were snorkelling in the swell around Pinnacle Rock, seeing turtles and white-tipped sharks swim around them, I was helping my seven-year old daughter do the same in the quieter water off a nearby beach. We had just seen a ray undulating in the sand, and some brightly striped fish float meditatively by, when a sea lion, svelte and shining, torpedoed towards us through the sunlit submarine shallows to inspect my daughter face to face through her mask. Ashore on several of the islands we had been inspected at close quarters by a number of these friendly inquisitive creatures, but in the water it is a different matter; here you are in their domain, and you feel their kindly disappointment that you are unable to shoot off with them into deeper water to play.

There is almost nowhere else on earth where the (sea) lion lies down with the lamb (my daughter); the Galapagos is heaven, and not just because it proves Darwin right.

AC Grayling's trip was arranged by Journey Latin America