The whole morning is like a lesson in natural selection. First, there is a sly competition to see who can get in the queue for the boats out to Urbina Bay on Isabela island. Walking in single file, some have a natural gift at being at the front, thus bagging all the expert commentary and being first to spot - and potentially frighten off - a Galapagos hawk, a finch, or the animal we are really after, a giant tortoise.
When one of the latter is spotted, it becomes a free-for-all: jostling for space, pushing aside wives, children and dinner pals from the night before to get close. Even the guide competes, pulling out his enormous telephoto, lying chest-down on the floor and trying to get the most saleable image of the tortoise's ... face, eyes, eyelashes?
The Galapagos brings out the Darwinian in visitors. The whole archipelago of 15 main islands, three smaller islands, and many rocks is a national park. So there are tight restrictions on tourists. You have to get what you can. Although there are several footpaths, landing areas and nature hot spots on most of the main islands, non-specialist visitors have access to only about 1 per cent of the land mass. There are similar restrictions in the marine reserve, were scuba diving is controlled and industrial fishing banned.
This allows the iconic species - marine and land iguanas, giant tortoises, sea turtles, Galapagos penguins, flightless cormorants, waved albatrosses - as well as less celebrated ones to go about their business as if humans hadn't shown up yet.
On Isabela I am so close to the tortoises that I can hear them breathing, and one friendly female comes up and pecks my lens. Little wonder sailors used to pick them up and store them, upside down, on their ships. Tortoises can survive months without water and are a great source of protein. As newswires reported in June, Lonesome George, the last of the Pinta Island giant tortoises, died, probably aged more than 100. Luckily, there are healthy numbers of 10 other subspecies on six islands, and no doubt their survival is partly a consequence of George's news-generating legacy.
One morning, when the heat and humidity get to me, I ask the guide if I can stay back and sit in the shade. I let the clamour of the group drift away and the insect and bird noises replace it. A Galapagos mocking bird comes up and, after a rummage around my feet, begins to clean my toenails. No doubt the past 20 years - in which tourist numbers have risen from 41,000 in 1991 to 160,000 now - have had a significant impact on animal behaviour, but to see a tiny, fragile bird being so gregarious is heart-rending.
During the seven-days on the luxurious Eclipse, I stay outside as much as possible. I make an exception at meal times, of course, when the Ecuadorean chefs ply us with fish ceviche, tender steaks, Mexican fajitas, grilled fish and a hog roast.
We see manta rays swimming beside our boat one evening, and a lava heron among the mangroves. Not surprisingly, the word 'lava' figures prominently in the names of Galapagos species. The archipelago is volcanic, the beaches black and spiky, the youngest of the islands, Fernandina, is a mere 700,000 years old.
In his journal, written while on board the Beagle during its landmark voyage, Charles Darwin was struck by what he called the 'antediluvian' character of the Galapagos. It looked pre-fallen, but also felt alienating and a bit scary. It is in his more polished Voyage of the Beagle that he remarks: 'The natural history of these islands is eminently curious... Most of the organic productions are aboriginal creations, found nowhere else; there is even a difference between the inhabitants of the different islands; yet all show a marked relationship with those of America, though separated from that continent by an open space of ocean, between 500 and 600 miles.'
It would be mistaken, however, to imagine the young - he was only 26 when he visited in 1835 - naturalist had a Damascus moment. In fact he failed to carefully classify the beaks of the birds that were to become associated with his theory, and it was left to ornithologist John Gould in England to announce that the specimens that Darwin had thought a mixture of different birds were, in fact, 12 species of finch. The Voyage of the Beagle was published in 1839, when he was back in England, and On the Origin of Species in 1859, and it took time for him to formulate his conclusions.
One thing we can learn from Darwin is to observe and be patient. And, in noting his own behaviour and that of the Beagle crew (killing birds to see how tame they were, vivisecting anything that moved, eating tortoises in a 'capital soup'), he shows us how humans are prone to upset the natural balance.
At Puerto Ayora, I see what colonisation does to the archipelago. Since 1991, the population has grown by 4 per cent every year and now numbers more than 25,000. The biggest settlement, with half of that number, Puerto Ayora, is a pretty ramshackle port town. After the fancy food of the Eclipse, it is a treat to have a pizza and beer, and chat to locals. The fish market is great, with a pelican, a blue-footed booby, a lava gull and a land iguana hanging around for scraps.
After four hours among humans and their cars and clutter, I am happy to be back on board the boat.
The next day we snorkel with Galapagos penguins and marine turtles. We spot small white-tipped reef sharks, emerging from the dark areas in the lava. There are flightless cormorants, a little penguin and dozens of Sally Lightfoot crabs on the rock. The crabs are everywhere, but they're not as numerous as they ought to be. Everything - everything beautiful, at least - is under threat.
On the last evening, on the upper deck, a passenger points to a flash of white or grey in the sea. We borrow a torch and it lights up a trio of sea lions darting and diving and showing off. Then, coming in at a pace, one, then two, then three, then four, reef sharks. Part of me feels like jumping in but the nature guide says they are potentially dangerous. For once the alpha-male photographers and queue-jumpers are stumped - this is one occasion when they aren't going to get too close for that close-up.
A trip to the Galapagos with Abercrombie & Kent costs from GBP4,795 (HK$57,600) per person. This includes one night in Quito, Ecuador, on arrival, seven nights on board the Eclipse and one night in Guayaquil, Ecuador, on return with all transfers, domestic and international flights from London. abercrombiekent.co.uk Tel: +44 (1242) 547 700
For more details on the conservation of the Galapagos Islands, go to savegalapagos.org
Read the e-text of Charles Darwin's Galapagos chapter (chapter 17) at literature.org