When Charles Darwin and the crew of The Beagle first saw the Galapagos Islands in 1835 the captain described them as 'the infernal regions'. Certainly the twisted black volcanic rocks and highlands hidden in mist come as a shock to anyone who expects your typical tropical island. And this tiny archipelago of volcanic islands inspired the theory of evolution which changed the world.

At the end of a one-week trip Darwin had catalogue so many new species his notebooks were almost full. What caught his attention was the fact that each island was home to its own custom-made creatures, adapted specifically to life in that one spot. The evidence he found on Galapagos was the foundation of his arguments for a theory of evolution in his book, Origins of Species. Going on for 200 years after his visit the islands are still a wildlife paradise. Protected by their isolation they are home to unique species; the seaweed-eating marine iguanas, the flightless cormorant, several different breeds of giant tortoise. The list goes on - and on.

As we dived into the water to look at the fish, they dive-bombed us. Like sleek-furred, jet-propelled missiles they raced between our legs. If that failed to attract attention they tugged at our flippers with their teeth or shoved their whiskery noses up against our snorkel masks as we goggled out at them - as inept in the water as they were elegant.

It would be a mistake, however, to think of these as 'tame' animals. These aren't performing sea lions. If they are playful it is because they want to be, not because it is the only way to get fish thrown at them. Anyone rash enough to get within fish-throwing distance of a bull sea lion protecting its territory would soon learn that these animals are wild.

On Fernandina Island the trombone-bellow of an attacking male stopped us in our tracks. The fight was serious stuff. Two huge males - heavyweight champions both - slapped all their bulk against each other and sunk their teeth deep into each others neck until blood smeared their coats. Afterwards, the victor lay in the shallow water like a stranded submarine attended by adoring cows that licked his wounds. The loser slunk off to pick a fight with a flyweight further down the beach, which he obviously thought he had more chance of beating.

Fernandina was one of my favourite stops. Not only because of the sea lions but because it was home to those strange marine iguanas, Darwin's 'Black imps'. The only iguana's in the world which swim. As the rubber pangas (dinghies) sped away towards the island the cruise ship behind us was a white ghost lost in a layer of mist which hung low over the water. Only its radio mast was still visible. The island in front of us was a black shadow.

As soon as we stepped ashore the iguanas came slithering towards us. Strange creatures from another age. Motionless, they were almost invisible, as if they were carved out of the rock. They lay on top of each other on the lava. It was almost impossible not to tread on their tails. When I found myself alone with a pack of them, their movements were somehow unsettling, macabre, like the folding of bat's wings.

My other favourite stop was the bird-infested Tower Island. At every step there were masked boobie birds with black stripes across their eyes, red-footed boobies looking like painted clowns and gripping the trees with their ungainly crimson feet. And best of all, the frigate birds with their bizarre inflatable chests, puffed out like red party balloons to attract the females in breeding season. So many of them nestled in the green bushes that from a distance they looked like berries.

Each island holds its own special secrets. On Isabela, you'll find the giant tortoises, on Hood, the massive waved albatrosses nest and dance towards each other in bobbing, flapping courtship rituals. Behind the rocky red sand beach of Jervis Island are pink flamingos. Beneath the waves that skirt Bartholemne Island are giant rays some 13 feet (four metres) across.

Unfortunately, I was headed to Baltra Island, where strange metal birds land and disgorge passengers at the tiny airport. My visit had come to an end.