OK, I thought, I've got the point. This is why the island is relatively unspoilt. I made one more lunge upwards, just as those on shore succeeded at last in pushing the boat away from me. This allowed me to get my head up above water and cling on to a crack in the rock. With a lot more slipping and sliding and thrashing in the heavy swell I managed finally to manoeuvre myself up to within arm's reach of Mark and the others, who yanked me urgently up and on to the rock. I sat in a spluttering, bleeding heap protesting that I was fine and all I needed was a quiet corner to go and die in and everything would be all right.

The sea had been swelling heavily for the two or three hours it had taken us to reach the island and it seemed as if my stomach had heaved something approaching my entire body weight into the sea, so by this time I was feeling pretty wobbly and strung out and my day on Round island passed in rather a blur. While Mark went with Wendy Strahm, the botanist, to try and find some of the species of plants and animals that exist only here on this single island, I went and sat in the sun near a palm tree called Beverly and felt dazed and sorry for myself.

I knew that the palm tree was called Beverly because Wendy told me that was what she had christened it. It was a bottle palm, so called because it is shaped like a Chianti bottle, and it was one of the eight that remain on Round Island , the only eight wild ones in the world.

Who on earth, I wondered, as I sat next to Beverly in a sort of companionable gloom, gets to name the actual islands?

I mean, here was one of the most amazing islands in the world. It looked utterly extraordinary, as if the moon itself was rising from the sea - except that where the moon would be cold and still, this was hot and darting with life. Though it appeared to be dusty and barren at first sight, the craters with which the surface was pocked were full of dazzling white-tailed tropicbirds, brilliant Telfair's skinks and Guenther's geckos.

You would think that if you had to come up with a name for an island like this you'd invite a couple of friends round, get some wine and make an evening of it. Not just say, oh it's a little bit round, let's call it ` Round Island '. Apart from anything else it isn't even particularly round. There was another island just visible on the horizon, which was much more nearly round, but that is called Serpent Island , presumably to honour the fact that, unlike Round island, it hasn't got any snakes on it. And there was yet another island I could see which sloped steadily from a peak at one end down to the sea at the other, and that, unaccountably, was called Flat Island . I began to see that whoever had named the islands probably had made a bit of an evening of it after all.

The reason that Round island has remained a refuge for unique species of skinks, geckos, boas, palm trees, and even grasses that died out long ago on Mauritius is not simply that it is hard for man to get on to the island, but that it has proved completely impossible for rats to get ashore. Round Island is one of the largest tropical islands in the world (at a bit over three hundred acres) on which rats do not occur.

Not that Round Island is undamaged - far from it. A hundred and fifty years ago, before sailors introduced goats and rabbits on to the island, it was covered in hardwood forest, which the foreign animals destroyed. That is why from a distance and to the untutored eye, such as mine, the island appeared to be more or less barren at first sight. Only a naturalist would be able to tell you that the few odd-shaped palms and clumps of grass dotted about the place on the hot, dry, dusty land were unique and unspeakably precious.

Precious to whom? And why?

Does it actually matter very much to anyone other than a bunch of obsessed naturalists that the eight bottle palm trees on Round Island are the only ones to be found in the wild anywhere in the world? Or that the Hyophorbe amarfcaulis (a palm tree so rare that it doesn't have any name other than its scientific one) standing in the Curepipe Botanic Gardens in Mauritius is the only one of its kind in existence? (The tree was only discovered by chance while the ground on which it stands was being cleared in order to construct the Botanic Gardens. It was about to be cut down.)

There is no `tropical island paradise' I know of which remotely matches up to the fantasy ideal that such a phrase is meant to conjure up, or even to what we find described in holiday brochures. It's natural to put this down to the discrepancy we are all used to finding between what advertisers promise and what the real world delivers. It doesn't surprise us much any more.

So it can come as a shock to realise that the world we hear described by travellers of previous centuries (or even previous decades) and biologists of today really did exist. The state it's in now is only the result of what we've done to it, and the mildness of the disappointment we feel when we arrive somewhere and find that it's a bit tatty is only a measure of how far our own expectations have been degraded and how little we understand what we've lost. The people who do understand what we've lost are the ones who are rushing around in a frenzy trying to save the bits that are left.

The system of life on this planet is so astoundingly complex that it was a long time before man even realised that it was a system at all and that it wasn't something that was just there.

To understand how anything very complex works, or even to know that there is something complex at work, man needs to see little tiny bits of it at a time. And this is why small islands have been so important to our understanding of life. On the Galapagos islands , for instance, animals and plants which shared the same ancestors began to change and adapt in different ways once they were divided from each other by a few miles of water. The islands neatly separated out the component parts of the process for us, and it was thus that Charles Darwin was able to make the observations which led directly to the idea of Evolution.

The island of Mauritius gave us an equally important but more sombre idea - extinction.

The most famous of all the animals of Mauritius is a large, gentle dove. A remarkably large dove, in fact: its weight is closest to that of a well-fed turkey. Its wings long ago gave up the idea of lifting such a plumpy off the ground, and withered away into decorative little stumps. Once it gave up flying it could adapt itself very well to the Mauritian seasonal cycle, and stuff itself silly in the late summer and autumn, when fruit is lying rich on the ground, and then live on its fat reserves, gradually losing weight, during the leaner, dry months.

It didn't need to fly anyway, since there were no predators that wished it any harm and it, in turn, is harmless itself. In fact the whole idea of harm is something it has never learnt to understand, so if you were to see one on the beach it would be quite likely to walk right up to you and take a look, provided it could find a path through the armies of giant tortoises parading round the beach. There's never even been any reason for humans to kill it because its meat is tough and bitter.