The great thing about being the only species that makes a distinction between right and wrong is that we can make up the rules for ourselves as we go along.

The fish were still hopping harmlessly up and down the tree. They were about three inches long, brown and black, with little bobble eyes set very close together on the top of their heads. They hopped along using their fins as crutches.

'Mudskippers,' said Mark, who happened along at that moment. He squatted down to look at them.

`What are they doing in the tree? I asked.

`You could say they were experimenting,' said Mark. 'If they find they can make a better living on the land than in the water, then in the course of time and evolution they may come to stay on the land. They absorb a certain amount of oxygen through their skin at the moment, but they have to rush back to the sea from time to time for a mouthful of water which they process through their gills. But that can change. It's happened before.'

'What do you mean?

'Well, it's probable that life on this planet started in the oceans, and that marine creatures migrated on to the land in search of new habitats. There's one fish that existed about 350 million years ago which was very like a mudskipper. It came up on to the land using its fins as crutches. It's possible that it was the ancestor of all land-living vertebrates.'

`Really? What was it called?

'I don't think it had a name at the time.'

'So this fish is what we were like 350 million years ago?'

`Quite possibly.'

'So in 350 million years time one of its descendants could be sitting on the beach here with a camera round its neck watching other fish hopping out of the sea?'

'No idea. That's for science fiction novelists to think about. Zoologists can only say what we think has happened so far.'

I suddenly felt, well, terribly old as I watched a mudskipper hopping along with what now seemed to me like a wonderful sense of hopeless, boundless, naive optimism. It had such a terribly, terribly, terribly long way to go. I hoped that if its descendant was sitting here on this beach in 350 million years time with a camera round its neck, it would feel that the journey had been worth it. I hoped that it might have a clearer understanding of itself in relation to the world it lived in. I hoped that it wouldn't be reduced to turning other creatures into horror circus shows in order to try and ensure them their survival. I hoped that if someone tried to feed the remote descendant of a goat to the remote descendant of a dragon for the sake of little more than a shudder of entertainment, that it would feel it was wrong.

I hoped it wouldn't be too chicken to say so.

We startled ourselves by arriving in Zaire on a missionary flight, which had not been our original intention. All regular flights in and out of Kinshasa had been disrupted by an outbreak of vicious bickering between Zaire and its ex-colonial masters, the Belgians, and only a series of nifty moves by Mark, telexing through the night from Godalming, had secured us this back door route into the country via Nairobi.

We had come to find rhinoceroses: northern white rhinoceroses, of which there were about twenty-two left in Zaire, and eight in Czechoslovakia. The ones in Czechoslovakia are not in the wild, of course, and are only there because of the life's work of a fanatical Czech northern white rhinoceros collector earlier in this century. There is also a small number in San Diego zoo, California. We had decided to go to rhino country by a roundabout route in order to see some other things on the way.

The aircraft was a sixteen-seater, filled by the three of us -Mark, Chris Muir our BBC sound engineer and myself - and thirteen missionaries. Or at least, not thirteen actual missionaries, but a mixture of missionaries, mission school teachers, and an elderly American couple who were merely very interested in mission work, and wore straw hats from Mia mi, cameras, and vacantly benign expressions which they bestowed on everyone indiscriminately, whether they wanted them or not.

We had spent about two hours in the glaring sun creeping sleepily around the dilapidated customs and immigration offices in a remote corner of Nairobi's Wilson airport, trying to spot which was to be our plane, and who were to be our travelling companions. It's hard to identify a missionary from first principles, but there was clearly something odd going on because the only place to sit was a small three-seater bench shaded from the sun by the overhanging roof, and everybody was so busy giving up their places on it to everybody else that in the end it simply remained empty and we all stood blinking and wilting in the burgeoning morning heat. After an hour of this, Chris muttered something Scottish under his breath, put down his equipment, lay down on the empty bench and went to sleep until the flight was ready. I wished I'd thought of it.

I knew from many remarks he had made that Mark had a particular dislike of missionaries, whom he has encountered in the field many times in Africa and Asia, and he seemed to be particularly tense and taciturn as we made our way out across the hot Tarmac and took our tiny, cramped seats. I then became rather tense myself as the plane started to taxi out to the runway, because the pre-flight talk from our pilot included a description of our route, an explanation of the safety features of the aircraft, and also a short prayer.

I wasn't disturbed so much by the 'O Lord, we thank Thee for the blessing of this Thy day', but 'We commend our lives into Thy hands, O Lord' is frankly not the sort of thing you want to hear from a pilot as his hand is reaching for the throttle. We hurtled down the runway with white knuckles, and as we climbed into the air we passed a big, old, cigar-fat Dakota finally coming in to land, as if it had been delayed by bad weather over the Great Rift Valley for about thirty years.

In contradiction of everything sensible we know about geography and geometry, the sky over Kenya is simply much bigger than it is anywhere else. As you are lifted up into it, the sense of limitless immensity spreading beneath you to infinitely distant horizons overwhelms you with excited dread.

The atmosphere on board the plane, on the other hand, was so claustrophobically nice it made you want to spit. Everyone was nice, everyone smiled, everyone laughed that terribly benign fading away kind of laugh which sets your teeth on edge, and everyone, strangely enough, wore glasses. And not merely glasses. They nearly all had the same sort of glasses, with rims that were black at the top and transparent at the bottom, such as only English vicars, chemistry teachers and, well, missionaries wear. We sat and behaved ourselves.

I find it very hard not to hum tunelessly when I'm trying to behave myself, and this caused, I think, a certain amount of annoyance in the missionary sitting next to me, which he signalled by doing that terribly benign fading away kind of laugh at me till I wanted to bite him.

I don't like the idea of missionaries. In fact the whole business fills me with fear and alarm. I don't believe in God, or at least not in the one we've invented for ourselves in England to fulfil our peculiarly English needs, and certainly not in the ones they've invented in America who supply their servants with toupees, television stations and, most importantly, toll-free telephone numbers. I wish that people who did believe in such things would keep them to themselves and not export them to the developing world. I sat watching the Mia mi hats as they gazed out of the window at Africa - between an immensity of land and an immensity of sky they sat there, incomprehensible, smiling at a continent. I think Conrad said something similar about a boat.