the gorillas. Ahead of us through the misty morning light reared the hump of Mikeno volcano.

The forest we plunged into was thick and wet and I complained about this to Mark.

He explained that gorillas like to live in 'montane' rain forest, or cloud forest. It was over 3,000 metres above sea-level, above the cloud level, and always damp. Water drips off the trees the whole time.

`It's not at all like lowland primary rain forest,' said Mark, `more like secondary rain forest, which is what you get when primary forest is burnt or cut down and then starts to regenerate.'

`I thought that the whole problem with rain forest was that it wouldn't grow again when you cut it down,' I said.

`You won't get primary rain forest again, of course. Well, you might get something similar over hundreds or thousands of years, we don't know. Certainly all of the original wildlife will have been lost for good. But what grows in the short term is secondary rain forest, which is far less rich and complex.

`Primary rain forest is an incredibly complex system, but when you're actually standing in it it looks half empty. In its mature state you get a very high, thick canopy of leaves, because of all the trees competing with each other to get at the sunlight. But since little light penetrates this canopy there will tend to be very little vegetation at ground level. Instead you get an ecological system which is the most complex of any on earth, and it's all designed to disseminate the energy which the trees have absorbed from the sun throughout the whole forest.

'Cloud forest, like this, is much simpler. The trees are much lower and more spaced out so there is plenty of ground cover vegetation as well, all of which the gorillas like very much because it means they can hide. And there's plenty of food within arm's reach.'

For us, however, all the thick, wet vegetation made the forest hard work to fight through. Murara and Serundori swung their machetes so casually through the almost impenetrable under growth that it took me a while to begin to see that there was more to it than just vague hacking.

Machetes are a very specific shape, a little like the silhouette of a banana with a fattened end. Every part of the blade has a slightly different curve or angle of cut to the line of movement, and a different weight behind, it as well. It was fascinating to watch the instinctive ways in which, from one slash to another, the guides would adapt their stroke to the exact type of vegetation they were trying to cut through - one moment it would be a thick branch, another moment it would be banks of nettles and another moment tangled hanging vines. It was like a very casual game of tennis played by highly skilled players.

Not only was the forest thick, it was also cold, wet and full of large black ants which bit all of us except for Helmut and Kurt who were wearing special ant-proof socks which they had brought with them from Latvia.

We complimented them on their foresight and they shrugged and said it was nothing. Latvians were always well prepared. They looked at our recording equipment and said that they were surprised that we thought it was adequate. They had much better tape recorders than that in Latvia. We said that that might very well be so, but that we were very happy with it and the BBC seemed to think it was fine for the job. Helmut (or was it Kurt?) explained that they had much better broadcasting corporations in Latvia.

The outbreak of outright hostilities was happily averted at this moment by a signal from our guides to keep quiet. We were near the gorillas.

'But of course,' said Kurt, with a slight smile playing along his thin Latvian lips, as if he'd known all the while that this was exactly where the gorillas would be.

But it wasn't a gorilla itself that had attracted our guides' attention, it was a gorilla's bed. By the side of the track along which we were walking there was a large depression in the undergrowth where a gorilla had been sleeping for the night.

Plant stems had been pulled down and folded under to keep the gorilla off the ground which was cold and damp at night.

One of the characteristics that laymen find most odd about zoologists is their insatiable enthusiasm for animal droppings. I can understand, of course, that the droppings yield a great deal of information about the habits and diets of the animals concerned, but nothing quite explains the sheer glee that the actual objects seem to inspire.

A sharp yelp of joy told me that Mark had found some. He dropped to his knees and started to fire off his Nikon at a small pile of gorilla dung.

'It's in the nest,' he explained once he had finished, 'which is very interesting, you see. The mountain gorillas, the ones that live here, actually defecate in their nests because it's too cold to get up at night. The western lowland gorillas, on the other hand, don't. They live in a warmer climate, so getting up in the middle of the night is less of a problem. Also, the western lowland gorillas live on a diet of fruit which is another incentive for not shitting in their nests.'

`I see,' I said.

Helmut started to say something, which I like to think was probably something about having far superior types of gorilla dung in Latvia, but I interrupted him because I suddenly had one of those strange, uncanny feelings that I was being watched by a truck.

We kept very quiet and looked very carefully around us. There was nothing we could see near us, nothing, in the trees above us, nothing peering furtively from the bushes. It was a moment or two before we saw anything at all, but then at last a slight movement caught our eyes. About thirty yards away down the track we were following, standing in plain view, was something so big that we hadn't even noticed it. It was a mountain gorilla, or perhaps I should say a gorilla mountain, standing propped up on its front knuckles so that it assumed the shape of a large and muscular sloping ridge tent.

You will have heard it said before that these creatures are awesome beasts, and I would like to add my own particular perception to this: these creatures are awesome beasts. It is hard to know how better to put it. A kind of humming mental paralysis grips you when you first encounter a creature such as this in the wild, and indeed there is no creature such as this. All sorts of wild and vertiginous feelings well up into your brain, that you seem to have no connection with and no name for, perhaps because it is thousands or millions of years since such feelings were last aroused.

I'm going to be a bit fanciful for moment, because it is very hard not to be when your rational, civilised brain (f use the words in the loosest possible sense) experiences things it has no way of recognising or accounting for but which are nevertheless very powerful.

I've heard an idea proposed, I've no idea how seriously, to account for the sensation of vertigo. It's an idea that I instinctively like and it goes like this.

The dizzy sensation we experience when standing in high places is not simply a fear of falling. It's often the case that the only thing likely to make us fall is the actual dizziness itself, so it is, at best, an extremely irrational, even self-fulfilling fear. However, in the distant past of our evolutionary journey towards our current state, we lived in trees. We leapt from tree to tree. There are even those who speculate that we may have something birdlike in our ancestral line. In which case, there may be some part of our mind that, when confronted with a void, expects to be able to leap out into it and even urges us to do so. So what you end up with is a conflict between a primitive, atavistic part of your mind which is saying, jump!' and the more modern, rational part of your mind which is saying, `For Christ's sake, don't.'

Certainly the dizzy experience of vertigo seems to have far more in common with feelings of oscillating mental conflict and confusion than it does with simple fear. If it is a fear, it's one we love to play with and tease ourselves with, which is how designers of big dippers and Ferris wheels make a living.