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Settle an argument, will you" said the tourist to the man in the craft shop. "Those fires on the mountain - have they been started on purpose?"

The man replied that the blazes were deliberate: shepherds were burning off old heather, as they always do in winter and early spring. Yet the tourist's curiosity was understandable, for the conflagrations were so numerous that half the hillsides seemed to be going up in smoke.

Out there in the Western Highlands, the weather was stunning. The sun blazed down, and only a cool breeze saved hill-walkers from overheating. For casual visitors, this seemed a perfect climate. But local people were all obsessed by one subject: drought.

Lochs were at an alarmingly low level, burns reduced to a trickle, and the surface of the land was as dry as tinder. Walking up into the great wilderness of Letterewe, north of Loch Maree, I felt a sense of unease, brought on by the way the dead grass crunched underfoot and only trickles of water wound their way down the rocky burn-channels.

It was in those very hills, that Frank Fraser Darling did the research for his celebrated book A Herd of Red Deer. In it he described how he spent the summer of 1935 living outdoors and walking barefoot - a practice which put him in far closer touch with the environment. Yet he also recorded how he found some places in the hills so alarming that he had to move out at nightfall, and how his sojourn in that "grey, broken country" made him intensely aware of "the ephemeral nature of individual man".

So it remains today. The immense antiquity of the mountains can strike dread into your soul - and worries about global warming heighten the sensation of man's impermanence. It so happened I had just read a report of an International Science Festival in Edinburgh, at which one speaker revealed that the remains of fossilised plants 520 millions of years old have been found in Scotland, and that the earliest known vertebrates crawled on to land in Morayshire 368 million years ago - developments brought about by the sea and air being tropically warm then.

Scientists dispute whether or not our climate is heading back in that direction; but hill-walkers are surrounded by evidence of relatively short-term meteorological change. The mountains of Wester Ross are now almost devoid of trees, but still in the sides of peat-hags you can see ancient roots, relics of the Caledonian forest which grew up after the glaciers of the last ice age had pulled back, perhaps 10,000 years ago.

Man played a part in the massacre of that forest. But the principal agent of destruction was the climate, which turned progressively colder and wetter, gradually degrading the soil and laying down a blanket of infertile peat.Which way are we heading now? I defy anyone to predict what Letterewe will look like 10,000 years hence.