Darwin's Island Part 8

The labour of the worms did a lot to improve the Down House garden. Its topmost layer is filled with channels, most of them thinner than a human hair, and around half filled with water. Below it lies a sheet of material with little air and no worms. A large part of the animals' contribution to fertility comes from their ability to open the ground to air and water. A hectare of rich and cultivated ground is riddled by ten million burrows - which, together, add up to the equivalent of a thirty-centimetre drainpipe. Half the air beneath the surface enters through burrows, and rain flows through a disturbed soil at ten times the rate of unperforated ground.

The surface of the Earth, when watched for long enough, is as unruly as the sea. Everywhere, soil is on the move. Gravity, water, frost and heat all play a part, but life disturbs its calm in many other ways. Living creatures - from bacteria to beetle larvae to badgers and to worms themselves - form and fertilise the ground. What lies beneath our feet forms the largest reservoir of diversity on the planet, with a thousand times more kinds of single-celled organism in a square metre than anywhere else. The soil contains more species than the Amazon rain forest. Its vast variety of inhabitants, large and small, burrow through the topmost layer, draw in air, digest its goodness, excrete into it and turn over so much material that the skin of our planet is in constant eruption. The 'biomantle', the organic layer near the surface, can be metres deep or be no more than a thin sheet. Its base is marked by a layer of pebbles that sinks to a depth at which the stones can no longer be disturbed by the animals that agitate the ground above. As the mantle churns, the relics of man's labour - from ancient tools in Africa to the pots of the first European settlers in Australia - sink through the topsoil, and acc.u.mulate, with the stones, just where the tillers abandon their efforts.

Darwin's subterranean subjects have many a.s.sistants. A shovelful of good earth contains more individuals than there are people on the planet. Most soils have hundreds of thousands of tiny mites and springtails in every square metre. Roots exude sugars and other substances that feed the millions of single-celled creatures that teem around them. They add their remains to the helpful productions of the worms' rear ends. Bacteria and fungi possess powerful enzymes that can break down material that even earthworms cannot digest. They feed roots, break down vegetation - and produce antibiotics. Until the 1930s, a diagnosis of tuberculosis was a death sentence. The disease had killed a billion people since Darwin's birth. Then it was found that a soil suspension attacked the bacteria responsible - and soon streptomycin was discovered and the disease was, at least temporarily, defeated. The microbial world beneath our feet is still almost unexplored and may have far more to offer. Molecular probes that pick up known genes in unknown species suggest it contains innumerable members of a very distinct group of creatures called Archaea. They look rather like ordinary bacteria, but in fact occupy a separate kingdom of life. Once seen as eccentric denizens of hot springs, we now know that there may be a hundred million of them in each gram of soil, many times more than bacteria. Each burns up ammonia and other waste products and helps maintain the Earth's fertility.

Worms stir their habitat without cease, and as roots grow they push barriers out of the way, and die to leave channels into which soil may collapse. As the roots suck in water, the soil settles, and as trees lash back and forth in storms they disturb the ground. A large tree can shatter solid rock as it falls, and the hole it leaves may take centuries to fill. Small animals do even more. Insects, mites, spiders and subterranean snails, together with the worms, may make up fifteen tons of flesh in a hectare of soil - an elephant and a half's worth (and a single pachyderm needs several times that area to feed itself).

The elephant under the gra.s.s is a voracious beast. Earthworms are earth-movers, but in the tropics ants and termites may do more, for they carry up material from several metres down. Alfred Russel Wallace was astonished by the richness of the ground in some parts of Brazil: 'a layer of clay or loam, varying in thickness from a few feet to one hundred . . . over vast tracts of country, including the steep slopes and summits . . . of a red colour, and is evidently formed of the materials of the adjacent and underlying rocks, but ground up and thoroughly mixed'. It had been mixed by ants.

Larger creatures also help to stir up the soil, and elephants themselves often paw away at the surface. Below ground, moles, prairie dogs, marmots, wombats, meerkats, badgers and other excavators join in, each in its own part of the world. They are helped by aardvarks, armadillos and anteaters as they scratch away in search of food. A colony of naked mole rats can build burrows a kilometre long. In the south-western United States, tens of thousands of symmetrical piles of earth up to two metres high and fifty metres across mystified historians for years. They were, they imagined, sacred sites of a lost tribe of Indians. The truth about the Mima Mounds is more prosaic. They are built by gophers, which over the millennia push tons of earth uphill to provide a dry refuge in a marshy place. Even the bottom of the sea is not safe, for manatees and narwhals dig up food, skates do the same and shrimps work away at the top few centimetres of mud. Enthusiasts for the process trace it back to the Cambrian explosion, around five hundred and forty million years ago, when the first animals with hard sh.e.l.ls emerged. They were able to dig into the thick layered mat of microbes that had until then covered the seabed. As they did, a whole new way of existence sprang into being. The revolution of the burrowers marked the origin of modern life, and their descendants are still essential to keep it healthy.

Today's worms are merchants as well as miners, for they are major players in the vast traffic in chemicals that pa.s.ses from the world of life to that of death and back again. Darwin knew as much when he wrote that 'All the fertile areas of this planet have at least once pa.s.sed through the bodies of earthworms.' In an English apple orchard they eat almost every leaf that falls - two tons in every hectare each year, and in the same area of pasture certain kinds munch their way through an annual thirty tons of cow dung. A few tropical species pile their casts into mounds twenty centimetres high and his book refers to the gigantic castings on the Nilgiri Hills of southern India as an indication of the vast amount the animals must chew. Most of their endless meal is ground down in the muscular gizzard. Rather little is absorbed. Even so, it undergoes chemical changes. The experimenter fed some of his subjects with soil laced with red iron oxide powder and noted that it lost its colour when excreted; proof that acid and enzymes had done the job. Their potent guts change the soil, for the chemistry of clay is much modified when pa.s.sed through their bodies. It is ground even finer than before, which helps it retain water and nutriments - and the tiny particles left after the worms have done their work mean that clay has ten thousand times the surface area of an equivalent volume of sand.

Some species of worm have the unexpected ability to draw - like certain plants - carbon from the air and convert it into soluble substances that can be recycled. Vegetable Mould Vegetable Mould suggests that the small grains of chalk found in the digestive glands are waste products. The truth is more remarkable. Radioactive labels show that the glands extract carbon from free carbon dioxide - abundant beneath the soil - at a considerable rate (an unusual talent for an animal) and combine it with salts of calcium. The particles of chalk so produced are excreted and also return to the earth when the creature dies. The worms hence do a lot to increase soil carbon and to improve fertility. suggests that the small grains of chalk found in the digestive glands are waste products. The truth is more remarkable. Radioactive labels show that the glands extract carbon from free carbon dioxide - abundant beneath the soil - at a considerable rate (an unusual talent for an animal) and combine it with salts of calcium. The particles of chalk so produced are excreted and also return to the earth when the creature dies. The worms hence do a lot to increase soil carbon and to improve fertility.

The constant flood of slime pumped out as they burrow recycles other minerals such as nitrogen. Plants and animals die, and farmers pour fertilisers, manure and treated sewage on to their lands and the worms do their bit to pull them into the earth. Their casts contain five times as much nitrogen and ten times as much pota.s.sium as does the soil itself. A large part of that emerges from their busy inner life; to the bacteria that live in the oxygen-free world of the gut. Each worm intestine is a tiny fermentation chamber in which bacteria chew up manure. They make useful fertiliser - but with the side-effect that they also pump out nitrous oxide, a greenhouse gas (and, as 'laughing gas', a primitive anaesthetic), which gives their hosts an unexpected role in global warming.

A simple experiment shows the power of the worm to disturb the underground world. A mouse carca.s.s was placed in a gla.s.s jar with some fine rubble and leaves, plus an added earthworm. In just three months, the bones had been scattered sideways across about ten centimetres and some had been dragged the same depth into the soil. In wormless jars, the corpse stayed undisturbed. Darwin, too, set out to test his subjects' powers of burial. On morning after morning, in the garden at Down House, he counted the number and size of casts - each the undigested remains of a worm's meal - and found dozens in a typical square yard. His cousin Francis Galton joined in and, ever keen to use statistics, counted the number of dead worms he saw on paths in Hyde Park. He found, on the average, a corpse every two and a half paces. The worms, he calculated, brought seven to twenty tons of earth to the surface in every acre of his local fields each year. At that rate, worms would lay down half a centimetre of top-soil in a twelvemonth. In fact, their labours are even more impressive, for most of what they excrete remained beneath the surface, invisible to the eye.

The number of worms is so huge, and their labours so sustained, that in time they can do great things. In a follow-up of his youthful observation at Maer, and soon after moving to his own grand house, Charles Darwin scattered quant.i.ties of broken chalk and brick over a field near Downe to test how fast it sank. Twenty-nine years later he dug a trench across the chalk site, and found most of the chalk buried some fifteen centimetres down. The bricks, on thinner soil, took longer but even they disappeared in the end. By 2005, the fragments of brick had sunk to the level of a solid band of flinty clay into which the worms could not penetrate, while the chalk had been dissolved away.

Darwin's garden had ten or more burrows in every square metre. Given the ability of each animal to chew through earth, if they acted with equal enthusiasm in every cubic centimetre the whole ma.s.s would be disturbed to a depth of a metre or so in about five thousand years. That was not at all the case, for stone tools of that age are often found at shallower levels. In addition, many species of worms reuse their burrows and that economical habit also reduces the extent to which they agitate the ground. As a result, an object that falls on the surface may sink quite fast in its first few decades, but then slow down.

In his final decade, Darwin started an experiment to test their sepulchral power. He placed a lump of rock - a hefty millstone forty centimetres across - in a corner of his lawn. A long bra.s.s rod was pushed deep into the soil through a hole in the centre. The movement of the rock in relation to the rod measured the efforts of the burrowers as they worked away below. In its first days, it sank by around twenty millimetres a year. Charles died before the experiment was complete, but his son Horace continued the study and found that the worm-stone sank by twenty centimetres in ten years. Today's stone, admired by the curious as it might be, is a copy of the original and has been moved since it was first put in place. Nowadays it sinks more slowly than it did. Sir Arthur Keith (who became wrapped up in the Piltdown Man scandal before writing an early biography of Charles Darwin) retired to live close to Down House in the 1930s, and re-examined the sites used in the chalk and brick experiments. Eighty years on, the marked stones had sunk little more than they had in the lifetime of those who set them there, as further proof that the worms are most active near the surface.

At Down House, the longest-running biological experiment in the world is still under way but, ancient as it might appear, the worm-stone has been in place for no more than an instant of geological history. Darwin realised that in the abyss of time his own life and the span of his own experiments were fleeting indeed. He saw that the remnants of ancient structures scattered over England gave him a better chance to test his subjects' powers. In late middle age, he began a tour of the stately ruins of England and - ever a busy correspondent - wrote to dozens of people who might give him information.

The head of excavations at Wroxeter, near Shrewsbury, came up with a strong hint of what worms could do, given time. The city had been founded by the Romans to act as the capital of a British tribe, the Cornovii. Viroconium, as it was called, at its peak held six thousand people. In time, it fell into decay and became, in legend, the site of King Arthur's court. Camelot, the archaeologist responded, was in some places buried under more than a metre of vegetable mould. Much of that was due, Darwin had no doubt, to the efforts of earthworms.

In 1877, men at work on the restoration of Abinger Hall in Surrey, the grand house of his friend Thomas Henry Farrer, who had earlier helped with the experiments on hops and other climbers, discovered the remains of a Roman villa. The Sage of Downe came to visit. He saw how the creatures crawled through the rotten concrete floor of the ancient structure, and brought up material from below. At the time, and for long afterwards, antiquarians a.s.sumed that the layers of earth found above decorated pavements and the like were the remnants of later and less civilised inhabitants, who had settled down in the houses of their erstwhile masters and left their household rubbish behind. The supposed squatters were, in truth, worms.

Darwin was impressed to discover burrows almost two metres beneath the modern surface. The animals could even mine into the ancient structure's thick walls. Farrer observed their activities for several weeks, and saw them hard at work as they heaved the soil. A quick sum showed that their labours were more than sufficient to bury a Roman house within a few centuries. At a villa with a mosaic floor on the Isle of Wight, Darwin's son William was told that so many castings were thrown up between the tiles that the ground had to be swept every day to keep the pattern in view. William also visited Beaulieu Abbey in Hampshire and found that as a result of their labours the bottom of a hole dug down to the ancient floor twenty years before was already covered.

The trip to Stonehenge was also part of the worm project. It showed that, active as the animals might be in rich soils, in some places they achieved rather less. Emma herself noted that they 'seem to be very idle' and in that thin soil the animals had done no more than enough to sink some of the 'Druidical' stones by twenty centimetres or so since they had toppled (they rested on the chalk layer beneath, into which the creatures could not penetrate). John Lubbock, who lived close to Down House, had dated the stones to the Bronze Age, which began in Britain around 2100 BC. The latest estimates push their masons further into antiquity at close to 2300 BC - a period when the Britons began to cut down their forests and replace them with fields. Some of the monoliths fell long ago, in part through the efforts of the worms themselves, whose work, and that of the rain, weathered away the soil that once supported them. Others fell - or were pulled down - within the past few centuries (one major collapse happened in 1797), which suggests that perhaps the burrowers were not as idle as Emma imagined. Indeed, they buried the stone chips left by the first modern excavators of the site in the 1920s to a depth of about five centimetres or so in thirty years, which was almost the same rate as that measured at Down House.

In Charles Darwin's sesquicentennial year of 1959 a plan was hatched for an improved version of his experimental millstone, built on a grander scale, to test the destructive effects of such creatures on the ancient monuments of England. The British a.s.sociation - that stamping ground of Victorian evolutionists - set up a Committee to Investigate by Experiment the Denudation and Burial of Archaeological Structures. A long pile of chalk, with a ditch alongside, of about the size and shape of a typical section of an English barrow or burial mound of three thousand years ago, was built at Overton Down, not far from Stonehenge itself. Plant spores and bits of broken flowerpot were scattered on the surface. Just thirty years later, natural weathering and the efforts of Darwin's favourite excavators had caused large parts of the wall to collapse into the ditch, and both were covered with a layer of gra.s.s and soil. The pieces of broken pottery moved by around three centimetres a decade, and the spores were carried several centimetres into the depths. The worms were at work; and a similar structure built at about the same time on an acid heath in Dorset, with far fewer of those animals, was far less disturbed. At Overton, the experimental barrow now looks much like others a hundred times older. Once again, most of the change in the soil took place in the first few years after it had been disturbed. The next survey is planned for 2024, when, no doubt, the British a.s.sociation Barrow will be almost impossible to tell from those built by the a.s.sociated British long before.

Life's underground frenzy soon blurs the record of the past. At Abinger Hall, several Roman coins were found; but among them was a halfpenny dated 1715. An incautious student would gain an odd view of British history if he took that observation literally. In a five-thousand-year-old Indian mound in Kentucky, the constant activity of soil animals has been enough to turn over and mix up the whole of the site fifty times over since the original inhabitants left. Sites with moist, rich soils are at more risk of disturbance than are deserts or cold uplands - but as men and worms have similar tastes in places to live, the news for those who hope to reconstruct ancient history is bad.

On Leith Hill, the highest in south-east England, Darwin tried to test the extent to which the material dug up would slide downwards to fill valleys and plains. He found that the castings soon rolled downhill and reckoned that for a steep slope a hundred metres long ten kilograms of earth would be washed to the bottom each year. His estimate is close to those made today, and is a tribute to the worms' importance as architects of the fertile fields of southern England - and of the hungry pastures on the hills above. The wind, too, can transport their excreta, to add another weapon to the animals' armoury as soil engineers. Wind-blown soils make up large parts of China, the Great Plains and the Rhine Valley. A strong gale moves stones and gravel, but such large elements soon fall to earth and the finest, and most nutrient-rich, particles - those of the worm-casts included - are blown furthest of all. That valuable powder can even cross the Atlantic. On the last leg of the Beagle Beagle voyage, the young naturalist noted a fall of white dust on to the deck of the ship as it sailed off South America. Some of that came from North Africa, thousands of kilometres away. Silt around Lake Chad - in part the product of worms and their fellows - is picked up by gales, and sifted finer and finer as it travels, until it becomes filled with valuable salts of nitrogen and phosphorus. More than ten million tons of the stuff fall on the Amazon rain forest each year and bring fertility to those thin and hungry lands. The good work of the worms can, it appears, cross great oceans. voyage, the young naturalist noted a fall of white dust on to the deck of the ship as it sailed off South America. Some of that came from North Africa, thousands of kilometres away. Silt around Lake Chad - in part the product of worms and their fellows - is picked up by gales, and sifted finer and finer as it travels, until it becomes filled with valuable salts of nitrogen and phosphorus. More than ten million tons of the stuff fall on the Amazon rain forest each year and bring fertility to those thin and hungry lands. The good work of the worms can, it appears, cross great oceans.

Nowhere is their power better seen than when they themselves traverse the seas. Some species - the 'peregrines' - are keen migrants. In New Zealand at the end of the nineteenth century, farmers found to their surprise that what had been thin pasture had been transformed into lush loam. The immigrants were at work as they broke down soil into compost. They can move into empty pastures at ten metres per year. In today's New Zealand, as they continue to spread, they can bury metal rings - a modern version of the worm-stone - at twice the speed measured in the Down House garden. Now, the Europeans are on every continent apart from Antarctica and in many places far outnumber the natives.

Their ability to improve the ground is so impressive that the animals are sometimes introduced to heal the damaged earth. After mining is finished, or all the peat has been stripped from a bogland, the intruders do a lot to help a landscape to recover. In the Kyzylk.u.m Desert of Kazakhstan and Uzbekistan vast numbers were moved in Soviet times to isolated oases, with salutary effects. Waterlogged Dutch polders, too, had their drainage improved by a hundred times after the animals were called in to help the engineers who had recovered the fields from the sea.

Long before they began to move, the worms helped make the landscapes of the agricultural regions of the world (southern England included), and as an incidental maintained innumerable farms and gardens in a fertile and healthy state. Now, those who till the ground - like their predecessors when farming began, but at a far greater rate - are undoing the animals' work. Like skin stretched too tight on an ageing face, the Earth's epidermis - the soil - has grown thinner with the years. Like age itself, the process is slow but impossible to resist, and like the signs of decay in a human body, the process speeds up with the years.

Man has flayed his home planet for ten millennia. Soil is hard to make, but easy to destroy. A modern plough shifts hundreds of tons a day, which is beyond the capacity of the most vigorous invertebrate. It digs down to no more than a metre or so, to make a solid and impermeable layer at just the depth of the blades. Another problem arises when fifteen-ton tractors roll across the surface. Their wheels compact the loose soil into a material almost like concrete, in which nothing will grow. In addition, continued ploughing breaks up the topmost layer and allows vast quant.i.ties to wash away. Every farm's raw material is on the move, from hill to plain, from plain to river and from land to sea. The evidence is everywhere. My parents' house overlooked the Dee Estuary (the Welsh rather than Scottish version). What was, a few centuries ago, a broad waterway has become a green field with a ditch in it and the local council is exercised about what to do about the sand that blows on to its roads. The reason lies in the fertile fields of Cheshire and North Wales. They have been ploughed again and again and their goodness has disappeared downstream.

The process is speeding up. The amount of organic carbon in Britain's lakes and streams has rocketed in the past twenty years and in some places has almost doubled. Waters that once ran clear now flow with the colour of whisky, which itself gains its hue from the carbon-rich streams that run through peat bogs to feed the stills. On the global scale, matters are even worse. Twenty-four billion tons of the planet's skin are washed away each year - four tons for every man and woman - and although some is replaced and some has always been lost to the rain and to gravity, the figure is far higher than once it was.

Man has long been careless of the deposits in his soil bank. Again and again, as a civilisation grows it empties its underground accounts, goes into decline and collapses. Usually it takes around a thousand years. Marx himself noticed, for he wrote: 'Capitalistic agriculture is a progress in the art, not only of robbing the worker, but of robbing the soil.'

Darwin compared the work of the worms with that of the plough. Since his day, farm machines have become far more powerful. His experiments showed how earth could slip and churn, but he had no more than primitive tools to measure how much movement there now is. A sinister spin-off of modern technology has come to the aid of science. From the first atom bomb in Nevada in 1945 to the last air-burst of a hydrogen bomb in 1968, vast quant.i.ties of radioactive fallout spilled across the world. Radioactive caesium, which has a half-life of around thirty years, binds to soil particles. In an undisturbed site, the element is most abundant near the surface, but after a plough has pa.s.sed the radioactivity is dispersed to the depth of its blade. As the disturbed ground is washed away, the irradiated soil is lost, to acc.u.mulate in places where the mud settles. In the Quantock Hills of Somerset, soil is, in this era of industrial agriculture, lost at around a millimetre every year. As the land sinks, the ploughs dig deeper, and the relics of the Romans that lie beneath will be smashed within a century - although, thanks to the worms, they have been preserved for two thousand years. Already, most of the treasures picked up by metal detectors come from ploughed fields, proof of how fast man's machines are stripping the fragile surface of the globe.

The Romans themselves paid the price for their abuse of the soil, for at the time of their decline and fall so much damage had been done to Italy's farmlands that a large part of the Empire's food had to be imported. The fertile fields around their capital lost their goodness and vast quant.i.ties of grain were shipped in from Libya, which in turn became a wasteland as its surface was stripped by ploughs. In Rome's last Imperial years, it took ten times more Italian ground to feed a single citizen than it had during its heyday.

The damage had begun long before. The first large towns appeared in the Middle East around eight thousand years ago. Quite soon their growing populations began to demand more food. The farmers exploited their precious mould with no thought of replacing its goodness or allowing their fields to rest. Instead they attacked it with ceaseless vigour. The plough was invented soon after oxen were domesticated. In a few centuries the topsoil was gone and many villages were abandoned. Within a couple of millennia, all the fertile land of Mesopotamia was under cultivation. Many irrigation ca.n.a.ls were built. The soil was soon washed away, and the ca.n.a.ls became blocked with mud. Enslaved peoples such as the Israelites were forced to clear it (and the abandoned city of Babylon is still surrounded by d.y.k.es of earth ten metres high, the remnants of their labours). Abraham's birthplace, Ur of the Chaldees, once a port, is now nearly two hundred and fifty kilometres from the sea, and the plain upon which it sits is the remnants of what were fine fields, lost to leave a desert. Salt poisoned the last of the land, and what had been the Fertile Crescent - and the civilisation it fed - collapsed.

The same happened in China, where the Great River was renamed the Yellow River two thousand years ago as swirling earth changed the colour of the water. The ancient Greeks, too, faced the problem when the country around Athens was stripped bare. Plato blamed the farmers.

The real disaster for the skin of the Earth came in the Americas. The Maya bled their landscape dry, as did the inhabitants of Chaco Canyon and Mesa Verde - the abandoned Native American settlements that now form part of the deserts of the American south-west. The Europeans were even worse. Virginia's 'l.u.s.ty soyle' was ideal for tobacco, but the plant sucks goodness from the ground as much as it does from the bodies of those who consume it. In three or four years of that hungry crop the soil was drained of goodness, but farmers saw no need for fertiliser ('They take but little Care to recruit the old Fields with Dung') and simply moved on to the next piece of land. George Washington himself complained that exhaustion of the soil would drive the Americans west, as it soon did. Charles Lyell, Darwin's geological mentor, used the huge gullies that scarred the devastated surface of Alabama and Georgia to examine the rocks below and commented that soon American agriculture would collapse.

The nemesis for vegetable mould - and a major threat to our own future - began in 1838 when John Deere invented the polished steel plough - 'The Plow that Broke the Plains', as the memorial at his childhood home calls it. Soon thousands of his devices were tearing up the prairies.

Since farmers began to work the Great Plains, the soil has lost half its organic matter. The Mississippi is what Mark Twain called America's 'Great Sewer', and the amount of silt that pours down it has doubled since John Deere's day. As the crew of the Beagle Beagle had noticed had noticed, yet more is taken by the wind. A thin layer of sod had kept the prairie soil in place and soon it began to blow away. The Dust Bowl followed. A huge gale in May 1934 blew a third of a billion tons of dust eastwards from Montana, Wyoming and the Dakotas. The dense cloud reached New York two days later, and petered out far out in the Atlantic. The finest material was blown furthest, which meant that the New Englanders gained the nutriment-filled dust that had pa.s.sed through the guts of Montana worms, while the unfortunate westerners were left with just a rough and hungry silt. The gales returned again and again until by the mid-1930s more than a million hectares of prairie had been replaced by desert. yet more is taken by the wind. A thin layer of sod had kept the prairie soil in place and soon it began to blow away. The Dust Bowl followed. A huge gale in May 1934 blew a third of a billion tons of dust eastwards from Montana, Wyoming and the Dakotas. The dense cloud reached New York two days later, and petered out far out in the Atlantic. The finest material was blown furthest, which meant that the New Englanders gained the nutriment-filled dust that had pa.s.sed through the guts of Montana worms, while the unfortunate westerners were left with just a rough and hungry silt. The gales returned again and again until by the mid-1930s more than a million hectares of prairie had been replaced by desert.

The situation elsewhere is worse. Worldwide, an area larger than the United States and Canada combined has already been despoiled. In Haiti, almost all the forest has gone and thousands of hectares of ground are now bare rock. Less than a quarter of the island's rice, its staple food, is now home-grown and over the past decade food production per head has gone down by a third. In China, the Great Leap Forward exhorted the peasants to 'Destroy Forests, Open Wastelands!' They did, and the soil paid the price. A parallel problem in Africa explains some of the continent's chronic instability. Across that landma.s.s, three-quarters of the usable land has been bled of its nutriment by farmers, who cannot afford fertiliser and whose fields are, as a result, no more than a third as productive as those elsewhere. Its earth still leaches its goodness into the water, or into dust. The Sahel, the area of thin soil to the south of the Sahara, is becoming a dust bowl and loses two centimetres of surface each year. Hundreds of millions of people go hungry as a result. A planet ploughed by man is far less sustainable than it was when tilled by Nature.

In 1937, after the Dust Bowl disaster, President Franklin D. Roosevelt said in a letter to state governors that 'The nation that destroys its soil destroys itself.' His Universal Soil Conservation Law became the first step to putting right the damage done to the precious fabric of his nation. It promoted careful ploughing, the use of windbreaks and a ban on the reckless destruction of forests. Within a few years, most of the American Dustbowl returned to a semblance of health. To plough with the contours, rather than against them, makes a real difference (and the Phoenicians had the same idea). In China, too, the Three Norths project plans a five-thousand-kilometre strip of trees in an attempt to stop the light earth from being taken by the wind. Even the Sahel has gained hope from low technology, with lines of stones set across the slopes to stop the thin earth from washing away. In Niger alone, fifty thousand square kilometres of land have been put back into cultivation.

Soil protection of this kind has a long and unexpected history. In most of the Amazon basin, the soil is bitter and thin for constant rain leaches goodness away, and the vegetation feeds on itself as it recycles nutrition from its own dead logs (which is why cleared forests are so infertile). Patches of the so-called Terra Preta ('black soil') lands, in contrast, are small islands of deep dark earth scattered among those starved soils. They were formed not long after the birth of Christ by native peoples who settled down, fed slow fires with rubbish and leaves, piled up excrement and added bones to the mix. They lived in large and scattered cities that recycled their rubbish and set up, in effect, precursors of the green belt that surrounds many modern towns. Over the centuries carbon levels shot up and the ground became filled with worms and their helpful friends. They chew up the ashes and excrete it as a muddy paste of carbon mixed with mucus. So fertile is terra preta, with ten times the average amount of nitrogen and phosphorus, that it is now sold to gardeners.

In Amazonia, the United States and elsewhere, the worm has begun to turn, although perhaps too late. The new concern for the soil is manifest in many ways. Organic gardeners use 'vermicompost' - the end-product of species such as the red wriggler fed with waste from farms or factories - as a powerful boost to the garden. Real enthusiasts make their own in bins into which they throw household rubbish and old newspapers to be transformed into fertiliser. They are part of a global movement which sees commercial agriculture of the kind that destroyed the prairies as an enemy and tries, in a small way, to replace what has been lost.

On the larger scale, too, the ploughman is going out of fashion and the burrowers and their fellows are allowed to work undisturbed. Farmers now scatter seed on undisturbed ground, or insert it through the previous year's stubble. The world has a hundred million hectares of such 'no-till' agriculture, and even in places where the plough still rules, the land is treated with more care than before. Brazil, in particular, with its deep ant-built landscapes, has been a pioneer. No-till farming does its best to leave the hard work to Nature. Instead of clearing the ground of dried stalks or leaves, the remains of a crop are left behind. They are soon dragged underground. As a result the earth can take up more carbon and become more fertile. Weeds are suppressed by the mat of dead vegetation that appears, water runs away more slowly and the temperature of the surface is less variable, and better for planted seeds, than in a bare ploughed field. Farmers who once saw worms as a pest now realise that to let them flourish undisturbed does more to preserve the ground - and to make profits - than to use machines.

The move away from technology and the return to Nature's tillers has been a real success. In some places it reduces soil loss by fifty times, and the habit is spreading fast. In Canada, two-thirds of the crops now grow on earth left unploughed, or ploughed in such a way as to reduce the damage. An era in which millions of tons of mud are lost from fields may be succeeded by an age in which plants and animals - their efforts feeble as individuals, but all-powerful en ma.s.se - are allowed to regenerate the soil. Darwin, with his pa.s.sion for the natural world, would be pleased; but there is still a lot to be done. The risk of a global Mesopotamia - a collapse in food production as the worms' precious but forgotten products are squandered - has not gone away.

The great naturalist himself, in old age, often spoke of joining his favourite animals in 'the sweetest place on earth', the graveyard at Downe. He was denied the chance to offer himself to their mercies for he was interred in Westminster Abbey, whose foundations kept them out. His remains may have been saved from the annelids but have no doubt been consumed by other creatures. Darwin's simple idea - of the importance of gradual change in forming the Earth and all that lives upon it - has replaced the power of belief with that of science. Perhaps today's science of the soil will return the compliment by allowing worms and their fellows to restore the damage done by his descendants to their most important product: the vegetable mould that keeps us all alive.

ENVOI THE DARWIN ARCHIPELAGO.

When Charles Darwin moved to Down House in 1842 the population of England was fifteen million and London, the largest city in the world, had some two million inhabitants. At the time of his death, forty years later, the number of Londoners had doubled and the capital's fringes were creeping towards his retreat. The city has multiplied by four times since then and his home has become a museum. It sits in a well-preserved enclave that pretends to be a village but is in fact part of the London Borough of Bromley. A few segments of countryside nearby have been preserved (and the 'tangled bank', the microcosm of life referred to at the end of The Origin The Origin, is almost unaltered) but the landscape around Down House has become suburban at best. A glance across the famous Sandwalk reveals the tailfins of planes parked a few hundred metres away. Biggin Hill Airport was a Battle of Britain fighter base whose pilots claimed to have shot down more than a thousand aircraft. Now the place is the most popular light aviation centre in England and its annual air fair attracts a hundred thousand visitors. A lot has changed in the Kentish countryside since Charles Darwin walked across what has become an oil-stained strip of tarmac.

Many other places a.s.sociated with the great man - and many of his subjects, from apes to earthworms and from insectivorous plants to h.o.m.o sapiens h.o.m.o sapiens himself - have been transformed since his demise. That would be a surprise to the patriarch of Down House. Darwin looked at the past to understand the present. He scarcely considered what the future might bring, for in his view evolution was so slow, and flesh so stable, that no real changes in the world of life were to be expected for many generations to come. In the long term, no doubt, the outlook was bleak: as he wrote in a letter to his old friend Joseph Hooker: 'I quite agree how humiliating the slow progress of man is, but every one has his own pet horror, and this slow progress and even personal annihilation sinks in my mind into insignificance compared with the idea or rather I presume certainty of the sun some day cooling and we all freezing. To think of the progress of millions of years, with every continent swarming with good and enlightened men, all ending in this, and with probably no fresh start until this our planetary system has again been converted into red-hot gas. himself - have been transformed since his demise. That would be a surprise to the patriarch of Down House. Darwin looked at the past to understand the present. He scarcely considered what the future might bring, for in his view evolution was so slow, and flesh so stable, that no real changes in the world of life were to be expected for many generations to come. In the long term, no doubt, the outlook was bleak: as he wrote in a letter to his old friend Joseph Hooker: 'I quite agree how humiliating the slow progress of man is, but every one has his own pet horror, and this slow progress and even personal annihilation sinks in my mind into insignificance compared with the idea or rather I presume certainty of the sun some day cooling and we all freezing. To think of the progress of millions of years, with every continent swarming with good and enlightened men, all ending in this, and with probably no fresh start until this our planetary system has again been converted into red-hot gas. Sic transit gloria mundi Sic transit gloria mundi, with a vengeance.'

A moment of hindsight on his two hundredth birthday shows that he was badly out in his timing of the coming apocalypse, at least when it came to biology. Every continent is indeed swarming with men, but 'progress' (if such it is) has not been slow, but meteoric. A great deal has happened in the evolutionary instant since 1809. In the next two centuries, plants, animals and people will see an upheaval greater than anything experienced for thousands of years. For most plants and animals the prospect of biological annihilation is far closer than the certainty of the heat death of the universe.

In the last few weeks of his voyage, in July 1836, the young explorer had a brief vision of what lay ahead. The Beagle Beagle dropped anchor at St Helena, halfway between Africa and South America. The island, first occupied by the Portuguese in the sixteenth century, is among the most isolated places in the world. Darwin was delighted by it: a hundred square kilometres of volcanic mountain rose 'like a huge black castle from the ocean'. He admired the 'English, or rather Welsh, character of the scenery' and noted to his surprise that St Helena's vegetation, too, had a British air, with gorse, blackberries, willows and other imports, supplemented by a variety of species from Australia. Over seven hundred plants had been described - but nine out of ten were invaders. They had driven the original inhabitants to extinction or to refuges high in the mountains. A sweep of thin pasture near the coast was known to locals as the 'Great Wood' - which is what it had been until a hundred years before, when the trees were felled and herds of goats and hogs consumed their seedlings and killed the forest. Plagues of rats and cats had come and gone as they ate themselves to extinction. On his first day ash.o.r.e, he found the dead sh.e.l.ls of nine species of 'land-sh.e.l.ls of a very peculiar form' (one of the few mentions of snails in his entire oeuvre) and - in an early hint of evolution - noted that specimens of a certain species 'differ as a marked variety' from others of the same species picked up a few kilometres away. All those molluscs apart from one had been wiped out and replaced by the common brown snail of English gardens. dropped anchor at St Helena, halfway between Africa and South America. The island, first occupied by the Portuguese in the sixteenth century, is among the most isolated places in the world. Darwin was delighted by it: a hundred square kilometres of volcanic mountain rose 'like a huge black castle from the ocean'. He admired the 'English, or rather Welsh, character of the scenery' and noted to his surprise that St Helena's vegetation, too, had a British air, with gorse, blackberries, willows and other imports, supplemented by a variety of species from Australia. Over seven hundred plants had been described - but nine out of ten were invaders. They had driven the original inhabitants to extinction or to refuges high in the mountains. A sweep of thin pasture near the coast was known to locals as the 'Great Wood' - which is what it had been until a hundred years before, when the trees were felled and herds of goats and hogs consumed their seedlings and killed the forest. Plagues of rats and cats had come and gone as they ate themselves to extinction. On his first day ash.o.r.e, he found the dead sh.e.l.ls of nine species of 'land-sh.e.l.ls of a very peculiar form' (one of the few mentions of snails in his entire oeuvre) and - in an early hint of evolution - noted that specimens of a certain species 'differ as a marked variety' from others of the same species picked up a few kilometres away. All those molluscs apart from one had been wiped out and replaced by the common brown snail of English gardens.

Two centuries or thereabouts after his visit, life on St Helena is worse. The island once had forty-nine unique species of flowering plant and thirteen of fern. Seven have been driven to destruction since the arrival of the Portuguese, two survive in cultivation and many more are on the edge. The last St Helena Olive died of mould in 1994. Parts of the tree-fern forest of the high mountains - still in robust health at the time of the Beagle Beagle - remain, but other unique habitats visited by Darwin, such as the dry gumwood, have gone and of the ebony thickets just two bushes remain. The island's giant earwig (at eight centimetres the world's largest), its giant ground beetle and the St Helena dragonfly, all common in the 1830s, have not been seen for years and Darwin's snail of peculiar form is now reduced to a population of no more than a few hundred. The St Helena Petrel is extinct and a solitary endemic feathered creature, the Wire Bird, is left. That too is under threat. - remain, but other unique habitats visited by Darwin, such as the dry gumwood, have gone and of the ebony thickets just two bushes remain. The island's giant earwig (at eight centimetres the world's largest), its giant ground beetle and the St Helena dragonfly, all common in the 1830s, have not been seen for years and Darwin's snail of peculiar form is now reduced to a population of no more than a few hundred. The St Helena Petrel is extinct and a solitary endemic feathered creature, the Wire Bird, is left. That too is under threat.

Three months after the farewell to St Helena, the naturalist's diary records that 'we made the sh.o.r.es of England; and at Falmouth I left the Beagle Beagle, having lived on board the good little vessel nearly five years'. His account of the expedition ends with a spirited enjoinder to all naturalists 'to take all chances and to start on travels by land if possible, if otherwise on a long voyage'. Charles Darwin never left British sh.o.r.es again.

He had no need to, for, as this book has shown, the plain landscapes of his own country gave him the raw material needed for a life filled with science. Darwin's fifty years of work on his homeland's worms, hops, dogs and barnacles changed biology for ever. Since then the British Isles have provided another useful lesson for students of wild Nature, for their modest archipelago is a microcosm of the global upheavals that have taken place since the naturalist came home.

Bartholomew's Gazetteer of the British Isles Gazetteer of the British Isles, in its edition published just after the great naturalist's death in 1882, describes Kent, 'the Garden of England', as a paradise: 'The soil is varied and highly cultivated . . . All cla.s.ses of cereals and root produce are abundant, as is also fruit of choice quality and more hops are grown in Kent than in all the rest of England. The woods are extensive . . . Fishing is extensively prosecuted . . . of which the oyster beds are especially famous.'

A lot has changed since then. The local farms bring in half what they did even a decade ago. The oysters are almost gone and the salmon fishery of the Thames Estuary, which fed the apprentices of London with such abundance that they refused to eat fish more than once a week, has collapsed. Bucolic pursuits have been replaced by that invaluable product, 'services', which accounts for three-quarters of the county's contribution to the nation's wealth. Kent is a dormitory of London and London has become a staging post for the world. The flow of people, power and cash has carved up its landscape with motorways, rail links and webs of power lines. The oast houses that once stored hops have become commuter homes and the hops themselves - the raw material of so many experiments - cover a fraction of the fields he knew. So far, the Great Wen has been kept in part at bay by the Green Belt, but plans for a 'Thames Gateway' mean that yet more of the Garden of England will soon be a bland suburb.

Much of Charles Darwin's work on insectivorous plants, on self-fertilisation and on orchids took place in Ashdown Forest in the adjacent county of Suss.e.x, where his cousin Sarah Wedgwood had a house and where he often walked, mused and botanised. It shows how fast the wild can retreat. In his day the forest was just one of several vast belts of English heath, successors of ancient tracts of trees felled thousands of years ago (Cobbett saw its thin soils as 'the most villainously ugly spot I ever saw'). Ashdown Forest was used by the Normans as a game preserve and was closed off with a thirty-kilometre bank. In time most of the trees were cleared and burned, in iron foundries as much as domestic hearths, and it became heathland, a semi-natural part of the semi-natural landscape that is England. Since the date of publication of The Origin The Origin, nine-tenths of the nation's heaths have been lost. The forest, at two and a half thousand hectares, is the largest piece left but even that is a shadow of what it was. The acid gra.s.s and marshes have been taken over by bracken or have dried out as water is pumped away to slake the thirst of millions. Many once familiar species - gentians, asphodels, sundews, orchids and more - are rare where once they were abundant and some of his favourite walks have become suburbs, farms or golf courses.

Ashdown Forest is a microcosm of the modern age. The Common Plants Survey keeps count of sixty-five of Britain's most abundant flowers, from primroses to bluebells and foxgloves, in five hundred random plots scattered across the nation. A century ago, those species were almost everywhere. By 2007, a quarter of the study sites had none of them at all. Most of the empty plots were in huge fields of corn or on wide pastures without hedgerows. Others were in woodlands. England's forests - preserved by the Woodland Trust, the Royal Society for the Protection of Birds and the National Trust as they may be - have lost large parts of their diversity. A stable ecology maintained by the labours of woodmen has been replaced by museums of elderly trees in which bluebells and foxgloves, sparrows, cuckoos and jackdaws are in decline. Across England - and across Europe - the fields are even more starved of life. Subsidies have made a desert and called it farming.

Kent's sorry tale is repeated in many of Darwin's favourite British places, from Stonehenge to Shrewsbury and from Wales to the Highlands. In a further blow to the products of evolution, the world has come to Kent and the animals - and people - of Kent have migrated to the world. Darwin's archipelago has been united with the globe, which has become a single giant continent rather than a series of islands, real or metaphorical. For his own county the Channel Tunnel makes that reality. Humankind, too, has been h.o.m.ogenised, for even genteel Bromley now has a tenth of its citizens from ethnic minorities. The struggle to exist for both man and beasts has become a worldwide conflict rather than a series of local skirmishes. No longer does evolution mould the natives of each corner of the planet to fit their own domain. Some creatures thrive in the international arena; but many more are doomed.

Evolution generates difference. One species and one alone has put the process into reverse. Man has inst.i.tuted a simplification almost as grand as that brought by the catastrophe that destroyed the dinosaurs. The Galapagos themselves are a stark reminder of what he has done in less than two centuries. HMS Beagle Beagle visited the island of James in 1835. Food was plentiful: 'We lived entirely on tortoise meat . . . the young tortoises make excellent soup.' In those inelegant creatures, Darwin saw, without realising it, his first hint of evolution, for the animals from that island were distinct from those on Indefatigable and Albemarle nearby. In a rare conjunction of taxonomy with gastronomy, he noted that the James specimens were 'rounder, blacker and had a better taste when cooked' - which at the time seemed little more than a curiosity but was in fact an introduction to the biology of change. visited the island of James in 1835. Food was plentiful: 'We lived entirely on tortoise meat . . . the young tortoises make excellent soup.' In those inelegant creatures, Darwin saw, without realising it, his first hint of evolution, for the animals from that island were distinct from those on Indefatigable and Albemarle nearby. In a rare conjunction of taxonomy with gastronomy, he noted that the James specimens were 'rounder, blacker and had a better taste when cooked' - which at the time seemed little more than a curiosity but was in fact an introduction to the biology of change.

Now, the tortoises of James and its fellows have been driven almost to extinction. From a quarter of a million in the Beagle Beagle's day, their numbers have dropped to fifteen thousand. Three of the fourteen unique races have gone and a solitary animal, the famous Lonesome George, is left from another (now, at the age of ninety or so, he has been persuaded to mate with a female of a different race in the hope of preserving his genes). Pigs, as much as men, have done the job, for they love to feast on tortoise eggs. Less obvious pests have also made their way to the archipelago. The cotton cushiony scale insect invaded twenty years ago. It has reached across the whole archipelago and attacks dozens of kinds of native plants.

Pigs and scale insects are dangerous because they have Epicurean tastes. They are happy to try anything once and - like the young explorer with the tortoises - will try a novel source of food if their usual diet is not available. They can, as a result, snack on the last specimens of an endangered species without eating themselves out of house and home. Such generalised predators, as they are called, are a real threat to diversity. On the Galapagos, goats and cats are a plague, pigeons have pushed out their feathered relatives and alien wasps have done terrible damage to the insects. The islands face an era in which specialists, evolved to fit their own small place in nature, have fallen to loutish strangers able to cope more or less anywhere. A tourist on the Galapagos today - and a hundred thousand arrive each year - has less to admire than did the crew of the Beagle Beagle. Next century's visitors will find the place more or less indistinguishable from South America for many of its natives will be gone. The products of millions of years of isolation have been destroyed by man, the most generalised predator of all.

The Galapagos are the icons of evolution and their problems get plenty of attention. Many other oceanic islets across the globe - rare, specialised and fragile as their natives are - face the same cataclysm or worse, but not many people notice. From St Helena to Tahiti and from Hawaii to the Cape Verdes, the alarm has at last been raised. It is too late to save the majority of such places, most of which began their decline long before the Many other oceanic islets across the globe - rare, specialised and fragile as their natives are - face the same cataclysm or worse, but not many people notice. From St Helena to Tahiti and from Hawaii to the Cape Verdes, the alarm has at last been raised. It is too late to save the majority of such places, most of which began their decline long before the Beagle Beagle arrived. arrived.

The fate of the giant earwig of St Helena or the tortoises of the Galapagos is sad enough but Charles Darwin's less spectacular subjects provide a more trenchant statement of the universal attack on the biosphere. They are both under threat and a threat to other places. The humble creatures he studied - the earthworms and bees, the primroses and orchids, the plants that climb and those that snap shut on their prey - all face an ecological earthquake, wherever they may live. In many ways the lessons to be learned from such modest beings are more alarming than are those from the spectacular inhabitants of distant Pacific isles. The crisis has moved well beyond the exotic, and what was once common, or even commonplace, has become rare.

The sundews of Kent and Suss.e.x are far from safe and many of the insectivores sent to Down House from across the globe are in deeper trouble. The wide fields of Venus flytraps and of pitcher plants that once covered parts of North and South Carolina have been destroyed. Agriculture and drainage tear up their homes and the gardeners who dig them up do not help. A more subtle threat comes from fire control, because such beings thrive best in places often burned - which in today's carefully managed countryside happens less than once it did.

Darwin's other subjects, the orchids, face the same problems. Their enemies are those of the insectivores: aggressive farmers, fragile habitats and greedy collectors. A third of the fifty British species are under threat, and several have populations of fewer than a hundred individuals - and one, the Lady's Slipper Orchid, was for a time reduced to a single plant in the Yorkshire Wolds (thousands of greenhouse specimens have now been sown in the hope that the species can be rescued). The Victorians suffered from 'Orchidelirium', and paid large sums for rare specimens. Traders destroyed whole beds to ensure that their own stock kept their price and Darwin's colleague the botanist Joseph Hooker noted how the area around Rio de Janeiro, visited by the young naturalist thirty years earlier, had even by then been pillaged of its orchids, which never reappeared. Unlike the Dutch tulip fever of the seventeenth century, which faded away, the orchid mania is still upon us, with a global trade worth ten billion dollars a year. Expensive specimens sell for thousands. Some of the business is legitimate and the plants are cultivated or cloned in huge numbers from cells taken from one or a few individuals. Plenty more is not, and many wild species from Thailand, China, Brazil, Guatemala and elsewhere are at risk. One orchid species in ten is threatened and the continued loss of tropical forests means that many more will disappear before they are known to science. Even those on 'Orchis Bank', near Down House, survive only through the vigilance of local naturalists.

Britain itself, like many other places, faces a mirror image of the loss of its insect-eaters, orchids and more: a revenge of the immigrants, a wave of creatures that have appeared from almost nowhere and have attacked its natives. A New Zealand flatworm introduced to Belfast in the 1960s has run wild and an Australian cousin has also begun to move. It kills earthworms as it wraps itself around them and digests them alive. The pest has spread through Scotland, northern England and Ireland and in some places worm populations have collapsed.

The losers in the post-Victorian battle have been replaced by others that have thrived in the new global economy. Many have migrated to new places. There, they cause havoc. Weedy plants are a nuisance but weedy animals are even worse.

Modern society has always depended on aliens, creatures moved from their native lands, be they maize, chickens or cattle. They evolved in the Middle East, in Asia or in the New World, but have been transported to all parts of the globe. Many have become pests in their new home and many more have hitched a ride with those who cultivate them. Darwin himself noticed the invasion of British plants into the United States and asked his American colleague Asa Gray, 'Does it not hurt your Yankee pride that we thrash you so confoundedly?' The New World soon got its own back on the Old, with grey squirrels that eat woodland birds' eggs and Canadian pondweed that blocks streams. The third millennium is the era of the weeds and the weediest species of all - h.o.m.o sapiens - h.o.m.o sapiens - is to blame. is to blame.

Plenty of weeds stay at home. They live in disturbed ground, flourish for a short time and move on to a new patch when conditions change. They do little damage except to the good temper of gardeners. When they escape, they are the botanical equivalents of pigs: they move in, exploit what is available and destroy the locals. Many imports from the Old World have thrived in the Americas. A common European roadside species, the knapweed, a small thistle with a pink or yellow flower, has covered tens of millions of hectares. It secretes a poison that kills native plants, which - unlike those at home - have not evolved resistance. As a sinister side-effect it also kills horses. The knapweed is now out of control. Others, such as the Brazilian water hyacinth, which has become a pest on Caribbean islands, find themselves in a place without their native pollinators and take up self-fertilisation. Some even hybridise with a relative, hijack its genes and gain renewed virulence as a result. The bright yellow and poisonous Oxford Ragwort common in disturbed ground in England is a hybrid between two Sicilian species brought to the Oxford Botanic Garden in the seventeenth century, which escaped and is still spreading.

Some of the most aggressive aliens are among the climbers. They are global pests. Even the hop has become a nuisance, with a j.a.panese variety that has spread across the United States. Kudzu, a climbing pea, is also native to j.a.pan. In a gesture of amity, it was transplanted into that nation's ornamental garden at the 1876 Philadelphia Exposition. Gardeners liked the flowers and it was dispersed across the country. At first sight, the immigrant seemed helpful. It lays down roots two metres long and in the South was used to reduce soil loss after the forests had been cut down. The railroads gave free kudzu to farmers in the hope that they would cultivate it for fodder that their trucks could then transport. That was a considerable mistake. The weed grows so fast that the locals recommend, with an attempt at wit, that windows be closed at night to keep it out. In some places, it extends by thirty centimetres a day - twenty metres a season - and can soon smother a huge tree. Kudzu is out of control over an area that straddles Alabama, Georgia and Mississippi, and has spread as far north as Ma.s.sachusetts and as far west as Texas. Attempts to subdue it cost half a billion dollars a year.

Other climbers are just as busy. Florida has 'air potatoes', yams from West Africa that sprawl over trees and block the light. It also suffers infestations of climbing ferns from Asia. English ivy has shaded out tracts of maple forest around Seattle. In Australia, the humble blackberry is a nuisance, as is the mile-a-minute vine, a morning glory introduced from the Old World tropics. Most are harmless at home, but a lifestyle that depends on a burst of growth when a sudden open s.p.a.ce appears in the forest is lethal when exported to a place not adapted to their wiles.

Some of the climbers' success emerges from another by-product of human activity. The effects of the carbon crisis on climate are familiar enough - but it has unexpected side-effects, for climbers thrive in the new and enriched atmosphere. Over the past two decades, the proportion of the Amazon jungle taken up by lianas has gone up and up; in part because the forest has been opened up by loggers, but also because of the increased carbon dioxide, which they can soak up and lay down as wood. As a result they flourish at the expense of trees. Ivy, too, now grows at an exceptional rate as it gains extra carbon from the