“Geography of Plants in Tropical Countries”, a drawing based on Alexander von Humboldt’s observations

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Most of us would agree nowadays that the natural world is the only world there is. Even if we think of our mental, moral and cultural lives as lying beyond the reach of natural science, we probably want to treat them as aspects of an all-embracing natural order rather than denizens of some transcendent elsewhere. It would not be much of an exaggeration to say that, philosophically speaking, we are all naturalists now.

But what is this thing called nature? The word itself connotes birth, inheritance and origins – it has the same root as native, nascent, innate, nation, natal and nativity – and etymologically it ought to cover everything that can ever come to being, as opposed to things that exist eternally. Yet its meaning has changed over time. In a cogent book on The Idea of Nature (published posthumously in 1945) the English philosopher R.G. Collingwood divided the story into three phases. First there was “Greek cosmology”, which treated nature as a mega-organism in a perpetual process of self-renewal; then there was the Renaissance view of it as a massive piece of machinery whose secrets would soon be laid bare by mathematical physics; and finally, thanks to idealist philosophers at the turn of the nineteenth century – notably Hegel – we arrive at the “modern idea of nature”, which treats it as an endless process encompassing the entire universe, including us and our consciousness of it. But Collingwood left the exact origin and character of the change unexplained.

That is where Andrea Wulf comes in. In The Invention of Nature, which has won several well-deserved prizes since it appeared in 2015, she argues that the modern concept of nature was not devised by philosophers or by scientists in the modern sense of the word, but by a colourful crew of adventurers who travelled the world, roaming forests, climbing mountains and diving into lakes and rivers in search of new forms of knowledge. They subjected themselves to every possible sight and sensation, measuring everything that could be measured. Unlike earlier generations of explorers, they were not keen on classification for its own sake: they were more interested in analogies, continuities and hidden links. They wanted to evoke them not in lists, catalogues and tables but through narratives, drawings, charts and poems.

Wulf starts with poets of the countryside like Johann Wolfgang von Goethe, William Wordsworth and Samuel Taylor Coleridge, before moving on to post-Christian transcendentalists like Ralph Waldo Emerson and Henry David Thoreau, and industrious data-gatherers such as Charles Lyell, Charles Darwin and Ernst Haeckel. She then turns to liberal anti-colonialists like Thomas Jefferson and Simón Bolivar, and shows how their politics were indebted to a passion for native landscapes, also shared by early environmentalists and national-park pioneers like George Perkins Marsh and John Muir. She finds room for the celebrated philosopher-diplomat Wilhelm von Humboldt, who inaugurated the discipline of “comparative linguistics”, but Wulf devotes most of her attention to Wilhelm’s younger brother Alexander, who who may not be so well known now, though in his time was world famous.

Alexander von Humboldt was an intrepid traveller whose exuberance, creativity and curiosity were an inspiration to the rest of Wulf’s pantheon of pioneers. He matched his brother’s comparative linguistics with his own “comparative climatology” and, as Wulf argues with energy and panache, he was the “hero of science” who invented modern attitudes to the natural world.

Humboldt was born into the Prussian aristocracy in 1769. As a boy he neglected his classical education in favour of reading books of travel and filling his pockets with roots, berries, flowers and insects. By the time he was 20, he had travelled round France, England and the Netherlands, admiring the landscapes, marvelling at the globalisation of trade and dreaming of sailing to the South Pacific. He found work as an inspector of mines and in his spare time he devised new mining lamps and breathing-masks, as well as publishing treatises on basalts and subterranean flora and conducting experiments on animal electricity. Soon he was being sought out by the leaders of German intellectual life, including Goethe.

In 1799, Humboldt set sail from La Coruña in Spain, accompanied by crates of books and scientific instruments. After six weeks he set foot in the Venezuelan city of Cumaná. He was appalled by colonial injustice and the slave system, but otherwise loved everything about South America. He revelled in the exotic flora and fauna, and the marvellous rivers, skies, rocks and soils. He was also aware of parallels with phenomena he had observed in Europe.

About a month after he arrived, something momentous happened: to the accompaniment of claps of thunder, the sky grew dark and the ground began to shake. Humboldt did his best to record the strength and frequency of the tremors and their direction of travel; but his metaphysical prejudices were collapsing along with the houses of Cumaná. Up to that moment, he had assumed that nature was stable and self-perpetuating, but from now on, as he put it, he “mistrusted the very soil on which we had always placed our feet.” The natural world was, it seems, as vulnerable to crises, catastrophes and revolutions as the human one.

A few weeks later, Humboldt set off for the interior. On the first phase of his journey he observed the spectacular agricultural enterprises that had started springing up in the lush valleys of Aragua; but he also spotted a lurking danger. He learned that the mighty Lake Valencia had started to shrink, and after taking measurements and making comparisons with European water systems, he realised what the problem was. The forests were being “decimated”, as he noted in his diary, and the soil was losing its capacity to hold water.

When forests are destroyed, the springs dry up entirely, or become less abundant. The beds of the rivers, remaining dry during part of the year, are converted into torrents when great rains fall. The sward and moss disappear with the brushwood from the sides of the mountains, and the waters falling as rain are no longer impeded in their course. Instead of slowly augmenting the level of the rivers by progressive filtrations, they furrow the sides of the hills, bear down the loosened soil, and form the sudden inundations that devastate the country.

Having documented the disastrous consequences of rapacious agriculture, Humboldt started to forge his way through the rainforest towards the Orinoco, making measurements by day and staying awake at night to listen to a natural chorus.

A confused noise issues from every bush, from the decayed trunks of trees, from the clefts of the rock, and from the ground undermined by lizards, millipedes, and cecilias … so many voices proclaiming to us, that all nature breathes, and that, under a thousand different forms, life is diffused throughout the cracked and dusty soil, as well as in the bosom of the waters, and in the air that circulates about us.

But on another night he heard an “ever-amplifying battle” of jaguars, tapirs and monkeys, which brought it home to him that the forest was not a haven of peace but a theatre of war.

After a quick round trip to Havana, Humboldt embarked on a journey of 2,500 miles through the Andes, and in the summer of 1802 tackled Mount Chimborazo, by some reckonings the world’s highest mountain. He climbed to unheard-of altitudes, taking measurements every few minutes.

As he ascended from tropical vegetation to Arctic-style wastelands he noted similarities with “climates the most distant” and started to think of the planet as a single unit tied together by “a thousand threads”. By the time he came down he had hit on the idea of expressing his intuition not in words but through a Naturgemälde: a “picture of nature” portraying a cross-section of Chimborazo divided into distinct climate zones, cross-referenced to similar forms of life and similar conditions all round the world.

When he got back to Europe in 1804, Humboldt settled in Paris, where he worked with artists, engravers and printers to bring his South American discoveries to an international public. His first volume – an “essay on the geography of plants”, which included a hand-coloured fold-out of the Naturgemälde – appeared in 1807, but the complete work, comprising 34 volumes, took 20 years to complete and brought him close to bankruptcy. He then moved back to Berlin, where he delivered popular lectures in which he invoked the “inner connections of natural forces”, arguing that science should not isolate different life forms in abstract taxonomic slots but show how they rely on each other for survival.

In 1829, Humboldt made another great journey of exploration, crossing Russia to Siberia and Mongolia, covering 12,000 miles in eight months. “At my age, nothing should be postponed,” he said (he was 60) and while the trip was not as adventurous as he would have liked, it generated four pioneering studies of the climate and geology of Asia. He went on to show how climatic geography could be mapped by means of “isotherms”, or lines that join points of equal temperature to form “long bands” encircling the globe. He made it clear that the natural world is in a state of endless flux and that the conditions for life on earth are under threat from human activity in the form of clearances, irrigation schemes and the emission of “great masses of steam and gas”.

When Humboldt died in Berlin, in 1859 at the age of 89, he had just completed a five-volume work called Kosmos, in which he sought to sum up his view of the universe as a “living whole” that enfolds us in a “never ending activity of animated forces”. Kosmos is sometimes regarded as his masterpiece, but if Wulf is right, his insights had already found perfect expression in the Naturgemälde, conceived on Chimborazo more than half a century before, perfected in Paris and then diffused all round the world as a luxurious print that functioned as a work of art as well as science. “This one drawing,” as she puts it, “would change the way future generations perceived the natural world.”