I was not educated at Sunnyhill Primary School. I just went there in the week. I was educated at the Natural History Museum. It was in the days before paedophiliaphobia, and I used to escape the oppressions of the ghastly Mrs Milford every Saturday and catch the No49 bus from Streatham Common.

I got off at South Kensington, a journey that cost eightpence, and I spent the day at the museum. Sometimes I went with a friend, but mostly I went by myself. I was 9 when it all started, I think. I would wander and wonder about the halls and passages, the naves and transepts of this wonderful, endlessly complex building: always something new, always old friends to return to.

I had not thought life had created so many. Long before the term biodiversity was coined, I understood every nuance of the concept. That was because, with my own eyes, I had seen immensity.

Mammals, yes, and birds: but everywhere you looked there were more and more different kinds of creatures: monstrous butterflies, gleaming tank-regiments of beetles, the giant Rafflesia flower, impossible crustacea: there really was no end to it.

Many of the creatures I visited were extinct: life is ever-changing and ever-productive. No visit was complete without walking the length of Diplodocus, a dinosaur skeleton, then displayed with the bones of his tail dragging across the polished floor. It has since been, as it were, politically corrected and stands majestically in the main hall with his tail thrashing the air above.

But what lay behind all this display? I wondered about that too and imagined more and more specimens, guarded and studied by a secret race of geniuses, people who searched for the secret of life and were pretty close to finding it. And that is exactly how it was and how it is.

Richard Fortey was the museum's trilobite man. He studied this wonderfully bio-diverse group - it contains more than 5,000 genera - of extinct arthropods, the vast phylum of quite astonishing diversity that includes insects, spiders, crabs and millipedes.

Fortey's task, and the task of all the others in this vast, rambling and impossible treasure house, is “to make known all the species on the Earth”. This is not a straightforward job: how many fungus gnats can you name? Did you know fungus gnats even existed?

There are 531 different species in this country alone: that is to say, 531 creatures, each of which has evolved to be as unique as Homo sapiens. Fortey writes: “The labyrinthine anatomy of the Natural History Museum might after all be appropriate to the world it represents, for the realm of nature is truly a castle of Gormenghast, with its half-explored wings, and obscure corners where few venture.”

These lost parts of the museum hide people of brilliance and eccentricity. There was the man who filed and catalogued string, with one part of his collection labelled “pieces of string too small to be of use”. There was the drunken whale man, who cut up huge cadavers in a Goya-esque dissecting-room and concealed bottles of Bushmills in the blubber. There was the womanising botanist - not inappropriately, he studied the madder family - who kept a card-index of his women, carefully attaching to each card a vigorous sprig of pubic hair.

Singular people carrying out singular work: knowledge for its own sake that sometimes carries huge importance in such matters as the state of the planet and the future of life.

A study of lichens, small stains found on old stones, reveals huge information on a century-long scale about the changing atmosphere and climate of the places where they grow.

A museum expert on mealy bugs saved West Africa from devastation greater than the Irish potato famine: as a result of his immense skill of naming minute and obscure scraps of life, he identified a bug that was attacking the staple food cassava. The bug was attacked and controlled by an introduced predator.

There are 28 million insect specimens at the Natural History Museum. This is a temple to life and its infinite possibilities, weirder than we imagine, weirder than we are capable of imagining. Those who work in fossils are working with stuff long gone; those that work with surviving species are frequently working against time.

The unique conditions of Lake Victoria have caused vast numbers of cichlid fishes to evolve into different species. An alien species, the Nile perch, was introduced into the lake in the 1950s because it grows fast to a great size: they are eating their way through this extraordinary and unique species-flock.

The study and naming of this extraordinary explosion of life has been the speciality of Humphrey Greenwood, a man regularly encountered by Fortey on al fresco smoking breaks. There, Greenwood told him that with many of the previously unknown fish he studied, he had no idea whether he was godfather or obituarist.

This book is worthy of the place it tells us about, and that is a pretty lofty chunk of praise. I walked the halls and learnt about wonder, which was a subject they didn't teach at Sunnyhill. Long before I read Darwin, I learnt that endless forms most beautiful and most wonderful have been and are being evolved.

Dry Store Room No1: The Secret Life of the Natural History Museum by Richard Fortey

HarperPress, £20; 352pp

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