“Oh good. Because that’s why we’ve come here, because of his theories. ”

Derek Ralph, head of science at Charles Darwin comprehensive school, Biggin Hill, is shepherding his Year 7 class around Down House, the Darwin Museum in Downe, Kent. Although Mr Ralph has taught at Charles Darwin for 10 years, this is his first visit to the pretty white house set on the edge of the North Downs, where Darwin wrote the books and thought the thoughts which changed the world.

One can’t blame Mr Ralph for that, since the house, administered by the Royal College of Surgeons since 1953, has hardly advertised itself. There is only one bus an hour from bustling Bromley South and resident curator Solene Morris works alone assisted only by a gardening contractor and two hours’ cleaning a week. A far cry from the vision of a seven-day a week education centre, free to all, which animated a distinguished London surgeon, Sir George Buckston Browne, when he bought and opened the house in 1929.

But looking at the eager young people who follow teacher Peter Skues around this pleasant Victorian dwelling, it is easy to see how the Natural History Museum’s plan for a Pounds 3.5 million development could realise and extend Buckston Browne’s original vision.

Everything delights the visiting children, just as everything - including children, Darwin had 10, seven of whom survivied - delighted the man who lived in this house. His endless experiments - visitors can still examine the “worm stone” in the garden, where Darwin measured how long it took an earthworm to cover a stone with earth - sprang from an endless curiosity and ability to ask the baldest questions. Can an earthworm hear? How would you find out? Try shouting at it and see what it does. How did snails get to the Galapagos islands? Put some in a bucket full of water and see how long they can stay afloat. What is the difference between apparently similar kinds of beetles? Pull them ever so carefully apart. Repeat every experiment more times than you or I could count. And all of this genius, an infinite capacity for taking pains, was developed in this house (after the voyage of the Beagle and a few years in the stench and hubbub of London).

Darwin moved to Down House in 1842 and between then and 1882, when he died, he wrote not just colossal but definitive works on volcanic islands, the geology of South America, barnacles, the variation of plants and animals under domestication, movements and habits of climbing plants, insectivorous plants, cross- and self-fertilisation and the formation of vegetable mould through the action of worms. He also published more reflective works on the descent of man, expression of emotions and, of course, most famously, On the Origin of Species by Means of Natural Selection, published in 1859, but first conceived in 1841.

Darwin agonised over his theory, which he well understood would rock the basis of established religious thought. Soon after he moved to Down House he parcelled up the original manuscript of the Origin and stashed it in a games cupboard under the stairs (now a door to the garden). His wife (and first cousin) the intensely religious Emma Wedgwood was to open and publish it in the event of his death. In the end, however, the bitter grief following the death of his first daughter Anne and the pressure of other thinkers of like mind, notably Alfred Russel Wallace, precipitated him into breaking cover at a meeting of the Linnean Society in 1858. The apparent indifference which greeted the reading of Darwin’s essay (he could not be there because his young son was dying of scarlet fever) belied the furore which it later caused and was to go on causing.

Unfortunately in Down House as it is now arranged, there is little space devoted to his thought apart from the “theory room”, a display of the history of the world and the evolution of its inhabitants. The display was done in l965 and is feeling its age. The rest is a fascinating tribute to the domestic life of genius: the study with its writing board, rolling chair, round table and microscope bench, stacks of books with spines broken for easy reading and retied with Darwin’s string and the lavatory corner where Darwin, famous for “hypochondria”, vomited three times a day; the drawing room with chaise longue, piano and card table, flooded with light from a veranda; the original hallway with a case of tropical birds (“are they real?” squeaked the children); the dining room with trophies from Darwin’s career; the Erasmus Darwin room recording the literary, philosophical and naturalist successes of Darwin’s famous grandfather. And then of course there is the garden.

You go down the gravel path, past the lawns with their huge mulberry and cedar, their scented herbaceous borders which visitors to the house enjoyed so much, past the greenhouse and Darwin’s laboratory, ruined now, then past the orchard and tennis court on the one hand and the home meadow on the other. Then through the green door and along the path, scented and humming with bees, to the Sandwalk Wood. Here, on the path now sandless but once sanded with sand from the same pit in which Darwin’s children played, walked Darwin, thinking his tremendous thoughts. On one side are the sunny fields, on the other the tangled woods which Mrs Darwin loved as a wild garden for her children. You return past the cricket field to see the gracious bay of the house.

All of this is now in bad repair, with upstairs - once nursery and bedrooms - damp and peeling, electricity scarcely functioning and servants’ quarters ramshackle. The Natural History Museum is putting in a bid for Lottery money (Pounds 3.2 million of which it has to raise a quarter) to restore the house, create displays which truly reflect Darwin’s theories, open the upstairs to students of taxonomy, echo aspects of Victorian social history in the reception rooms and the servants’ quarters and finally realise Buckston Browne’s dream of an inspiration to scientific education.

Meanwhile the curator Solene Morris is carrying on the tradition of scientific enquiry. Daniel Bates, aged 11, from the Charles Darwin comprehensive, has 100 dinosaur figures and Pounds 100 worth of dinosaur books. He is wrestling with Solene’s question. How can we find out the noise dinosaurs made?

“We don’t really know what dinosaurs were like,” Daniel points out. “How would we find out?” asks Solene. “Would we look at living animals?” “But there aren’t any living dinosaurs.” “So what animals would we look at?” “Like alligators?” “Yes, and what noise do they make?” “They roar.” “So did dinosaurs roar?” “We don’t know, we can’t see their voice-boxes.” “But what can we see of them?” “Their bones.” “And what about their muscles.” “There aren’t any muscles left.” “But muscles attach to bones, and they leave scars where they are attached. And the blood vessels leave marks too. ” You can see the light break slowly. “So you mean like you can make the muscles by looking at the bones?” “Yes - and that’s how they’ve made the models at the Natural History Museum.”

Solene turns to me. “And we could do so much more if the project goes ahead.”

A child rushes past. “Is this a museum?” “Yes.” “Oh. It just seems, like, interesting.”

The Darwin Museum, Down House, Downe, Kent is open 1-5.30 pm, Wednesday to Saturday. Ring 01689-859119. To make a donation to the appeal ring Nancy Giles, development manager, at the Natural History Museum on 0171 938 9123.