In all its rounds of Birtist "restructurings", the BBC has never once suggested a cut in funding for its Natural History Unit. The Corporation knows it wouldn't be worth even thinking about. Middle England would revolt. Only the axing of The Archers could cause a comparable furore.
The Natural History Unit reaches its 40th birthday this autumn, which it is celebrating with two major new series, The Animal Zone, and The Wildlife Specials. The individual most responsible for giving the Natural History Unit this untouchable status is, of course, Sir David Attenborough. In his timelessly unfashionable safari suits, he has done more than anyone else to bring, say, a termite's nest into our living rooms and demonstrate why it is uniquely magical. We trust him to take us on epic journeys without frightening the horses (or your granny). One of only a handful of stars beyond sniping, he is up there with Michael Palin as an unquestioned television treasure. His appeal is so broad that he is one person both your parents and your children have heard of.
It's not just we Brits who love him, either. The BBC's biggest-selling programme ever, which has been taken by 82 countries, is his Living Planet. Meanwhile, Life on Earth, his first blockbuster (or "blue-chip series" in BBC-speak), has been seen by an estimated 500 million viewers worldwide. He is the BBC's in-house Midas; everything he touches turns to ratings gold.
As you might expect, he is typically unassuming about all this. He tries to brush off his status as a national institution. "When I first started, there was no one else doing it," he shrugs. "I've now been doing it for so long, I've acquired a reputation. In some senses I'm a dinosaur."
He has presented natural history programmes for the BBC since 1952. Now 71, he shows absolutely no sign of losing his famous enthusiasm (affectionately parodied by every aspirant impressionist in the land). "It would be a sad day if that ever happened," he tells me between edits for his new series. "There's so much still to see."
Approaching the interview with the same infectious relish he brings to every job, he goes on to explain our long-lasting love affair with natural history. "It's not trying to sell you anything, it's not after your vote, it's not about man's inhumanity to man, and it's full of drama. What more do you want?"
Natural history is also a wonderful way of seeing the more inhospitable corners of the globe without having to leave your armchair. "The British climate is very beneficent," Attenborough reckons. "It's not seen by most of us as hostile. Even if you live in Italy, nature can be very hostile. But in the balmy, lush English meadows and woodlands, nature doesn't threaten you."
We also have an unquenchable thirst for knowledge. Many Britons, says Attenborough, "are fascinated by the extraordinary variety and beauty of butterflies, for instance. There is a stamp-collecting element to it."
Alastair Fothergill, the head of the BBC's Natural History Unit, traces our affinity with the subject back to "the amateur naturalist tradition, the collectors in colonial days. It's Darwin, it's Wallace; that inquiring mind has been part of British culture for generations. People in Britain like their natural history. The sort of people who work in this Unit had an earthworm in their pocket at school."
The tradition looks set to continue at the BBC. The reason the Natural History Unit has lasted so long, according to Attenborough, "stems from the BBC's interpretation of its public service remit in the charter. They have a responsibility to cover the full spectrum of programmes. They don't wait for four million people to bang on the door to make something. When it started in the 1950s, it was by no means certain that the Natural History Unit was going to be a success. It wasn't for many years in the US where the programmes were all 'bring it back alive, and hand-to-hand wrestling with rhinos'."
There remain carpers who accuse natural history of being intellectually untaxing, but Attenborough has no time for them. "People say it's escapist, but it isn't because it's true. It's about an aspect of which we're part. The moment we think the natural world is something other than ourselves, God help us all."
'The Animal Zone' starts on BBC2 tomorrow.
The Wildlife Specials, presented by Sir David Attenborough, begin on BBC1 next month.