Homer Simpson may be dumb. But sometimes it is survival of the stupidest.

The Simpsons have hit their 300th episode after a TV career of 13 years and counting.

And the cartoon family that became global icons will run until at least 2005, its creators say.

Clueless dad Homer, wise mother Marge, and the Simpson children - tearaway Bart, genius Lisa and clumsy infant Maggie - show no signs of falling out of favour.

"I hope it goes on forever" Animated Encounters director Kieran Argo

What is the Simpson's secret? How have they triumphed when live action shows like Frasier and Friends start to sag, or see their audiences deserting when the actors aren't quite so fresh faced?

Part of the reason is its writing, says the director of Britain's leading animation festival.

Kieran Argo, festival director of Animated Encounters, brought creator Matt Groening to the UK last year.

He says the show is still consistently one of the most creative on television.

"One of the most striking things about it is the quality of its scripts. They have a very intensive scriptwriting session, and they use a lot of writers.

"It's also the fact the they've broadened the show to be less about Bart and more about Homer," Argo says.

The show's minor characters, like the cynical children's show host Krusty The Clown or Bart's friend Milhouse, are fleshed out as much as the Simpson family.

"You really do get to know each character, even the minor ones like Itchy and Scratchy [the violent cartoon show beloved of Bart and Lisa]," he says.

"There is an irreverence there, and I think it's always healthy to have a bit of irony, some slight cynicism. And at the same time I love the way touches on serious subjects like ecology.

"I hope it goes on forever," he says.

Animated shorts

There was very little hint of how big The Simpsons would become when they started life.

The crudely drawn shorts which started on Tracey Ullmann's US TV show in 1987 were an animated oddity amidst the British comedienne's sketches.

Groening, a cartoonist best known for his dark, foreboding strip Life Is Hell, started the short animations as a parody of American family life.

Those first shorts were much less surreal than the full-length show, which first aired in 1990.

Homer was based heavily on Groening's own father, and seemed not so far removed from a normal family man.

Now, poor old Homer is, as his voice actor Dan Castellaneta once said, "a dog trapped in a man's body".

The first seasons had Bart centre-stage. The Simpson' was a vehicle for the mischievous 10-year old's Dennis The Menace-styled antics.

Now, Homer and his bizarre situations are the focus.

Argo believes that change in emphasis has also breathed fresh life into the show.

Bart used to be the focus of the show

"I don't think it's a derivative of anything that's gone before," he says. "It's so strong because it is unique."

In fact, the madcap style of later Simpson has been a big influence not only one other animations, but on new live actions comedies like Malcolm In The Middle.

Even rock stars and Hollywood actors cannot resist the programme.

Mel Gibson, Michelle Pfeiffer, Alec Baldwin, three Beatles and U2 have been among the celebrities clamouring to play themselves.

"The fact that Hollywood celebrities are so desperate to get on the show shows how cool it is," says Argo. "It seems that it's the thing to get - they're not someone till they've been on The Simpsons."

Audiences for its new series - its 14th - are up by 12% in the US.

Recent figures show that nearly a quarter of Sky One subscribers - the channel that first airs new Simpsons episodes in the UK - do so simply to watch the show.

And Channel 4 recently won a bidding war to show the programme on terrestrial TV - at the tune of £700,000 an episode.

As far as Argo is concerned, it's money well spent.

"The show is worth every penny," he says.