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ingly strangled, by taking up all available space in the computers’ memory.

Worse, when they attempted to restart one of the computers, the worm was reawakened and proceeded to strangle the machine again. To destroy the worm, they had to write another program – a killer program. Unlike Brunner’s tapeworm, the Xerox worm was not indestructible. Shoch and Hupp later said the worm’s behaviour was ‘rather puzzling’ and they abandoned their experiment, leaving unsolved the problem of ‘controlling [the worm’s] growth while maintaining stable behaviour’.

In the early 1980s a number of computer science students succeeded in writing self-replicating programs for the new Apple II computers. Joe Dellinger, in 1981 a student at Texas A&M University, became intrigued by the idea that computer programs can become modified when copied. The modifications, he decided fancifully, were a kind of evolution, as the copied program mutates into something different from the original. He had no trouble writing a self-replicating program for the Apple II, even though he didn’t think of himself as a particularly clever programmer. His biggest problem was in writing a program that wouldn’t cause damage.

Though Dellinger was fascinated by the way that programs modify when copied – when transferred from computer to computer – there is nothing metaphysical about this. It’s simply computer error. The longer and more complex a program is, the more likely it is that a line of instruction or a command will be skipped or mangled in the copying process. These tiny modifications rarely cause problems – but the potential for error is there.

What is more important is the difficulty Dellinger had writing a program that wouldn’t cause damage to other computers. He discovered that any self-replicating program, no matter how benign, carried with it the potential for damage – just like a fruit fly buzzing about a room carries with it the possibility of disease. Unlike software from commercial houses, self-replicating programs are untested, untried and generally unstable. The modifications made when copying these programs from machine to machine can cause them to be damaging; equally, their presence on a computer is inherently risky.

The most intriguing thing Dellinger discovered was how quickly

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