'One of the reasons why the Calcutta chromosome can't be found by normal methods is because unlike the standard chromosomes it isn't present in every cell. Or if it is, it's so deeply encrypted that our current techniques can't isolate it. And the reason why it isn't present in every cell is because unlike the other chromosomes it's not symmetrically paired. And the reason why it's not paired is because it doesn't split into eggs and sperm. And guess why that is? I'll tell you: it's because this is a chromosome that is not transmitted from generation to generation by sexual reproduction. It develops out of a process of recombination and is particular to every individual. That's why it's only found in certain kinds of cells: it simply isn't present in regenerative tissue. It only exists in non-regenerating tissue: in other words, the brain.

'Let me put it like this: if there really is such a thing as the Calcutta chromosome only a person like Mangala, someone who's completely out of the loop, scientifically speaking, would be able to find it – even if she didn't know what it was and didn't even have a name for it. For what we have here is a biological expression of human traits that is neither inherited from the immediate gene pool nor transmitted into it. It's exactly the kind of entity that would be hardest for a conventional scientist to accept. Biologists are under so much pressure to bring their findings into line with politics: right-wing politicians sit on them to find genes for everything, from poverty to terrorism, so they'll have an alibi for castrating the poor or nuking the Middle East. The left goes ballistic if they say anything at all about the biological expression of human traits: it's all consciousness and soul at that end of the spectrum.

'But if you think about it, it figures that certain kinds of traits would have a biological correlate. But who said they have to be determined by biology? Maybe it even works the other way around – that they leave their imprint on biology. Who knows?

'And just because those biological correlates aren't transmitted by sexual reproduction, it doesn't mean that they can't be transferred between individuals by other methods. And that's where Mangala comes up to bat. Remember that she started at the deep end, by stumbling upon the process of transmission, rather than the chromosome itself – after all she didn't know what a chromosome was. No one did back then. Remember that it was malaria that led her to it. Remember that one of the extraordinary things about the malaria bug is that it has the capacity to 'cut and paste' its DNA – unlike any creature we know of except the trypanosome. Remember that's one of the reasons why it's been so hard to develop a malaria vaccine. Because what's special about the malaria bug is that as it goes through its life cycle it keeps altering its coat-proteins. So by the time the body's immune system learns to recognize the threat, the bug's already had time to do a little costumechange before the next act.

'Perhaps what Mangala chanced upon was just this: that the malaria bug, because of its recombinatory powers, can actually digest this bit of DNA by splitting it up and redistributing it. Then, when it's reintroduced in a patient whose bloodlbrain barrier's been made spongy, perhaps it can carry the information back and make some tiny little rewirings in the host's wetware.

'I reckon that once she stumbled on the process she dropped everything else and began to concentrate on refining it – in two directions. One was in trying to figure out some way of side-stepping the syphilis step. And the other was in trying to stabilize the chromosome during the process of transference. Because what was happening till then was that the bug was breaking it up in the weirdest ways and she wanted to be able to control the kinds of traits that were being transmitted.

'It's my guess that by about 1897 Mangala had run into a dead end, and she'd come to the conclusion that the existent strains of malaria wouldn't let her go any further. That's why she was so desperate to have Ronnie figure the whole thing out and publish it. Because she actually believed that the link between the bug and the human mind was so close that once its life cycle had been figured out it would spontaneously mutate in directions that would take her work to the next step. That was what she believed, I think: that every time she reached a dead end, the way ahead was by provoking another mutation.'

Pushing away her empty plate, Urmila said: 'How?'

'By trying to make certain things known.'

'So did she succeed?' she asked.

Murugan smiled. 'I think we're going to find out.'

'How?'

'My guess is that that's what this experiment is about.'

'But why in this way? Why not…?'

'Don't you get it?' said Murugan. 'She's not in this because she wants to be a scientist. She's in this because she thinks she's a god. And what that means is that she wants to be the mind that sets things in motion. The way she sees it, we can't ever know her, or her motives, or anything else about her: the experiment won't work unless the reasons for it are utterly inscrutable to us, as unknowable as a disease. But at the same time, she's got to try and tell us about her own history: that's part of the experiment too.'

'Why are you talking about her as if she was still alive?' said Urmila. 'Are you really trying to say that she is? That she somehow managed to…?'

Murugan smiled. 'Well,' he said. 'What do you think?'

Crossing her arms, Urmila hugged herself, suddenly cold. 'I don't know what to think,' she said. She took hold of the booth's curtain and tweaked it back.

The moment she looked into the cabin, everything seemed to stop; it was as though everyone in the room had turned to stare – the other customers, the waiters, the dishevelled college students at the next table – as though they had been waiting all along to see her face.

She pushed the curtain quickly shut.

'But what about Lutchman?' she said. 'Nothing you've told me proves any connection between Mangala and Lutchman. For that matter who was Lutchman? What was his past?'

'You've got me there, Calcutta,' Murugan said. 'That's where I keep coming up short. All I have is bits and pieces – no beginning, no middle and definitely no end.'

'Give me examples,' said Urmila. 'What are these bits and pieces you're talking about?'

'Farley's letter is the main source,' said Murugan. 'Farley says there was another guy working with Mangala at Cunningham's lab. He seems to have been about the same age as Lutchman and he fits the same general profile.'

'That's not much to go on,' said Urmila.

'That's true,' Murugan acknowledged, 'except that a couple of references in the letter seem to suggest that this assistant was the same guy who turned up at Ross's door on May 25 1895.'

'Like what?' said Urmila.

'Well, one thing we know about Lutchman, from another source, is that he was digitally challenged – that is, his left hand was missing a thumb. It doesn't seem to have made any difference to his manual skills. He was probably born that way, because his index finger seems to have retrained itself to do the thumb's job… '

Something stirred in Urmila's mind, a distant memory.

'What's up?' said Murugan. 'Why're you frowning?'

She bit her lip: 'I thought I'd remembered something, but I can't place it. Anyway, go on. Does Farley say anything about the assistant's hand?'

'Nothing explicit,' said Murugan. 'But there's a sentence where he says: "he was surprisingly deft given the circumstances". Something like that, anyway. My guess is that the "circumstances" he's referring to had something to do with the guy's hand.'

'Is that all?' Urmila said in disappointment.

'There's just one other thing. At the end of the letter Farley said that the assistant had been using an assumed name.'