‘Of course.’ Meredith blinked behind his glasses. ‘This was the sort of thing we hoped for when we planned this visit. At least – we didn’t quite know what to expect. We certainly did not expect a poison bottle could have lain undisturbed to the present day. Yes, we’ll search.’

‘But you don’t really think that bottle’s still there, do you?’ Paul’s mind shrank from the implications.

‘Why not?’ Bourne replied. ‘If it happened as it has been told to us, then Mary Rolfe could not have removed it for she died that afternoon.’

‘Not unless,’ Pollard said slowly, ‘she remembered her conversation with you, Dr Bourne, later that morning and removed it before she went out boating with William Metcalfe.’

There was dead silence. Bourne looked blankly at Pollard.

‘Oh come now – ’ Then he stopped, his eyes narrowing. ‘If I get your train of thought, Captain Pollard, you’re speculating on the possibility that by talking with the entity claiming to be Mary Rolfe we are in some way actually putting thoughts into the mind of Mary Rolfe living in the late nineteenth century?’

Pollard shrugged. ‘I know it sounds totally against common sense but is it?’

‘What you’re saying,’ said Meredith, ‘is that although Mary Rolfe in common-sense parlance lived in the late nineteenth century, and we are living in the late twentieth, Ann Parish as Mary Rolfe could actually be living simultaneously in eighteen-ninety-two.’

‘Something like that. She remembered you, Dr Bourne, talking to her. You remember when you asked her what day it was she said: “I’ve told you that before”? Now, did that memory register in Ann Parish’s brain or did it in some way become part of Mary Rolfe’s memory store as well? If that is so, perhaps your concern about the bottle will cause her’ – he grinned – ‘we need new tenses for this, don’t we? – will make her get rid of it, before she goes boating this afternoon.’

Bourne sighed.

‘Ingenious, and intriguing, but quite without foundation, I’m afraid. One can quite easily dismiss it.’

‘How?’

‘Because if it were true, we could change the past. Suppose for the sake of argument I convince Mary Rolfe that she will be drowned by William if she goes out in the boat with him this afternoon. She does not go and therefore survives, perhaps to live another forty years. Suppose she marries and has children, one of them becoming a doctor who discovers penicillin ten years before Alexander Fleming. Then most of the thousands who died in the thirties from septicaemia would have lived. And so on. No. It would become a snowball rolling down the slope of time altering history in ever-growing measure with each passing second. It cannot be true. We know Mary Rolfe drowned in eighteen-ninety-two. Nothing can alter that.’

‘But,’ queried Meredith, ‘would we know?’

‘What do you mean?’