‘You suggest then,’ Mark said evenly, ‘that the murder was done some time between ten to eight when my grandmother went home and a quarter-past eight when I went home.’ He thought for a moment and then said: ‘I suppose that’s quite possible. The murderer might have heard or caught sight of me, thrown down the rag in a panic and taken to the nearest cover only to emerge after I’d picked up the sketching gear and gone on my way.’

Lady Lacklander said after a long pause: ‘I find that a horrible suggestion. Horrible.’

‘I dare say,’ Alleyn agreed dryly. ‘It was an abominable business after all.’

‘You spoke of scientific evidence,’ Mark said.

Alleyn explained about the essential dissimilarities in individual fish scales. ‘It’s all in Colonel Cartarette’s book,’ he said, and looked at George Lacklander. ‘You had forgotten that perhaps.’

‘Matter of fact, I – ah – I don’t know that I ever read poor old Maurice’s little book.’

‘It seems to me to be both charming,’ Alleyn said, ‘and instructive. In respect of the scales it is perfectly accurate. A trout’s scales, the Colonel tells us, are his diary in which his whole life-history is recorded for those who can read them. Only if two fish have identical histories will their scales correspond. Our two sets of scales, luckily, are widely dissimilar. There is Group A; the scales of a nine-or ten-year-old fish who has lived all his life in one environment. And there is Group B, belonging to a smaller fish who, after a slow growth of four years, changed his environment, adopted possibly a sea-going habit, made a sudden spurt of growth and was very likely a newcomer to the Chyne. You will see where this leads us, of course?’

‘I’m damned if I do,’ George Lacklander said.

‘Oh, but, yes, surely. The people who, on their own and other evidence are known to have handled one fish or the other are Mr Phinn, Mrs Cartarette and the Colonel himself. Mr Phinn caught the Old ’Un, Mrs Cartarette tells us she tried to take a fish away from Thomasina Twitchett. The Colonel handled his own catch and refused to touch the Old ’Un. Lady Lacklander’s paint rag with the traces of both types of fish-scales, tells us that somebody, we believe the murderer, handled both fish. The further discovery of minute bloodstains tells us that the spike of the shooting-stick was twisted in the rag after being partially cleaned in the earth. If, therefore, with the help of the microscope we could find scales from both fish on the garments of any one of you, that one will be Colonel Cartarette’s murderer. That,’ Alleyn said, ‘was our belief.’

‘Was?’ Mark said quickly and Fox, who had been staring at a facetious Victorian hunting print, refocused his gaze on his senior officer.