'You do not misquote me. Are you saying that George Edalji slit the bellies of horses because that's what his ancestors did five centuries ago in Persia or wherever they were then?'

'I have no idea whether barbaric or ritual practices were involved. Perhaps so. It may well be that Edalji himself did not know what impelled him to act as he did. An urge from centuries back, brought to the surface by this sudden and deplorable miscegenation.'

'You truly believe that this is what happened?'

'Something like it, yes.'

'Then what about Horace?'

'Horace?'

'Horace Edalji. Born of the same mixture of bloods. Currently a respected employee of His Majesty's Government. In the tax inspectorate. You are not suggesting Horace was part of the gang?'

'I am not.'

'Why not? He has as good credentials.'

'Again, you are being facetious. Horace Edalji lives in Manchester, for a start. Besides, I am merely proposing that a mixing of the blood produces a tendency, a susceptibility under certain extreme circumstances to revert to barbarism. To be sure, many half-castes live perfectly respectable lives.'

'Unless something triggers them…'

'As the full moon may trigger lunacy in some gypsies and Irish.'

'It has never had that effect on me.'

'Low-born Irish, my dear Doyle. Nothing personal intended.'

'So what is the difference between George and Horace? Why, in your belief, has one resorted to barbarism and the other not – or not as yet?'

'Do you have a brother, Doyle?'

'I do indeed. A younger one. Innes. He's a career officer.'

'Why has he not written detective stories?'

'I am not tonight's theorist.'

'Because circumstances, even between brothers, vary.'

'Again, why not Horace?'

'The evidence has been staring you in the face, Doyle. It was all brought out in court, by the family itself. I'm surprised you overlooked it.'

It was a pity, Doyle thought, that he had not booked into the White Lion Hotel over the road. He might have the need to kick some furniture before the evening was finished.

'Cases like this, which seem baffling as well as repugnant to the outsider often turn, in my experience, on matters which are not discussed in court, for obvious reasons. Matters which are normally confined to the smoking room. But you are, as you have indicated with your tales of Mr Oscar Wilde, a man of the world. You have a medical training too, as I recall. And you have travelled in support of our army in the South African War, I believe.'

'All that is true.' Where was the fellow leading him?

'Your friend Mr Edalji is thirty years old. He is unmarried.'

'As are many young men of his age.'

'And is likely to remain so.'

'Especially given his prison sentence.'

'No, Doyle, that's not the problem. There's always a certain low sort of woman attracted by the whiff of Portland. The hindrance is other. The hindrance is that your man's a goggling half-caste. Not many takers for that, not in Staffordshire.'

'Your point?'

But Anson did not seem especially keen to reach his point.

'The accused, as was noted at the Quarter Sessions, did not have any friends.'

'I thought he was a member of the famous Wyrley Gang?'

Anson ignored this riposte. 'Neither male comrades nor, for that matter, friends of the fairer sex. He has never been seen with a girl on his arm. Not even a parlourmaid.'

'I did not realize you had him followed quite so closely.'

'He does not engage in sporting activities either. Had you noticed that? The great manly English games – cricket, football, golf, tennis, boxing – are all quite foreign to him. Archery,' the Chief Constable added; and then, as an afterthought, 'Gymnastics.'

'You expect a man with a myopia of eight dioptres to enter the boxing ring, otherwise you'll send him to gaol?'

'Ah, his eyesight, the answer to everything.' Anson could feel Doyle's exasperation building, and sought to incite it further. 'Yes, a poor, bookish, solitary boy with bulging eyes.'

'So?'

'You trained, I think, as an ophthalmologist?'

'I had consulting rooms in Devonshire Place for a short while.'

'And did you examine many cases of exophthalmus?'

'Not a great number. To tell the truth, I had few patients. They neglected me to such an extent that I was able to give my time there to literary composition. So their absence was to prove unexpectedly beneficial.'

Anson noted the ritual display of self-satisfaction, but pressed ahead. 'And what condition do you associate with exophthalmus?'

'It sometimes occurs as a consequence of whooping cough. And, of course, as a side-effect of strangulation.'

'Exophthalmus is commonly associated with an unhealthy degree of sexual desire.'

'Balderdash!'

'No doubt, Sir Arthur, your Devonshire Place patients were altogether too refined.'

'It's absurd.' Had they descended into folk traditions and old wives' tales? This from a Chief Constable?

'It is not, of course, an observation that would be put up in evidence. But it is generally reported among those who deal with a certain class of criminal.'

'It's still balderdash.'

'As you wish. Further, we need to consider the curious sleeping arrangements at the Vicarage.'

'Which are absolute proof of the young man's innocence.'

'We have agreed we shall not change each other's minds one jot or one tittle tonight. But even so, let us consider those sleeping arrangements. The boy is – what? ten? – when his little sister falls ill. From that moment, mother and daughter sleep in the same room, while father and elder son also share a common dormitory. Lucky Horace has a room of his own.'

'Are you suggesting – are you suggesting that something dastardly happened in that room?' Where on earth was Anson heading? Was he completely off his head?

'No, Doyle. The opposite. I am absolutely certain that nothing whatever happened in that room. Nothing except sleep and prayers. Nothing happened. Nothing. The dog did not bark, if you will excuse me.'

'Then…?'

'As I said, all the evidence is in front of you. From the age of ten, a boy sleeps in the same locked room as his father. Through the age of puberty and into early manhood, night after night after night. His brother leaves home – and what happens? Does he inherit his brother's bedroom? No, this extraordinary arrangement continues. He is a solitary boy, and then a solitary young man, with a grotesque appearance. He is never seen in the company of the opposite sex. Yet he has, we may presume, normal urges and appetites. And if, despite your scepticism, we believe the evidence of his exophthalmus, he was prey to urges and appetites stronger than customary. We are men, Doyle, who understand this side of things. We are familiar with the perils of adolescence and young manhood. How the choice often lies between carnal self-indulgence which leads to moral and physical enfeeblement, even to criminal behaviour, and a healthy diversion from base urges into manly sporting activities. Edalji, by his circumstances, was happily prevented from taking the former path, and chose not to divert himself with the latter. And while I admit that boxing would hardly have been his forte, there were, for instance, gymnastics, and physical culture, and the new American science of bodybuilding.'

'Are you suggesting that on the night of the outrage there was… some sexual purpose or manifestation?'

'Not directly, no. But you are asking me what I believe happened and why. Let us admit, for the moment, much of what you claim about the young man. He was a good student, a son who honoured his parents, who prayed in his father's church, who did not smoke or drink, who worked hard at his practice. And yet you in return must accept the likelihood of another side to him. How could there not be, given the peculiarity of his breeding, his intense isolation and confinement, his excessive urges? By day he is a diligent member of society. And then by night, every so often, he yields to something barbaric, something buried deep within his dark soul, something even he probably does not understand.'