“I think so,” Roberts blinked nervously. He had that trick of blinking hard, twice—it reminded Alleyn of a highly strung boy. “Of course,” he added uneasily, “I realise, inspector, that it would probably be to my advantage if I said that I thought the lethal dose was given when the patient was on the table. That, however, is, in my opinion, most improbable.”

“I must here trot out my customary cliché that it is always to an innocent person’s advantage to tell the truth,” Alleyn assured him. “Do you know, it’s my opinion that at least two-thirds of the difficulties in homicidal cases are caused by innocent asses lying for all they’re worth.”

“Indeed? I suppose there is no possibility of suicide in this instance?”

“It seems very unlikely so far. Why? How? Where’s the motive?”

“There need not necessarily be any usual motive.” Roberts hesitated and then spoke with more assurance than he had shown so far. “In suggesting this,” he said, “I may be accused of mounting my special hobbyhorse. As you have seen, I am greatly interested in hereditary taints. In Sir Derek O’Callaghan’s family there is such a taint. In his father, Sir Blake O’Callaghan, it appeared. I believe he suffered at times from suicidal mania. There has been a great deal of injudicious inbreeding. Mark you, I am perfectly well aware that the usual whole-hearted condemnation of inbreeding is to be revised in the light—”

He had lost all his nervousness. He lectured Alleyn roundly for ten minutes, getting highly excited. He quoted his own works and other authorities. He scolded the British public, in the person of one of their most distinguished policemen, for their criminal neglect of racial problems. Alleyn listened, meek and greatly interested. He asked questions. Roberts got books from his shelves, read long passages in a high-pitched voice, and left the volumes on the hearthrug. He told Alleyn he should pay more attention to such things, and finally, to the inspector’s secret amusement, asked him flatly if he knew, if he had taken the trouble to find out, whether he himself was free from all traces of hereditary insanity.

“I had a great-aunt who left all her money to a muffin-man with coloured blood,” said Alleyn. “She was undoubtedly bats. Otherwise I have nothing to tell you, Dr. Roberts.”

Roberts listened to this gravely and continued his harangue. By the time it was over Alleyn felt that he had heard most of the theories propounded at the International Congress on Sex Reform and then some more. They were interrupted by the man-servant, who came in to announce dinner.

“Inspector Alleyn will dine,” said Roberts impatiently.

“No—really,” said Alleyn. “Thank you so much, but I must go. I’d love to, but I can’t.” The man went out.

“Why not?” asked Roberts rather huffily.

“Because I’ve got a murder to solve.”