“It was clever of you to steal Rodney’s surgical knives and break into Lazarus’ shop to sharpen them. That way you thought there would be no record that you had ever sharpened a knife or had one in your possession. But there was a record. First, the night before Ingelow’s murder you had a cut on your forefinger and Lazarus tells me such cuts are characteristic of knife-grinders whether professional or amateur. Secondly, each time you were in Lazarus’ shop you set the canary free. You even did so a third time this evening, when you broke into Milhau’s office to get another knife from his safe and saw that the canary’s cage was there. What sort of man would feel an irrational, irresistible impulse to set a caged bird free every time he saw one, regardless of consequences to himself or the bird? Only a man who knew how agonizing the sense of forced confinement can be—an ex-convict who had served a prison term that he considered unjust. You were the only ex-convict among the suspects. As Wanda knew about your prison sentence she began to suspect you when she first heard that a burglar who sharpened a knife had also freed a canary. She must have wondered then if you knew Ingelow. But she couldn’t be sure, and she was too frightened to talk. None of the others suspected you even after the murder because they didn’t know about your prison sentence or your motive. Of course, you never dreamed we would associate the freeing of the bird with your prison sentence, because you had never associated the two yourself. An irresistible, irrational impulse is neurotic, and a neurosis is by definition a failure to associate consciously an act with its emotional cause.”
Leonard Martin laughed. “Cobwebs and moonshine! Psychology is a joke to the layman, and juries are made up of laymen. The only things they believe are eye witnesses and material evidence—bloodstains and fingerprints. You haven’t got anything like that!”
“Murder is rarely performed before an eye witness. But we have material evidence far more conclusive than most fingerprints and bloodstains. That’s where the fly comes in.”
“The fly?”
“Do you know how the Hindus diagnose diabetes mellitus? There is an account of their procedure in Dr. Heiser’s autobiography. They set the patient’s urine outdoors in the sun. If it attracts flies they know it contains sugar and is therefore diabetic urine. The same sugar is present in the perspiration of a diabetic. When I first met you at the art gallery I saw you were a sick man. As I happen to be a doctor of medicine, I soon noticed the principal external symptoms of diabetes in you, just as I noticed that Pauline was anemic. You had the bronze skin of the diabetic in place of the prison pallor one expects to see in a man just out of prison. You had the extreme emaciation of the diabetic and his avoidance of sweet food. You refused French pastry at the art gallery, and you didn’t even take sugar in your coffee at Wanda’s this morning. Finally, there was the peculiar sweet, fruity odor of your breath which indicates butyric acid in the lungs.
