On the journey back to London Holmes explained. "I had come to a wrong conclusion at first, my dear Watson. The gypsies in the park and the idea of a spotted band led me astray. Then I saw that the danger could not come from the window or the door, and I began to think about the ventilator and the bell-rope. The rope was a bridge for something passing through the hole to the bed. A snake at once suggested itself. The doctor had a supply of creatures from India. The idea of a poison that no chemical test could find would occur to a clever and cruel man who had worked in the East. The speed of the death would be in his favour, and only a very sharp coroner would notice the two little dark punctures. Then I thought of the whistle. He had to call the snake back before daylight, so he trained it with milk. He put it through the ventilator at the hour he thought best, knowing that it would crawl down the rope and reach the bed. It might not bite the sleeper for many nights, but sooner or later she must fall a victim. I had come to these conclusions before I ever entered his room. When I examined the chair, I saw that he had been in the habit of standing on it to reach the ventilator. The metallic sound your sister heard was the safe closing upon its terrible occupant. I am no doubt responsible for Dr. Grimesby Roylott's death, but I cannot say that it will weigh very heavily upon my conscience."