“Yes?”

“To Cayley the clothes seemed an enormously important clue.  I didn’t quite see why, but I did realize that to a man in Cayley’s position the smallest clue would have an entirely disproportionate value.  For some reason, then, Cayley attached this exaggerated importance to the clothes which Mark was wearing on that Tuesday morning; all the clothes, the inside ones as well as the outside ones.  I didn’t know why, but I did feel certain that, in that case, the absence of the collar was unintentional.  In collecting the clothes he had overlooked the collar.  Why?”

“It was the one in the linen-basket?”

“Yes.  It seemed probable.  Why had Cayley put it there?  The obvious answer was that he hadn’t.  Mark had put it there.  I remembered what you told me about Mark being finicky, and having lots of clothes and so on, and I felt that he was just the sort of man who would never wear the same collar twice.”  He paused, and then asked, “Is that right, do you think?”

“Absolutely,” said Bill with conviction.

“Well, I guessed it was.  So then I began to see an x which would fit just this part of the problem—­the clothes part.  I saw Mark changing his clothes; I saw him instinctively dropping the collar in the linen-basket, just as he had always dropped every collar he had ever taken off, but leaving the rest of the clothes on a chair in the ordinary way; and I saw Cayley collecting all the clothes afterwards—­all the visible clothes—­and not realizing that the collar wasn’t there.”

“Go on,” said Bill eagerly.

“Well, I felt pretty sure about that, and I wanted an explanation of it.  Why had Mark changed down there instead of in his bedroom?  The only answer was that the fact of his changing had to be kept secret.  When did he change?  The only possible time was between lunch (when he would be seen by the servants) and the moment of Robert’s arrival.  And when did Cayley collect the clothes in a bundle?  Again, the only answer was ’Before Robert’s arrival.’  So another x was wanted—­to fit those three conditions.”

“And the answer was that a murder was intended, even before Robert arrived?”

“Yes.  Well now, it couldn’t be intended on the strength of that letter, unless there was very much more behind the letter than we knew.  Nor was it possible a murder could be intended without any more preparation than the changing into a different suit in which to escape.  The thing was too childish.  Also, if Robert was to be murdered, why go out of the way to announce his existence to you all—­even, at the cost of some trouble, to Mrs. Norbury?  What did it all mean?  I didn’t know.  But I began to feel now that Robert was an incident only; that the plot was a plot of Cayley’s against Mark—­either to get him to kill his brother, or to get his brother to kill him—­and that for some inexplicable reason Mark seemed to be lending himself to the plot.”  He was silent for a little, and then said, almost to himself, “I had seen the empty brandy bottles in that cupboard.”