“Exactly.  But in that case you have to build up a new theory entirely.”

“You mean that it makes it seem more deliberate?”

“Yes; that, certainly.  But it also seems to make Mark out an absolute idiot.  Just suppose for a moment that, for urgent reasons which neither of you know anything about, he had wished to get rid of his brother.  Would he have done it like that?  Just killed him and then run away?  Why, that’s practically suicide—­suicide whilst of unsound mind.  No.  If you really wanted to remove an undesirable brother, you would do it a little bit more cleverly than that.  You’d begin by treating him as a friend, so as to avoid suspicion, and when you did kill him at last, you would try to make it look like an accident, or suicide, or the work of some other man.  Wouldn’t you?”

“You mean you’d give yourself a bit of a run for your money?”

“Yes, that’s what I mean.  If you were going to do it deliberately, that is to say and lock yourself in before you began.”

Cayley had been silent, apparently thinking over this new idea.  With his eyes still on the ground, he said now:  “I hold to my opinion that it was purely accidental, and that Mark lost his head and ran away.”

“But what about the key?” asked Bill.

“We don’t know yet that the keys were outside.  I don’t at all agree with Mr. Gillingham that the keys of the down-stairs rooms are always outside the doors.  Sometimes they are, no doubt; but I think we shall probably find that these are inside.”

“Oh, well, of course, if they are inside, then your original theory is probably the correct one.  Having often seen them outside, I just wondered that’s all.  You asked me to be quite frank, you know, and tell you what I thought.  But no doubt you’re right, and we shall find them inside, as you say.

“Even if the key was outside,” went on Cayley stubbornly, “I still think it might have been accidental.  He might have taken it in with him, knowing that the interview would be an unpleasant one, and not wishing to be interrupted.”

“But he had just told you to stand by in case he wanted you; so why should he lock you out?  Besides, I should think that if a man were going to have an unpleasant interview with a threatening relation, the last thing he would do would be to barricade himself in with him.  He would want to open all the doors and say, ‘Get out of it’”

Cayley was silent, but his mouth looked obstinate.  Antony gave a little apologetic laugh and stood up.

“Well, come on, Bill,” he said; “we ought to be stepping.”  He held out a hand and pulled his friend up.  Then, turning to Cayley, he went on, “You must forgive me if I have let my thoughts run on rather.  Of course, I was considering the matter purely as an outsider; just as a problem, I mean, which didn’t concern the happiness of any of my friends.”

“That’s all right, Mr. Gillingham,” said Cayley, standing up too.  “It is for you to make allowances for me.  I’m sure you will.  You say that you’re going up to the inn now about your bag?”