down the letter.

"I thought you'd say that," murmured Antony.

"Tony, do you mean to say that you knew all this?"

"I guessed some of it. I didn't quite know all of it, of course."

"Good Lord!" said Bill again, and returned to the letter. In a moment he
was looking up again. "What did you write to him? Was that last night?
After I'd gone into Stanton?"

"Yes."

"What did you say? That you'd discovered that Mark was Robert?"

"Yes. At least I said that this morning I should probably telegraph to
Mr. Cartwright of Wimpole Street, and ask him to--"

Bill burst in eagerly on the top of the sentence. "Yes, now what was all
that about? You were so damn Sherlocky yesterday all of a sudden. We'd
been doing the thing together all the time, and you'd been telling me
everything, and then suddenly you become very mysterious and private and
talk enigmatically--is that the word?--about dentists and swimming and
the 'Plough and Horses,' and--well, what was it all about? You simply
vanished out of sight; I didn't know what on earth we were talking
about."

Antony laughed and apologized.

"Sorry, Bill. I felt like that suddenly. Just for the last half-hour;
just to end up with. I'll tell you everything now. Not that there's
anything to tell, really. It seems so easy when you know it--so obvious.
About Mr. Cartwright of Wimpole Street. Of course he was just to
identify the body."

"But whatever made you think of a dentist for that?"

"Who could do it better? Could you have done it? How could you? You'd
never gone bathing with Mark; you'd never seen him stripped. He didn't
swim. Could his doctor do it? Not unless he'd had some particular
operation, and perhaps not then. But his dentists could--at any time,
always--if he had been to his dentist fairly often. Hence Mr. Cartwright
of Wimpole Street."

Bill nodded thoughtfully and went back again to the letter.

"I see. And you told Cayley that you were telegraphing to Cartwright to
identify the body?"

"Yes. And then of course it was all up for him. Once we knew that Robert
was Mark we knew everything."

"How did you know?"

Antony got up from the breakfast table and began to fill his pipe.

"I'm not sure that I can say, Bill. You know those problems in Algebra
where you say, 'Let x be the answer,' and then you work it out and find
what x is. Well, that's one way; and another way, which they never give
you any marks for at school, is to guess the answer. Pretend the