"Very simply.When those packets were sealed up, Jonas Oldacre got McFarlane to secure one of the seals by putting his thumb upon the soft wax.It would be done so quickly and so naturally that I dare say the young man himself has no recollection of it.

Very likely it just so happened, and Oldacre had himself no notion of the use he would put it to.Brooding over the case in that den of his, it suddenly struck him what absolutely damning evidence he could make against McFarlane by using that thumb-mark.

It was the simplest thing in the world for him to take a wax impression from the seal, to moisten it in as much blood as he could get from a pin-prick, and to put the mark upon the wall during the night, either with his own hand or with that of his housekeeper.If you examine among those documents which he took with him into his retreat I will lay you a wager that you find the seal with the thumb-mark upon it.""Wonderful!" said Lestrade."Wonderful! It's all as clear as crystal, as you put it.But what is the object of this deep deception, Mr.Holmes?"It was amusing to me to see how the detective's overbearing manner had changed suddenly to that of a child asking questions of its teacher.

"Well, I don't think that is very hard to explain.A very deep, malicious, vindictive person is the gentleman who is now awaiting us downstairs.You know that he was once refused by McFarlane's mother? You don't! I told you that you should go to Blackheath first and Norwood afterwards.Well, this injury, as he would consider it, has rankled in his wicked, scheming brain, and all his life he has longed for vengeance, but never seen his chance.

During the last year or two things have gone against him --secret speculation, I think -- and he finds himself in a bad way.

He determines to swindle his creditors, and for this purpose he pays large cheques to a certain Mr.Cornelius, who is, I imagine, himself under another name.I have not traced these cheques yet, but I have no doubt that they were banked under that name at some provincial town where Oldacre from time to time led a double existence.He intended to change his name altogether, draw this money, and vanish, starting life again elsewhere.""Well, that's likely enough."

"It would strike him that in disappearing he might throw all pursuit off his track, and at the same time have an ample and crushing revenge upon his old sweetheart, if he could give the impression that he had been murdered by her only child.It was a masterpiece of villainy, and he carried it out like a master.

The idea of the will, which would give an obvious motive for the crime, the secret visit unknown to his own parents, the retention of the stick, the blood, and the animal remains and buttons in the wood-pile, all were admirable.It was a net from which it seemed to me a few hours ago that there was no possible escape.But he had not that supreme gift of the artist, the knowledge of when to stop.He wished to improve that which was already perfect --to draw the rope tighter yet round the neck of his unfortunate victim -- and so he ruined all.Let us descend, Lestrade.

There are just one or two questions that I would ask him."The malignant creature was seated in his own parlour with a policeman upon each side of him.

"It was a joke, my good sir, a practical joke, nothing more,"he whined incessantly."I assure you, sir, that I simply concealed myself in order to see the effect of my disappearance, and I am sure that you would not be so unjust as to imagine that I would have allowed any harm to befall poor young Mr.McFarlane.""That's for a jury to decide," said Lestrade."Anyhow, we shall have you on a charge of conspiracy, if not for attempted murder.""And you'll probably find that your creditors will impound the banking account of Mr.Cornelius," said Holmes.

The little man started and turned his malignant eyes upon my friend.

"I have to thank you for a good deal," said he."Perhaps I'll pay my debt some day."Holmes smiled indulgently.

"I fancy that for some few years you will find your time very fully occupied," said he."By the way, what was it you put into the wood-pile besides your old trousers? A dead dog, or rabbits, or what? You won't tell? Dear me, how very unkind of you!

Well, well, I dare say that a couple of rabbits would account both for the blood and for the charred ashes.If ever you write an account, Watson, you can make rabbits serve your turn."