"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.