“Well, sir, you certainly got equal with me on that. But how in the world did you know that he was in the house at all?”
“The thumb-mark, Lestrade. You said it was final; and so it was, in a very different sense. I knew it had not been there the day before. I pay a good deal of attention to matters of detail, as you may have observed, and I had examined the hall, and was sure that the wall was clear. Therefore, it had been put on during the night.”
“But how?”
“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 daresay 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.”