"Now, don't you see that in order to use this entrance it is necessary to know what combination was used to lock the safe from the study and vice versa?" he questioned.
"Yes, that's plain enough. To use the entrance the criminal had to know the combination. Well, what of it? A clever man would hardly be balked by so small a thing."
"You still don't get what I'm driving at," he returned. "I'll try to explain. You have arrived at the conclusion that I held a while ago; namely, that the criminal came in and went out by the secret entrance. Am I right?"
"Yes, that is my opinion."
"Now we come to my point," he said, rising and beginning to pace the room. "If the criminal entered by the safe, he must have been cognizant of three things: first, that there was such an entrance; secondly, that three of the doors were opened by a key of a certain size and make; thirdly, that the safe door was unlocked by a certain combination, that combination being the one which Philip Darwin himself had used. That the criminal should know of one, or perhaps of two of these facts, yes. But that he should be aware of all three of them seems incredible!"
"Why incredible?" I objected. "He may have known of the entrance. He could easily then take an impression of the outer lock and have a key made, and Philip Darwin himself may have revealed the combination to him."
"Very good, but not carried quite far enough," he said with his quizzical smile. "Before I show you where you are at fault, answer me a question. How do you suppose that entrance came to be there so very handy for the criminal's purpose?"
"I presume it was built with the house," I answered.
"Precisely. When?"
"Almost a hundred years ago—1830, to be exact."