When Jenkins arrived with the keys, McKelvie looked them over critically, selected a couple, and tried them on the door. The first was too large, but the second turned the trick. Cautioning us to stoop to avoid the shelves, McKelvie pushed open the back of the safe, which swung away from him into the darkness beyond. With the flash to guide him he stepped through the opening, then beckoned us to follow him. Though it was too dark to see, I knew I was in a room of some sort, for I felt the velvet softness of a carpet beneath my feet, and I also tripped over some article of furniture. By this time McKelvie had located the light and I saw that my room was really an alcove fitted up with a luxurious divan heaped high with pillows, beside which stood a small smoking-stand. But ornate and sumptuous as the alcove was I should not personally have cared for it, since the atmosphere was close and smoke-laden and there was no means of letting in the light of day.

McKelvie glanced hastily about and then striding to the divan he bent down and sniffed at it critically. Instantly I imitated him. To my amazement the same fragrance clung to the Persian cover of the couch that I had detected on the blood-stained handkerchief. I smelled it again to make sure and then as my memory still played me false I turned to ask McKelvie what it was. He was trying his key in the lock of a door at the rear of the room, and if he heard my question he failed to reply to it.

With less difficulty this time he unlocked this second door, which swung inwards and stood at the head of a flight of rather steep and dark stairs. As before, McKelvie preceded Jenkins and myself, but we kept as close as possible to him that his flash might guide us as well. At the bottom of the steps was another door of similar make, which also opened inwards, and to my astonishment it gave exit onto the garden at the side of the house between the first study window and the corner. So skillfully had it been cut in the masonry, however, that only one initiated into the secret of the entrance would have known it was there.

McKelvie examined the ground around the door and as at this point also the cement walk reached clear to the wall, I wondered what he hoped to discover. Whatever it was, his scrutiny satisfied him, for he stood up with a smile and applied his lens to the key-hole of the door. Then he nodded his head in a contented manner and remarked that we had better return to the study. I noticed that he locked all the doors scrupulously behind him, leaving the secret entrance exactly as he had found it, even to replacing the round disk which counterfeited the knot-hole.

Once in the room he knelt down and examined minutely the dial of the safe.

"Interesting and unique," he commented. "Look here, Mr. Davies!" He pointed to the inside of the door, and I noticed to my astonishment that the dial was duplicated within. "Do you get the significance?" he asked quickly.

"Why, that safe can be opened or closed by combination from the inside as well as the outside," I hazarded.

"Naturally, to be of any use as an entrance it would have to be capable of being opened from the inside," he said caustically. "No, what I meant was this. Supposing we want to lock the safe. Give me a combination."

"I gave him 'Darwin,' the first word that occurred to me, for it was one of those old style safes with the six-letter combination. He twirled the knob of the dial on the outside and pointed as he did so to the inside. Just as the inside handle of a door will revolve when the outer one is turned, so the inner knob of the dial duplicated the revolutions of the outer.

"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."

"Exactly, and old Elias Darwin, the great-grandfather of Philip, who was a firm believer in the established order of affairs, modeled his home in the country (for this stretch of land was country then) on that which was built by his ancestors in pre-revolutionary days, secret entrance and all; for, of course, in those times secret entrances were indispensable for the concealment of friends, whether Tories or Whigs."

"Where did you learn all this?" I asked in amazement.

"I have a book home which details the histories of various mansions in New York," he replied.

"That accounts for the entrance. But what about the safe?" I continued.

"The safe is decidedly more recent. Doubtless the secret entrance had been blocked up, if it was ever cut through, and no one knew of its existence until Philip Darwin stumbled on the knowledge. I looked up the family history of the Darwins this morning while I was awaiting your arrival. Who's Who describes Mr. Frank Darwin, the father, as having been a strait-laced, Puritanical man, and you yourself know what the son was. Can't you imagine the clash between them?"

In view of Mr. Trenton's story concerning Dick's mother I could well believe that father and son had not agreed.

"In 1906 there is record that Frank Darwin went to Europe for a year. Of course, this is mere conjecture, but it is reasonable to suppose that Philip, who was then twenty-one, took the occasion to have the safe built, and the secret entrance unblocked."

"Mason should know," I said.

"I don't think so, or he would have mentioned it at the inquest. However, there is no harm in questioning him. Go and get him, Jenkins."

When Mason stood before us McKelvie said quietly, though his eyes sparkled: "You testified that you had been with the Darwin family thirty years. Did you remain in the house when Mr. Frank Darwin went to Europe in 1906?"

"Yes, sir. I remained as caretaker."

"Then you can tell us when that safe was built?"

"Yes, sir. It was that same year, sir. Mr. Phil complained he had no private safe and his father told him to have one built while he was gone. He chose that place, sir, because he liked the study. His father used the den upstairs."

"Why did he build such a large safe?"

"I don't know, sir. He sent me away to visit some of my folks, sir, while it was being built. He told his father it was to hold his fortune, sir."

McKelvie looked across at me with a triumphant expression which said as plainly as words, "Notice how accurately I deduced the truth," but his voice was subdued enough as he continued his questions.

"He did not get along with his father, I understand?"

"No, sir. They had different ideas on every subject, sir."

"Why didn't Philip Darwin live at his club then, when he came of age?" McKelvie inquired.

"Because his father told him, sir, that if he left the house it would be for good, and not one penny of his money would he get, sir. Mr. Phil knew that his father always carried out his threats, sir."

"That is all, Mason."

"Yes, sir."

The moment the door closed behind the old butler McKelvie said, with a smile, "Just as I thought. And what came in handy when his father was alive was doubly useful after his marriage. And thus we come back to the original discussion, whether the criminal would know the three necessary facts to enter by the safe."

"A member of the family might," I said.

"Yes, a member of the family. Lee, for instance, or even Orton might discover that there was such a passage and secure a key to it. Would either of them know the combination?"

"Orton was Darwin's private secretary."

"As far as his business down-town went, but not his secretary, as far as his personal affairs were concerned. Besides, recall Mason's testimony. He was surprised to find Orton in the study because Darwin always kept it religiously locked, to preserve his secret, of course. Then, too, Orton was Darwin's creature and, therefore, he would be doubly careful not to place himself in the fellow's power. He evidently considered he was running no risk, since he let Orton into the study that night. Besides, if you did not want anyone prying into your safe, what precaution would you take to prevent it?"

"I'd change the combination frequently."

"Exactly; and there you have an answer to my problem. Granted that the criminal knew the first two facts, was he going to depend on a combination that might be changed five minutes before he wished to use the entrance? No, no, we're dealing with a person too clever not to foresee that contingency. Besides, as far as I could detect, no one has recently taken an impression of the outer lock."

"Then we get back where we started and the entrance is of no value to us at all," I pointed out.

"You jump back too far. It merely shows that the criminal did not enter by the safe. That he left that way is proved by the fact that he vanished from the study without using door or windows, and that he very evidently took Darwin's key with him."

"But—the combination?"

"The safe was open, for Darwin had just removed the will from it. Even if it had been closed, a clever man could find an excuse for making his victim open the safe. Once inside any combination of six letters would close the door effectually against intruders."

"I suppose you are right, but how did he get in then?"

"Darwin let him in himself, either through the window or the door. Most probably through the window, since you would have otherwise heard steps in the hall. Recall Orton's testimony. He went to the garage to follow the maid. When he returned he heard voices in the study."

"And when he went in at eleven-thirty, Philip Darwin was alone," I remarked with a smile.

"Yes, to be sure, Philip Darwin was alone," he repeated, crestfallen.