“But why this elaborate deception?”

“Because he foresaw that she would be very much more useful to him in the character of a free woman.”

All my unspoken instincts, my vague suspicions, suddenly took shape and centred upon the naturalist. In that impassive, colourless man, with his straw hat and his butterfly-net, I seemed to see something terrible - a creature of infinite patience and craft, with a smiling face and a murderous heart.

“It is he, then, who is our enemy - it is he who dogged us in London?”

“So I read the riddle.”

“And the warning - it must have come from her!”

“Exactly.”

The shape of some monstrous villainy, half seen, half guessed, loomed through the darkness which had girt me so long.

“But are you sure of this, Holmes? How do you know that the woman is his wife?”

“Because he so far forgot himself as to tell you a true piece of autobiography upon the occasion when he first met you, and I dare say he has many a time regretted it since. He was once a schoolmaster in the north of England. Now, there is no one more easy to trace than a schoolmaster. There are scholastic agencies by which one may identify any man who has been in the profession. A little investigation showed me that a school had come to grief under atrocious circumstances, and that the man who had owned it - the name was different - had disappeared with his wife. The descriptions agreed. When I learned that the missing man was devoted to entomology the identification was complete.”

The darkness was rising, but much was still hidden by the shadows.

“If this woman is in truth his wife, where does Mrs. Laura Lyons come in?” I asked.

“That is one of the points upon which your own researches have shed a light. Your interview with the lady has cleared the situation very much. I did not know about a projected divorce between herself and her husband. In that case, regarding Stapleton as an unmarried man, she counted no doubt upon becoming his wife.”

“And when she is undeceived?”

“Why, then we may find the lady of service. It must be our first duty to see her - both of us - to-morrow. Don’t you think, Watson, that you are away from your charge rather long? Your place should be at Baskerville Hall.”

The last red streaks had faded away in the west and night had settled upon the moor. A few faint stars were gleaming in a violet sky.

“One last question, Holmes,” I said as I rose. “Surely there is no need of secrecy between you and me. What is the meaning of it all? What is he after?”

Holmes’s voice sank as he answered:

“It is murder, Watson - refined, cold-blooded, deliberate murder. Do not ask me for particulars. My nets are closing upon him, even as his are upon Sir Henry, and with your help he is already almost at my mercy. There is but one danger which can threaten us. It is that he should strike before we are ready to do so. Another day - two at the most - and I have my case complete, but until then guard your charge as closely as ever a fond mother watched her ailing child. Your mission to-day has justified itself, and yet I could almost wish that you had not left his side. Hark!”

A terrible scream - a prolonged yell of horror and anguish burst out of the silence of the moor. That frightful cry turned the blood to ice in my veins.

“Oh, my God!” I gasped. “What is it? What does it mean?”

Holmes had sprung to his feet, and I saw his dark, athletic outline at the door of the hut, his shoulders stooping, his head thrust forward, his face peering into the darkness.

“Hush!” he whispered. “Hush!”

The cry had been loud on account of its vehemence, but it had pealed out from somewhere far off on the shadowy plain. Now it burst upon our ears, nearer, louder, more urgent than before.