`This is most important,' said he when I had concluded. `It fills up a gap which I had been unable to bridge in this most complex affair. You are aware, perhaps, that a close intimacy exists between this lady and the man Stapleton?'

`I did not know of a close intimacy.'

`There can be no doubt about the matter. They meet, they write, there is a complete understanding between them. Now, this puts a very powerful weapon into our hands. If I could only use it to detach his wife'

`His wife?'

`I am giving you some information now, in return for all that you have given me. The lady who has passed here as Miss Stapleton is in reality his wife.'

`Good heavens, Holmes! Are you sure of what you say? How could he have permitted Sir Henry to fall in love with her?'

`Sir Henry's falling in love could do no harm to anyone except Sir Henry. He took particular care that Sir Henry did not make love to her, as you have yourself observed. I repeat that the lady is his wife and not his sister.'

`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.