“ ‘Never,’ said I.
“ ‘I suppose that you could not possibly whistle, yourself, in your sleep?’
“ ‘Certainly not. But why?’
“ ‘Because during the last few nights I have always, about three in the morning, heard a low, clear whistle. I am a light sleeper, and it has awakened me. I cannot tell where it came from—perhaps from the next room, perhaps from the lawn. I thought that I would just ask you whether you had heard it.’
“ ‘No, I have not. It must be those wretched gipsies in the plantation.’
“ ‘Very likely. And yet if it were on the lawn, I wonder that you did not hear it also.’
“ ‘Ah, but I sleep more heavily than you.’
“ ‘Well, it is of no great consequence, at any rate.’ She smiled back at me, closed my door, and a few moments later I heard her key turn in the lock.”
“Indeed,” said Holmes. “Was it your custom always to lock yourselves in at night?”
“Always.”
“And why?”
“I think that I mentioned to you that the doctor kept a cheetah and a baboon. We had no feeling of security unless our doors were locked.”
“Quite so. Pray proceed with your statement.”
“I could not sleep that night. A vague feeling of impending misfortune impressed me. My sister and I, you will recollect, were twins, and you