Could Mr. Hornby himself be the thief? His failure appeared sudden to
the outside world, but he must have seen difficulties coming. There,
indeed, was the thumb-mark on the leaf which he had torn from his
pocket-block. Yes! but who had seen him tear it off? No one. The fact
rested on his bare statement.

But the thumb-mark? Well, it was possible (though unlikely)--still
possible--that the mark might have been made accidentally on some
previous occasion and forgotten by Reuben, or even unnoticed. Mr. Hornby
had seen the "Thumbograph," in fact his own mark was in it, and so would
have had his attention directed to the importance of finger-prints in
identification. He might have kept the marked paper for future use, and,
on the occasion of the robbery, pencilled a dated inscription on it, and
slipped it into the safe as a sure means of diverting suspicion. All
this was improbable in the highest degree, but then so was every other
explanation of the crime; and as to the unspeakable baseness of the
deed, what action is too base for a gambler in difficulties?

I was so much excited and elated by my own ingenuity in having formed an
intelligible and practicable theory of the crime, that I was now
impatient to reach home that I might impart my news to Thorndyke and see
how they affected him. But as I approached the centre of the town the
fog grew so dense that all my attention was needed to enable me to
thread my way safely through the traffic; while the strange, deceptive
aspect that it lent to familiar objects and the obliteration of
landmarks made my progress so slow that it was already past six o'clock
when I felt my way down Middle Temple Lane and crept through Crown
Office Row towards my colleague's chambers.

On the doorstep I found Polton peering with anxious face into the blank