This is an entry from Matthew E. Bunson's _Encyclopedia Sherlockiana_
(Macmillan, 1994), to which I have added publication dates:

FINGERPRINT.  The mark left by the fingertip provides a nearly infallible
means of identification.  Holmes noted that a letter from Neville St. Clair
to his wife had been posted by a man with a dirty thumb in "The Man with the
Twisted Lip" (December 1891); he finds two thumb marks on "The Cardboard
Box" (January 1893) mailed to Susan Cushing; and dismissed the thumb mark on
the envelope of the letter sent to Mary Morstan as probably that of the
postman in _The Sign of Four_ (February 1890).  A thumb mark made in blood
that was found at Deep Dene House seemed to provide the final, absolutely
damning proof that Jonas Oldacre had been murdered by the unfortunate John
Hector McFarlane, but Homles found it to be crucial in proving his innocence
in "The Norwood Builder" (October 1903).  Interestingly, the "advances" made
in fingerprint analysis by Holmes were a model for Scotland Yard, which
began using the fingerprint for identification starting in 1901.

David Rachels
Virginia Military Institute