Mark Twain also discussed *fingerprints* in LIFE ON THE MISSISSIPPI and
produced an illustation of fingerprints to go along with the text.  Of
course,
his work with fingerprints was more sophisticated by the time he wrote
PUDD'NHEAD.  There are several articles on where he received his background
information on this new technique.  When I am near my resources -- and if
someone else hasn't already answered, I'll give you documentation.