See Mike Rogin's piece in Gillman and Robinson, eds. _Mark Twain's PW:
Race, Conflict and Culture (Duke UP, 1990) on the connection between
_PW_ and Francis Galton's _Fingerprints_ published in 1892.
I think it's worth noting, though, that _LOM_'s Karl Ritter story turns
on the identification of a murderer on the basis of a thumbprint. And
Poe's detective Dupin used pantograph tracings of the finger marks left
on the victim's throat in "Rue Morgue" to identify the murderer as an
orangutan. This takes nothing away from Rogin's thesis regarding
Dalton, but it does suggest that Twain's interest in fingerprints and the
idea of their significance long before that was scientifically
confirmed no doubt whetted Twain's interest in Galton all the more.