The point that Thorndyke is making is that in order for a fingerprint to show that a particular person committed the crime, two things need to be established:
- The print was left by that person.
- Only the criminal could have left the print.
But often the only evidence presented is:
- The print matches that person’s finger.
(3) goes some way towards establishing (1), but it does not help with (2), which needs to be established by other means. However, people can be so dazzled by the uniqueness of fingerprints that they completely forget the need for (2).
In ‘The Case of the White Footprints’ the fingerprints on the bottle of poison were assumed by Superintendent Platt to be those of the murderer, but in fact they belonged to Dr Foxton, who had picked up the bottle when he examined the body:
Foxton placed his fingers on the blackened plate and then pressed them on the paper pad, leaving on the latter four beautifully clear, black fingerprints. These Superintendent Platt scrutinized eagerly, and as his glance travelled from the prints to the photographs he broke into a sheepish grin.
“Sold again!” he muttered. “They are the same prints.”
“Well,” said Miller, in a tone of disgust, “you must have been a mug not to have thought of that when you knew that Dr. Foxton had handled the bottle.”
“The fact, however, is important,” said Thorndyke. “The absence of any fingerprints but Dr. Foxton’s not only suggests that the murderer took the precaution to wear gloves, but especially it proves that the bottle was not handled by the deceased during life. A suicide’s hands will usually be pretty moist and would leave conspicuous, if not very clear, impressions.”
R. Austin Freeman (1924). ‘The Case of the White Footprints’. In The Blue Scarab, pp. 82–83. New York: Dodd, Mead & Co.
Freeman’s first novel, The Red Thumb Mark, is also concerned with the evidential value of fingerprints. He wrote in the preface:
It may happen that the book may serve a useful purpose in drawing attention to certain popular misapprehensions on the subject of finger-prints and their evidential value; misapprehensions the extent of which may be judged when we learn from the newspapers that several Continental commercial houses have actually substituted finger-prints for signed initials.
R. Austin Freeman (1924). The Red Thumb Mark, p. v. New York: Dodd, Mead & Co.
The Red Thumb Mark describes a case in which (3) and (2) are well-established and this seems to be conclusive as to the criminal’s identity, but Thorndyke is able to demonstrate that (1) does not follow.