Why would the subject of fingerprints make everybody lose their common sense?

Question

In "The Case of the White Footprints" in Dr. Thorndyke's Case-Book (1923) by R. Austin Freeman, Dr. Jervis said to his friend that the chief officer found fingerprints in the crime scene after Dr. Jervis himself found peculiar footprints, saying:

"I think so, excepting that I learned from Foxton that Superintendent Platt has obtained the complete fingerprints of a right hand."

Thorndyke raised his eyebrows. "Fingerprints!" he exclaimed. "Why, the fellow must be a mere simpleton. But there," he added, "everybody—police, lawyers, judges, even Galton himself—seems to lose every vestige of common sense as soon as the subject of fingerprints is raised. But it would be interesting to know how he got them and what they are like. We must try to find that out. However, to return to your case, since your theory and the police theory are probably the same, we may as well consider the value of your inferences.

Why would the subject of the fingerprints make everybody lose their common sense?

Does it mean that "because the footprints are already peculiar, there are no logical use for the fingerprints"?

And does "the fellow" mean "the murderer or the Superintendent Platt"?

Answer 1

Fingerprinting was still a relatively new and "cutting edge" technique:

1901 - Sir Edward Henry, an Inspector General of Police in Bengal, India, develops the first system of classifying fingerprints. ... This system of classifying fingerprints was first adopted as the official system in England, and eventually spread throughout.

Thus, the police are likely to jump onto fingerprint evidence the same way we often do DNA evidence, or digital signatures, as absolute proof. Dr. Thorndyke is annoyed because he feels they will ignore the other evidence in favor of the fingerprints.

And yes, I think "the fellow" does refer to the Superintendent.

Answer 2

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:

1. The print was left by that person.
2. Only the criminal could have left the print.

But often the only evidence presented is:

3. 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.