Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more, bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent. Only human failure to find it, study and understand it, can diminish its value. Paul Leland Kirk, 1953
That does not sound unfamiliar. It sounds like something you might hear a Crown Attorney tell a jury after having provided for them an entertaining "analysis" by an expert "microscopist" proving beyond a shadow of a doubt that a fibre from a sweater owned by the accused was found near the body of the victim. Or that a tiny, invisible pin-head-sized dot on a car door is actually, possibly, almost certainly, human blood, maybe. Or that the baby could only have died as a result of incredibly forceful shaking by the only person with the baby at the time of death, with a 99% degree of certainty.
I have a feeling this is prevailing mythology around police investigators and prosecutors in North America. As such, it is extravagantly misleading. It answers a question that does not need to be asked-- the real question is, does a "matching" fibre really provide an exclusive match to the material in question? Does it prove that the suspect and only the suspect could have committed the crime?
What exactly is a "match" anyway? You might be surprised to know that the answer is somewhat religious: I know because I know. There is no objective criteria for what constitutes a match because there is no "science" of microscopy-- check it out: there isn't. There is no research or systematic investigation that shows how and where and why fibres travel or are found. There is no statistical proof that any particular fibre is unlikely to match any other fibre from any other article of clothing of similiar characteristics...
Locard is right in one sense: at any given crime scene there will be multitudes of "evidence", of fibres, of blood, hairs, saliva, skin flakes, whatever. The question is, what does any particular sample prove?
About the silliest comment in the entire quote is "physical evidence cannot be wrong". No one said it could. Indeed, no one ever accused a fibre of lying. But when a "microscopist" tells a jury that he has some kind of rational calculation to tell them about the odds of that particular fibre coming from someone else's sweater-- he is lying.
If you are intrigued by this, you might want to read the blood spatter testimony of Dr. Paul Leland Kirk. Yeah, I used to eat this stuff up too, but when I read it now it almost sounds farcical. Well, all right-- it's not entirely farcical. At least he did some experiments and testing which, even if it was ridiculously specious and capricious, at least gave some empiricle heft to his testimony. I'm not hard to please: I would have loved to see a well-funded defense team record on film some of their own experiments and then try to persuade Dr. Kirk to analyze the results and compare his conclusions to the actual record.
I'm not ridiculing science here. In fact, it is precisely because I value real science highly that I think our criminal justice system has to start filtering out the junk science that tries to pass for the real thing. Kirk's testimony at the Sam Sheppard trial is a classic case: he mixes in real scientific facts and research with rather startling conclusions that don't really have a tight connection to the evidence. His statements about the arc of the swing of the attacker's arms, the angle from which she was struck, and the source of various blood spatters strike me as dubious at best. It provides a patina of "science" to a lot of conjecture, the signal conjecture being that Marilyn Sheppard was murdered by someone who was left-handed (Dr. Sheppard was right-handed).
The truth is there may have been an element to the crime that no one has thought of yet. If you think Dr. Sheppard's explanation of what happened is a little difficult to believe, that is precisely because we are not likely to imagine, before hand, the sequence of events he describes. And we all know how often the police create suspects out of people who do not behave the way they expect after a traumatic event, including Lindy Chamberlain.
There are good sciences, like DNA analysis, that do inspire confidence. So far, it is most famous for exonerating people who were convicted based on evidence like that given by Paul Kirk.