Fingerprints. A single print, being unique -- not even identical twins have the same fingerprints -- that single print found in the right place at the right time is sufficient to disclose the perpetrator of a crime.
Actually, there are those who claim that identifying and matching fingerprints is not sufficiently scientific and so the fingerprint doesn't carry the weight in court that it did only a decade ago. DNA is now the sexy evidence.
In The Red Thumbmark by R Austin Freeman, published in 1907, a single fingerprint is found at the scene of a crime. When the police are able to identify that fingerprint, the case seems closed.
But Dr Thorndyke, the detective/barrister/medical doctor who takes on defense of this suspect, thinks he can disprove the prosecution's case, based on that same fingerprint.
It does not take Dr Thorndyke to figure out who the criminal is. The mystery in this wonderful detective tale is who the lovely heroine is in love with. The answer may surprise you.