Two black brothers, identical twins, are accused of a grisly murder in Missouri. The weapon is a bloody knife found at the scene of the crime. At the trial, the defense lawyer shows the jury that the murderer's fingers have each left their own characteristic prints on the weapon, and those prints match not the twins, but another person in the courtroom. The court is stunned: clearly, the wrong people are on trial!
The story is Mark Twain's Pudd'nhead Wilson, first published in 1893 by Century Magazine. Wilson's address to the jury gave many Americans their first introduction to the science of fingerprints:
Every human being carries with him from his cradle to his grave certain physical marks which do not change their character, and by which he can always be identified—and that without shade or doubt or question. These marks are his signature, his physiological autograph, so to speak, and this autograph cannot be counterfeited, nor can he disguise it or hide it away, nor can it become illegible by the wear and mutations of time.
Our understanding of fingerprints has changed little to this day. Determined by a combination of genetics and random processes inside the womb, fingerprints are fixed by birth and remain fixed for life. The marks truly are a unique signature: there is so much room for variation that no two people ever have shared, or ever will share, the same pattern.
Perhaps most importantly, fingerprints are permanent. I learned this firsthand ...