1891: Galton's fingerprint classification system.

The online exhibition "Trailblazing: Three and a Half Centuries of Royal Society Publishing" includes "1891: The Proof That Fingerprints Are Unique," which discusses the still-used classification system for fingerprints developed by Sir Francis Galton, a cousin of Charles Darwin.

About the image: Galton's fingerprints, from the cover of his book Finger Prints (1892)