Usage Note: The 19th-century Danish archaeologist Christian Thomsen proposed an innovative classification system based on the assumption of a chronological progression in human technology from stone to bronze to iron. Once empirical study of archaeological collections began, Thomsen's Three Age system was modified into four ages by the subdivision of the Stone Age into the Old Stone (now Paleolithic) and New Stone (Neolithic) ages. Subsequent refinement has added Mesolithic (Middle Stone) and Chalcolithic (Copper and Stone) to the original terms, which are now known as periods rather than ages. Use of the full terminology—Paleolithic, Mesolithic, Neolithic, Chalcolithic, Bronze, and Iron—is appropriate only for Europe, the Middle East, and Egypt, and even there it is not uniformly accepted among archaeologists today.