In sociology, social mechanics, social change, I am working with particular changes and movements back through centuries of antiquity. These include stone age developments, the agricultural revolution, agrarian developments, and the industrial revolution. Each of these will be subdivided further. For now, since I am interested in beginnings, In the 19th century, European archaeologists began to classify artifacts as belonging to the Stone age, Bronze age, and Iron age. The Stone age was further subdivided into the Paleolithic (Old Stone Age), Mesolithic (Middle Stone age) and Neolithic (New Stone age). The Paleolithic was subdivided into include Lower Paleolithic, Middle Paleolithic, and Upper Paleolithic, referring to the depth at which artifacts were found. There were regional variants. Outside Europe, Mesolithic is often called Epipaleolithic, and African finds are often labeled differently (Early Stone Age, Middle Stone Age, and Later Stone Age). Dating of cultures and cultural developments is only very approximate, and the various stone ages began and ended in different times and in different places. Most of these cultures and stages of development are only known through archaeology. I refer to those discovered and described by explorers and anthropologists, chiefly in modern times, as the historical Stone Age.