The story of the Tigris and Euphrates civilizations, of which we have given as yet only the bare outline, is a story of conquest following after conquest, and each conquest replaces old rulers and ruling classes by new; races like the Sumerian and the Elamite are swallowed up, their languages vanish, they interbreed and are lost, the Assyrian melts away into Chaldean and Syrian, the Hittites become Aryanized and lose distinction, the Semites who swallowed up the Sumerians give place to Aryan rulers, Medes and Persians appear in the place of the Elamites, the Aryan Persian language dominates the empire until the Aryan Greek ousts it from official life. Meanwhile the plough does its work year by year, the harvests are gathered, the builders build as they are told, the tradesmen work and acquire fresh devices; the knowledge of writing spreads, novel things, the horse and wheeled vehicles and iron, are introduced and become part of the permanent inheritance of mankind; the volume of trade upon sea and desert increases, men’s ideas widen, and knowledge grows. There are set-backs, massacres, pestilence; but the story is, on the whole, one of enlargement. For four thousand years this new thing, civilization, which had set its root into the soil of the two rivers, grew as a tree grows; now losing a limb, now stripped by a storm, but always growing and resuming its growth. After four thousand years the warriors and conquerors were still going to and fro over this growing thing they did not understand, but men had now (330 B.C.) got iron, horses, writing and computation, money, a greater variety of foods and textiles, a wider knowledge of their world. The time that elapsed between the empire of Sargon I and the conquest of Babylon by Alexander the Great was as long, be it noted, at the least estimate, as the time from Alexander the Great to the present day. And before the time of Sargon, men had been settled in the Sumerian land, living in towns, worshipping in temples, following an orderly Neolithic agricultural life in an organized community for at least as long again. “Eridu, Lagash, Ur, Uruk, Larsa, have already an immemorial past when first they appear in history.”[112] One of the most difficult things for both the writer and student of history is to sustain the sense of these time intervals and prevent these ages becoming shortened by perspective in his imagination. Half the duration of human civilization and the keys to all its chief institutions are to be found _before_ Sargon I. Moreover, the reader cannot too often compare the scale of the dates in these latter fuller pages of man’s history with the succession of countless generations to which the time diagrams given on pages 14, 60, and 89 bear witness. § 3