"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."