The innovations of Gerzean can best be explained as the effect of a permeation of Upper Egypt by people who had affinities with their Asiatic neighbours and derived from them certain features of their culture.[74] We know that in historical times a similar gradual but continuous drift of people from Lower Egypt into Upper Egypt can be observed.[75] During the Gerzean period the country seems to have become more densely populated; and it has been suggested that the reclamation of the marshland was begun.[76] Such work presupposes co-operation between neighbouring groups and organization of men in some numbers. We may assume that this took place, but on a strictly limited scale. For there are no signs of large political units. There are no ruins of great size, no monuments of an exceptional nature; and if it is objected that these may have existed but may not have been discovered yet, we must insist on the significant fact that among the many thousands of predynastic graves which have been found, there is not a single one which by its size or equipment suggests the burial of a great chief.[77] The Gerzean innovation did not change the general character of the country’s culture; the remains suggest a prosperous homogeneous population, fully exploiting its rich environment and loosely organized in villages and rural districts. It was in this setting that the efflorescence of Pharaonic civilization occurred. In Mesopotamia the corresponding change took place in the extreme south, in the marshy plain between the head of the Persian Gulf and the higher ground which stretches north from Samarra and Hit.[78] This older diluvial part of the country had been farmed already for many centuries before the south was inhabited. The northern farmers had passed through three phases which can be distinguished by their material equipment (see chronological table at end of book).[79] When the third was predominant in the north, men from the Persian plateau entered the southern marshes. Under present conditions it would be inconceivable that highlanders would elect to do so, or even that they would be able to survive there. But in the fifth and fourth millennia B.C. the Iranian plateau had not yet become a salt desert. Many rivers, descending from the surrounding mountains, ended in upland seas without an outlet and ringed by swamps. Even to-day, in eastern Iran, marsh dwellers are found on the shores of the great lake of the river Hamun.[80] Like the Marsh Arabs of southern Iraq, they build boats and huts of reeds, fish and keep water buffaloes and cattle. Similar conditions must have prevailed over much of Persia in the period we are discussing, and immigrants from such regions would be well prepared to face life in the delta of the Euphrates and Tigris.