Another type of barrier is found in densely populated areas, the inhabitants of which, if strong enough, would prevent a migra tion across their territory. The same applies to a country which had a strong military force or contained a warrior population. There are many examples of this. Artificial barriers such as dykes and ramparts or walls may prevent movements for, at all events, some time. As Ujfalvy suggests: "The building of the great wall of China was an event fraught with the greatest consequences, and one may say without exaggeration that it contributed powerfully to the premature downfall of the Roman Empire." (C. de Ujfalvy, Les Aryens au nord et an sud de l'Hindou-Kouch, p. 24, 1896.) Barriers are thus relative, and only in rare cases are they insurmountable.
There are certain means of transport which greatly facilitate movements of peoples, such as beasts of burden and boats. The domestication of the few animals that could be utilized for riding and drawing of vehicles is confined to the Old World, though in Peru llamas are used to carry burdens and to a slight extent for riding, and dogs were utilized in North America to drag travaux on the plains and sledges in the arctic regions. The domestication of the reindeer for hauling sledges is confined to the arctic regions of the Old World and possibly this idea arose from the example of draught cattle. Only to a limited extent were these animals aids to movements of peoples. Probably somewhere in central western Asia before 5000 B.c. took place the domestication of camels, cattle, horses and asses. Although invaluable for arid areas the camel has a restricted use, and it is not suitable for and does not thrive in most countries or climates. Cattle can be employed almost everywhere. except where there is intense cold or great scarcity of water. Their rate of progress is slow, but we know that ox carts or wagons were used for human transportation in the Bronze age, and in our own time the trekking of the Boers in South Africa was accomplished in the same manner. A pastoral or semi-pastoral people can only migrate along a country which affords sufficient pasturage and water—mountains, deserts and swamps are practically insuperable for such peoples. A further barrier for migration is found in the tsetse fly, ticks and other insect pests, which afford intermediate hosts of the parasites of various kinds of cattle diseases ; these have been especially operative in Africa. The use of the yak and sheep in Tibet as beasts of burden has had no effect on migrations. The horse on
account of its speed has rendered possible rapid raids of nomad peoples, which would prepare the way for more leisurely mass movements, since communities moving with their flocks and herds cannot travel faster than the slowest of these. The horse reached Sumeria from the north about the time of Hammurabi and later the conquest of Egypt by the Hyksos was greatly facilitated by their horse-drawn war chariots, for it was not till subsequently that cavalry were employed in warfare. Wherever the horse has been taken it has facilitated human movements, and so, to far less extent, has the ass.
We may well believe that for many thousands of years large stretches of water proved very effective barriers to migration. Rivers might be crossed on floating logs and rafts ; later would come dug-outs and such craft as the coracle and allied skin boats of the Old World and the bull-boat of North America, which are practically skin-covered baskets; certainly very early there were floats and incipient boats made of bundles of reeds, etc., and doubtless also bark canoes and skin floats; but none of these would be efficient for long distances on open seas. It seems im probable that the Tasmanians, for example, could have crossed Bass strait as it now is in the floats of bundles of bark that they possessed when first discovered. It was not until canoes were provided with side-planks that seaworthy vessels were available for oversea commerce, warfare and colonization. The invention of outriggers to steady such craft occurred in the Indian ocean, either in the East Indian archipelago or in India, but nowhere else; and it was by this discovery, together with that of lashing two canoes together to form a double canoe, that the migrations of the Polynesians all over the Pacific became possible. Some maintain that the invention of sea-going craft was due to the ancient Egyptians, but there is also a case to be made out for at least an independent invention in the Persian gulf or possibly in India. Peake and Fleure are of opinion that "between 400o and 3500 B.C. the Sumerians arrived probably by sea, at the head of the Persian gulf, bringing with them for the first time into Meso potamia herds of dairy cattle, an advanced copper culture, and a habit of decorating objects by means of inlaid slabs of mother-of pearl and other materials." (The Corridors of Time, III., p. 95, 1927.) But there were two earlier civilizations in Mesopotamia.