But in order to ascertain how the Romans obtained the mass of information which was summarised for them by Ptolemy in his great work, we have merely to concentrate our attention on the remarkable process of continuous expansion which ultimately led to the existence of the Roman Empire.
All early histories of kingdoms are practically of the same type.
A certain tract of country is divided up among a certain number of tribes speaking a common language, and each of these tribes ruled by a separate chieftain.

One of these tribes then becomes predominant over the rest, through the skill in war or diplomacy of one of its chiefs, and the whole of the tract of country is thus organised into one kingdom.

Thus the history of England relates how the kingdom of Wessex grew into predominance over the whole of the country; that of France tells how the kings who ruled over the Isle of France spread their rule over the rest of the land; the history of Israel is mainly an account of how the tribe of Judah obtained the hegemony of the rest of the tribes; and Roman history, as its name implies, informs us how the inhabitants of a single city grew to be the masters of the whole known world.
But their empire had been prepared for them by a long series of similar expansions, which might be described as the successive swallowing up of empire after empire, each becoming overgrown in the process, till at last the series was concluded by the Romans swallowing up the whole.

It was this gradual spread of dominion which, at each stage, increased men's knowledge of surrounding nations, and it therefore comes within our province to roughly sum up these stages, as part of the story of geographical discovery.
Regarded from the point of view of geography, this spread of man's knowledge might be compared to the growth of a huge oyster-shell, and, from that point of view, we have to take the north of the Persian Gulf as the apex of the shell, and begin with the Babylonian Empire.

We first have the kingdom of Babylon--which, in the early stages, might be best termed Chaldaea--in the south of Mesopotamia (or the valley between the two rivers, Tigris and Euphrates), which, during the third and second millennia before our era, spread along the valley of the Tigris.