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.  But in the fourteenth century B.C., the Assyrians to the north of it, though previously dependent upon Babylon, conquered it, and, after various vicissitudes, established themselves throughout the whole of Mesopotamia and much of the surrounding lands.  In 604 B.C. the capital of this great empire was moved once more to Babylon, so that in the last stage, as well as in the first, it may be called Babylonia.  For purposes of distinction, however, it will be as well to call these three successive stages Chaldaea, Assyria, and Babylonia.

Meanwhile, immediately to the east, a somewhat similar process had been gone through, though here the development was from north to south, the Medes of the north developing a powerful empire in the north of Persia, which ultimately fell into the hands of Cyrus the Great in 546 B.C.  He then proceeded to conquer the kingdom of Lydia, in the northwest part of Asia Minor, which had previously inherited the dominions of the Hittites.  Finally he proceeded to seize the empire of Babylonia, by his successful attack on the capital, 538 B.C.  He extended his rule nearly as far as India on one side, and, as we know from the Bible, to the borders of Egypt on the other.  His son Cambyses even succeeded in adding Egypt for a time to the Persian Empire.  The oyster-shell of history had accordingly expanded to include almost the whole of Western Asia.

The next two centuries are taken up in universal history by the magnificent struggle of the Greeks against the Persian Empire—­the most decisive conflict in all history, for it determined whether Europe or Asia should conquer the world.  Hitherto the course of conquest had been from east to west, and if Xerxes’ invasion had been successful, there is little doubt that the westward tendency would have continued.  But the larger the tract of country which an empire covers—­especially when different tribes and nations are included in it—­the weaker and less organised it becomes.  Within little more than a century of the death of Cyrus the Great the Greeks discovered the vulnerable point in the Persian Empire, owing to an expedition of ten thousand Greek mercenaries under Xenophon, who had been engaged by Cyrus the younger in an attempt to capture the Persian Empire from his brother.  Cyrus was slain, 401 B.C., but the ten thousand, under the leadership of Xenophon, were enabled, to hold their own against all the attempts of the Persians to destroy them, and found their way back to Greece.