From their upland farms they must have looked eastward upon yet higher mountains, rising impenetrable above the snowline; but to north and south and west they might turn to lower regions; and by degrees, perhaps as they grew too numerous for comfort, a few families wandered off along the more inviting routes. Whichever way they started, their adventurous spirit led them on. We find no trace of a single case where hearts failed or strength grew weary and the movement became retrograde, back toward the ancient home. Spreading out, radiating in all directions, it is they who have explored the earth, who have measured it and marked its bounds and penetrated almost to its every corner. It is they who still pant to complete the work so long ago begun. Before B.C. 2000 one of these exuded swarms had penetrated India, probably by way of the Indus River. In the course of a thousand years or so, the intruders expanded and fought their way slowly from the Indus to the Ganges. The earlier and duskier inhabitants gave way before them or became incorporated in the stronger race. A mighty Aryan or Hindu empire was formed in India and endured there until well within historic times. Yet its power faded. Life in the hot and languid tropics tends to weaken, not invigorate, the sinews of a race. Then, too, a formal religion, a system of castes[8] as arbitrary as among the Egyptians, laid its paralyzing grip upon the land. About B.C. 600 Buddhism, a new and beautiful religion, sought to revive the despairing people; but they were beyond its help.[9] Their slothful languor had become too deep. From having been perhaps the first and foremost and most civilized of the Aryan tribes, the Hindus sank to be degenerate members of the race. We shall turn to look on them again in a later period; but they will be seen in no favorable light. [Footnote 8: See _The Formation of the Castes_, page 52.] [Footnote 9: See _The Foundation of Buddhism_, page 160.] Meanwhile other wanderers from the Aryan home appear to the north and west. Perhaps even the fierce Tartars are an Aryan race, much altered from long dwelling among the yellow peoples. One tribe, the Persians, moved directly west, and became neighbors of the already noted Semitic group. After long wars backward and forward, bringing us well within the range of history, the Persians proved too powerful for the whole Semite group. They helped destroy Assyria,[10] they overthrew the second Babylonian empire which Nebuchadnezzar had built up, and then, pressing on to the conquest of Egypt, they swept the Hamites too from their place of sovereignty.[11] [Footnote 10: See _Destruction of Nineveh_, page 105.] [Footnote 11: See _Conquests of Cyrus_, page 250.]