As mentioned above, the great master fact that explains almost all we know of the ancient history of India is our knowledge that two or three thousand years before the birth of Christ a Sanscrit-speaking nation migrated from the valleys of the Oxus and Jaxartes. They crossed the Indus in such numbers as to impress their civilisation and their language on the whole of the north of India, and this to such an extent as practically to obliterate, as far as history is concerned, the original inhabitants of the valley of the Ganges, whoever they may have been. At the time when this migration took place the power and civilisation of Central Asia were concentrated on the lower Euphrates, and the Babylonian empire never seems to have extended across the Carmanian desert to the eastward. The road, consequently, between Bactria and India was open, and nations might pass and re-pass between the two countries without fear of interruption from any other people. If any of the ancient dynasties of Babylonia extended their power towards the East, it was along the coast of Gedrosia, and not in a north-easterly direction. It is, indeed, by no means improbable, as hinted above, that the origin of the Dravidians may be found among the Accadian or in some of the Turanian peoples who occupied southern Babylonia in ancient times, and who may, either by sea or land, have passed to the western shores of India. Till, however, further information is available, this is mere speculation, though probably in the direction in which truth may hereafter be found. When the seat of power was moved northward to Nineveh, the Assyrians seem to have occupied the country eastward of the Caspian in sufficient force to prevent any further migration. At least, after that time--say B.C. 1000--we have no further trace of any Aryan tribe crossing the Indus going eastward, and it seems mainly to have been a consequence of this cutting off of the supply of fresh blood that the purity of their race in India was so far weakened as to admit of the Buddhist reform taking root, and being adopted to the extent it afterwards attained. During the period of the Achemænian sway, the Persians certainly occupied the countries about the Oxus in sufficient strength to prevent any movement of the peoples. So essentially indeed had Bactria and Sogdiana become parts of the Persian empire, that Alexander was obliged to turn aside from his direct route to conquer them, as well as the rest of the kingdom of Darius, before advancing on India. Whether it were founded for that purpose or not, the little Greek kingdom of Bactria was sufficiently powerful, while it lasted, to keep the barbarians in check; but when about the year 127-126 B.C., the Yuechi and other cognate tribes invaded Sogdiana, and finally about 120 B.C. conquered the whole of Bactria,[23] they opened a new chapter in the history of India, the effects of which are felt to the present day.