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.

It is not yet quite clear how soon after the destruction of the Bactrian kingdom these Turanian tribes conquered Cabul, and occupied the country between that city and the Indus. Certain it is, however, that they were firmly seated on the banks of that river before the Christian Era, and under the great king Kanishka had become an Indian power of very considerable importance. The date of this king is, unfortunately, one of those small puzzles that still remain to be solved. Generally, it is supposed he reigned till about twenty to forty years after Christ.[24] Evidence, however, has lately been brought to light, which seems to prove that he was the founder of the Saka era, A.D. 79, and that his reign must be placed in the last quarter of the first century of our era, instead of in the earlier half.[25]

Be this as it may, it seems quite certain that the power of these Turuska kings spread over the whole Punjab, and extended as far at least as Muttra on the Jumna, in the first century of the Christian Era.

At the same time another horde, known to us only from the coins and inscriptions in which they call themselves Sahs or Sah kings, crossed the Indus lower down, and occupied the whole of the province of Gujerat. It is not quite clear whether the first of them, Nahapana, was only the Viceroy of one of these northern kings—probably of Kanishka himself—though he and his successors afterwards became independent, and founded a kingdom of their own. They seem to date their coins and inscriptions from the Saka era, A.D. 79, and the series extends from that date to A.D. 349, or at latest to 371.[26] It thus happens that though Gautamiputra, the Andra king (312-333), boasts of having humbled them,[27] they were only in fact finally disposed of by the rise of the Guptas.

No other foreign race, so far as we know, seems to have crossed the Lower Indus into India. But the whole external history of northern India, from the time of Kanishka to that of Ahmed Shah Durani (1761) is a narrative of a continuous succession of tribes of Scythian origin, pouring across the Upper Indus into India, each more Turanian than the one that preceded it, till the whole culminated in the Mogul conquest of India, in the 15th century, by a people as distinct in blood from the Aryans as any that exist.

Of the older races, it seems probable that the Yavanas must be distinguished from the Turanians. It will hardly now be contended that they were pure Greeks, though their name may be merely a mispronunciation of Ionian. The term seems to have been applied by Indian authors to any foreign race coming from the westward who did not belong to one of the acknowledged kingdoms known to them. As such it would apply to any western adventurers, who during the existence of the Bactrian kingdom sought to establish settlements in any part of India, and would also apply to the expatriated Bactrians themselves when driven from their homes by the Yuechi, 120 or 130 years B.C. It is only in this sense that we can explain their presence in Orissa before and about the Christian Era, but in the west the term may have been more loosely applied. The Cambojas seem to have been a people inhabiting the country between Candahar and Cabul, who, when the tide was setting eastward, joined the crowd, and sought settlements in the more fertile countries within the Indus.