The answer to the problem of immigration is probably simple. There was a time when the great rivers of India did not follow their courses as they do now. This was most recently the case as regards the Indus and the rivers of Central India. In the days when there was no Indus delta and the Indus emptied itself into the great sandy depression of the Rann of Katch, another great lost river from the north-east, the Saraswati, fed the Indus, and between them the desert area was immensely reduced if it did not altogether disappear. Then, possibly, could the cairn-erecting stone-monument building Dravidian sneak his way along the west coast within sight of the sea, and there indeed has he left his monuments behind him. Otherwise the Dravidian element of Central Southern India could only have been gathered from beyond the seas; a proposition which it is difficult to believe. However, never since that desert strip was formed which now flanks the Indus to the east can there have been a right-of-way to the heart of India by the gateways of the west. The earliest exploration of these western roads, of which we can trace any distinct record, was once again due to the enterprise of the Greeks. We need not follow Alexander's victorious footsteps through India, nor concern ourselves with the voyage of his fleet down the Indus, and from the mouth of the Indus to Karachi. General Haig, in his pamphlet on the Indus delta, has traced out his route[3] with patient care, demonstrating from observations taken during the course of his surveys the probable position of the coast-line in those early days.
From Karachi to the Persian Gulf, a voyage undertaken 300 years B.C., of which a log has been kept from day to day, is necessarily of exceeding interest, if only as an indication of a few of the changes which have altered the form of that coast-line in the course of twenty-two centuries. This old route from Arabia to the west coast of India can hardly be left unnoticed, for it illustrates the earliest beginning of those sea ways to India which were destined finally to supplant the land ways altogether. I have already pointed out that, judged by the standard of geographical aptitude only, there is no great difficulty in reaching Persia from Karachi. But geographical distribution of mountain, river, and plain is not all that is necessary to take into account in planning an expedition into new territory. There is also the question of supplies. This was the rock on which Alexander's enterprise split. In moving out of India towards Persia he adopted the same principle which had stood him in good stead on the Indus, viz. the maintenance of communication between army and fleet. Naturally he elected to retire from India by a route which as far as possible touched the sea. This was his fatal mistake, and it cost him half his force.