This is obviously a most heterogeneous mixture, but to fully appreciate the diversity of its origin the several elements should be traced farther back towards their sources. Thus, the Moors are principally descendants of Arabs, who flooded the northern provinces of Africa in successive waves of emigration eastwards, both before and after the Hegira, partly combining with the Berbers as they went, and partly displacing them from the littoral districts and driving them to the oases of the Sahara, whence they in their turn displaced the Negro population, whom they drove down to the Soudan. The Gypsies, according to Sir Henry Rawlinson,[16] came from the Indo-Scythic tribes who inhabited the mouths of the Indus, and began to migrate northward, from the fourth century onward. They settled in the Chaldean marshes, assumed independence and defied the caliph. In A.D. 831 the grandson of Haroun al-Raschid sent a large expedition against them, which, after slaughtering ten thousand, deported the whole of the remainder first to Baghdad and thence onwards to Persia. They continued unmanageable in their new home, and were finally transplanted to the Cilician frontier in Asia Minor, and established there as a military colony to guard the passes of the Taurus. In A.D. 962 the Greeks, having obtained some temporary successes, drove the Gypsies back more into the interior, whence they gradually moved towards the Hellespont under the pressure of the advancing Seljukians, during the twelfth and thirteenth centuries. They then crossed over to Europe and gradually overspread it, where they are now estimated to number more than three millions. [Footnote 16: _Proceedings of the Royal Geographical Society_, vol. i. This account of the routes of the Gypsies is by no means universally accepted, nor, indeed, was offered as a complete solution of the problem of their migration, but it will serve to show how complex that problem is.]