century of our own era, when the armies of Islam, after conquering Western Asia and Northern Africa, penetrated even to South-Western Europe. It was by far the most extensive of the four in the area it covered, and, in spite of being the last of the series, it illustrates the character and methods of the earlier movements in their initial stages, when the desert nomad, issuing in force from his own borders, came within the area of settled civilization. It is true that great tracts of Central Arabia are to-day quite uninhabitable, but there is reason to believe that its present condition of aridity was not so marked in earlier periods. We have definite proof of this in the interior of Southern Arabia, where there is still a belt of comparatively fertile country between the flat coastal regions and the steep mountain range, that forms the southern boundary of the central plateau.[1] On the coast itself there is practically no rainfall, and even on the higher slopes away from the coast it is very scanty. Here the herds of goats frequently go without water for many weeks, and they have learnt to pull up and chew the fleshy roots of a species of cactus to quench their thirst. But further still inland there is a broad belt of country, which is marvellously fertile and in a high state of cultivation. The rainfall there is regular during a portion of the year, the country is timbered, and the main mountain range, though possessing no towns of any size, is thickly dotted with strong fighting towers, which dominate the well-farmed and flourishing villages. To the north of the range, beyond the cultivation, is a belt roamed over by the desert-nomads with their typical black tents of woven goat-hair, and then comes the central desert, a region of rolling sand. But here and there the ruins of palaces and temples may still be seen rising from the sand or built on some slight eminence above its level. At the time of the Sabæan kingdom, as early as the sixth century B.C., this region of Southern Arabia must have been far more fertile than it is at the present day. The shifting sand, under the driving pressure of the simoom, doubtless played its part in overwhelming tracts of cultivated country; but that alone cannot account for the changed conditions. The researches of Stein, Pumpelly, Huntington and others have shown the results of desiccation in Central Asia,[2]and it is certain that a similar diminution of the rainfall has taken place in the interior of Southern Arabia.[3] To such climatic changes, which seem, according to the latest theories, to occur in recurrent cycles,[4] we may probably trace the great racial migrations from Central Arabia, which have given their inhabitants to so many countries of Western Asia and North Africa.