A not improbable cause has been suggested of these migrations. It is known that, in spite of the immense volume of water which the Volga is daily pouring into it, the Caspian Sea is gradually drying up, and it has been conjectured as highly probable that hundreds of years ago the Caspian was not only joined to the Sea of Aral, but extended over a large district which is now sandy desert. The slow shrinking in its bed of this sea would, by decreasing the rainfall, turn what was once a fertile country into a desert; and if we suppose this result taking place while the Aryan nations were gradually increasing in numbers, the effect would be to drive them, in despair of finding subsistence in the ever-narrowing fertile tract between the desert and the mountains, to seek for new homes elsewhere. This, at any rate, is what they did. First among them, in all probability, started the Kelts or Celts, who, travelling perhaps to the south of the Caspian and the north of the Black Sea, found their way to Europe, and spread far on to the extreme west. At one time it is most likely that the greater part of Europe was inhabited by Kelts, who partly exterminated and partly mingled with the stone-age men whom they found there. As far as we know of their actual extension in historic times we find this Keltic family living in the north of Italy, in Switzerland, over all the continent of Europe west of the Rhine, and in the British Isles; for the Gauls, who then inhabited the northern part of continental Europe west of the Rhine, the ancient Britons, and probably the Iberians, the ancient inhabitants of Spain, belonged to this family.[32] The Highland Scotch, who belong to the old blood, call themselves Gaels, and their language Gaelic, which is moreover so like the language of the old Irish (who called themselves by practically the same name--Gaedhill) that a Highlander could make himself understood in Ireland; perhaps he might do so in Wales, where the inhabitants are likewise Kelts. These words Gael and Gaedhill are of the same origin and meaning as Gaul. In the early days of the Roman republic the Gauls, as we know, inhabited all the north of Italy, and used often to make successful incursions down to the very centre of the peninsula. Beyond the Alps they extended as far as into Belgium, which formed part of ancient Gaul. So much for the Kelts.