Districts of refuge located centrally to several natural regions of migration receive immigrants from many sides, and are therefore often characterized by a bizarre grouping of populations. The cluster of marshy islands at the head of the Adriatic received fugitives from a long semi-circle of north Italian cities during the barbarian invasions. Each refugee colony occupied a separate island, and finally all coalesced to form the city of Venice. Central mountain districts like the Alps and Caucasus contain "the sweepings of the plains." The Caucasus particularly, on the border between Europe and Asia, contains every physical type and representative of every linguistic family of Eurasia, except pure Aryan. Nowhere else in the world probably is there such a heterogeneous lot of peoples, languages and religions. Ripley calls the Caucasus "a grave of peoples, of languages, of customs and physical types."[172] Its base, north and south, and the longitudinal groove through its center from east to west have been swept by various racial currents, which have cast up their flotsam into its valleys. The pueblos of our arid Southwest, essentially an area of asylum, are inhabited by Indians of four distinct stocks, and only one of them, the Moquis, show clearly kinship to another tribe outside this territory,[173] so that they are survivals. The twenty-eight different Indian stocks huddled together in small and diverse linguistic groups between the Pacific Ocean and the eastern slope of the Sierra Nevada and Cascade Range[174] leave the impression that these protected valleys, similar to the Caucasus in their ethnic diversity, were an asylum for remnants of depleted stocks who had fled to the western highlands before the great Indian migrations of the interior.[175] Making their way painfully and at great cost of life through a region of mountain and desert, they came out in diminished bands to survive in the protection of the great barrier. Of the twenty-one Indian linguistic stocks which have become extinct since the arrival of the white man, fifteen belong to this transmontane strip of the Pacific slope[176]--evidence of the fragmentary character of these stocks and their consequently small power of resistance, [See map page 54.] [Sidenote: Emigration and colonization.]