Benin art has been practiced without interruption for centuries, and Von Luschan says that it is "of extraordinary significance that by the sixteenth and seventeenth centuries a local and monumental art had been learned in Benin which in many respects equaled European art and developed a technique of the very highest accomplishment."[24] Summing up Yoruban civilization, Frobenius concluded that "the technical summit of that civilization was reached in the terra-cotta industry, and that the most important achievements in art were not expressed in stone, but in fine clay baked in the furnace; that hollow casting was thoroughly known, too, and practiced by these people; that iron was mainly used for decoration; that, whatever their purpose, they kept their glass beads in stoneware urns within their own locality, and that they manufactured both earthen and glass ware; that the art of weaving was highly developed among them; that the stone monuments, it is true, show some dexterity in handling and are so far instructive, but in other respects evidence a cultural condition insufficiently matured to grasp the utility of stone monumental material; and, above all, that the then great and significant idea of the universe as imaged in the Templum was current in those days."[25] Effort has naturally been made to ascribe this civilization to white people. First it was ascribed to Portuguese influence, but much of it is evidently older than the Portuguese discovery. Egypt and India have been evoked and Greece and Carthage. But all these explanations are far-fetched. If ever a people exhibited unanswerable evidence of indigenous civilization, it is the west-coast Africans. Undoubtedly they adapted much that came to them, utilized new ideas, and grew from contact. But their art and culture is Negro through and through. Yoruba forms one of the three city groups of West Africa; another is around Timbuktu, and a third in the Hausa states. The Timbuktu cities have from five to fifteen hundred towns, while the Yoruba cities have one hundred and fifty thousand inhabitants and more. The Hausa cities are many of them important, but few are as large as the Yoruba cities and they lie farther apart. AH three centers, however, are connected with the Niger, and the group nearest the coast--that is, the Yoruba cities--has the greatest numbers of towns, the most developed architectural styles, and the oldest institutions. The Yoruba cities are not only different from the Sudanese in population, but in their social relations. The Sudanese cities were influenced from the desert and the Mediterranean, and form nuclei of larger surrounding monarchial states. The Yoruba cities, on the other hand, remained comparatively autonomous organizations down to modern times, and their relative importance changed from time to time without developing an imperialistic idea or subordinating the group to one overpowering city.