The early history of maritime colonization shows in general two geographic phases: first, the appropriation of the islet and headland outskirts of the seaboard, and later--it may be much later--an advance toward the inner edge of the coast, or yet farther into the interior. Progress from the earlier to the maturer phase depends upon the social and economic development of the colonizers, as reflected in their valuation of territorial area. The first phase, the outcome of a low estimate of the value of land, is best represented by the Phoenician and earliest Greek colonies, whose purposes were chiefly commercial, and who sought merely such readily accessible coastal points as furnished the best trading stations on the highway of the Mediterranean and the adjacent seas. The earlier Greek colonies, like those of the Triopium promontory forming the south-western angle of Asia Minor, Chalcidice, the Thracian Chersonesus, Calchedon, Byzantium, the Pontic Heraclea, and Sinope, were situated on peninsulas or headlands, that would afford a convenient anchor ground; or, like Syracuse and Mitylene, on small inshore islets, which were soon outgrown, and from which the towns then spread to the mainland near by. The advantages of such sites lay in their accessibility to commerce, and in their natural protection against the attack of strange or hostile mainland tribes. For a nation of merchants, satisfied with the large returns but also with the ephemeral power of middlemen, these considerations sufficed. While the Phoenician trading posts in Africa dotted the outer rim of the coast, the inner edge of the zone was indicated by Libyan or Ethiopian towns, where the inhabitants of the interior bartered their ivory and skins for the products of Tyre.[436] So that commercial expansion of the Arabs down the east coast of Africa in the first and again in the tenth century seized upon the offshore islands of Zanzibar, Pemba, and Mafia, the small inshore islets like Mombasa and Lamu, and the whole outer rim of the coast from the equator southward to the Rovuma River.[437] The Sultan of Zanzibar, heir to this coastal strip, had not expanded it a decade ago, when he had to relinquish the long thread of his continental possessions. [Sidenote: Inland advance of colonies.]