As Leopold was busying himself casting around for colonies, far reaching changes were taking place in the relationship between Europe and Africa. Trading links had existed between the two continents since medieval times, when gold from West Africa was carried across the Sahara by desert caravans and used as the basis of much of Europe's coinage. The Africa to which the Portuguese arrived was, from every point of view, immensely varied. In pursuit of the growing trade, the Portuguese, later to be followed by the British, Dutch, French, Danes and others, seldom ventured inland. In West Africa in particular, to do so was found to be highly dangerous, not only on account of the deadly fevers the land contained, but also because of the difficulty of much of the terrain and the threat from increasingly well armed inhabitants. In West Africa, palm oil replaced slaves as the main item of export and used for the manufacture of soap and lubricants.