During the Dark ages, Greco-Roman science survived in the eastern Roman Empire and the most important advances in pharmacology and pharmacy were made in Byzantium. As the Arab empires spread in the 7th and 8th centuries, they incorporated earlier learning, and the most important contribution of Arabic medical writers was probably the introduction of formularies to aid in the preparation of medicines. In turn, the later spread of Arabic knowledge to the West introduced little-known plants and fostered an interest in collecting and cultivating them, and also introduced the palatable dose forms preferred by the Arabic doctors. In the West, however, the Christian Church taught a doctrine of unquestioning faith, and despite the centers of learning, e.g. at Salerno, most ordinary people depended on the healing power of faith, religious relics and traditional folk medicine. Hydrology was also well developed in the Middle Ages. The formularia that survive describe many indigenous plants, but with few illustrations. Their gathering and preparation is generally guided by magic ceremonies and ritual, and plants often took their properties from their habitat, e.g. the wayside plantain was thought good for tired or wounded feet. Concepts of therapeutic plants were also influenced by alchemy and were linked to related metals and planets.