As a scientific discipline medicinal chemistry could only emerge after it became generally accepted that the properties of the active ingredients of medicinal products determine the pharmacological and consequently the therapeutic effects of the active principle. Until synthetic organic chemistry developed, coincidently almost during the same period in which pharmacology became an experimental science, medicinal products were mainly obtained from natural materials, from plants especially; to a certain extent mineral products were also used. It was the shape, the colour of (parts of) plants which were considered to be indicative for the biological activities.