Aristotle, Plato’s pupil, concerned himself with the whole field of science, and his influence, especially during the Middle Ages, had a most profound effect on European thought. The greater part of his botanical writings, which belong to the fourth century before Christ, are unfortunately lost, but, from such fragments as remain, it is clear that his interest in plants was of an abstract nature. He held that all living bodies, those of plants as well as of animals, are organs of the soul, through which they exist. It was broad, general speculations, such as these, which chiefly attracted him. He asks _why_ a grain of corn gives rise in its turn to a grain of corn and not to an olive, thus raising a plexus of problems, which, despite the progress of modern science, still baffle the acutest thinkers of the present day. Aristotle bequeathed his library to his pupil Theophrastus, whom he named as his successor. Theophrastus was well fitted to carry on the traditions of the school, since he had, in earlier years, studied under Plato himself. He produced a ‘History of Plants’ in which Botany is treated in a somewhat more concrete and definite fashion than is the case in Aristotle’s writings. Theophrastus mentions about 450 plants, whereas the number of species in Greece known at the present day is at least 3000. His descriptions, with few exceptions, are meagre, and the identification of the plants to which they refer is a matter of extreme difficulty. In various points of observation, Theophrastus was in advance of his time. He noticed, for instance, the distinction between centripetal and centrifugal inflorescences—a distinction which does not seem to have again attracted the attention of botanists until the sixteenth century. He was interested in the germination of seeds, and was aware, though somewhat dimly, of the essential differences between the seedling of the Bean and that of the Wheat. In the Middle Ages, knowledge of Aristotelian botany was brought into western Europe at two different periods,—the ninth and the thirteenth centuries. In the ninth century of the Christian era, Rhabanus Magnentius Maurus, a German writer, compiled an encyclopædia which contained information about plants, indirectly derived from the writings of Theophrastus. Rhabanus actually based his work upon the writings of Isidor of Seville, who lived in the sixth and seventh centuries—Isidor having obtained his botanical data from Pliny, whose knowledge of plants was in turn borrowed from Theophrastus.