During the years 1714 to 1721, Richard Bradley, who was later to become the first Professor of Botany at Cambridge University, proposed a unified, unique, living agent theory of the cause of infectious diseases of plants and animals and the plague of humans. Bradley's agents included microscopic organisms, revealed by the studies of Robert Hooke and Antony van Leeuwenhoek. His theory derived from his experimental studies of plants and their diseases and from microscopic observation of animalcules in different naturally occurring and artificial environments. He concluded that there was a microscopic world of "insects" that lived and reproduced under the appropriate conditions, and that infectious diseases of plants were caused by such "insects." Since there are structural and functional similarities between plants and animals, Bradley concluded that microscopic organisms caused human and animal infectious diseases as well. However, his living agent cause of infectious diseases was not accepted by the contemporary scientific society.