Not one of Virchow's numerous pupils and friends can appreciate more than I do his real services to medical science. His _Cellular Pathology_ (1858), his thorough application of the cell-theory to the science of disease, is, in my opinion, one of the greatest advances made by modern medicine. I had the good fortune to begin my medical studies at Würzburg in 1852, and to spend six valuable terms under the personal guidance of four biologists of the first rank--Albert Kölliker, Rudolf Virchow, Franz Leydig and Carl Gegenbaur. The great stimulus that I received from these distinguished masters in every branch of comparative and microscopic biology was the starting-point of my whole training in that science, and enabled me subsequently to follow with ease the higher intellectual flight of Johannes Müller. From Virchow especially I learned, not only the analytic art of careful observation and judicious appreciation of the detailed facts of anatomy, but also the synthetic conception of the whole human frame, the profound conviction of the _unity_ of our nature, the inseparable connection of body and mind, to which Virchow gave a fine expression in his classic essay on "The Efforts to bring about Unity in Scientific Medicine" (1849). The leading articles which he wrote at that time for the Journal of Pathological Anatomy and Physiology, which he had founded, contain much new insight into the wonders of life, and a number of excellent general reflections on their significance--pregnant ideas that we can make direct use of for Monistic purposes. In the controversy that broke out between empirical rationalism and materialism and the older vitalism and mysticism, he took the side of the former, and fought together with Jacob Moleschott, Carl Vogt, and Ludwig Büchner. I owe the firm conviction of the unity of organic and inorganic nature, of the mechanical character of all vital and psychic activity, which I have always held to be the foundation of my Monistic system, in a great measure to Virchow's teaching and the exhaustive conversations I had with him when I was his assistant. The profound views of the nature of the cell and the independent individuality of these elementary organisms, which he advanced in his great work _Cellular Pathology_, remained guiding principles for me in the prolonged studies that I made thirty years afterwards of the organisation of the radiolaria and other unicellular protists; and also in regard to the theory of the cell-soul, which followed naturally from the psychological study of it.