The union of these two chief tendencies into the physico-chemical aspects of physiology has established the modern way of looking upon vital activities. These vital activities are now regarded as being, in their ultimate analysis, due to physical and chemical changes taking place within the living substratum. All along, this physico-chemical idea has been in contest with that of a duality between the body and the life that is manifested in it. The vitalists, then, have had many controversies with those who make their interpretations along physico-chemical lines. We will recollect that vitalism in the hands of the immediate successors of Haller became not only highly speculative, but highly mystical, tending to obscure any close analysis of vital activity and throwing explanations all back into the domain of mysticism. Johannes Müller was also a vitalist, but his vitalism was of a more acceptable form. He thought of changes in the body as being due to vitality--to a living force; but he did not deny the possibility of the transformation of this vital energy into other forms of energy; and upon the basis of Müller's work there has been built up the modern conception that there is found in the human body a particular transformation-form of energy, not a mystical vital force that presides over all manifestations of life. The advances in physiology, beginning with those of William Harvey, have had immense influence not only upon medicine, but upon all biology. We find now the successful and happy union between physiology and morphology in the work which is being so assiduously carried on to-day under the title of experimental morphology. The great names in physiology since Müller are numerous, and perhaps it is invidious to mention particular ones; but, inasmuch as Ludwig and Du Bois-Reymond have been spoken of, we may associate with them the names of Sir Michael Foster and Burdon-Sanderson, in England; and of Brücke (one of Müller's disciples) and Verworn, in Germany, as modern leaders whose investigations have promoted advance, and whose clear exposition of the facts and the theories of physiology have added much to the dignity of the science. CHAPTER X VON BAER AND THE RISE OF EMBRYOLOGY Anatomy investigates the arrangement of organic tissues; embryology, or the science of development, shows how they are produced and arranged. There is no more fascinating division of biological study. As Minot says: "Indeed, the stories which embryology has to tell are the most romantic known to us, and the wildest imaginative creations of Scott or Dumas are less startling than the innumerable and almost incredible shifts of rôle and change of character which embryology has to entertain us with in her histories."