The most valuable fruit of these comprehensive studies of Johannes Müller was his _Manual of Human Physiology_. This classical work contains much more than the title indicates; it is the sketch of a comprehensive "comparative biology." It is still unsurpassed in respect of its contents and range of investigation. In particular, we find the methods of observation and experiment applied in it as masterfully as the philosophic processes of induction and deduction. Müller was originally a vitalist, like all the physiologists of his time. Nevertheless, the current idea of a vital force took a novel form in his speculations, and gradually transformed itself into the very opposite. For he attempted to explain the phenomena of life mechanically in every department of physiology. His "transfigured" vital force was not _above_ the physical and chemical laws of the rest of nature but entirely bound up with them. It was, in a word, nothing more than life itself--that is, the sum of all the movements which we perceive in the living organism. He sought especially to give them the same mechanical interpretation in the life of the senses and of the mind as in the working of the muscles; the same in the phenomena of circulation, respiration, and digestion as in generation and development. Müller's success was chiefly due to the fact that he always began with the simplest life phenomena of the lowest animals, and followed them step by step in their gradual development up to the very highest, to man. In this his method of _critical comparison_ proved its value both from the physiological and from the anatomical point of view. Johannes Müller is, moreover, the only great scientist who has equally cultivated these two branches of research, and combined them with equal brilliancy. Immediately after his death his vast scientific kingdom fell into four distinct provinces, which are now nearly always represented by four or more chairs--human and comparative anatomy, pathological anatomy, physiology, and the history of evolution. This sudden division of Müller's immense realm of learning in 1858 has been compared to the dissolution of the empire which Alexander the Great had consolidated and ruled.