The further development of Darwin's theory of selection in the last four decades, and the increasing support which has been given to the theory of descent in the great advance of ontogeny, phylogeny, comparative anatomy, and physiology, did much to establish the monistic conception of life. It took the shape more and more of a definite anti-vitalism. Hence it is strange to find that in the course of the last twenty years the old vitalism that everybody had thought dead has lifted up its head once more, though in a new and modified form.[4] This modern vitalism comprises two essentially different tendencies. The partisans of the modern vital force are divided into two groups, which may be designated the sceptical and the dogmatic. Sceptical Neovitalism was first formulated by Bunge, of Basle (1887), in the introduction to his _Manual of Physiological Chemistry_. While he granted the possibility of a full explanation of one part of the vital phenomena by mechanical causes, or the physical and chemical forces of lifeless nature, he rejected it for the other half, especially for psychic activities. He insists that the latter cannot be explained mechanically, and that there is nothing analogous to them in inorganic nature; only a supra-mechanical vital force can produce them, and this is transcendental and beyond the range of scientific inquiry. Much the same was said later by Rindfleisch (1888), more recently by Richard Neumeister in his _Studies of the Nature of Vital Phenomena_ (1903), and by Oscar Hertwig in the lecture on "The Development of Biology in the Nineteenth Century," which he delivered at Aachen in 1900. This sceptical Neovitalism is far surpassed by the dogmatic system, the chief actual representatives of which are the botanist Johannes Reinke and the metaphysician Hans Driesch. The vitalist writings of the latter, which are devoid of any grasp of historical development, have gained a certain vogue through the extraordinary arrogance of their author and the obscurity of his mystic and contradictory speculations. Reinke, on the other hand, has presented his transcendental dualism in clever and attractive form in two works which deserve notice on account of their consistent dualism. In the first of these, _The World as Reality_ (1899), Reinke gives us "the outline of a scientific theory of the universe." The second work (1901) has the title, _Introduction to Theoretical Biology_. The two works have the same relation to each other as my _Riddle of the Universe_ and the present supplementary volume. As our philosophic convictions are diametrically opposed in the main issues, and as we both think ourselves consistent in developing them, the comparison of them is not without interest in the great struggle of beliefs. Reinke is an avowed supporter of dualism, theism, and teleology. He reduces all the phenomena of life to a supernatural miracle. SECOND TABLE ANTITHESIS OF THE MONISTIC AND DUALISTIC THEORIES OF ORGANIC LIFE