The magnificent reform of biology which Darwin initiated in 1859 by his epoch-making _Origin of Species_ gave a fresh impulse to natural philosophy. As this work not only used the rich collection of facts already made in proof of the theory of descent, but gave it a new foundation in the theory of selection (Darwinism properly so called), everything seemed to call for the embodiment of the new conception of nature in a monistic system. I made the first effort to do this in my _General Morphology_ (1866). As this found few supporters among my colleagues, I undertook in my _History of Creation_ (1868) to make the chief points of the system accessible to the general reader. The remarkable success of this book (a tenth edition of it appearing in 1902) emboldened me at the end of the nineteenth century to state the general principles of my monistic philosophy in my _Riddle of the Universe_. About the same time (1899) there appeared the work of the Kiel botanist, Johannes Reinke, _The World as Reality_; and two years afterwards he followed it up with a supplementary volume, _Introduction to Theoretic Biology_. As Reinke treats the general problems of natural philosophy from a purely mystic and dualistic point of view, his ideas are diametrically opposed to my monistic and naturalistic principles. The history of philosophy describes for us the infinite variety of ideas that men have formulated during the last three thousand years on the nature of the world and its phenomena. Überweg has given us, in his excellent _History of Philosophy_, a thorough and impartial account of these various systems. Fritz Schultze has published a clear and compendious "tabulated outline" of them in thirty tables in his genealogical tree of philosophy, and at the same time shown the phylogeny of ideas. When we survey this enormous mass of philosophic systems from the point of view of general biology, we find that we can divide them into two main groups. The first and smaller group contains the monistic philosophy, which traces all the phenomena of existence to one single common principle. The second and larger group, to which most philosophic systems belong, constitutes the dualistic philosophy, according to which there are two totally distinct principles in the universe. These are sometimes expressed as God and the world, sometimes as the spiritual world and material world, sometimes as mind and matter, and so on. In my opinion, this antithesis of monism and dualism is the most important in the whole history of philosophy. All other systems are only variations of one or the other of these, or a more or less obscure combination of the two.