As the fourth and last stage in the history of Christianity we oppose our nineteenth century to all its predecessors. It is true that the enlightenment of preceding centuries had promoted critical thought in every direction, and the rise of science itself had furnished powerful empirical weapons; yet it seems to us that our progress along both lines has been quite phenomenal during the nineteenth century. It has inaugurated an entirely new period in the history of the human mind, characterized by the development of the monistic philosophy of nature. At its very commencement the foundations were laid of a new anthropology (by the comparative anatomy of Cuvier) and of a new biology (by the _Philosophie Zoologique_ of Lamarck). The two great French scientists were quickly succeeded by two contemporary German scholars--Baer, the founder of the science of evolution, and Johannes Müller, the founder of comparative morphology and physiology. A pupil of Müller, Theodor Schwann, created the far-reaching cellular theory in 1838, in conjunction with M. Schleiden. Lyell had already traced the evolution of the earth to natural causes, and thus proved the application to our planet of the mechanical cosmogony which Kant had sketched with so much insight in 1755. Finally, Robert Mayer and Helmholtz established the principle of energy in 1842--the second, complementary half of the great law of substance, the first half of which (the persistence of matter) had been previously discovered by Lavoisier. Forty years ago Charles Darwin crowned all these profound revelations of the intimate nature of the universe by his new theory of evolution, the greatest natural-philosophical achievement of our century.