While human history was thus being invaded by natural science and had to defend its autonomy against the naturalistic principle of science, natural science on the other hand was in the nineteenth century invaded by the historical principle of evolution. Natural science becomes a history. We have seen that in the Romantic circle there was great interest in the origin and development of law, philology, art, etc. In the beginning and middle of the nineteenth century this interest spread to an investigation of the origin of animal life. This investigation has been the most notable in this century, because (1) it included in its scope the source and means of progress of the human race; and because (2) it advanced a new conception of development. Development now becomes evolution. Up to the nineteenth century the world was looked upon as a graded series of types, but no type was supposed to evolve into another. (See vol. i, pp. 180, 193; vol. ii, p. 306.) The theory of historical evolution of the nineteenth century is notable because it advanced the conception, based upon empirical investigation, that types are changed into others. This theory, among those of the century, comes the nearest to an original philosophical doctrine. The book that became the centre of scientific interest for many years was Darwin’s _Origin of Species_, published in 1859. The name most prominently linked with that of Darwin is that of Herbert Spencer, who attempted to make universal the principle of development and to formulate its law. The modern theory of the historical evolution of animal life has reinforced the mechanical principle of nature, which had its origin in the minds of the philosophers of the Renaissance. It has antagonized the theological doctrine of creation; it has related the animal and man by filling in the supposably impassable gulf between them; it has advanced the doctrine of chemical synthesis against the hylozoistic notion of a vital principle; it has pushed forward with great assurance its theories of transformation and equivalence of forces, and of the action of electricity as a substitute for thought-activity; it has shown a wonderful parsimony in giving a value to all the facts of history which had hitherto been conceived as trivial; and on the other hand it has reduced the conception of mighty cosmic cataclysms to a geological series of gradual gradations. Darwin’s place in this movement of the nineteenth century was this: he tried to show that animal life can be explained without the aid of final causes. In other words, the adaptation of the structure of animals can be accounted for mechanically. The factors involved in the development of organic life upon the earth were, according to Darwin, infinite differentiation, adaptation, natural selection, and the survival of the fittest.