It is true that, notwithstanding his encyclopaedic knowledge, Herbert Spencer has not made a really profound study of political economy, or that at least he has not furnished us the evidence of the _facts_ to support his assertions in this field as he has done in the natural sciences. This does not alter the fact, however, that socialism is, after all, in its fundamental conception only the logical application of the scientific theory of natural evolution to economic phenomena. It was Karl Marx who, in 1859 in his _Critique de l'economie politique_, and even before then, in 1847, in the famous _Manifesto_ written in collaboration with Engels, nearly ten years before Spencer's _First Principles_, and finally in _Capital_ (1867) supplemented, or rather completed, in the social domain, the scientific revolution begun by Darwin and Spencer. The old metaphysics conceived of ethics--law--economics--as a finished compilation of absolute and eternal laws. This is the conception of Plato. It takes into consideration only historical times and it has, as an instrument of research, only the fantastic logic of the school-men. The generations which preceded us, have all been imbued with this notion of the absoluteness of natural laws, the conflicting laws of a dual universe of matter and spirit. Modern science, on the contrary, starts from the magnificent synthetic conception of monism, that is to say, of a single substance underlying all phenomena--matter and force being recognized as inseparable and indestructible, continuously evolving in a succession of forms--forms relative to their respective times and places. It has radically changed the direction of modern thought and directed it toward the grand idea of universal evolution.[47] Ethics, law and politics are mere superstructures, effects of the economic structure; they vary with its variations, from one parallel (of latitude or longitude) to another, and from one century to another. This is the great discovery which the genius of Karl Marx has expounded in his _Critique de l'economie politique_. I will examine further on the question as to what this sole source or basis of the varying economic conditions is, but the important point now is to emphasize their constant variability, from the pre-historic ages down to historical times and to the different periods of the latter. Moral codes, religious creeds, juridical institutions both civil and criminal, political organization:--all are constantly undergoing transformation and all are relative to their respective historical and material environments. To slay one's parents is the greatest of crimes in Europe and America; it is, on the contrary, a duty enjoined by religion in the island of Sumatra; in the same way, cannibalism is a permitted usage in Central Africa, and such it also was in Europe and America in pre-historic ages.