units of like kinds tend to gather together, and the units of unlike kinds to separate, everywhere coöperates in aiding Evolution. Yet a further universal law is recognized and developed—the law of equilibration. The question is asked, "Can these changes which constitute Evolution continue without limit?" and the answer given is that they cannot; but that they universally tend in each aggregate toward a final state of quiescence, in which all the forces at work have reached a state of balance. Like the other universal processes, that of equilibration is traced out in all divisions of phenomena. But the most important development given to the doctrine of Evolution in this volume was its affiliation upon the ultimate principle underlying all science—the persistence of force. It was shown that from this ultimate law there result certain universal derivative laws, which are dealt with in chapters on "The Correlation and Equivalence of Forces," "The Direction of Motion," and "The Rhythm of Motion," and it was demonstrated that these derivative laws hold throughout all changes from the astronomical to the psychical and social. It is then shown that "the Instability of the Homogeneous," "The Multiplication of Effects," "Segregation" and "Equilibration," are also deducible from this ultimate principle of the persistence of force. So that Evolution, having been first established inductively as universal, is further shown to be universal by establishing it deductively as a result of the deepest of all knowable truths.
The first edition of "First Principles" was published, but another important step in elucidating the philosophy of Evolution required to be taken. In dealing with the classification of the sciences, from the point of view to which his philosophy has brought him, Mr. Spencer had occasion to seek for that aspect of all physical phenomena which forms the most general division of physical science. He found that what he sought must be some general fact respecting the redistribution of matter and motion. The law was soon arrived at that integration of matter results from decrease of the contained motion, while disintegration of matter results from increase of the contained motion. It is at once manifest that the law thus reached was deeper than the principle of Evolution, for it is conformed to by mineral bodies which do not exhibit the phenomena of Evolution as Mr. Spencer had interpreted them. In short, it became clear that a law had been reached holding of all material things whatever, whether they are those which do, or those which do not, increase in heterogeneity. It was now first possible to judge of the relative value and importance of the several factors of the evolutionary process. In Von Baer's conception of organic development, it is made to consist essentially and solely in the change of increasing heterogeneity in the evolving body. But Mr. Spencer had shown that Evolution is a double process—a tendency to unity as well as to diversity, an integration as well as a differentiation. It was now found that the process of integration, as it applies