which, for a number of years, he was slowly feeling his way to an answer. In his earliest publication—the Letters on the Proper Sphere of Government—there was already implied the belief that societies are not manufactured, but grow; and it was from the side of natural law, therefore, that this question of progress was at once approached. It was in the pages of Social Statics that he elaborated his first reply. There, borrowing from Coleridge the theory that Coleridge in turn had derived from German speculation—that life is "a tendency toward individuation"—he undertook to show that it is in the fulfillment of this tendency that all progress will be found to consist. Individuation, then, was the master-principle of his thought. But, examined closely, this tendency toward individuation resolves itself into two closely related processes: one making for more and more sharply defined separateness; the other for increasing unity of organization. Universal specialization, with resulting development of complexity, represents one side of the movement we call progress; increasing interdependence among the specialized parts of the organism represents the other.
Progress, therefore—or, to substitute the proper word, evolution—was already recognized by Mr. Spencer as a double-sided process, comprising differentiation, with consequent growth in complexity, and integration, with consequent growth in unification. But though this second-named element—unification—was never entirely lost sight of by him, and is given clear statement, for example, in the essays on The Philosophy of Style and The Genesis of Science, it was upon the former element—differentiation—that for a time his attention was fixed. Taking this principle by itself, and detaching it from all other considerations, he attempted, in his essay on Progress: its Law and Cause, to expand it into a complete theory of universal evolution. In this he was helped by von Baer's law, with which he had become acquainted in 1852—"that the series of changes gone through during the development of a seed into a tree, or an ovum into an animal, constitute an advance from homogeneity of structure to heterogeneity of structure." Overlooking the principle of integration, Mr. Spencer announces this generalization as his text. "We propose," he writes, in the early part of his essay, "to show that this law of organic progress is the law of all progress." In other words, evolution is made to consist wholly in the increase of complexity—in the transformation of the homogeneous into the heterogeneous by successive differentiations.
Satisfied that he had now reached not only a law of evolution, but also the law of evolution, Mr. Spencer, when he began work on the Synthetic Philosophy, proceeded to elaborate his thesis in the first edition of First Principles. Further thought, however,
