IV. Spencer's psychological discoveries were corollaries to that great principle of evolution of which he made the following announcement as early as 1857 in the _Westminster Review_. After declaring his belief in "that divergence of many races from one race which we inferred must have continually been occurring during geologic time," he stated that "The law of all progress is to be found in these varied evolutions of the homogeneous into the heterogeneous," or in other words, "out of the simple into the complex." The discoveries of Darwin and Wallace were not announced before 1858, but Spencer avowed in 1852 his belief in "the theory of evolution" or "development hypothesis," according to which "complex organic forms may have arisen by successive modifications out of simple ones." It was without any aid or suggestion from Darwin that Spencer's statement of the law of evolution was brought into the final form published in 1862. Evolution was then described as change, not only from the simple to the complex, but also from the chaotic to the concentric and consolidated, or, in Spencer's own words, "from an indefinite incoherent homogeneity to a definite coherent heterogeneity." Progress, he says, consists in integration as well as differentiation. There is an increase in permanence and definiteness as well as in variety. Higher forms are not only more complex and unlike than lower ones, but also more stable and more strongly marked. Spencer has been represented by some Transcendentalists as Darwin's pupil; but the whole system just described would, in all probability, have been built up in substantially its present form, if both Darwin and Wallace had kept their discoveries to themselves. The only difference would have been that Spencer could not have been sustained by such a great mass of evidence. All these facts were collected by Darwin merely to prove the physical development of men and other animals from lower forms of life; but Spencer showed that all the phenomena of thought and feeling, as well as of astronomy, geology, and chemistry, are results of the great laws of integration and differentiation. All human history and social relations can be accounted for in this way. And if this extension had not been given to the principle of evolution, Darwin's discoveries might soon have ceased to have much interest, except for students of natural history. Each of the two great evolutionists helped the other gain influence; but their co-operation was almost as unintentional as that of two luminaries which form a double star. V. Spencer has done much to diminish intolerance, by teaching, as early as 1862, that all religions are necessary steps in the upward march of evolution.