influence of Copernicus was rather different. From the point of view of the history of Science his importance is that he made Newton possible; from our present point of view his importance is that he made Darwin possible. Copernicus’ destruction of the isolated position of man’s planet in the solar system prepares the mind for Darwin’s destruction of the isolated position of man in the animal kingdom. They each shocked the same set of prepossessions. The “materialistic philosophy” which was so marked a feature of the latter part of the nineteenth century, and which still forms, we believe, the prevalent intellectual complexion, owed the whole of its plausibility to its supposed scientific backing. Its basis was not merely biological; physics played quite as great a part as biology. The notion of determinism derived its strength, as we have said, chiefly from physics; biology was not in a position to demonstrate the exact correspondences required. The ultimate grandiose vision of the purely natural and inevitable march of evolution from the atoms of the primitive nebula to the British Association for the Advancement of Science, as outlined by Tyndall in his Belfast Address, assumed the results of physics and astronomy as much as Darwin’s _Origin of Species_. It was because biology was not the only science involved that it was possible to found a “materialistic” philosophy on Darwinism. One primary assumption of that philosophy, that life arises from “dead” matter, not only had no biological support, but had been decisively refuted by the experiments of Pasteur. But, as related to the general movement of Science, the hypothesis had the necessary plausibility. Considering the then existing evidence, this hypothesis, together with the hypothesis that mental states are produced by atomic movements in a strictly determinist manner, are, indeed, striking instances of the way in which the _Zeitgeist_, as much as the evidence, determines the direction of our thinking.