My reference to the French materialism of the eighteenth century, in its contrast with the theism of Voltaire, is intended to point the stronger contrast between the feeble survivals of that materialism in our time and the unshakable theism which is in harmony with the Doctrine of Evolution. When some naturalist like Haeckel assures us that as evolutionists we are bound to believe that death ends all, it is a great mistake to hold the Doctrine of Evolution responsible for such a statement. Haeckel's opinion was never reached through a scientific study of evolution; it is nothing but an echo from the French speculation of the eighteenth century. Such a writer as La Mettrie proceeded upon the assumption that no belief concerning anything in the heavens above, or the earth beneath, or the waters under the earth, is worthy of serious consideration unless it can be demonstrated by the methods employed in physical science. Such a mental attitude was natural enough at a time when the mediæval theory of the world was falling into discredit, while astronomy and physics were winning brilliant victories through the use of new methods. It was an attitude likely to endure so long as the old-fashioned fragmentary and piecemeal habits of studying nature were persisted in; and the change did not come until the latter half of the nineteenth century. The encyclopædic attainments of Alexander von Humboldt, for example, left him, to all intents and purposes, a materialist of the eighteenth century. But shortly before the death of that great German scholar, there appeared the English book which heralded a complete reversal of the attitude of science. The "Principles of Psychology," published in 1855 by Herbert Spencer, was the first application of the theory of evolution on a grand scale. Taken in connection with the discoveries of natural selection, of spectrum analysis, and of the mechanical equivalence between molar and molecular motions, it led the way to that sublime conception of the Unity of Nature by which the minds of scientific thinkers are now coming to be dominated. The attitude of mind which expressed itself in a great encyclopædic book without any pervading principle of unity, like Humboldt's "Kosmos," is now become what the Germans call _ein ueberwundener Standpunkt_, or something that we have passed by and left behind.