Towering above all the achievements and discoveries of the century we have the great comprehensive "law of substance," the fundamental law of the constancy of matter and force. The fact that substance is everywhere subject to eternal movement and transformation gives it the character also of the universal law of evolution. As this supreme law has been firmly established and all others are subordinate to it, we arrive at a conviction of the universal unity of nature and the eternal validity of its laws. Accordingly we are to conclude with Goethe that all proceeds by iron law to the fulfilling of inevitable destiny; or as an ardent disciple proclaims, who undertakes to expound the new creed to the people,[21] We rest in sure and certain hope that no force and no combination of forces can stop the process of Evolution, which from a speck of jelly has developed such living forms as Charles Darwin and Herbert Spencer, and which has produced the beauty of the earth and the heavens from formless ether. This outline of the Evolutionary system in its widest and fullest sense will enable us to judge upon what grounds it can claim the sanction of Science. Various points here present themselves for consideration, which demand separate treatment. V WHAT IS A "LAW OF NATURE"? As we have seen, the doctrine of Evolution is presented by its advocates as being based upon the existence of a "Law of Evolution," or "Law of Substance," which both brings about evolutionary processes, and certifies us of their occurrence, so that we may appeal to it as an authority for our belief in the facts of evolution themselves. Thus as Professor Milnes Marshall told the British Association,[22] The doctrine of descent, or of evolution, teaches us that as individual animals arise, not spontaneously, but by direct descent from pre-existing animals, so also is it with species, with families, and with larger groups of animals, and so also has it been for all time. It is not said, be it observed, that the establishment of such facts teaches us the doctrine of evolution, but that the doctrine assures us of the facts; and the utterances constantly met with, of which the above is a fair sample, have no signification if they do not mean this. In the same way Professor Haeckel declares[23] that his fundamental cosmic law "establishes" the eternal persistence of matter and force, and their unvarying constancy throughout the entire universe, becoming thus "the pole-star that guides our Philosophy through the mighty labyrinth to a solution of the world problem," and the key to this supreme problem, he further tells us,[24] is found in one magic word--Evolution.