Why was it that the 'Origin of Species' possessed this extraordinary vitalising and kinetic power, this germinal energy, this contagious force, beyond all other forms of evolutionism previously promulgated? Why did the world, that listened so coldly to Lamarck and Chambers, turn so ready an ear to Charles Darwin and natural selection? Partly, no doubt, because in the fulness of time the moment had come and the prophet had arisen. All great movements are long brewing, and burst out at last (like the Reformation and the French Revolution) with explosive energy. But the cause is largely to be found, also, I believe, in the peculiar nature of the Darwinian solution. True, a thoroughly logical mind, a mind of the very highest order, would have said even before Darwin, 'Creation can have no possible place in the physical series of things at all. How organisms came to be I do not yet exactly see; but I am sure they must have come to be by some merely physical process, if we could only find it out.' And such minds were all actually evolutionary even before Darwin had made the _modus operandi_ of evolution intelligible. But most people are not so clear-sighted. They require to have everything proved to them by the strictest collocation of actual instances. They will not believe unless one rise from the dead. There are men who rejected the raw doctrine of special creation on evidence adduced; and there are men who never even for a moment entertained it as conceivable. The former compose the mass of the scientific world, and it was for their conversion that the Darwinian hypothesis was so highly salutary. As Professor Fiske rightly remarks, 'The truth is that before the publication of the "Origin of Species" there was no opinion whatever current respecting the subject that deserved to be called a scientific hypothesis. That the more complex forms of life must have come into existence through some process of development from simpler forms was no doubt the only sensible and rational view to take of the subject; but in a vague and general opinion of this sort there is nothing that is properly scientific. A scientific hypothesis must connect the phenomena with which it deals by alleging a "true cause;" and before 1859 no one had suggested a "true cause" for the origination of new species, although the problem was one over which every philosophical naturalist had puzzled since the beginning of the century. This explains why Mr. Darwin's success was so rapid and complete, and it also explains why he came so near being anticipated.' To put it briefly, _a priori_, creation is from the very first unbelievable; but, as a matter of evidence, Lamarck failed to make evolution comprehensible, or to give a rationale of its mode of action, while Darwin's theory of natural selection succeeded in doing so for those who awaited _a posteriori_ proof. Hence Darwin was able to convert the world, where Lamarck had only been able