The unparalleled success of Charles Darwin is well known. It shows him to-day, at the close of the century, to have been, if not the greatest, at least the most effective of its distinguished scientists. No other of the many great thinkers of our time has achieved so magnificent, so thorough, and so far-reaching a success with a single classical work as Darwin did in 1859 with his famous _Origin of Species_. It is true that the reform of comparative anatomy and physiology by Johannes Müller had inaugurated a new and fertile epoch for the whole of biology, that the establishment of the cellular theory by Schleiden and Schwann, the reform of ontogeny by Baer, and the formulation of the law of substance by Robert Mayer and Helmholtz were scientific facts of the first importance; but no one of them has had so profound an influence on the whole structure of human knowledge as Darwin's theory of the natural origin of species. For it at once gave us the solution of the mystic "problem of creation," the great "question of all questions"--the problem of the true character and origin of man himself. If we compare the two great founders of transformism, we find in Lamarck a preponderant inclination to _deduction_, and to forming a completely monistic scheme of nature; in Darwin we have a predominant application of _induction_, and a prudent concern to establish the different parts of the theory of selection as firmly as possible on a basis of observation and experiment. While the French scientist far outran the then limits of empirical knowledge, and rather sketched the programme of future investigation, the English empiricist was mainly preoccupied about securing a unifying principle of interpretation for a mass of empirical knowledge which had hitherto accumulated without being understood. We can thus understand how it was that the success of Darwin was just as overwhelming as that of Lamarck was evanescent. Darwin, however, had not only the signal merit of bringing all the results of the various biological sciences to a common focus in the principle of descent, and thus giving them a harmonious interpretation, but he also discovered, in the principle of selection, that direct cause of transformation which Lamarck had missed. In applying, as a practical breeder, the experience of artificial selection to organisms in a state of nature, and in recognizing in the "struggle for life" the selective principle of natural selection, Darwin created his momentous "theory of selection," which is what we properly call Darwinism.