It was a vast work. A single man had at first the whole kingdom in his hands, had to reject the old lines of demarcation and create new ones. There was a certain advantage at the time. Since Cuvier’s time an immense quantity of new discoveries had accumulated for the construction of a system of living things. Müller, Siebold, Leuckart, Vogt, and many others, had done a great deal of preparatory work. All this was of great assistance to the man who now came forward with courage and a talent for organisation. Nevertheless it needed real genius, together with almost boundless knowledge, to accomplish the task. We must remember how reactionary (even apart from the question of evolution) was the systematic work of distinguished and assuredly learned zoologists like Giebel at that time; they worked on in a humdrum way as if the more advanced students did not exist. How different it has all become since Haeckel’s thorough reform of classification! We are astounded to-day at the skill with which he drew lines in his very first sketch that were so near to the permanent truth. I need only point to the new scheme of the classification of the vertebrates. A good deal of his work was, of course, bound to be defective, because the facts were not yet known; for instance, in fixing the point at which the vertebrates may have evolved from the invertebrates. It was not until a year later that the discovery of the embryonic development of the ascidia by Kowalewsky threw light on this. Again, there was the solution of the problem of the ultimate root-connection of the great parallel animal stems. In this matter Haeckel himself brought illumination by his gastræa-theory. On the whole this systematic introduction to the second volume would have sufficed of itself to secure for Haeckel a prominent position in the history of zoology and botany. He himself was chiefly proud of the fact that it was the first natural-philosophical system on the new lines to meet the rigorous demands of academic science, and indeed to revolutionise academic science. This enhances his complete triumph in the last two books of the volume. First man is introduced, with absolute clearness and decisiveness, into the system of evolved natural beings, as crown of the animal world, but subject to the same laws as the animal: a vertebrate, a mammal, whose nearest relatives are the anthropoid apes. Thus at last the “system of nature” was complete. It embodied the unity of nature. It formed the framework of facts for a unified natural philosophy, Monism. The _monon_, the “one,” embracing all things, that included nature in itself and itself in nature, became the last scientific definition of what people called “God.”