What does Johannes Müller say? “Imagination is an indispensable servant; it is by means of it we make the combinations that lead to important discoveries. The man of science needs, in harmonious co-operation, the discriminating force of the analytic intelligence and the generalising force of the synthetic imagination.” That is spoken from the depths of Haeckel’s heart, and he drives it home. Nothing is more amusing than to find Haeckel’s later opponents saying, _apropos_ of any particular question, that his statement springs from his “imagination,” as if it were something wholly unscientific that the naturalist must shun like the pest; or again, that Haeckel here or there falls a victim to the deadly enemy of all scientific research, natural philosophy. It is pointed out to him as a great discovery which he must approach in a proper penitential spirit—to him who has discussed these matters so unequivocally in his first theoretical work. As a fact, these methodological chapters in the first volume are as clear as crystal. The titles will seem strange to the man who thinks he can do without any philosophical instruction in zoology and botany, and wants to hear only of cells, tissues, stalks, leaves, bones, scales, and so on, in a general morphology. One chapter has the heading: “Empiricism and Philosophy (Experience and Knowledge).” Another heading runs: “Analysis and Synthesis.” Then there are: “Induction and Deduction,” “Dogmatism and Criticism,” “Teleology and Causality (Vitalism and Mechanism),” “Dualism and Monism.” The last three antithetic headings are united under a general title as “Critique of Scientific Methods that are Mutually Exclusive.” Such a title illumines the whole situation like a flash of lightning. Many years afterwards Haeckel himself said of his _General Morphology_ that it was a comprehensive and difficult work that had found few readers. At least the whole of this first and most difficult part of the book must be defended against the criticism of its parent. If it is far from adequately appreciated to-day, especially by professional philosophers, that is certainly not due to its style, which is a model of clearness in the eyes of any one with the least philosophical culture. The real evil was that people did not look to it for instruction from the philosophical side. The title, “Morphology of Organisms,” had a technical sound. The empty space between professional philosophy and professional zoology is wide enough to-day, but it was far wider thirty-four years ago. Books like Büchner’s superficial and popular _Force and Matter_, or Haeckel’s own later work, the _History of Creation_, that can only be regarded as a brief and incomplete popular extract in comparison with the _General Morphology_, with all its peculiar literary charm, stole into the philosophy of the time like foxes with burning straw tied to their tails. Professional philosophers have written whole libraries on them. The matter recalls a fundamental