Haeckel himself, at the time he was producing his greatest work, saw in Darwin the absolute “open Sesame” to all the doors of philosophic morphology. With this Sesame came an entirely new impulse, namely, to write the natural history of the animal and plant form. It was just the same as when æsthetics perceives a new world, a world that alone is worthy of it, the moment it passes from the making of a mere catalogue of the world’s art-treasures to the knowledge of even one single law of artistic creation, in virtue of which one single work of art has been actually built up. It is impossible to begin with more general considerations than this book does. The method of scientific research generally is explained in order to give an idea of the new Darwinian morphology. With a calmness that must have made most of the contemporary zoologists and botanists shiver, the discredited idea of natural philosophy is restored from the lumber-room. “All true science is philosophy, and all true philosophy is science. And in this sense all true science is natural philosophy.” The various periods in the development of morphology are coolly schematised. These epochs are characterised by the vicissitudes of the struggle between the simple description of forms in the animal and plant worlds and the philosophic exposition of the laws that lie behind these forms. In the eighteenth century, under Linné, there is a period of purely external description and classification. It is succeeded in the first third of the nineteenth century by a triumph of the philosophic treatment of animal and plant forms. This increases with Goethe and Lamarck, and grows into the older (and now generally abused) imaginative natural philosophy. Then there is a general reaction; with Cuvier comes the least philosophical of methods, though at the time it is a real advance. While Linné only gave an external description of forms and catalogued them, Cuvier’s epoch penetrated to the inner structure, the inner world of forms, and thus rendered great service. The last and greatest workers of the period, Müller, Schleiden, &c., give the signal for a reaction in the hour of its chief triumph. Haeckel now follows this up as “the element of fact in their ideas.” With Darwin he inaugurates the fourth epoch, the triumph of natural philosophy for the second time. But it is now far deeper and clearer; it embodies all the good that preceded, all that Cuvier and his followers have done, without the irresolution of earlier days. Now that we have studied the living form in its innermost structure, as was never done before, in the earliest stages of embryonic development in the ovum and womb, in the past geological periods of the earth’s history, we will _think_ over this form, think with all the means at our command, reason, synthesis—even _imagination_, when it is necessary to press on to the great final conclusion, a new synthesis of the defective positive data.