We may instance Cuvier for the clearness with which he separates the predominant and subordinate characteristics: “If,” he says, “we consider the animal kingdom on the principles just laid down, regarding only the organisation and nature of the animals, instead of their size and utility, according to our knowledge of them, or the sum of accessory circumstances, we find that there are four principal forms, four general plans, if we may so express ourselves, on which all animals seem to have been modelled,” etc. These four branches (a new word created by him), which he held to be irreducible, were the Vertebrata, Articulata, Mollusca, and Radiata. Finally, since the progress of consecutive abstraction and generalisation consists in incessantly seeking out extracts of extracts, and simplifications of simplifications, the natural movement of the mind tends fatally towards pure unity as its supreme end. This last phase belongs to the nineteenth century, and still more to the contemporary epoch. It comes from various sources, and has assumed different forms: 1. Speculative in the school of Schelling. To Oken, the highest representative of this view, man is the prototype and measure of animal organisation; all other animals are constructed after his pattern. “Their body is in some sort the analysed body of a man; the human organs live, whether in isolation, or in different combinations, in the state of independent animals. Each such combination constitutes a class.” 2. Embryological, according to the labors of Von Baer. While Cuvier, in classification, brought anatomy and morphology to the front, a new system now appears, founded upon development only; the science of embryology. To be accurate, Baer’s conception was not unitary, since it admitted four types: peripheral (radiate), massive (molluscan), longitudinal (articulated), bi-symmetrical (vertebrate). But little by little, the oft-substantiated principle asserted itself and found firm footing among his successors: the animal with the highest organisation passes, during its individual development, through phases which, in less highly evolved beings, are permanent states; or, more briefly, among the higher animals, ontogenesis is a repetition of phylogenesis. 3. Transformist. The boldest partisans of this view, e. g., Haeckel, adopt a rigorously unitary conception: all the innumerable examples of the animal kingdom have issued from one common stock. In all there is a fundamental trend of the mind towards the idea of original unity. It is unimportant for the moment to examine whether this concept of ideal unity (we might also recall the vegetable ideal of Goethe, and the vertebrate ideal of Richard Owen) is a delusion, or a true apprehension: we shall return to this later, in discussing the _objective_ value of the notions of genus and species (Chap. V. § vi). At this point, the _subjective_ psychological process alone is relevant to our purpose.