In sum, there is co-existence of two processes: one scientific, implying a preliminary analysis; the other of common observation, which does not sensibly differ from concrete-abstract classifications; and the idea of a hierarchy formed by abstraction of abstracts, by a systematic arrangement of the animal kingdom, has not yet made its appearance. Yet Aristotle’s work, just by reason of its composite nature, is interesting to the psychologist who studies the evolution of the faculty of abstracting and generalising.
We may pass over two thousand years, during which no progress was made, till we come to Linnæus. “He was the first man who distinctly conceived the idea of expressing, under a definite formula, what he believed to be the system of nature.” His nomenclature is fixed. Under the names of classes (genus summum), orders (genus intermedium), genera (genus proximum), species, varieties, he proposes subdivisions of decreasing value, embracing a greater or less number of animals which all present in common more or less general attributes. He pursues the research after fundamental characteristics, and essential similarities, incessantly correcting his first results. Thus it is only at the eleventh edition of his Systema naturæ that the class of “Quadrupeds” is converted into Mammals: the Cetacea are included in this class, and no longer placed among the fish, as also bats, which were formerly classified with birds, etc.[79] Whatever their objective value, we have here a true system of rational concepts.
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.
This review has no pretensions at being even an abridged history of zoölogical classifications. It merely aims at showing by facts, (1) how a hierarchy of concepts is constituted, and in the travail of centuries passes from the period of generic images to the ideal of embryological unity, common to all beings; (2) how the work of dissociation and analysis has always gone on, and multiplied, in quest of similarities more and more difficult to discover—often indeed fragile or dubious—to stop at unity only, the supreme abstraction.
We are now at the threshold of the last period of abstraction, that of complete symbolism, and it is not without interest to note that what passes in the theoretical order has its equivalent in another form of human activity—the practical order—where the mechanism of exchange is again developed by the aid of an ever-increasing substitution. Thus, at the lowest stage, all commercial transactions are reduced to truck, to exchange by barter. The concrete for the concrete is the method of primitive peoples. An immense step is taken when this rudimentary process is succeeded by the employment of precious metals. A substitutory value is taken as the common measure of other values. At the outset, silver and gold, in the form of powder or of small bullion, were weighed out by the contractors for each particular transaction. Next, this inconvenient procedure was replaced by coined money, issued under the control of an officer, or of the social aggregate, thus conferring a general value on the instrument of exchange. Lastly, at a much later period, bills of exchange, bank-notes, and numerous forms of letters of credit, were substituted for gold and silver; so that a sheet of paper worth less than a centime may become the symbol of millions and tens of millions.