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. This resemblance of the two cases is by no means fortuitous. It is based upon identity of psychological process, namely a substitution of ascending degrees, an ever increasing simplification, whether in the order of speculative research, or in the department of commercial transaction: and just as paper tokens, unless financially convertible into objects of consumption, for use or luxury, are nonentities that can accumulate in the bank without the gain of anything more than a simulacrum—so, if the highest symbols of abstraction cannot be reduced to the data of experience, we may, as too often occurs, accumulate, manipulate, build up concepts, and still be in a state of permanent intellectual bankruptcy. CHAPTER IV. THE HIGHER FORMS OF ABSTRACTION. THEIR NATURE.