“Individuals are the support, at the actual moment, of the characters not merely of species, but of all other divisions. As representative of _genus_, they have certain details of a definite and specific structure, identical with those possessed by the representatives of other species. As representative of _family_, they have a definite constitution, expressive of a distinct and specific model, in forms resembling those of the representatives of other genera. As representative of _order_, they take definite rank, as compared with the representatives of other families. As representative of _class_, they manifest the structural plan of their ramifications by the aid of special means, and according to specific directions. As representative of _branches_ the individuals are all organised on a distinct plan which differs from the plan of other branch-lines.”[134] It was shown above (Ch. III.) that the contemporary classifications, which are radically embryological, transformist, and generic, proceed otherwise, and have a different aim. Their ideal is to draw up the genealogical tree of living beings, with its multiple ramifications, marking the principal moments of evolution. But if, leaving aside the material of these (animal or vegetable) classifications, we consider only the psychological labor by which they are constituted, we find that the transformists and their adversaries have at least one common point which is of cardinal importance. The notion of fundamental types—conceived as fixed or provisory—is for the one as for the other a compass, a guide in research, a normal, by means of which deviations are appreciated. Hence, these concepts have a practical value, and it is true that we find abstraction and generalisation in their principal rôle, which is, not to discover, but to simplify, above all to be useful. In effect, the one side, yielding to the natural tendency of the mind to reify abstractions, admit the permanence and objectivity of types: they believe firmly that they have in certain concepts the possibility of an ideal reconstruction of the entire world of living beings. This faith sustains them and urges them on to more and more exact determinations. Their opponents, the transformists of every degree, are guided by a different ideal; they search after continuity, transition, forms of passage. Species, genera, families, etc., are but provisory starting-points, with intermediate lacunæ which they endeavor to bridge over. Although the animal order, the chain of life, is itself only a theoretical construction, a natural abstraction, many fine works could be quoted which are inspired by this faith in continuity. Such, e. g., are Huxley, Cope, and others upon the genus _Equus_, establishing the filiation of the four-fingered _Eohippus_ of the old Tertiary epoch, with the Hipparion of the new Tertiary epoch, and with the Horse of the Quaternary period.