Even supposing species to be the sole natural division, we may accept the superior combinations as means to aid us to a gradually extending survey of the whole. Perhaps did we possess all the links of the vast chain of beings we should find genera, and every other superior combination, melt away through the intimate alliance of the succession of species that would obliterate the lines of separation, by making the sutures imperceptible; but what mind could compass the detail of such a limitless unbroken series? Their subdivision may therefore be accepted as a positive necessity, to enable us to compass their investigation. As it at present stands, with our imperfect knowledge of the entire series of species, these higher groups are indispensably requisite.
The specific diagnosis being the only sure basis upon which all our knowledge can rest, its accuracy is all-important, and requires a few observations. It comprises two parts—the specific _character_, and the specific _description_. The difference between these is, that the first is constructed with the extremest brevity consistent with its utility, is fluctuating and not permanent. The latter permits all the diffuseness needful to embrace a full description of the creature.
The object of the first is to establish the _present_ identity of the species amongst all its known congeners—those associated in the same genus;—and that of the second to secure it in its perpetual identity, and segregate it from all future and contingent discoveries. The specific character admits, consequently, modifications to suit any extension of the genus, and in fact exacts it at the hands of all who describe new species. This many naturalists undertake without any apparent consciousness of the scientific responsibilities that attach to it, and whence results the confusion so much to be deplored, of the synonymy that prevails, constituting, as it does, such a Dædalian labyrinth. The describer of a new species is bound to cast around, and endeavour to know all that has been previously done upon the subject of the genus. He has to revise all the specific characters within the genus, and mould them to those he introduces, and he must insert these closest to their evident affinities. Thus, therefore, the describer’s labour is not light, if to be of any value. The specific character, although thus varying, becomes a permanent utility, and only so fulfils its object,—that of rapidly showing, at a glance, the known species of a genus, and thereby permitting the speedy determination of the identity or distinctness of a compared object. If doubt should exist from this brevity, the specific description is at hand to solve it, by the amplitude and completeness of its details. Of course this mode of treatment is only suitable to monographs, or portions of the science discussed separately, and not to a general or universal survey.