Specific differences result from many characteristics,—from colour, clothing, size, and sometimes from peculiarities of structure; but these last are usually of a higher order, tending to indicate an aberration, slight though it be, from the normal generic character which holds the group together, thus implying a distinctive economy. This is sometimes called a subgeneric attribute, and there might be a reason, certainly, for not elevating such species to the full rank of genera, were genera equivalents, which they are not, and it merely remains an evasive admission of the doubt that attaches, except for the sake of convenience, to any subdivision, but the specific. The species is thus the very last term of subdivision, the very elemental principle itself, which unites together as one, solely for the purposes of perpetuation, the two sexes of similar individuals, and without whose intercourse the kind or species would die out. That some species greatly abound in individuals, as before observed, whilst others appear to be extremely limited, is an absolute fact, and not merely suggested by a defective observation of their occurrence, resulting from their rapid dispersion. It is verified by being noticed to occur where we know they would resort, as is exemplified in the case of some of the parasitical species of the insects herein treated of, and which are sometimes rare, even in the vicinity of the metropolis of their sitos, and where this also greatly abounds. In other cases, other species absolutely swarm where the similar attraction lies. 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.