This fact has led, in a great measure, to the complicating and confounding the science of Natural History, by the undue multiplication of names, species, and genera, where no specific differences exist; rendering the science infinitely difficult to the beginner, and causing the unlearned to undervalue the lore of the naturalist, and to deny the reality of all scientific distinctions whatsoever. On differences of structure, such as the situation and texture of the fins, the number of spines or soft rays in each, the form of the gill covers, the character and position of the teeth, perfect reliance may be placed, as indicating unchangeable specific characteristics, by observation of which the educated naturalist will name at a glance the species, genus and subgenus of any fish, unseen before; and will unerringly determine his habits, his food, and in some degree his habitation. Thus of the _Percoid_ family we distinguish the subgenera _Perca_, perch proper, from _Gristes_ and _Centrarchus_, to which are referred the types black basse of the lakes, and the little rock basse of the St. Lawrence basin, by the fact that the _Percæ_ have one spine to the ventrals and two to the anal. The _Gristes_ one to the ventrals and three to the anal. The _Centrarchi_ one to the ventrals and six to the anal. And in like manner, by the number of spines supporting the first dorsal, we are enabled to pronounce on the truth or untruthfulness of the many subdivisions of the perch family, as predicated by the fishermen of various regions, and insisted on by credulous naturalists, such as Dr. Smith, of Massachusetts, whose book is rendered absolutely valueless by the readiness which he displays in adopting every local legend concerning new varieties, and classifying new species; until, if we believe him at all, we must believe that every several stream and pool from Maine to Minnesota has its own distinct variety of perch; nor of perch only, but of trout, and, more or less, of every finny tenant of the waters. The truth appears to have been at length firmly established, and to be this—that there is but one clearly defined and distinct perch, _perca flavescens_, the yellow perch, found in the United Stales—the _perca fluviatilis_, common river perch of Europe, does not exist at all in American waters, though it is so closely connected with our fish that a casual observer would pronounce them identical—that the supposed subgenera of _perca granulata_, or rough-headed perch, _perca argentea_, silver perch, _perca acuta_, or sharp-nosed perch, and _perca gracilis_, said to be peculiar to the small lake of Skaneateles, in the interior of New York, are not sufficiently made out as permanent varieties; and that the variations of color from dark green, and greenish brown, to bright yellow, silvery, and something nearly approaching to orange, are merely local, casual, and individual differences, and not general, permanent, specific distinctions.