If one had a tree, all in fragments, pieces of twig and stem, some of them lost, some destroyed, and some not yet separated from the mass not yet picked over, and wished to place each part where he could find it, he would be forced to adopt some system of natural classification. In such a scheme he would lay those parts together which grew from the same branch. If he were compelled to arrange all the fragments in a linear series, he would place together those of one branch, and when these were finished, he would begin with another. If all this were a matter of great importance, extending over years or over many lifetimes, with many errors to be made and corrected, a set of names would be adopted—for the main trunk, for the chief branches, the lesser branches, and on down to the twigs and buds.
A task of tills sort on a world-wide scale is the problem of systematic zoology. There is reason to believe that all animals and plants sprang from a single stock. There is reasonable certainty that all vertebrate animals are derived from a single origin. These vertebrate animals stand related to each other, like the twigs of a gigantic tree, the lowermost branches are the aquatic forms to which we give the name of fishes, with their still more primitive fish-like relatives.
The aquatic vertebrates, reasonably called by the names of fishes, constitute about three classes, or larger lines of descent. There are lampreys, sharks and true fishes. If we include the extinct forms, we may perhaps add two more, but this is uncertain, while below the fishes are the protochordate classes of Enteropneustans, Tunicates and Lancelets, which stand nearer to fishes than to anything else. Each of these groups differs from the others in varying degree.
Each of these again is composed of minor divisions called orders, each containing many species. The different species, or ultimate kinds of animals are again grouped in genera. A genus is an assemblage of closely related species grouped around a central species as type. The type of a genus is, in common usage, that species with which the name of the genus was first associated. The name of the genus, as a noun, taken with that of the species, which is an adjective in signification, if not in form, constitutes the scientific name of the species. Thus Petromyzon is the genus of the common large lamprey; marinus is its species, and the scientific name of the species is Petromyzon marinus. Petromyzon means stone-sucker; marinus of the sea; thus distinguishing it from a species called fluviatilis, of the river.
In like fashion all animals and plants are named in scientific record or taxonomy.
A family in zoology is an assemblage of related genera. The name of a family, for convenience, always ends in the patronymic idæ, and it is always derived from the leading genus, that is, the one best
