A task of this 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 of which the lowermost branches are the aquatic forms to which we give the name of fishes. The fishes are here regarded as composed of six classes or larger lines of descent. Each of these, again, is composed of minor divisions called orders. The different species or ultimate kinds of animals are 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, often 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. Technical names are necessary because vernacular names fail. Half a million kinds of animals are known, while not half a thousand vernacular names exist in any language. And these are always loosely used, half a dozen of them often for the same species, one name often for a dozen species. In the same way, whenever we undertake an exact description, we must use names especially devised for that purpose. We cannot use the same names for the bones of the head of a fish and those of the head of a man, for a fish has a different series of bones, and this series is different with different fishes.