TEE CLASSIFICATION OF FISHES. 9 



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 lower- 

 most 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 Lan- 

 celets, 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 distinguish- 

 ing 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 idee, 

 and it is always derived from the leading genus, that is, the one best 



