37 2 Classification of Fishes 



pelled to arange 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 and 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 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 compsed 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 ani- 

 mals 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 adjec- 

 tive in signification if not in form, constitutes the scientific 

 name of the species. Thus Petromyzon is the genus of the com- 

 mon 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 ani- 

 mals 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 descrip- 

 tion, 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. 



