CHAPTER XVIII 

 CLASSIFICATION 



Phylogenetic trees. Species and their origin. Races, sub-species, and 

 varieties. Genera and sub-genera. Nomenclature. Classification of 

 Marsipobranchs, of Selachians, of Bony Fishes. 



Classification is the sorting of different kinds of individuals 

 into groups of varying size and importance, and, as has been 

 already pointed out, the classifications of most of the older 

 naturalists were necessarily of an artificial nature. They were 

 well aware of the existence of natural affinities, and of the fact 

 that individuals fall naturally into groups, which in turn may 

 be linked together into larger categories, and so on, but the 

 units of classification which they used remained convenient 

 pigeon-holes and little more. The doctrine of evolution has 

 changed all this, and the modern systematist endeavours to 

 arrange his animals into groups which are in accord with their 

 natural relationships. Such a classification may be visualised 

 as a tree, and the mere fact that living organisms can be 

 arranged in this manner provides striking evidence in support 

 of the view that they themselves have been produced by tree- 

 like evolution. The roots of such a genealogical or phylogenetic 

 tree are deeply buried far back in geological time, and in each 

 succeeding period of the earth's history its branches have 

 become more and more ramified. The existing fishes are 

 represented by the topmost and youngest twigs of the fish 

 "tree," and in spite of the knowledge of the past provided 

 by a study of the fossils, most of the branches connecting the 

 living twigs with the lower parts of the tree may be said to 

 have died out, leaving little, if any, trace of their former existence. 

 It is this dying away of the older parts of the tree, the connecting 

 links, as it were, that makes it possible to sort the living fishes 

 into groups, for if it were possible to examine at one time all 

 the individuals, both past and present, existing and extinct, 

 each would be found to be linked up with the others by a com- 

 plete series of small gradations. In the accompanying diagram 

 of a typical "tree" the continuous black lines represent existing 

 species, the dotted lines extinct stems and unsuccessful branches 



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