CHAPTER II 



PRINCIPLES OF CLASSIFICATION 



14. Classification. It is plain that natural relations of 

 some sort exist among living organisms. A dog is more 

 like a cat than it is like a sheep. A dog is more like a 

 sheep than either is like a butterfly. The very existence 

 of such terms as animals and plants, insects and fishes, 

 implies various grades of relationship. Classification is the 

 process of reducing our knowledge of these grades of like- 

 ness and unlikeness to a system. By bringing together 

 those which are alike, and separating those which are 

 unlike, we find that these rest on fixed and inevitable laws. 

 Classification is thus defined as " the rational, lawful dis- 

 position of observed facts." 



15. Homology, All rational classification of plants or 

 animals concerns itself with homologies. Homology means 

 fundamental identity of structure, as distinguished from 

 analogy, which means incidental resemblance in form or 

 function. Thus the arm of a man is homologous with the 

 fore leg of a dog, because in either we can trace throughout 

 deep-seated resemblances or homologies with the other. 

 In every bone, muscle, vein, or nerve the one corresponds 

 closely with the other. The " limb " of a tree, the " arm " 

 of a starfish, or the fore leg of a grasshopper shows no such 

 correspondence. In a natural classification, or one founded 

 on fact, those organisms showing closest homologies are 

 placed together. An artificial classification is one based on 

 analogies. Such a classification would place together a 



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