CHAPTER XXI 

 CLASSIFICATION OF FISHES 



AXONOMY. Classification, as Dr. Elliott Coues has 

 well said,* is a natural function of " the mind which 

 always strives to make orderly disposition of its 

 knowledge and so to discover the reciprocal relations and 

 interdependencies of the things it knows. Classification pre- 

 supposes that there do exist such relations, according to 

 which we may arrange objects in the manner which facilitates 

 their comprehension, by bringing together what is like and 

 separating what is unlike, and that such relations are the 

 result of fixed inevitable law. It is therefore taxonomy (rocZts, 

 away ; rojuos, law) or the rational, lawful disposition of observed 

 facts." 



A perfect taxonomy is one which would perfectly express 

 all the facts in the evolution and development of the various 

 forms. It would recognize all the evidence from the three ances- 

 tral documents, palaeontology, morphology, and ontogeny. It 

 would consider structure and form independently of adaptive 

 or physiological or environmental modifications. It would 

 regard as most important those characters which had existed 

 longest unchanged in the history of the species or type. It 

 would regard as of first rank those characters which appear first 

 in the history of the embryo. It would regard as of minor 

 importance those which had arisen recently in response to 

 natural selection or the forced alteration through pressure of 

 environment, while fundamental alterations as they appear one 

 after another in geologic time would make the basal characters 

 of corresponding groups in taxonomy. In a perfect taxonomy 

 or natural system of classification animals would not be divided 

 into groups nor ranged in linear series. We should imagine 



* Key to North American Birds. 



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