CHAPTER II 



THE GENERAL PRINCIPLES OF ZOOLOGICAL 



CLASSIFICATION 



In classifying plants and animals, as in the classification of 

 any objects, we group together those which resemble one another 

 in one or more respects ; but we must be careful to select as a 

 basis for our grouping characteristics which have a real, funda- 

 mental, and not merely a superficial, significance. Thus two 

 animals, a shark and a whale, for example, may both inhabit the 

 water, but their structure, method of development, and food 

 may differ widely ; the habitat, then, would be, not a natural but 

 an artificial basis for classification. All biological classification 

 is based upon structure and development and at times on func- 

 tion as well ; in many cases the structure of the adult plant or 

 animal is sufficient to show its relationship to other plants and 

 animals ; in some instances it is necessary to consider the devel- 

 opment also, and at times the latter forms the only criterion. 

 Thus our present system of classification rests upon anatomy 

 and embryology, the sciences of structure and development; but 

 even then there must necessarily be some difference of opinion 

 among scientists as to the amount of importance to be attached 

 to the various morphological details, i.e. the details of form in 

 its broadest sense ; furthermore, our knowledge of the structure 

 and development of organisms is constantly increasing, new 

 facts are being discovered, and hence it is not surprising that 

 our systems of classification are undergoing continual changes, 

 and that there is much diversity of opinion among specialists on 

 this subject. For the present there can be, therefore, no fixed, 

 final classification of plants and animals ; but our knowledge of 

 the biological sciences, the sciences of life, is now sufficiently 

 broad so that further changes are not likely to affect the larger 



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