INTRODUCTION. 



§ I. General Definitions. 



It will tend to a better understanding of the sub- 

 ject of this treatise to begin the volume with some 

 general definitions respecting the classification and 

 structure of birds, and some explanation of the techni- 

 cal terms used in describing them. 



According to Agassiz's principles of classification, 

 the animal kingdom may be arranged in the following 

 divisions : — 



Branches : Characterized by the general plan of 

 structure. 



Classes : By the manner in which that plan is exe- 

 cuted. 



Orders : By the complication of structure. 



Families: By form. 



Genera : By details of execution in special parts. 



Species : By the relation of individuals to each other 

 and to their conditions of environment. 



Each of these groups may be again divided into in- 

 termediate ones, distinguished by the prefix sub-. The 

 main divisions of classes, orders, families, genera, and 

 species, are in practice still recognized by all natural- 

 ists, however differently they interpret the significance 



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