206 ESSAY ON CLASSIFICATION. 



therefore, be fully expressed in a natural classification ; 

 and, though structure furnishes the most direct indication 

 of some of these relations, always appreciable under every 

 circumstance, other considerations should not be neglected, 

 which may complete our insight into the general plan of 

 creation. 



In characterizing the great branches of the animal 

 kingdom, it is not enough to indicate the plan of their 

 structure in all its peculiarities ; there are possibilities of 

 execution which are at once suggested, to the exclusion of 

 others, and which should also be considered, and so fully 

 analyzed, that the various modes in which such a plan 

 may be carried out shall at once be made apparent. The 

 range and character of the general homologies of each 

 type should also be illustrated, as well as the general con- 

 ditions of existence of its representatives. In character- 

 izing classes, it ought to be shown why such groups con- 

 stitute a class, and not merely an order or a family ; and, 

 to do this satisfactorily, it is indispensable to trace the 

 special homologies of all the systems of organs which are 

 developed in them. It is not less important to ascertain 

 the foundation of all the subordinate divisions of each 

 class ; to know how they differ, what constitutes orders, 

 what families, what genera, and upon what characteristics 

 species are based in every natural division. This we shall 

 examine in the next chapter. 



