138 ESSAY ON CLASSIFICATION 



be carried out shall at once be made apparent. The range and char- 

 acter of the general homologies of each type should also be illustrated, 

 as well as the general conditions of existence of its representatives. 

 In characterizing 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 im- 

 portant 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. 



