136 ESSAY ON CLASSIFICATION. Part 1. 



deliberately. In the beginning of this chapter, I have already stated that Classi- 

 fication seems to me to rest upon too narrow a foundation when it is chiefly based 

 upon structure. Animals are linked together as closely by their mode of develop- 

 ment, by their relative standing in their resjDective classes, by the order in which 

 they have made their ajjpearance upon earth, by their geographical distribution, and 

 generally by their connection with the world in which they hve, as by their 

 anatomy. All these relations should, 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 

 shoidd not be neglected, wliich 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 pecuharities ; there are possibilities 

 of execution wliich 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 conditions of existence of its representatives. In characterizing 

 classes, it ought to be shown why such groups constitute 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 famiUes, what 

 genera, and upon what characteristics species are based in every natural division. 

 This we shall examine in the next chapter. 



