224 ESSAY ON CLASSIFICATION. 



in their class, and assign to the latter a higher rank than 

 to the former. 



It follows, then, from the preceding remarks, that classes 

 are to be distinguished by the manner in which the plan 

 of their type is executed, by the ways and means by which 

 this is done, or, in other words, by the combinations of 

 their structural elements ; that is to say, by the combina- 

 tions of the different systems of organs building up the 

 body of their representatives. We need not consider here 

 the various forms under which the structure is embodied, 

 nor the ultimate details, nor the last finish which this 

 structure may exhibit, as a moment's reflection will con- 

 vince any one that neither form nor structural details can 

 ever be characteristic of classes. 



There is another point to which I would call attention, 

 respecting the characteristics of classes. These great di- 

 visions, so important in the study of the animal kingdom, 

 that a knowledge of their essential features is rightly con- 

 sidered as the primary object of all investigations in com- 

 parative anatomy, are generally represented as each exhi- 

 biting some essential modification of the type to which they 

 belong. This view, again, I consider to be a mistaken 

 appreciation of the facts, to which Cuvier has already 

 called attention, though his warning has remained un- 

 noticed. 1 There is, in reality, no difference in the plan of 

 animals beloneino- to different classes of the same branch. 



o o 



The plan of structure of the Polyps is no more a modifica- 

 tion of that of the Acalephs, than that of the Acalephs or 

 Echinoderms is a modification of the plan of the Polyps ; 

 the plan is exactly the same in all three. It may be re- 

 presented by one single diagram, and may be expressed 

 in one single word, radiation ; it is the manifestation of one 



1 CUVIER, Regn. An., 2d edit, p. 48. 



