146 ESSAY ON CLASSIFICATION 



were supposed to agree in the plan of their structure with the true 

 Radiata that Cuvier placed them in that division, but simply because 

 he allowed himself to depart from his own principle and to add an- 

 other consideration, besides the plan of structure, as characteristic of 

 Radiata — the supposed absence of a nervous system and the great 

 simplicity of structure of these animals; — as if simplicity of execu- 

 tion had any necessary connection with the plan of structure. An- 

 other remarkable instance of the generally approved removal of a 

 class from one of the types of Cuvier to another was the transfer of 

 the Cirripeds from among the Mollusks to the branch of Articulata. 

 Imperfect knowledge of the plan of structure of these animals was 

 here the cause of the mistake, which was corrected without any op- 

 position as soon as they became better known. 



From a comparison of what is stated here respecting the different 

 plans of structure characteristic of the primary divisions of the ani- 

 mal kingdom with what I have to say below about classes and orders, 

 it will appear more fully, that it is important to make a distinction 

 between the plan of a structure and the manner in which that plan 

 is carried out, or the degrees of its complication and its relative per- 

 fection or simplicity. But even after it is understood that the plan of 

 structure should be the leading characteristic of these primary 

 groups, it does not yet follow, without further examination, that the 

 four great branches of the animal kingdom, first distinguished by 

 Cuvier, are to be considered as the primary divisions which Nature 

 points out as fundamental. It will still be necessary, by a careful and 

 thorough investigation of the subject, to ascertain what these primary 

 groups are; but we shall have gained one point with reference to our 

 systems — that whatever these primary groups, founded upon differ- 

 ent plans, which exist in nature may be, when they are once defined, 

 or whilst they are admitted as the temporary expression of our pres- 

 ent knowledge, they should be called the branches of the animal 

 kingdom, whether they be the Vertebrata, Articulata, Mollusca, and 

 Radiata of Cuvier, or the Artiozoaria, Actinozoaria, and Amorpho- 

 zoaria of Blainville, or the Vertebrata and Invertebrata of Lamarck. 

 The special inquiry into this point must be left for a special paper. 

 I will only add that I am daily more satisfied that in their general 

 outlines the primary divisions of Cuvier are true to nature, and that 

 never did a naturalist exhibit a clearer and deeper insight into the 



