Natural History of the United States. 243 



graphic nerve and contractile muscle"; and these are produced 

 and maintained to subserve its psychical endowments of sensation 

 and voluntary motion. 



This being the general and essential nature of the animal, type 

 or pattern may be discovered in any one of these three leading 

 peculiarities : in the psychical nature of the animal; in the arrange- 

 ment of its nerve and muscle, or, subordinate to them, in the ar- 

 rangement of the hard parts which protect the former or serve 

 for the attachment of the latter ; or lastly, in that of the apparatus 

 employed for nutrition, respiration and circulation. It must hap- 

 pen, that, to a certain extent, these will agree as grounds of ar- 

 rangement. Thus if the nerve matter be arranged on a given 

 plan, this must indicate something corresponding in the psychical 

 endowments, and may probably require something corresponding 

 in the apparatus for motion, protection, and nutrition. Still, some 

 of these points may be more important than others. For instance,, 

 psychical characters, not being material, cannot be accurately 

 measured ; apparatus for nutrition has a broad similarity amount- 

 ing almost to general identity of plan, over the whole animal 

 kingdom, while again it is subject to modification in nearly re-' 

 lated species, intended to consume different articles of food. For 

 such reasons, when we study the types of animals, we prefer to 

 take as our chief guide that part of the physical structure which 

 is most independent of the accidents of outward relations, and 

 which is most nearly connected with the intelligence, which is the 

 essence of the animal. Hence Agassiz very justly traces the old 

 division of Aristotle into enaima and anaima, and that of La- 

 marck into vertebrata and invertebrata ; not so much to the per- 

 ceived difference in blood or skeleton, as to the perception, perhaps 

 unconsciously, that there is an essential difference between the 

 plans of structure in those animals that have the nervous matter 

 protected in separate cavities of skull and spinal column, and 

 those that have it confounded, as it were, with the organs of vege- 

 tative life. Hence also Cuvier, examining more minutely the 

 nature and value of these differences, proposed the four branches 

 of the Vertebrata, Mollusca, Articulata and Radiata, based on the 

 arrangement of the structures protective of the nervous matter 

 or subserving voluntary motion. Hence also Owen, penetrating 

 more deeply into the real philosophy of the subject, names these 

 branches from the arrangement of the nerve matter itself, Mye_ 

 lencephala, Heterogangliata, Homogangliata, and Nematoneura ;. 



