Gradation of the Vertebrata. 143 



nation of nervous action become predominant, the greater the 

 concentration, tlie more efficient the action. This co-ordination 

 will depend partly upon positional and partly upon commis- 

 sural relations. One element of position is degree of separation. 

 We should then, beforehand, expect to find those portions of 

 the nervous system which are devoted to local and, in a 

 measure, independent functions of the simple reflex nature 

 scattered more or less, their position being determined rather 

 by that of the organs whose action they control, than by their 

 relations to other parts of the nervous system ; while on the 

 other hand we should expect to find those parts whose office 

 is the organization of thought, feeling and co-ordinated motion, 

 or whose function is closely related to these, collected together, 

 forming a great nervous center, more and more concentrated 

 and compacted as the co-operation of parts becomes more 

 prompt and efficient. We find it to be so. It is but a com- 

 mon observation of naturalists, and to a greater or less extent 

 its gradational value has been recognized. 



The scattered situation of the sympathetic ganglia is marked 

 as well as their separation from the encephalic centres. So of 

 articulata, mollusca and radiata. The spinal cord, regarded 

 as a nervous centre and performing functions a gi-ade higher, 

 has a better relationship but not a close one. In the lowest 

 vertebrata the encephalic ganglia are markedly separate. As 

 we proceed upward the separation is less and less marked, till 

 in man aggregation reaches its highest state. 



The less the separation the higher the rank. 



4:th. Relative position of the parts. 



This is intimately connected with the preceding principle 

 and has the same general import, but yet is in a measure dis- 

 tinct from it. In the amphioxus the arrangement is purely 

 linear, and in all the cyclostomes there is little variation from 

 it. In the higher fishes there begins to appear what may be 

 termed a two-ranked arrangement, the cerebrum and cerebel- 



