THE NERVOUS SYSTEM 303 



reacts and that this reaction gives birth to a perception which there- 

 after becomes an idea. 



This does not appear to me to be based on a study of the faculties 

 of the cerebral pulp ; and I cannot convince myself that so soft a 

 substance is really active, or that it can truly be said to react and 

 give rise to perception when agitated by the movements transmitted 

 to it. 



This mistake arises, in the first place, from the fact that Cabanis 

 took no note of the nervous fluid, and was obhged in his mind to 

 attribute the functions of that fluid to the nervous tissue in which it 

 moves ; and in the second place, from the fact that he confused 

 sensations with intelUgence, whereas the nature of these two organic 

 phenomena is essentially different, and demands in each case an 

 individual system of organs for its production. 



Thus there are four very different kinds of functions carried on by 

 the perfected nervous system, that is, when it is completely developed 

 and provided with its accessory organ ; but seeing that the organs 

 which give rise to these various functions are not the same, and seeing 

 that they have only come into existence successively, nature formed 

 those which are adapted to muscular movement before those which 

 give rise to sensation, and these latter before setting up the means 

 for producing emotions of the inner feeling ; she at length completed 

 the perfection of the nervous system by making it capable of produc- 

 ing the phenomena of intelUgence. 



We shall now see that all animals neither have nor can have a nervous 

 system, and, moreover, that those which possess this system do not 

 necessarily derive from it the four kinds of faculties named. 



The Nervous System is limited to certain Aijimals. 



Doubtless it is only in animals that the nervous system can exist ; 

 but does it follow thence that they must all possess it ? There are 

 certainly many animals whose organisation is such that they could 

 not possibly have this system of organs ; for the system consists 

 necessarily of two kinds of parts, viz. : a main medullary mass, and 

 various nervous threads which unite with it. Now this cannot exist 

 in the elementary organisations of a great many known animals. 

 It is obvious, moreover, that the nervous system is not essential to Ufe, 

 since aU Uving bodies do not possess it, and it would be vainly sought 

 among plants. This system, then, can only have become necessary 

 to those animals in which nature was able to estabUsh it. 



In Chapter IX. of Part II., p. 273, I have abready shown that the 

 nervous system is pecuUar to certain animals : I shall now give a 

 further proof of it by showing the impossibihty that all animals should 



