306 ZOOLOGICAL PHILOSOPHY 



we shall see that none of them can be produced except when the whole 

 animal shares in the general effect, by reason of the complexity of its 

 nervous system and of the single common nucleus for the nerves. 

 If this is the case, it follows that in animals with the most elementary 

 nervous systems, where there are different nuclei for the nerves, no 

 effect or agitation can become generalised through the individuals, 

 no sensation can be produced, nor can the separate medullary masses 

 give rise to any special sense. If these separate medullary masses 

 communicate together by threads, it is in order to secure the free 

 distribution of the nervous fluid within them. 



Yet as soon as the nervous system exists, however simple it may 

 be, it must be capable of performing some function ; we may therefore 

 hold that it has an effective action, even when it cannot yet give rise 

 to feeling. 



If we reflect that, for the excitation of muscular movement, which is 

 the least of the faculties of the nervous system, a lower degree of 

 complexity and less extent of its parts are required than for the pro- 

 duction of feeling, and moreover that separate centres of communica- 

 tion are no bar to the nervous fluid conveying its influence to the 

 muscles from the individual nuclei, it will then appear very probable 

 that the animals with the simplest nervous systems derive from it the 

 faculty of muscular movement, while yet being destitute of feeling. 



Thus on starting the nervous system, nature appears to have formed 

 at first only separate gangUa, communicating together by threads and 

 dispatching other threads to the muscular organs. These ganglia are the 

 main medullary masses ; and although they communicate by threads, 

 the separation of the nuclei prohibits the general effect necessary for 

 constituting sensation, though it is not opposed to the excitation of 

 muscular movement : hence the animals which possess such a nervous 

 system are devoid of any special sense. 



Having now seen that the nervous system in its extreme simplicity 

 can only produce muscular movement, we shall go on to show that 

 when nature has developed, compounded, and further perfected this 

 system, she proceeded to endow it not only with the faculty of exciting 

 muscular action but also with that of producing feeling. 



The Nervous System on reaching a Higher Complexity pro- 

 duces BOTH Muscular Movement and Feeling. 



Of all the systems of organs the nervous system is doubtless that 

 which confers upon animals the most lofty and the most marvellous 

 faculties ; but unquestionably it only reaches this point after having 

 acquired its highest possible complexity and development. Prior to 

 this stage, the animals which have nerves and a main medullary 



