312 ZOOLOGICAL PHILOSOPHY 



some traces of hearing : such again is the nervous system of annelids 

 and cirrhipedes, some of which possess eyes, while others are destitute 

 of them for the reasons named in Chapter VII. of Part I. 



The molluscs, although having a higher organisation than the 

 animals just mentioned, are in the midst of a change of plan on nature's 

 part, and have no ganghonic longitudinal cord nor spinal cord ; but 

 they have a brain and some of them seem to possess the most perfect 

 of the simple brains, that is, of brains without a hypocephalon : since 

 the nerves of several special senses terminate in them. If this is the 

 case, then the nervous system produces muscular movement and feeHng 

 in all animals from the insects to the molluscs inclusive, but it does not 

 permit of the formation of ideas. 



Lastly, at a far more perfect stage, the nervous system of vertebrates 

 presents a spinal cord, nerves and a brain, of which the superior and 

 anterior part is provided with two accessory wrinkled hemispheres 

 whose development is proportional to the stage of progress of the new 

 plan. This system then gives rise not only to muscular movement, 

 feehng and inner emotions, but also to the formation of ideas, the 

 clearness and number of which are proportional to the development 

 of the hemispheres. 



How can any one suppose that nature, who in all her productions 

 invariably proceeds by gradual stages, could have endowed a nervous 

 system at once with all the faculties which it possesses when it has 

 attained its ultimate completion and perfection ? 



Moreover, since the faculty of feeling is not the property of any 

 substance of the body, we shall see that the mechanism necessary for 

 its production is so complex that the nervous system in its extreme 

 simphcity coidd have had no other faculty than that of exciting 

 muscular movement. 



I shall endeavour to ascertain in Chapter IV. what is the power that 

 causes and directs the emissions of nervous fluid to the hemispheres 

 or other parts of the body. I shall merely say here that the dispatch 

 of this fluid to the cerebral hemispheres arouses in them functions very 

 different from those aroused in the muscles and vital organs. 



I have now given a brief general accoimt of the nervous system, the 

 nature of its parts, the conditions that were required for its formation, 

 and the four kinds of functions that it fulfils when it has attained its 

 perfection. 



Without undertaking any enquiry as to how nervous influence may 

 set the muscles in action and cause the performance of their functions 

 by various organs, I may observe that the explanation is probably 

 to be found in a stimulus to the irritabiUty of the parts. 



But in the case of that function of the nervous system through which 



