THE NERVOUS SYSTEM 305 



never acted suddenly or by a single leap, but has always worked by 

 degrees towards a gradual and imperceptible development : conse- 

 quently all her products and transformations are everywhere clearly 

 subject to this law of progress. 



If we follow the operations of nature, we shall indeed see that she 

 created by successive stages all the tissues and organs of animals, 

 that she gradually brought them to completion and perfection, and that 

 in the same way by slow degrees she modified, animalised, and com- 

 pounded all the internal fluids of the animals she had brought into 

 existence ; so that in course of time they were brought to the condition 

 in which we now see them. 



The nervous system at its origin is assuredly in its greatest simplicity 

 and least perfection. This kind of origin is common to it, as to all 

 the other special organs, which also began in their most extreme state 

 of imperfection. Now it cannot be doubted that, in its greatest 

 simplicity, the nervous system gives to the animals possessing it less 

 numerous and lofty faculties than it bestows on the more perfect 

 animals, where it has reached its highest complexity and acquired 

 its accessories. We only have to observe the facts to recognise the 

 truth of this statement. 



I have already proved that when the nervous system is in its greatest 

 simplicity, it necessarily has two kinds of parts, viz. : a main medullary 

 mass and nervous threads which run into this mass ; but this same 

 medullary mass may at first exist without giving rise to any special 

 sense, and it may be divided into separate parts, to each of which 

 run nervous threads. 



Such appears to be the case in animals of the class of radiarians, 

 or at least in those of the division of echinoderms in which a nervous 

 system is supposed to have been discovered ; the system would be 

 reduced to separate ganglia, communicating together by threads and 

 sending out others to the parts. 



If the observations, which afiSrm this state of the nervous system, 

 are well-founded, we have here the system in its greatest simplicity. 

 It possesses several centres of communication for the nerves, that is 

 to say, as many nuclei as there are separate ganglia ; lastly, it does 

 not give rise to any of the special senses, not even to sight, which is 

 certainly the first to show itself unequivocally. 



By special senses I mean those which result from special organs 

 such as sight, hearing, smell, and taste ; as to touch, it is a general 

 sense, a type no doubt of all the rest, but needing no special organ and 

 incapable of being yielded by the nerves until they are competent to 

 produce sensations. 



When I come to describe in Chapter III. the mechanism of sensations, 



