THE NERVOUS SYSTEM 311 



indicate their existence in the brain. But surely the only conclusion 

 to be drawn from this, is the extreme fineness of the marks, and the 

 Umitations of our own faculties. Will any one say that nothing 

 exists but what man can perceive ? It is enough for us that 

 memory is a certain testimony of the existence of these impressions 

 in the brain. 



If it is true that nature does nothing suddenly or at a single swoop, 

 she must have created successively all the organs which give rise to the 

 faculties observed in the most perfect animals ; and this is just what 

 she has done, with the help of time and favourable conditions. 



This assuredly has been her procedure, and we cannot substitute 

 any other for it \mless we abandon the positive ideas that we derive 

 from the observation of nature. 



Thus in the animal organisation, the nervous system was created 

 in its turn Uke the other special systems, and this can only have occurred 

 when the organisation was sufficiently developed for the three sorts 

 of substances composing this system to have been formed and 

 deposited in their proper situations. 



It is therefore absurd to expect to find this system with its 

 dependent faculties in animals so simply organised and so imperfect 

 as the infusorians and polyps ; for it is impossible that such complex 

 organs could exist in these creatures. 



Let me repeat that just as the special organs in animals were formed 

 one after the other, so too each of them was gradually compounded, 

 completed and perfected in correspondence with the increasing com- 

 plexity of organisation ; so that the nervous system presents in different 

 animals the three following principal stages. 



At its origin, when it is in its highest imperfection, the system 

 appears to consist merely of various separate gangha, which have 

 communicating threads and from which issue other threads to certain 

 parts of the body : it then shows no brain and cannot give rise either 

 to sight, hearing, or possibly any true sensation ; but it already 

 possesses the faculty of exciting muscular movement. Such apparently 

 is the nervous system of the radiarians, if there is any truth in the 

 observations cited in Part I. of this work (Chapter VIII., p. 138). 



At its next stage, the nervous system presents a gangUonic longitudi- 

 nal cord and nervous threads which terminate in the ganglia of that 

 cord : henceforward the gangUon at the anterior extremity of the cord 

 may be regarded as a rudimentary brain, since it gives rise to the organ 

 of sight and subsequently to that of hearing ; but this small brain is 

 still simple and has no hypocephalon or wrinkled hemispheres with 

 special functions. Such is the nervous system of insects, arachnids, 

 and crustaceans,— animals which have eyes and in the latter case even 



