48 ZOOLOGICAL PHILOSOPHY 



(1) Some only move themselves or their parts when their irrita- 

 bility has been stimulated ; but they experience no feeling : these 

 are the most imperfect animals ; 



(2) Others, in addition to the movements that their parts can 

 undergo through stimulated irritability are capable of experiencing 

 sensations, and possess a very vague inner feeling of their existence ; 

 but they only act by the internal impulse of an inclination which 

 leads them towards some or other object ; so that their will is always 

 dependent and controlled ; 



(3) Others again not only exhibit in some of their parts movements 

 resulting from their stimulated irritability ; not only are they capable 

 of receiving sensations, and possess an inner feehng of their existence, 

 but they have besides the faculty of forming ideas, although confused 

 ones, and of acting by a free will, subject however to inclinations which 

 lead them exclusively towards certain special objects ; 



(4) Finally, others (and these are the most perfect) possess in a 

 high degree all the aforementioned faculties ; in addition they are 

 able to form clear and precise ideas of the objects which affect their 

 senses and attract their attention ; to compare and combine their 

 ideas up to a certain point ; to form judgments and complex ideas ; 

 in short to think, and to have a will that is less boimd down and permits 

 them to introduce more or less variation into their activities. 



Life in the most imperfect animals has no energy of movement ; 

 and irritabihty alone suffices for the execution of vital movements. 

 But since vital energy increases in proportion to complexity of organisa- 

 tion, there arrives a time when nature has to improve her methods 

 in order to provide for the necessary activity of vital movements ; 

 for this purpose she has utiUsed muscular activity in estabhshing the 

 circulatory system, so that the fluids can move with greater rapidity. 

 This rapidity itself is increased in proportion to the increase of the 

 muscular power which works it. Finally, since no muscular activity 

 can take place without nervous influence, this has become every- 

 where necessary for the acceleration of 1;he fluids in question. 



Thus nature has been able to add muscular activity and nervous 

 influence to an irritabihty which was no longer adequate. But this 

 nervous influence, which gives rise to muscular activity, never does 

 so by means of feehng, as I hope to show in Part II. I shall then 

 prove that sensibility is by no means necessary to the execution 



_vital movements, even in the most perfect animals. 



The various animals which exist are thus clearly distinguished ' 

 from one another, not only by pecuharities of external shape, con- 

 sistency of body, size, etc., but, in addition, by the faculties which 

 they possess. Some, such as the most imperfect, are extremely Umited 



