ACTIONS OF ANEVIALS 349 



of an invisible fluid, which we cannot perceive owing to the limita- 

 tion of our available resources ? 



The consumption and exhaustion of the subtle fluid of the nerves 

 in prolonged or painful actions, is certainly a fact that can never be 

 disputed, since reason and organic phenomena afford the strongest 

 proof of it. 



Although it is true that one part of an animal's nervous fluid is 

 constantly employed, independently of its will, in the maintenance of 

 its vital movements and of such functions as are necessary to its 

 existence, yet when the individual uses up too much of the free fluid 

 available to it, the functions of its vital organs suffer. For in these 

 circumstances the unavailable part of the nervous fluid fills the place 

 of the available fluid that has been dissipated. It is therefore run too 

 low, and provides inadequately for the operations of the vital organs ; 

 thereupon the functions of these organs flag to some extent, and are 

 imperfectly fulfilled. 



It is chiefly man — who is aUied to animals by his organisation — 

 that uses up his physical strength in this way ; for, of all his actions, 

 those that use the most nervous fluid are the prolonged acts of his 

 understanding, thoughts, meditations, and, in short, the continued 

 efforts of his intellect. The digestion is then impaired, and the physical 

 strength is proportionally diminished. 



The consumption of nervous fluid in the movements and actions 

 of animals is so well known that I need not dwell upon it longer ; 

 but I may say that this by itself would be a sufi&cient proof of the 

 existence of the fluid in the most perfect animals, if there were not 

 many others of equal cogency. 



Of the Origin of the Propensity towards Repeating the Same 

 Actions, and also of Instinct in Animals. 



It is assuredly worth while to investigate the cause of the well- 

 known phenomenon which compels nearly all animals always to repeat 

 the same actions, and which gives rise even in man to a propensity 

 towards any action that has become habitual. 



If the principles expounded in this work are really well-founded, the 

 causes which we seek may be very easily deduced from them ; so 

 that we shall cease to be astonished by phenomena seemingly so 

 mysterious, when once we have recognised the simphcity of their 



origin. 



Let us see, then, how we can account for the phenomena in question, 

 in accordance with the principles enunciated above. 



Every action is caused by some movement in the fluid of the nerves. 

 Now when this action has been several times repeated there is no doubt 



