CHAPTER V. 



OF THE FORCE WHICH PRODUCES THE ACTIONS OF ANIMALS, 

 AND OF CERTAIN PECULIAR FACTS RESULTING FROM THE 

 USE OF THIS FORCE. 



Animals, independently of their organic movements and of the 

 functions essential to life, carry out yet other movements and 

 actions of which it is very important to ascertain the cause. 



We know that plants can satisfy their needs without changing their 

 position or making any sudden movement : the reason of this is that 

 every plant in a favourable situation finds in its neighbourhood the 

 substances which it requires for food ; so that it only has to absorb 

 them and profit by some among them. 



The case is different with animal.^ : for, with the exception of 

 the most imperfect animals at the beginning of the animal chain, 

 the food on which they live is not always at hand, and they are obliged 

 to carry out movements and actions in order to procure it. Moreover, 

 most of them have other wants to be satisfied as well, which in their 

 turn demand other movements and other actions. 



Now the question at issue was how to identify the source from which 

 animals derive the faculty of making comparatively sudden movements 

 of their parts ; — in short, of carrying out the various activities by means 

 of which they satisfy their needs. 



I noted, in the first place, that every action is a movement, and that 

 every movement necessarily has some cause which produces it : the 

 question was therefore reduced to ascertaining the nature and origin 

 of this cause. 



Thereupon, when I reflected that the movements of animals are 

 never communicated nor transmitted, but merely exited, their cause 

 appeared to be revealed to me in the clearest and most decisive manner ; 

 and I became convinced that these movements are really in every case 

 the product of some power which excites them. 



Indeed in certain animals, muscular activity is a force fully adequate 



