188 THE NATURE AND ORIGIN OF LIFE 



and in man the nervous system is the most wonderful part 

 of his mechanism ; and so a few words have to be given 

 to the consequences which flow from the existence of large 

 nerve centres. 



An influx arriving at the body of a neuron from a surface 

 cell spreads through the neuron according to the actual 

 distribution of the colloid states of its protoplasm. It is 

 propagated, therefore, with variable intensity in the different 

 prolongations of the neuron hairs. Thence it follows the 

 points of least resistance and passes into the other neigh- 

 bouring neurons to be distributed in their bodies in con- 

 formity with the actual distribution of the colloid states of 

 their protoplasm and so on. To be able to foresee the 

 real distribution of a nerve influx in a centre composed of a 

 great number of neurons, we should need an exact know- 

 ledge of the actual colloid state of the whole centre and of 

 the relations of continuity or contiguity existing between 

 the neighbouring neurons at the given moment. Now, all 

 this varies every instant. There is nothing constant either 

 in the relation of neighbouring neuron hairs or in the nature 

 of the colloid equilibrium of each one of them. In fact, 

 the previous influx modified everything where it passed 

 and it will be the same for the following. 



It is, therefore, impossible for an outside observer, when 

 he determines in a higher animal a certain stimulus, to 

 foresee how the animal will react to the stimulus. He 

 knows that a nerve influx will be produced, but he cannot 

 guess how the influx will be distributed in the centres and 

 what motor apparatus it will reach and finally set in motion. 



The outside observer's sole possibility of foreseeing is 

 based on the law of habit. If he has remarked that, in 

 given conditions, an animal often reacts in the same way 

 to a certain stimulus, he is obliged to think that, by func- 



