FACTS 159 



nal conditions capable of intervening in the determination 

 of A's activity. 



It is difficult enough to demonstrate this fact in an animal 

 so complex as man, whose mechanism as a whole comprises 

 several trillion cells co-ordinated by a highly perfected ner- 

 vous system. In him, as we have seen, there may exist 

 accumulated provisions of energy, either in the chemical 

 form of reserves or in the physical form of colloid tensions. 

 There are cases where it might be believed that he acted by 

 himself because those portions of the medium with which he 

 reacts are situated in his interior. But even then he still 

 borrows from the exterior. We shall never see a man func- 

 tioning without heat or oxygen. 



The study of the agents of movement is easier in unicellular 

 beings ; among the simplest of them the movement pro- 

 voked by an agent may have direct relation with the 

 situation of that agent. On the contrary, with man the 

 transformation in his brain of impressions received through 

 the sense organs prevents our verifying any relation what- 

 ever between the direction of the movement and its causes. 



As for that, the causes of movement in unicellular beings 

 are so numerous and varied that, without the help of experi- 

 mental methods, we should be tempted to believe in the 

 spontaneity of their displacements. When, with our eye at 

 a microscope, we observe a drop of an infusion of hay thick 

 with protozoa and verify that two near neighbours among 

 them are executing movements different from each other, 

 our first idea is that these living corpuscles have the total 

 cause of their activity within themselves. 



The heterogeneousness of the drop of hay infusion is 

 not apparent to the observer at the microscope. In the 

 transparent liquid we do not remark the variations in 

 oxygen, in salts of every kind, not to speak of local modifi- 



