THE MEANING OF EVOLUTION 281 



consciousness waxes and wanes with our activities. 

 It is absent in normal sleep, when bodily activity in 

 the real sense ceases almost absolutely, when the 

 cerebral cortex becomes inactive, and when the only 

 movements performed are those truly automatic ones 

 of parts of the body which are analogous to the move- 

 ments of the plant organism. Such movements are the 

 rhythmic ones of the heart and lungs, the movements of 

 the blood, and so on, in general the movements leading 

 to constructive metabolism. Consciousness is most 

 intense in difficult unfamiliar actions : the lad learning 

 to row ; the child learning scales on the piano, or the 

 fingering of the violin ; the engineer assembling to- 

 gether the parts of a nevv^ machine ; or the artist 

 engaged on a picture. In each of these cases the worker 

 is acutely conscious, in a deliberative manner, of his 

 own bodily actions. But with the habitual exercise 

 of these movements, and with the ease and facility 

 with which they are performed, consciousness that they 

 are being performed fades towards nothingness. 



What does this mean but that degrees of conscious- 

 ness are parallel to degrees of complexity of deliberated 

 and purposeful bodily movements or actions ? Or 

 degrees of consciousness are also parallel to the attempt 

 of the organism to perform these actions. What is 

 pain, the most acutely felt of all our mental states ? 

 It is, Bergson says, the consciousness of the persistent 

 and unsuccessful effort of the tissues to respond purpose- 

 fully to a persistently renewed stimulus. But complex 

 actions require for their performance systems of 

 skeletal and muscular parts capable of moving in the 

 m^ost varied ways, and a system of afferent and efferent 

 nerves with all their connections in the central nervous 

 system : that is, a sensori-motor system. Therefore 

 just as the sensori-motor system is more or less 



