80 HEREDITY AND ENVIRONMENT 



an interesting fact that in man, and in several 

 other animals which may be assumed to have 

 a sense of identity, the nerve cells, especially 

 those of the brain, cease dividing at an early 

 age, and these identical cells persist through- 

 out the remainder of life. If nerve cells con- 

 tinued to divide throughout life, as epithelial 

 cells do, there would be no such persistence 

 of identical cells, and one is free to speculate 

 that in such cases there would be no persis- 

 tence of the sense of identity. 



Organization includes both structure and 

 function, and continuity of organization im- 

 plies not only persistence of protoplasmic and 

 cellular structures but also persistence of the 

 functions of sensitivity, reflexes, memory, in- 

 stincts, intelligence, and will ; the continuity of 

 consciousness is associated with the continuity 

 of these activities, as well as with the struc- 

 tures of the body in general and of the brain in 

 particular. It is well known that things which 

 interrupt or destroy these functions or struc- 

 tures interrupt or destroy consciousness. 

 Lack of oxygen, anesthetics, normal sleep 

 cause in some way a temporary interruption 



