54 Heredity and Environment 



tors, which by a long series of combinations and transformations, 

 due to interactions with one another and with the environment, 

 give rise to the fully developed condition. 



Continuity of Consciousness. Finally there seems good rea- 

 son for believing that the continuity of consciousness, the con- 

 tinuing sense of identity, is associated with the continuity of or- 

 ganization, for in spite of frequent changes of the materials of 

 which we are composed our sense of identity remains undis- 

 turbed. However, the continuity of protoplasmic and cellular 

 organization generally remains undisturbed throughout life, and 

 the continuity of consciousness is associated with this continuity 

 of organization, especially in certain parts of the brain. It is an 

 interesting fact that in man, and in several other animals which 

 may be assumed to have a sense of identity, .the nerve cells, espe- 

 cially those of the brain, cease dividing at an early age, and these 

 identical cells persist throughout the remainder of life. If nerve 

 cells continued 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 persistence 

 of the sense of identity. 



Organization includes both structure and function, and con- 

 tinuity of organization implies not only persistence of protoplas- 

 mic and cellular structures but also persistence of the functions 

 of sensitivity, reflexes, memory, instincts, intelligence, and will ; 

 the continuity of consciousness is associated with the continuity 

 of these activities as well as with the structures of -the body in gen- 

 eral and of the brain in particular. It is well known that things 

 which interrupt or destroy these functions or structures interrupt 

 or destroy consciousness. Lack of oxygen, anesthetics, normal 

 sleep cause in some way a temporary interruption of these func- 

 tions and consequently temporary loss of consciousness; while 

 certain injuries or diseases of the brain which bring about the 

 destruction of certain centers or association tracts may cause 

 permanent loss of consciousness. 



