250 DEVELOPMENT AND HEREDITY. 



seems to be the state which the Hving body is con- 

 stantly approaching after it has finished its develop- 

 ment. The conditions of life, however, are too com- 

 plex to offer more than a general simile, but it is 

 enough to give an idea of the nature of life and 

 death. Development and life consist in internal 

 changes ; death is a cessation of change. In the 

 living body changes are most frequent and pro- 

 nounced in the earliest stages and gradually decrease 

 as life approaches its end. By the long continuation 

 of the same stimuli the reaction to them gradually 

 becomes less. This phenomenon is familiarly known 

 as the ability to grow accustomed to things. We 

 may become unmindful or even unconscious of im- 

 pressions which at first affected us either pleasantly 

 or painfully. When growth has ceased, new co-or- 

 dinations and new adaptations to external changes 

 are not easily formed. Also, as the energy of inter- 

 nal changes is lessened, the restorative and trophic 

 processes become weaker, and all the phenomena of 

 senescence follow, the earliest and strongest associa- 

 tions remaining to the last, — second childhood in 

 man. Finally the weakest wheel in the machine 

 breaks, and dissolution takes place. 



The causes of death, then, may be summed up as, 

 — first, the inability of the specialised body (soma) 

 of the protozoa to form new adaptive co-ordinations. 



