46 BIOLOGY OF DEATH 



form its special function properly, it upsets the delicate 

 balance of the whole associated community; of cells and tis- 

 sues. Because of the differentiation and specialization of 

 function, the parts are mutually dependent upon each 

 other to keep themselves and the whole going. Conse- 

 quently any disturbance in the balance which is not 

 promptly righted by some regulatory process must even- 

 tually end in death. 



Since the publication of this material in serial form 

 an objection to the foregoing statement has been sug- 

 gested on the ground that differentiation per se does not 

 appear to the critic to have much to do with, the question of 

 natural death in the Metazoa. To quote ; ' i rather it is the 

 failure after differentiation to keep up indefinitely the 

 state reached. If, from any internal or external accident, 

 the differentiated part suffers injury, the injury cannot 

 be made good any more, since in certain organs this power 

 has been lost. Hence, in time, loss after loss occurs and 

 the machine wears out. The protozoan is as highly differ- 

 entiated as any cell of a metazoan (or much more so) ; but 

 since it "multiplies by dividing," it has retained the 

 power to make good any loss. Therefore, it is not the 

 differentiation per se, but the loss of power to repair 

 that produces senescence.' 



This seems to me to be in the main onlv a somewhat 



v 



different form of statement of precisely the idea that I 

 have endeavored to express. When I have used the term 

 "differentiation" in this connection, I have always had 

 in mind, as one of its most important physiological con- 

 comitants, just the thing spoken of above. Furthermore, 

 whether the protozoan cell is as highly differentiated as 

 a metazoan cell, is not to the point at all. For, to have 

 any pertinence so far as the present issue is concerned, 

 the comparison must be between the differentiated proto- 



