BOOK V. 



SENESCENCE AND DEATH. 



Chap. I. The different points of view from which death may be 

 regarded. — Chap. II. Constitution of the organisms — 

 Partial death — Collective death. — Chap. III. Physical 

 and chemical characteristics of cellular death — Necrobiosis. 

 Chap. IV. Apparent perennity of complex individuals. — 

 Chap. V. Immortality of the protozoa and of slightly 

 differentiated cells. 



We grow old and we die. We see the beings 

 which surround us grow old and disappear. At first 

 we see no exceptions to this inexorable law, and we 

 consider it as a universal and inevitable law of 

 nature. But is this generalization well founded ? 

 Is it true that no being can escape the cruel fate of 

 old age and death, to which we and all the repre- 

 sentatives of the higher animality are exposed ? Or, 

 on the other hand, are any beings immortal ? Biology 

 answers that, in fact, some beings are immortal. 

 There are beings to whose life no law assigns a 

 limit, and they are the simplest, the least differentiated 

 and the least perfect. Death thus appears to be a 

 singular privilege attached to organic superiority, the 

 ransom paid for a masterly complexity. Above these 

 elementary, monocellular, undifferentiated beings, 

 which are protected from mortality, we find others, 

 higher in their organization, which are exposed to 



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