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 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 representatives 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