As we do not know the source or nature of the stimulus

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responsible for cell division we cannot give a molecular account of death in the higher organisms.  However we shall now see that, philosophically, we are entitled to consider reproduction as a saving factor in this case also; and to regard the death of the individual much as we regard the fall of the leaf from the tree:  i.e. as the cessation of an outgrowth from a development extending from the past into the future.  The phenomena of old age and natural death are, in short, not at variance with the progressive activity of the organism.  We perceive this when we come to consider death from the evolutionary point of view.

Professor Weismann, in his two essays, “The Duration of Life,” and “Life and Death,"[1] adopts and defends the view that “death is not a primary necessity but that it has been secondarily acquired by adaptation.”  The cell was not inherently limited in its number of cell-generations.  The low unicellular organisms are potentially immortal, the higher multicellular forms with well-differentiated organs contain the germs of death within themselves.

He finds the necessity of death in its utility to the species.  Long life is a useless luxury.  Early and abundant reproduction is best for the species.  An immortal individual would gradually become injured and would be valueless or even harmful to the species by taking the place of those that are sound.  Hence natural selection will shorten life.

[1] See his Biological Memoirs. Oxford, 1888.

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Weismann contends against the transmission of acquired characters as being unproved.[1] He bases the appearance of death on variations in the reproductive cells, encouraged by the ceaseless action of natural selection, which led to a differentiation into perishable somatic cells and immortal reproductive cells.  The time-limit of any particular organism ultimately depends upon the number of somatic cell-generations and the duration of each generation.  These quantities are “predestined in the germ itself” which gives rise to each individual.  “The existence of immortal metazoan organisms is conceivable,” but their capacity for existence is influenced by conditions of the external world; this renders necessary the process of adaptation.  In fact, in the differentiation of somatic from reproductive cells, material was provided upon which natural selection could operate to shorten or to lengthen the life of the individual in accordance with the needs of the species.  The soma is in a sense “a secondary appendage of the real bearer of life—­the reproductive cells.”  The somatic cells probably lost their immortal qualities, on this immortality becoming useless to the species.  Their mortality may have been a mere consequence of their differentiation (loc. cit., p. 140), itself due to natural selection.  “Natural death was not,” in fact, “introduced from absolute intrinsic necessity inherent in the nature of living matter, but on grounds of utility,