THE PERIODICITY OF THE ORGANISM AND THE LAW OF PROGRESSIVE
ACTIVITY
The organic system is essentially unstable. Its aggressive
attitude is involved in the phenomenon of growth, and in
reproduction which is a form of growth. But the energy absorbed
is not only spent in growth. It partly goes, also, to make good
the decay which arises from the instability of the organic unit.
The cell is molecularly perishable. It possesses its entity much
as a top keeps erect, by the continual inflow of energy.
Metabolism is always taking place within it. Any other condition
would, probably, involve the difficulties of perpetual motion.
The phenomenon of old age is not evident in the case of the
unicellular organism reproducing by fission. At any stage of its
history all the individuals are of the same age: all contain a
like portion of the original cell, so far as this can be regarded
as persisting where there is continual flux of matter and energy.
In the higher organisms death is universally evident. Why is
this?
The question is one of great complexity. Considered from the more
fundamental molecular point of view we should perhaps look to
failure of the power of cell division as the condition of
mortality. For it is to this phenomenon—that of cell
division—that the continued life of the protozoon is to be
ascribed, as we have already seen. Reproduction is, in fact, the
saving factor here.
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.