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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 ent.i.tled 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 quant.i.ties 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,

[1] Biological Memoirs, p. 142.

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that is from necessities which sprang up, not from the general conditions of life, but from those special conditions which dominate the life of multicellular organisms."

On the inherent immortality of life, Weismann finally states: "Reproduction is, in truth, an essential attribute of living matter, just as the growth which gives rise to it.... Life is continuous, and not periodically interrupted: ever since its first appearance upon the Earth in the lowest organism, it has continued without break; the forms in which it is manifest have alone undergone change. Every individual alive today--even the highest--is to be derived in an unbroken line from the first and lowest forms." [1]

At the present day the view is very prevalent that the soma of higher organisms is, in a sense, but the carrier for a period of the immortal reproductive cells (Ray Lankester)[2]--an appendage due to adaptation, concerned in their supply, protection, and transmission. And whether we regard the time-limit of its functions as due to external constraints, recurrently acting till their effects become hereditary, or to variations more directly of internal origin, encouraged by natural selection, we see in old age and death phenomena ultimately brought about in obedience to the action of an environment. These are not inherent in the properties of living matter. But, in spite

[1] Loc. cit., p. 159

[2] Geddes and Thomson, The Evolution of s.e.x, chap. xviii.

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of its mortality, the body remains a striking manifestation of the progressiveness of the organism, for to this it must be ascribed. To it energy is available which is denied to the protozoon. Ingenious adaptations to environment are more especially its privilege. A higher manifestation, however, was possible, and was found in the development of mind. This, too, is a servant of the cell, as the genii of the lamp. Through it energy is available which is denied to the body. This is the masterpiece of the cell. Its activity dates, as it were, but from yesterday, and today it inherits the most diverse energies of the Earth.

Taking this view of organic succession, we may liken the individual to a particle vibrating for a moment and then coming to rest, but sweeping out in its motion one wave in the continuous organic vibration travelling from the past into the future. But as this vibration is one spreading with increased energy from each vibrating particle, its propagation involves a continual accelerated inflow of energy from the surrounding medium, a dynamic condition unknown in periodic effects transmitted by inanimate actions, and, indeed, marking the fundamental difference between the dynamic att.i.tudes of the animate and inanimate.

We can trace the periodic succession of individuals on a diagram of activity with some advantage. Considering, first, the case of the unicellular organism reproducing by subdivision and recalling that conditions, definite and inevitable, oppose a limit to the rate of growth, or, for our

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present purpose, rate of consumption of energy, we proceed as follows:

{Fig. 1}

Along a horizontal axis units of time are measured; along a vertical axis units of energy. Then the life-history of the amoeba, for example, appears as a line such as A in Fig. 1.

During the earlier stages of its growth the rate of absorption of energy is small; so that in the unit interval of time, t, the small quant.i.ty of energy, e1, is absorbed. As life advances, the activity of the organism augments, till finally this rate attains a maximum, when e2 units of energy are consumed in the unit of time.[1]

[1] Reference to p. 76, where the organic system is treated as purely mechanical, may help readers to understand what is involved in this curve. The solar engine may, unquestionably, have its activity defined by such a curve. The organism is, indeed, more complex; but neither this fact nor our ignorance of its mechanism, affects the principles which justify the diagram.

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On this diagram reproduction, on the part of the organism, is represented by a line which repeats the curvature of the parent organism originating at such a point as P in the path of the latter, when the rate of consumption of energy has become constant. The organism A has now ceased to act as a unit. The products of fission each carry on the vital development of

{Fig. 2}

the species along the curve B, which may be numbered (2), to signify that it represents the activity of two individuals, and so on, the numbering advancing in geometrical progression. The particular curvature adopted in the diagram is, of course, imaginary; but it is not of an indeterminate nature. Its course for any species is a characteristic of fundamental physical importance, regarding the part played in nature by the particular organism.

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In Fig. 2 is represented the path of a primitive multicellular organism before the effects of compet.i.tion produced or fostered its mortality. The lettering of Fig. 1 applies; the successive reproductive acts are marked P1, P2; Q1, Q2, etc., in the paths of the successive individuals.

{Fig. 3}

The next figure (Fig. 3) diagrammatically ill.u.s.trates death in organic history. The path ever turns more and more from the axis of energy, till at length the point is reached when no more energy is available; a tangent to the curve at this point is at right angles to the axis of energy and parallel to the time axis.

The death point is reached, and however great a length we measure along the axis of time, no further consumption of energy is

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indicated by the path of the organism. Drawing the line beyond the death point is meaningless for our present purpose.

It is observable that while the progress of animate nature finds its representation on this diagram by lines sloping _upwards_ from left to right, the course of events in inanimate nature--for example, the history of the organic configuration after death, or

{Fig. 4}

the changes progressing--let us say, in the solar system, or in the process of a crystallisation, would appear as lines sloping downwards from left to right.

Whatever our views on the origin of death may be, we have to recognise a periodicity of functions in the life-history of the successive individuals of the present day; and whether or not we trace this directly or indirectly to

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a sort of interference with the rising wave of life, imposed by the activity of a series of derived units, each seeking energy, and in virtue of its adaptation each being more fitted to obtain it than its predecessor, or even leave the idea of interference out of account altogether in the origination or perpetuation of death, the truth of the diagram (Fig. 4) holds in so far as it may be supposed to graphically represent the dynamic history of the individual. The point chosen on the curve for the origination of a derived unit is only applicable to certain organisms, many reproducing at the very close of life. A chain of units are supposed here represented.[1]

THE LENGTH OF LIFE

If we lay out waves as above to a common scale of time for different species, the difference of longevity is shown in the greater or less number of vibrations executed in a given time, _i.e._ in greater or less "frequency." We cannot indeed draw the curvature correctly, for this would necessitate a knowledge which we have not of the activity of the organism at different periods of its life-history, and so neither can we plot the direction of the organic line of propagation with respect to the

[1] Projecting upon the axes of time and energy any one complete vibration, as in Fig. 4, the total energy consumed by the organism during life is the length E on the axis of energy, and its period of life is the length T on the time-axis. The mean activity is the quotient E/T.

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axes of reference as this involves a knowledge of the mean activity.[1]

The group of curves which follow, relating to typical animals possessing very different activities (Fig. 5), are therefore entirely diagrammatic, except in respect to the approximate

{Fig. 5}

longevity of the organisms. (1) might represent an animal of the length of life and of the activity of Man; (2), on the same scale of longevity,