by     Reginald O. Kapp


Chapter II - The Characteristics of Living Bodies

We generally have no difficulty in recognizing whether a thing is alive or not. Often a mere casual glance suffices. We can tell a plant or an animal from a lifeless thing even if we have not seen its like before. Were we to find ourselves on some distant planet on which evolution had followed a different course from that on this earth, we should not expect to have much difficulty in knowing what was alive and what was not. The conclusion we have to draw is that we are all of us well acquainted with something which is a pronounced and conspicuous characteristic common to all living things, but not to be found in matter that is not living. The most untutored minds are evidently familiar with this characteristic. Life's imprint must be apparent on the mere external shape of a living thing.

In spite of this, few of us could say instantly what this characteristic is. When several persons discuss the question many suggestions tend to be made. Some of them appear to contain a measure of truth. Properties are mentioned which undoubtedly help us to recognize living bodies. But most of these properties will not bear examination as criteria. They have to be rejected because they apply to non-living bodies as well, or because they are not common to all living things, or because they are not readily observed.

Among criteria of life often suggested in such a discussion is movement. In doubtful instances it sometimes helps us to decide that a thing is alive when we see it move. But our judgment can never be guided by mere movement alone. Many, we might even say most, living things do not move perceptibly, and many non-living ones do move most conspicuously. Clouds scurry before the wind; rivers hasten to the sea; pebbles roll over the beach in the wash of the breaking waves. We do not think that these things even look alive. Their movement is not "lifelike". Evidently it is not movement in general but some special kind of movement which helps us to recognize when a thing is living. We are no nearer to defining a characteristic of living bodies if we cannot say what property of their movement justifies the epithet "life-like". When, therefore, we say that we knew a thing to be alive because we saw it move we do not say what we mean. We probably first recognized a characteristic property which led us to surmise that the thing was alive. We then recognized not any sort of movement but some characteristic property in its movement which turned our surmise to certainty. When we say that we recognize a living thing by its movement we evidently know more about the characteristics of living bodies than we have expressed.

In discussion of this question another person may point out how useful a test softness is. We prod a thing if we doubt what it is. If it is hard we conclude it to be probably a stone (though it may be a crustacean), if soft we decide that it is a living body. Someone else will suggest that the presence of moisture is a characteristic of practical use. We break a twig to find out if it is moist or dry. That decides whether it is alive or dead. But a more critical member of the party will have no difficulty in showing that softness and moisture are inadequate criteria. These properties may be readily found in the non-living world. The earth is soft and moist after rain. In so far as softness and moisture are characteristics of .living matter it can only be in conjunction with other circumstances.

Capacity for reproduction is so important a criterion of living bodies that it is bound to receive serious attention in any discussion. It is not known in the non-living world. There are occasions on which it is employed as a test. For instance there are certain diseases of which it is surmised that they are due to ultra-microscopic organisms called viruses. As these are so small that they pass through the finest filters, they cannot be isolated, and. as they cannot be seen with certainty under the most powerful magnification, little is known about them. But there is some evidence that they reproduce. That is taken by most expert investigators as evidence that they are living bodies though they may each contain but a few thousand molecules.

The criterion of capacity for reproduction is, however, one employed only exceptionally. We do not usually wait to find out if things have offspring before we decide that they live. And, moreover, we should reach absurd conclusions if we had to do so. Cut flowers kept alive in a vase would have to be called dead. We should have to conclude that a mule was not a living thing because it had no hope of posterity.

Capacity to heal wounds and capacity to grow are inadequate as criteria for the same reason that capacity to reproduce is inadequate. They cannot form the basis of our common judgments because we recognize living bodies without waiting to verify these properties. Besides, processes faintly analogous to the healing of wounds can be imitated in non-living substance, and various non-living bodies grow. Crystals, stalactites.. icicles do so under suitable circumstances. Moreover living bodies do not always grow. When they are starved they actually dwindle.

Other participants in the discussion may mention yet further properties of living bodies. The exhalation of carbon-dioxide is universal and the absorption of oxygen is performed by all living things (with the exception of anaerobic bacteria). But the fire on the hearth gives out carbon-dioxide and iron absorbs oxygen when it rusts. Moreover even if these properties or others of like nature could be proved to be sound criteria we should have to reject them, because they are not obvious to the casual observer.

That a body is warmer than its surroundings does sometimes serve as a criterion of life to the most untutored among us. But, like movement, softness and moisture, warmth is only a useful test when taken in conjunction with other circumstances. When we say we knew a body to be alive because it was warm., we do not say what we really mean, any more than when we say we knew it to be alive because it moved. We do not think the hearthstone is alive because it is warm.

When these and similar properties of living matter have been examined and rejected as inadequate, the participants in the discussion are likely to become impatient of the subject. Some of them will reach the conclusion that living matter has no specific characteristics at all. They will say that there is no essential difference between living and non-living bodies.

Others may still hold to the view that living bodies possess distinctive properties not to be found elsewhere. But they will maintain that these properties cannot be perceptible to the common man. They will conclude that they must be so deeply hidden in the innermost tissues that only the most refined scientific methods will ever be able to discover them. These participators in the discussion will believe that the distinguishing mark of Life, like its cause and purpose, is among the great mysteries.

Both these views are denied by our common experience. If living bodies had no essential distinguishing characteristics we should not so often be able to distinguish them from non-living ones, and if these characteristics were not conspicuous even to the untutored observer we should not usually be able to distinguish them so readily.

Probably the reason why discussions of this subject so frequently end inconclusively is that the most fundamental and noticeable characteristics of living bodies are of such a kind that they cannot easily be expressed in words. We know quite well what they are and we act accordingly. But our knowledge is hardly conscious. If that theory is correct our first task must be the comparatively light one of finding means of expressing some observations which we have all made about living things. If we consider it a task of science to discover what we do not know, we must consider our present immediate task to be rather a literary than a scientific one, namely, to discover what we do know but have not yet expressed.

In attempting this task we may begin by considering what common properties are possessed by the various forms plant life assumes. At first we are bewildered by the limitless variety that occurs here. There is the delicate fern, the sturdy oak, the squat cactus, the ragged bindweed, and the stately lily. Some plants grow in symmetry and others indulge in tousled untidness. Some, such as grasses and certain mosses or lichen, are rather uniform in colour, while others blaze forth in every conceivable richness of hue. Some grow into spikes and others into broad flat leaves. Some, like the mycelia of fungi, are no more than fine threads beneath the earth, while others look like a green scum across the surface of a pond.

Moreover all the diversity in the forms of plants comprises but one part of what living matter is capable of. The forms assumed by animals are, if anything, even more manifold. Some animals are short and some are tall; some have rounded shape, and others are extremely elongated; there are things on four legs and on two legs; on many legs and on no legs at all. Some are covered with fur; some with feathers; some with scales; some with a chitinous armour; some have nothing but a bare skin. There are shells twisted into spirals and almost transparent jelly fish; there are creatures which fly, which walk, which crawl. There are those which swim, and others which remain permanently attached to one place. There are all gradations of articulated detail, from insects with their somewhat rigid perfection to superficially featureless things like slugs and earthworms. On fossils we find imprints of other shapes that have long since vanished from this earth; but yet we know that those shapes were once owned by living things. What common features can there be in this rich profusion of form to reveal to us so surely the fashioning power of Life?

One such feature seems to be a quality of symmetry or regularity. In the majority of animals the left half is a close, though not usually perfect, counterpart of the right. In certain others, such as starfish, the structure is on a circular plan, and according to which arrangement is present, we speak of bilateral or radial symmetry. This property is possessed to an even higher degree by a great many flowers and leaves. Apart from such simple mirroring or repetition of form, we can observe in the contours of an animal, and even more clearly in those of a plant, a recurrence at regular or irregular intervals of the same forms and patterns. Flowers, leaves, buds, are scattered over a plant, one closely resembling the others of its kind. If in a world untouched by man we were to find something possessing bilateral symmetry, we should suspect it was an animal even if it was different in every particular of structure from any animal we had ever seen before; and if we met something on which a number of identical shapes were attached at irregular intervals, we should consider the possibility that it was a plant, unless some distinctive circumstance made the surmise impossible.

This patterning of living matter is very detailed. We can realize how much so, even if we confine our attention to the mere surface. Let anyone examine a flower and he will see that a thorough scheme of structure is before him. In a tulip, let us say, there are three external petaloid sepals and three internal petals, making six altogether. There are six stamens and a pistil of roughly triangular shape. Each petal is symmetrical about a centre line faintly marked and allowing a slight bulge to either half. It is as if the structure of the whole flower were ringing the changes on the numbers two and three and their product six, or as if each of these numbers were struggling for supremacy. The surface of each petal is covered with very fine lines, suggesting the threads in a piece of finely woven silk. There is regularity in the spacing of the lines, as in their curvature. The theme of slightly curved parallel lines is repeated in the leaves, but here they are more widely spaced and coarser; they almost amount to ribs. An examination of any other flower would show similar repetitions of what can be most conveniently called a structural idea. The term is not scientific and may not be justified, but it is a convenient one for descriptive purposes.

Structure, highly systematized, finely detailed structure, is evidently a fundamental characteristic of living matter. But it is not an adequate criterion. It can only be part of the observation on which our untutored judgment is based. Rhythmical patterns are traced by inanimate nature too. The waves of the sea, the ripples left by the receding tide on the sand are such. The curve and sequence of colours in a rainbow give nearly as good an impression of structural system, as a flower does, though the system is less elaborate. Complete symmetry may be found in crystals. Nevertheless, detailed structure must be one of the properties that make things appear lifelike. We tend to compare things possessing such structure to living things. Snowflakes viewed under a magnifying glass show a beautiful hexagonal symmetry of most elaborate patterns. We say they look like flowers. A surface of ice crystals on the window-pane is likened to ferns.

Patterns found in the inanimate world are, however, only somewhat lifelike. They do not look completely like those formed by living substance. They are justly called patterns, because they contain repetitions of what for convenience we have called a structural idea. But the patterns formed by life are repetitions with a difference. We might say that successive recurrences of the same theme in living bodies are graded or proportioned. Thus the feathers on a bird's body are of graded length. The scales on a fish are of graded size and gradually change shape from place to place. Our fingers are similar, but each of a different length. We recognize this presence of system when we speak of a creature as being "well proportioned". We mean that the proportions appear to obey some law which we can appreciate at least instinctively. What law of proportion operates in each type of plant or creature has not been much studied. It is possible that if it were, new light would be thrown on the course of evolution. We might expect the principle underlying the proportioning of nearly related species to be the same, and to differ from that appertaining to more distant ones. In the case of man the principle has been investigated and expressed in mathematical terms. For instance, it has been suggested that the ratio of the lengths of neighbouring long bones in the hmbs of a human body approximates to that of the side of a regular pentagon to the radius of the circumscribed circle. Such a law, even though probably an over-simphfication, is something far more complicated than the theme in the tulip flower which involves the simple numbers two or three. But in the tulip, too, we recognize the existence of more elaborate laws in the proportions between the graded sizes and graded spacings of recurrent patterns.

The general effect of these laws of proportion can be expressed most easily in non-scientific terms. A word frequently employed in art criticism best describes the pattern manifested by a plant or an animal. This word is rhythm". This points to a certain analogy between a living body and a work of pictorial art. But the comparison must not be over-stressed. It holds in so far as a general relationship between parts, a structural scheme, found in one place may be found again in another. But it does not hold when the examination is minute. On close investigation of details a work of art looks chaotic. The closest study of living matter shows that, however great the detail, it still has graded patterning.

The word rhythm implies that, at intervals determined by some law the existence of which is appreciated by the observer, structural features recur. These features may not be identical in each recurrence but they have some property in common. In the bark of a tree, the plumage of a bird, or the back of a man's hand, in the graceful lines of a greyhound or the awkward ones of a toad; everywhere and anywhere we can observe this elusive quality, this thorough detailing of every minute morsel of the substance, this subtle regularity revealed in uneven but consistent spacing of patterns.

We have to conclude that the characteristic for which we have been looking that by which we recognize living bodies so often on a casual glance is their rhythmical detailed structure. We know, without usually expressing our knowledge in words, that living bodies are structures built to some law of proportion, and that they consist of patterns repeated many times but with slight modifications at each repetition. We know that neighbouring modifications are often such as to give an impression of grading, and that the structure of these patterns is so detailed that the impression of patterning is retained in the smallest parts of the surface that our eyesight can distinguish.

That description must apply to inner structure as well as to the surface. If a body were completely lifelike but found to look homogeneous inside like a marble statue we should all decide that it was not alive. We expect to find, and always do find, a lack of homogeneity, a patterning in the inner tissues of living bodies, quite as detailed as in their surfaces.

But the characteristic for which we have just found expression cannot by itself be the whole of the specific property of living bodies, which makes them radically different from all non-living ones. It is true that it is not found in inanimate nature any more than in the works of man. Neither stones, nor mountains, nor clouds, nor machines show this high degree of characteristic patterning. But substance that has once been alive and is so no longer, like dead wood, does show it. We must look for some further property which has to be considered in conjunction with characteristic structure in order to give the complete nature of living substance which we all appreciate from our common experience.

The further property we are seeking appears to be one which may be most aptly described by the word "vulnerability". It is a property that is often as evident to the casual observer as is finely featured structure. A portion of a living thing is seen to be weak, slender, brittle or soft; in other words we realize at a glance that it is vulnerable. It is only when we consider pattern in conjunction with vulnerability that its significance as a distinguishing characteristic becomes fully apparent.

What is fundamental about regularity and symmetry in living structures is not that these properties are present, but that they are retained in spite of the violence of the surrounding forces. That is a characteristic of all living matter which is never present in the inanimate world. When we recognize that a given shape belongs to something that is alive, it is because we appreciate that this shape must have some quahty other than mere mechanical strength by which it is immune from destruction.

We can appreciate the force of this generalization more fully if we picture our earth entirely depleted of all living things. We should then find in it seas and rivers, clouds and mountains, boulders, pebbles, and grains of sand. These latter three would be about the only movable solid objects. They would all be approximately spherical as the world itself, the sun and the stars are all approximately spherical. They could not be otherwise, for anything that was of a different shape would be broken and ground or pulled together by gravity until it conformed to the common roundness. In this shape it would be least subject to further destruction. Soft things would long ago have been crushed and things of irregular shape smashed. Rocks would have been reduced to boulders, boulders ground to pebbles, and these to grains of sand. Only in the shelter of caves might occasional stalactites have been preserved from the general roundness. We may conclude that a most significant thing about living forms is that they are not sheltered and they are not hard, and yet they assume the greatest diversity of shape, the widest conceivable departure from a safe sphere.

This characteristic of living matter which we have termed vulnerability, appears to be the only superficial and generally apparent one which rigidly conforms to the conditions to be fulfilled by a characteristic that may sharply distinguish the work of Life from the inanimate world. In a world without Life it is inconceivable that any structures could persist which were mechanically unable to resist the violence of their surroundings. The structures constituting living bodies have some capacity to do so.

There is a further feature of the patterns formed by living bodies that requires our attention. It does not generally help towards their immediate recognition, but is yet one with which we are all familiar. This is the fact that the patterns formed by living matter are not constant. The shape of animals changes as a result of muscular contractions, the colour of the skin or fur may alter under the influence of sunhght, the general appearance is modified in. accordance with the general wellbeing of a creature. An individual may be fat or lean, according to the amount of food it has been able to obtain. A plant may grow this way or that. Its general outline is the outcome of environment, of such factors as the direction of the prevailing wind, the places where the soil holds most nourishment or moisture, the direction from which the sunlight reaches it. Thus living forms are not rigidly fixed. There is some latitude, but not unhmited latitude. The pattern of a living individual may depart from the average by a certain amount, but by no more. The departure is never great enough to make the pattern characteristic of the species to which it belongs unrecognizable.

These variations of pattern are imposed by the environment. We might call them distortions. They may be regarded as accidental. But there is another type of change in structure that is not wholly dependent on environment. The shape of every individual alters radically in the course of its existence. Every animal starts as a single cell (or possibly a bud or a gemmula) and displays in succession a series of forms characteristic of the embryo, the young individual and the adult. Such changes are at times extremely drastic, as is shown by the metamorphosis of the higher insects. Less extreme modifications of shape are normal to all living organs. The heart undergoes them with every beat, the muscles are permanently and rapidly alternating between contraction and relaxation, the sap rises anew in a tree at every springtime. Such rhythmical changes are of the very essence of all living matter. Living patterns are not static but cyclical. They are not merely patterns in space as are those formed by crystals. They are patterns in four dimensions, of which three are in space and one is in time.

As regular recurrences observable in the spatial dimension may be distorted, without destruction of the individual if the departure from the average is not too great, so may those along the time component. Breathing can be retarded or accelerated by certain drugs, as can also the heart-beat. The rise of sap in trees is later if winter weather is prolonged. But if the cold spell lasts too long the sap will rise in spite of it. The migration of birds, the breeding of animals, the flowering of plants, sleeping and waking, are only partially influenced by external circumstances. There can be no doubt that cyclical changes in living matter are a part of its natural pattern. They may sometimes or always be related to cyclical changes in the external world. But cyclical changes in living patterns are at most distorted by the environment, they are not produced by it. Later, when we come to consider the causes of the patterning of living matter, we shall have to give as much importance to their time as to their spatial components. We shall have to remember that any explanation of the phenomenon of the characteristic patterning shown by living substance can only be satisfactory if it accounts for its occurrence in all four dimensions.

Science sometimes discovers that the opinions held by the man in the street are wrong. But the conclusions about living bodies which we have reached on the basis of common experience and observation are more than confirmed by science. Living patterns are found by microscopic examination to be far more detailed than the unaided eye could perceive; these patterns are found to be a great deal more vulnerable than superficial observation suggests, cyclical changes in time are found to be more important, more detailed and more universal than untutored observers would suspect.

Portions of living matter are not like a mosaic, constructed of a number of pieces each of which is itself homogeneous. In any living substance each feature or marking is found when examined by a microscope, an X-ray analyser, or any other searching scientific device to be in itself a structure richly marked. This lack of homogeneity can be pursued down to molecular dimensions, and is present in internal tissues as much as on the surface. At least in the most vital tissues of the cell substances the detailing is so great that perhaps no two adjoining molecules are alike. One end of a molecule does not even resemble the other. One may be alkaline and the opposite one acid, one end have an affinity for water and the other end an affinity for fats. When a structure is as finely sub-divided as that, one cannot speak of cell substance as a chemical compound. It is a mixture of compounds in which each may be represented by a single molecule.

But in the internal tissues, as in the surface marking, this lack of homogeneity is not the same as chaos. A group of molecules, each different from its neighbour, may form a cluster which is reflected at a little distance by another similar cluster, and yet another and another. There is as much repetition as in a tesselated pavement, although the law governing the repetitions is less easily stated. A pattern formed by clusters of molecules may in turn form a part of another larger pattern. This larger pattern may in its turn form part of another yet larger one. As this process continues we reach structures made up of structures, and others formed by groups of the latter. Finally we proceed from the structures known as cells to those formed by groups of cells until the organs of a living body are reached; these again, are sometimes arranged in pairs to form the simple pattern known as bilateral symmetry. We might use the expression "serial patterning" to describe the way living structures are made up.

If the common man observes that the entire living body lives in an environment of destructive forces and escapes them in spite of its vulnerability, the scientist is able to tell us that internal parts of the body also live in an environment of some violence and escape destruction, although their vulnerability is almost unbelievably great. The environment of internal tissues is provided by the surrounding substance of the same individual. The forces this substance exerts are generally spoken of in thermal, chemical or electrical terms. Thus, a rise in temperature causes molecules to collide together a little more forcibly. That is enough to knock bits off them. The nature of the substance they form is then altered as may be seen when white of egg is heated. Thus very slight changes in temperature, weak chemical reagents, minute electrical currents, produce radical changes in the substances of which living tissues are formed. The neighbouring tissues are for ever creating such changes. Thereby they form what has been described above as an environment of some violence.

Substances that are highly susceptible to external influences are spoken of as chemically unstable. Many such substances are employed in the service of man and persist when they are carefully guarded from destructive influences, though none of these are even approximately as unstable as some of those to be found in living matter. In a world containing no living things such unstable chemical compounds could not possibly persist. Any substance that could easily be altered would long ago have been acted upon by the environment until it had been turned into something more capable of withstanding the influences to which it was subjected. One would not even find any pure iron in such a world, because if it had been present at one time it would have rusted and formed the ferric and ferrous oxides. Hence it is inevitable that substances in the inanimate world are generally far more stable than those found in living tissues.

For a long while it was not appreciated that the chemical compounds found in plant and animal matter differed from those found elsewhere only in this one particular, namely, in the greater degree of chemical instabihty they possessed. It was believed that there was some further unknown intrinsic difference, and consequently a sharp distinction was made between organic and inorganic chemistry. Since the year 1828 when Wohler succeeded in producing one of the compounds of organic chemistry, namely Urea, in his laboratory, this distinction has become increasingly blurred. Since that early experiment a large number of substances have been produced synthetically of which it was at one time beheved that the agency of a living body was required for their creation. It is now thought that if any compound occurring in vegetable or animal matter cannot be synthecized in a laboratory, it is only because of its inherent instability.

Urea is a comparatively simple substance. Its molecule consists of only eight atoms. They hold together quite firmly. But the more atoms go to make up a molecule, the weaker as a rule is the force that keeps them in position. An analogy is provided by a house of cards. The first and second stories of such a house are fairly firm, but as each successive card is added the whole structure becomes a little more precarious until eventually the addition of one card or a slight jolt of the table is enough to bring the whole flimsy structure down. If one builds very carefully and takes precautions to prevent the table from being in the least shaken, one may succeed in constructing a high house. Some organic molecules are like a very high house of cards, but the strange thing is that they are like one built on a table that is persistently and violently jolted. Such molecules are shaken by the chemical influence of and mechanical collisions with the active neighbouring molecules.

How unstable the fine structure of living matter is and how violent the surrounding forces in comparison with its delicate constitution can be appreciated when we consider the sequence of events following death. These are quite analogous to the collapse of a house of cards. In an animal, muscles which were previously flaccid, contract. Their substance, which was previously translucent, supple and extensible, becomes opaque, rigid and inextensible, their chemical reaction changing from slightly alkaline to acid. These changes are due to molecular causes probably associated with the disappearance of glycogen. Another set of changes is directly due to the action of the chemical compounds in the cell substance on each other. These effects are known as autolysis or digestive softening of the cellular tissues. The active agents are substances which are always present in living cells and which normally assist metabolism. They are known as enzymes. It is they which may be said to help "shake the table" and bring down the card houses represented by each protein molecule. The fate of each fragment of matter is thus what one would expect of any highly vulnerable substance in a turbulent inanimate world, namely a gradual change into something more stable, and composed of a larger number of smaller entities. The enormous, complex, unstable molecules of living matter are broken, ground and scattered by the chemical onslaughts of the surrounding enzymes as surely as rocks and boulders are broken, ground and scattered by the onslaughts of the breakers beating on them in a December storm.

From the combination of the two characteristics of living substance we have been considering, namely, a most extreme complexity of structure and a most extreme degree of vulnerability, a number of problems arise. What makes it possible for such great, elaborate and unstable molecules always to be present in the cell just where required to complete the intricate pattern of the tissues and for them to escape from the destructive violence of the surrounding forces? What makes it possible for animals and plants to be found alive and well on a sea-shore after raging storms have torn huge boulders out of the face of the cliffs and ground the surface of hard flints until they are no more than bright, smooth, round pebbles?

How this happens is not at all mysterious. There are two principal means of preserving living forms and we can observe both of them in operation at any time. One of them is movement performed in obedience to an instinct of self-preservation and the other the replacement of lost substance.

A bird spreads its wings and steers a course at a safe distance from rocks and trees against which the wind might hurl it; fish swim out to sea where they are secure from shattering breakers; some creatures seek refuge in nooks and crannies or burrow under the earth; others, more bold, go out to attack their enemies; even flowers fold their petals, protectively, against the cold night sky. Thus may a living thing fly, or fight, or hide, according to its nature.

Such movements occur, because the creature perceived a dreaded or desired object, or for some other reason which may be called the cause of the movement. The movements of non-living bodies also occur through some cause. But there any obvious resemblance ceases. It is apparent to everyone that the movements of inanimate bodies are governed by the mechanical forces which act on them. These are always expressible as pushes and pulls, and the laws by which they act can be found in any text-book on dynamics. According to these laws a body moves in a direction given by the resultant of the polygon of forces acting on it, while its acceleration is proportional to the magnitude of the resultant force divided by the mass of the body.

The external circumstances which we observe as causes of the movements of living bodies when these follow the instinct of self-preservation are quite different. A faint noise, a smell, some barely perceptible evidence of the vicinity of a creature's natural prey or enemy may occasion the most pronounced and energetic movement. We could not draw a polygon of forces from the sense data on which the creature acts. No one can relate such causes as perceptions with the resultant displacements in terms of dynamics. The stimulus to a creature's behaviour can only be described in terms of form; the cause of the movement of an inanimate body can only be described in terms of force.

The results of the movements of non-living and living bodies can be expressed in a similar sharp antithesis. A non-living body will sooner or later collide with other matter and become chipped, broken, scoured or in some other way altered. The ultimate result of its movement is a change in its shape. A living body moves in such a way as to avoid anything that might cause an ultimate change in its shape. It moves, for instance, to escape danger or starvation. It may be an oversimplification, but it is sufficiently near the truth and serves to emphasize this antithesis if we say that the law governing the movement of living bodies is the law of preservation of pattern.

But are we right in saying that the faint physical forces accompanying a perception cannot be related to the movements of a creature by dynamical laws alone? Would a sufficiently complete polygon of forces give the necessary resultant in the direction of the creature's motion and of the requisite magnitude to account for its acceleration? Many biologists believe that if all physical forces including molecular and atomic ones, electrical fields and chemical attractions, are considered, it would. This belief expresses a philosophy known, as "mechanism". Scientific proof of this view has not been found in any instance, not in the simplest forms of behaviour nor the most primitive unicellular creatures. Belief in mechanism, therefore, depends on the hope that science will some day provide the justification. It is based on faith and not on fact.

The other means mentioned already by which living bodies preserve their form, namely, replacement of lost substance, is at least as important as movement, and far more general. If living bodies had to depend on movement alone they would not last long. Their surfaces do not escape from being worn away any more than pebbles escape from being ground smooth. The inner tissues are subject to chemical alteration, just as iron is subject to rusting. But as fast as the pattern of living matter is destroyed by mechanical or chemical means it is restored by the addition of new matter. This dual process is called metabolism. Matter that is no longer of the right pattern is removed in the breath, the sweat, the excreta. New matter enters an animal through the respiratory and alimentary system and enters a plant through the leaves and roots. Some- times the rate of replacement is slightly, but only very slightly, greater than the rate of removal. We then observe the phenomenon known as growth. Sometimes there occurs a localized removal of rather much substance resulting in a portion of the living matter being taken away, out of its turn as it were. We then speak of a wound and call the replacement healing. In spite of certain differences, metabolism, growth and the healing of wounds all appear as processes obeying the same fundamental law of living matter; the law that pattern must be preserved.

In the continuous replacement of substance we thus find a further pronounced and complete distinction between living and non-living matter. Under the influence of the environment the shape of non-living bodies changes, but the substance remains the same. Under the influence of the environment the substance of living bodies changes, but the shape remains recognizably the same. Considered as three-dimensioned forms at any one moment of time, living bodies are material objects in the same sense in which non-living ones are. But considered as four-dimensioned patterns having structure in time as well as in space living bodies are forms through which matter is in continuous passage. The most fundamental and distinctive characteristic of living matter, from which all the others we have been considering may be deduced, is that it conforms to the Principle of Preservation of Pattern.

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