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 and 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 "life-like". 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 it is 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 each may 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 in 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 like them 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 a useful test only 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 said we knew 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.
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 lichens, 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, 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 jelly fish are almost transparent; 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?
This patterning of living matter is very detailed. We can realize this 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, let us say, a tulip, 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 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, and we say they look like flowers. A pattern of ice crystals on a window-pane may be likened to ferns.
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 limbs 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-simplification, 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 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 and whereby we so often recognize living bodies on a casual glance is their rhythmical detailed structure. We know, usually without 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, like a marble statue, found to look homogeneous inside, 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 describe the complete nature of living substance which we all appreciate from our common experience.
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 quality 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.
These variations of pattern are imposed by the environment. We might call them distortions and 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 every springtime. Such rhythmical changes are of the very essence of all living matter.
Portions of living matter are not like a mosaic, constructed of a number of pieces each of which is itself homogeneous. When examined by a microscope, an X-ray analyser, or any other searching scientific device, each feature or marking in any living substance is found 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 end may be alkaline and the opposite end 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, and 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. They thereby form what has been described above as an environment of some violence.
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 the greater degree of chemical instability 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 [Friedrich Wohler, 1800-82, German chemist. — Ed.] succeeded in producing urea, one of the compounds of organic chemistry, in his laboratory, this distinction has become increasingly blurred. Since that early experiment, a large number of substances whose creation was formerly believed to require the agency of a living body, have been produced synthetically. It is now thought that if any compound occurring in vegetable or animal matter cannot be synthesized in a laboratory, it is only because of its inherent instability.
Urea is a comparatively simple substance. Its molecule consists of only eight atoms which hold together quite firmly. But as a rule, the more atoms that go to make up a molecule, the weaker 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.
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 house 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.
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; the other is 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 perceives a dreaded or desired object, or for some other reason which may be called the cause of the movement.
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 be described only in terms of form; the cause of the movement of an inanimate body can be described only 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 over-simplification, 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 could be considered, such an account could be given. This belief expresses a philosophy known as mechanism. Scientific proof of this view has not been found in any instance: neither in the simplest forms of behaviour nor in 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.
Replacement of lost substance, the other means by which living bodies preserve their form, 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 systems, and enters a plant through the leaves and roots. The rate of replacement is sometimes 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.