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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