THE ORGANISM AS A MECHANISM
We propose now to consider the organism purely as a physico-chemical
mechanism, but before doing so it may be useful to summarise the
results of the discussions of the last chapter. Let us, for the moment,
cease to regard the organism as a structure--a “constellation of
parts”--and think of it as the physiologist does: it is a machine; it
is essentially “something happening.” What, then, is the object of its
activity? Whatever else the study of natural history shows us, it shows
us this, that the immediate object of the activity of the organism is
to adapt itself to its surroundings. It must master its environment,
and subdue, or at least avoid whatever in the latter is inimical. It
must avoid accident, disease, and death, it must find food and shelter;
it must seek for those conditions of the environment which are most
favourable to its prolonged existence. Ultimate aims--the preservation
of its race, ethical ideals--do not concern us in the meantime. The
main object of the functioning of the individual organism is that it
may dominate its environment, and obtain mastery over inert matter.
Consciously or unconsciously it acts towards this end.
All those actions which we call reflex, or automatic, or instinctive,
have this in common, that the organism in performing them comes into
relation with only a very limited region of its environment. But
knowing that region intuitively, its actions have a completeness
that an intelligent action does not exhibit until it has become so
habitual as to approach to automatic acting. The relations between
the organism and that part of its world on which it acts, intuitively
or instinctively, is something like that between a key and the lock
to which it is fitted: it opens this lock, perhaps one or two others
which resemble it, but no more. Now just because of this perfect, but
restricted, adjustment of the instinctive or automatically acting
organism to the objects on which it operates, knowledge of all else in
the environment becomes of little consequence.
It is clear that intelligent acting involves deliberation. The almost
inevitable motor response to a stimulus, which is characteristic of
the reflex or instinct, does not occur in the intelligent action:
instead of this we find that we choose between two or more responses
to the same stimulus. We reply to the latter by doing _this_ now,
and _that_ another time; and we see at once what results flow from
acting differently upon the same part of our environment, or acting in
the same way upon different parts. Perception, that is, knowledge of
the world, arises from acting; and as our actions, when carried out
intelligently, become almost infinitely varied, the environment appears
to us in very many aspects. In every action we modify that part of our
surroundings on which we operate. We can produce many modifications
that are of no use to us: these we do not attend to. We produce others
that are useful, and then we note the sequences of events involved in
our actions. Thus we discover or invent natural law--an environment
which is an orderly one. We can calculate and predict what will happen:
we produce, for instance, a _Nautical Almanac_, at once the type
of useful knowledge and of knowledge of sequences of events rigidly
determined--knowledge in short that is mechanistic; and which has been
engendered by the necessity for acting on our environment in our own
interests.
All this, the reader may note, is Bergson’s theory of intellectual
knowledge, a theory which, new and paradoxical at first, becomes more
and more convincing the longer we think about it, until at last it
seems so obvious that we wonder that it ever seemed new. Our modes of
thinking become constrained into certain grooves, just because these
modes of thinking have been those that were generated by our modes
of acting. So long as our thinking relates only to our acting, its
exercise is legitimate. But if its object is pure speculation its
results may be illusory, for a method has been applied to objects other
than those for which it was evolved. Let us now extend our intellectual
methods to the investigation of the organism. Necessarily we must
reason about the latter as a mechanism if we reason about it at all.
If it is a mechanism it must conform to the laws of energetics,
for science, so far as it is quantitative, whether its results are
expressed in the form of equations or inequalities, is based on these
principles.
The first principle of energetics,[6] or the first law of
thermodynamics, is that of the conservation of energy. Let us think
of an isolated system of parts such as the sun with its assemblage
of planets, satellites, and other bodies: in reality these do not
form an isolated system, but we can regard them as such by supposing
that just as much energy is received by them from the rest of the
universe as is radiated off by them to the rest of the universe. In
this system, then, the sum of a certain entity remains constant, and
no conceivable process can diminish or increase its quantity. We call
this entity energy, and we usually extend the principle of its absolute
conservation to matter, though this extension is unnecessary, for we
must think of matter in terms of energy. Stated more generally the
principle is that whatever exists must continue to exist, if we are to
regard this existence as a real one.[7]
[6] See appendix, p. 356.
[7] The principal reason why we do not believe in phantasms is that
these appearances _are not conserved_.
It is not at all self-evident to the mind that energy must be
conserved, for we see that, to all appearance, it may disappear. A
golf-ball driven up the side of a hill possesses energy while in
flight, kinetic energy or the energy of motion; but this apparently
is lost when the ball alights on the hill-top and comes to rest. We
say, however, that it now possesses potential energy in virtue of its
position; for if the hill is a steep one a little push will start the
ball rolling down with increasing velocity, and when it reaches the
spot from which it was originally impelled it possesses kinetic energy.
This is described as one-half of the mass of the ball multiplied by
the square of its velocity. Now the kinetic energy of the ball at
the instant when it left the head of the driver ought to be equal to
its kinetic energy when it reached the same horizontal level on its
downward roll. Yet it can easily be shown that this is not the case,
and we account for the lost kinetic energy by saying that it has been
dissipated by the friction of the ball against the atmosphere in its
flight, and against the side of the hill on its roll back. We cannot
verify this quantitatively, but we are quite certain that it is the
case. If we take a clock-spring and wind it up, the energy expended
becomes potential in the spring, and when the latter is released most
of it is recovered. But we may dissolve the spring in weak acid without
allowing it to uncoil. What then becomes of the energy imparted to it?
We are compelled to say that it has changed the physical condition of
the solution into which it passes, either becoming potential in this
solution, or becoming dissipated in some way. Yet again we cannot trace
this transformation experimentally though we may be quite sure that all
the energy potential in the coiled spring is conceivably traceable.
Suppose, again, we burn some hundredweights of coal in a steam-boiler
furnace. Heat is evolved which raises steam in the boiler, and the
steam actuates an engine, and the latter exhibits measurable kinetic
energy. Where did this come from? It was potential in the coal, we
say, though no method known to physics enables us to prove this by
mere inspection of the coal. We must cause the latter to undergo some
transformation. But by rigid methods we can estimate very exactly the
potential energy of the coal, and we can calculate the kinetic energy
equivalent to this. Yet again we find that the kinetic energy of the
steam-engine is only a fraction of that which calculation shows us
is the equivalent of the kinetic energy of the coal. What becomes
of the balance? We can be quite certain that it has been dissipated
in friction, radiation, loss of heat by conduction, loss of heat in
the condenser, and so on, although we cannot prove this rigidly by
experimental methods.
Think of the universe as an isolated system. It contains an invariable
quantity of energy. This energy may be that of bodies in motion--suns,
planets, cosmic dust, molecules, etc.--when it is kinetic energy;
or it may be the energy of electric charges at rest or in motion;
or any one of the many kinds of potential energy. It may pass
through numerous transformations--the chemical potential energy of
coal may be transformed into the kinetic energy of water molecules
(steam at high temperature), and this into the kinetic energy of the
revolving armature of a dynamo, and this again into the energy of
moving electrons (the current of electricity in the circuit of the
dynamo), and then again into the energy of ethereal vibration (light,
heat, X-rays, or other electro-magnetic waves), and these again into
mechanical or kinetic energy, and so on. When we say that we can
control energy we say that we can produce these transformations; we can
cause things to happen, we bring becoming into being. In this sense
energy is causality. But while the sum-total of energy in the universe
remains constant, the sum of causality continually diminishes. Energy
is the power, or condition, of producing _diversity_, but while energy
can suffer no diminution of quantity, diversity tends continually to
decrease.
In the last two sentences we state, in one way, the second law of
thermodynamics--in some respects the most fundamental result of our
experience in the physical investigation of the universe. In its most
technical form, as enunciated by Clausius, this law states that the
value of a certain mathematical function, called _entropy_,[8] tends
continually towards a maximum, when it is applied to the universe as a
whole. When we say the “universe,” we mean all that comes within our
power of physical investigation. Let us now see what this statement
means.
[8] See appendix, p. 369. Entropy is a shadowy kind of concept,
difficult to grasp. But again we may point out that the reader who
would extend the notion of mechanism into life simply _must_ grasp it.
The energy of the solar system is in part the kinetic energy of
those parts of it which are in motion--planets, planetesimals,[9]
and satellites. This quantity of energy is enormously great. In the
case of our earth it is 1/2(_mv_^2), _m_ being the mass of the earth,
and _v_ its velocity. Translated into numerical symbols we find this
quantity almost inconceivable. The greater part of this energy is
_unavailable_, that is, it can undergo no transformations. But because
the earth is in rotation at the same time as it revolves round the sun,
and because the moon revolves round the earth, there are tides in the
watery and atmospheric envelopes of the earth. The energy of the tides
is the kinetic energy of water or air in motion, and we can employ
this energy in the production of transformations, and it is therefore
_available_. But well-known investigations have shown that the tides
produce friction, and that the period of rotation of the earth is
slowly becoming greater. Ultimately the earth will rotate on its own
axis in the same time that it revolves round the sun--then a year and
day will be of the same length. When that occurs, the sun, earth, and
moon will be in equilibrium, and tidal phenomena due to the sun will
cease. The kinetic energy of the earth, rotating once in 24 hours is
obviously greater than its kinetic energy when rotating in the period
which will then be its year. What has become of the balance? It has
been transformed into the mechanical friction of the tides against the
surface of the earth,[10] and this friction has been transformed into
low-temperature heat, and this heat has been radiated off into space.
[9] Meteorites, cosmic dust, and other small particles moving in the
solar system within influence of the sun’s gravity.
[10] Not entirely, of course, but whatever be the transformation it
ends in heat production.
The solar system also contains energy in the form of the heated sun and
planets, and in the form of chemical potential energy of the substances
of which those bodies are composed. Let us think of the system, sun
and earth. The sun contains enormous heat energy, its temperature
being some 6000° C. absolute.[11] It contains enormous chemical energy
in the shape of compounds existing beneath its outer envelopes, and
it contains energy in the form of its own gravity--its contraction
together produces heat. But this heat is being continually radiated
away: chemical reactions must occur in which the potential chemical
energy of its substances must become transformed into heat, and this
heat is also radiated away; contraction of its mass must occur up to a
point when the materials are as closely packed together as possible;
heat is developed during the contraction, and this also passes away
by radiation. Suppose that modern speculations are well founded and
that radio-active substances are present in the sun: in the atomic
disintegration of these substances heat is produced and again radiated.
Therefore in whatever form energy exists in the sun, it transforms into
heat and this radiates. The ultimate fate of the sun is to cool down
and solidify. It will then move through space as a body having a cool,
solid crust, and an intensely heated interior. Slowly, very slowly,
this heated interior will cool down by the conduction of its heat from
the core to the outer shell, and by the radiation of this heat from the
shell into space. For incredibly long periods radio-active substances
in the interior must generate heat, but even this process must reach an
end.
[11] Absolute temperature is Centigrade temperature +273. This is, of
course not a full definition, but it is sufficient for our present
discussion.
The energy received by the earth is that of solar and stellar
radiation. Stellar radiation is minute, the absolute temperature
of cosmic space (or ether) being about -263° C. The absolute
temperature of the earth is about +17° C., so that it radiates off
more heat into space (other than that represented by the sun) than
it receives. All energy-transformations on the earth (except tidal
effects, and energy-conduction from the heated core, and possibly
radio-active effects) are transformations of this solar energy
received by radiation. We see these in oceanic and atmospheric
circulations (currents, winds, rainfall, etc.). We see them also in
the transformations of the chemical potential energy of coal and other
products of life--products in which the contained potential energy has
been absorbed from solar radiation.
Let us follow the transformations of this energy. Oceanic currents
transport heat from the equatorial sea-areas to the colder temperate
and polar areas, and compensatory polar currents flow towards the
equator, absorbing heat from the waters of temperate and equatorial
areas. Winds act in an analogous way. Water is evaporated where the
solar radiation is intense, and heat is absorbed in the transformation
of water into aqueous vapour. Then this water vapour is transported in
the winds into regions where it becomes condensed and precipitated as
rain or snow, heat being emitted in this condensation. In all these
movements there is friction, and this friction transforms to heat. In
all the effect is the general distribution over the earth of the heat
which the equatorial regions receive in excess of that which the polar
regions receive. Other mechanical effects are also produced by oceanic
and atmospheric circulations--the denudation of the coasts by tides and
storms, the erosion of the land by rivers, rains, snow, and ice, the
transport of dust in winds, etc. In all these friction is produced,
and this friction passes into heat.
The potential chemical energy which results from absorption of solar
radiation by plants is principally accumulated as coal. Apart from
the interference of man, this coal would slowly accumulate, perhaps
it would more slowly disappear by bacterial action, or by physical
transformations. In these transformations the energy of the coal
would become heat energy and the potential energy of the gas produced
by bacterial activity. By man’s agency the coal suffers other
transformations, and in the present phase of civilisation it is his
chief source of energy. It is available for doing work of many kinds,
and in all these forms of work it becomes transformed by chemical
action (burning) into high temperature heat.
We can cause this potential energy of coal to transform into mechanical
energy of machines, vehicles, and ships in motion by causing it to pass
into heat. In the steam-engine, or gas-engine, a highly heated gas
(steam, or the mixture resulting from the explosion of coal gas and air
in the cylinder of the engine) expands and propels a piston or rotates
a turbine. (Obviously in the petrol engine the same essential process
takes place.) We employ this kinetic energy directly in transport, or
we cause it to undergo other transformations. In the dynamo, kinetic
energy of machinery in motion transforms to electrical energy; and this
may transform to radiant energy (light, heat in electric radiators,
wireless telegraphy radiations), or it may transform to chemical
energy (the manufacture of carborundum in the electric furnace, for
instance), or it may transform again to the kinetic energy of bodies
in motion (electric traction). In innumerable ways the human power of
direction causes transformation of this accumulated potential energy,
and the reader will notice the analogy of all this with the essential,
unconsciously expressed activity of the animal organism in its own
metabolism--a point to which we return later.
Notice now that all the energy-transformations we have noticed are
_irreversible_. This is a matter of deep philosophical importance,
and we must devote some time to it. Consider first of all the working
of the steam-engine; what occurs is this--coal is burned in the
boiler-furnace, that is to say, potential chemical energy passes
into heat and this vaporises water in the boiler, producing a gas
at high temperature (steam). This gas expands in the high-pressure
cylinder of the engine, driving forward a piston; it expands further
in the intermediate cylinder, propelling its piston also, and again
in the low-pressure cylinder. It is then cooled by passing through
the condenser, and in the contraction further mechanical energy
is obtained. The train of events thus begins with a gas at a high
temperature and ends with the same gas at the temperature of the water
in the condenser. The heat lost is transformed into the mechanical
energy of the engine. But not all of it. A certain quantity is lost
by radiation from the boiler walls, the walls of the steam-pipes, the
cylinders, and other parts of the engine; also some of the energy is
transformed to friction, and this again to heat. In this way a very
considerable part of the energy contained in the coal is frittered away
in unavoidable heat-conduction and radiation, and a last residue of it
goes down the drain, so to speak, with the condenser water. This loss
is inherent in the nature of the mechanism of the engine.
Suppose that the energy of the engine is employed to drive a dynamo.
The armature of the latter rotates against the constraint of powerful
electro-magnets, and in so doing a current of electricity is generated.
By the law of conservation this current should contain as much energy
as was put into the rotation of the armature; as a matter of fact it
does not, and the deficiency is represented by the friction of the
parts of the machine against each other, by imperfect conductivity of
electricity in the wires, and by imperfect insulation of the current.
Friction, imperfect conductivity, and imperfect insulation all
transform to heat, and this radiates away. Suppose now that the current
is used for lighting purposes: to do this it must heat the metallic
filaments in the lamps, or the points of the carbons in an arc. This
heat then transforms to light, but along with the light, which was the
object of the transformation, heat is produced, and this heat radiates
away.
The actual process in which the particular form of energy required is
generated may or may not be reversible in theory. That employed in
the steam-engine is not, for if we start with a cold boiler and then
work the engine backwards we could not raise steam. The process in the
dynamo is theoretically reversible: if we send a current of electricity
into a dynamo the machine will begin to rotate, and become a motor, so
that we can obtain mechanical work from it. Now in theory all forms of
energy are mutually convertible, and all can be expressed in terms of
a common unit. The unit of mechanical energy is called the _erg_: let
a current, the energy of which is equal to _N_ ergs, be sent into the
dynamo, then we ought to obtain from the latter mechanical energy equal
to _N_ ergs. Conversely, if _N_ ergs of mechanical energy be employed
to rotate the dynamo, we should obtain electrical energy equal to this
amount. Now as a matter of fact we do not obtain these theoretical
conversions, for some of the electrical energy is dissipated when we
employ the machine as a motor, and some of the mechanical energy is
likewise dissipated when we employ it as a dynamo.
The entity that we call energy is the product of two factors, a
capacity-factor and an intensity-factor. Thus:--
Mechanical energy = quantity of water × height at which it is
of water power situated above the water-motor.
Energy of an = quantity of electricity × electrical potential.
electric current
Chemical energy = equivalent weight of the substance ×
chemical potential.
What is it that determines whether or not an energy-transformation will
occur? It is the condition that a difference of the intensity-factors
of the energy in different parts of a system exists. Water will
flow from a higher to a lower level, doing work as it flows, if
it is directed through a motor. Electricity will flow if there is
a difference of electrical potential. A chemical reaction will
occur if two substances before interacting possess greater chemical
potential than do the products which may possibly be formed during the
interaction. Coal and oxygen possess greater chemical potential than do
carbon dioxide and water, therefore they will combine, forming carbon
dioxide and water. Energy-transformations will therefore occur wherever
it is possible that differences of intensity or potential can become
abolished. The energy that may thus flow from a condition of high to a
condition of low potential, undergoing a transformation as it flows, is
the available energy of the system of bodies in which it is contained.
A closed vessel surrounded by an envelope impervious to heat, and
containing a mixture of oxygen and hydrogen, is an isolated system
containing available energy. Let the mixture be fired by an electric
spark, and heat is evolved. The total energy of the system is unaltered
in amount, but the available energy has disappeared, since the heated
water vapour is incapable of undergoing further transformations while
it forms part of its isolated system.[12]
[12] It is really necessary to lay stress on the distinction between
available and unavailable energy, as it is one which many biologists
appear to ignore. Thus, a popular book on the making of the earth
attempts to argue that essential distinctions between living and
inorganic matter are non-existent. One of these distinctions is that
organisms absorb energy, and this author points to the absorption
of “latent heat” by melting ice as an example of the absorption of
energy in a purely physical process. Consider a system consisting
of a block of ice and a small steam boiler. We can obtain work
from this by the melting of the ice--that is, its “absorption of
latent heat.” The system, ice at 0° C. + steam at 100° C., possesses
available energy, but the system, melted ice + condensed steam, both
at the same temperature, contains none. The molecules of water at
0° C. “absorb energy,” that is to say, their kinetic energy becomes
greater, but their available energy in the system has disappeared. In
saying that the organism absorbs energy, we mean, of course, that it
accumulates available energy, that is, the power of producing physical
transformations. (See further, appendix, p. 366.)
All physical processes are therefore _irreversible_, that is to say,
proceed in one direction only. Either a process is irreversible in
the sense that it cannot proceed both in the positive and negative
directions (a steam-engine, for instance), or it is irreversible in
the sense that while it proceeds the energy involved in it becomes
less capable of being transformed into other conditions. (In the
theoretically reversible dynamo, energy becomes dissipated in the form
of heat.) The following statements may be regarded as axioms[13]:--
[13] Bryan, _Thermodynamics_: Teubner, Leipzig, 1907, p. 40.
(1) “If a system can undergo an irreversible change, it will do so.”
(2) “A perfectly reversible change cannot take place by itself.”
In the phenomena studied by physics we see only irreversible
changes. In all these processes energy descends the incline, and some
(considerable) fraction of the amount involved passes into conditions
in which it is incapable of further transformation; in all, energy
becomes less and less available. Expressed in its most technical form,
the second law of thermodynamics states that entropy tends continually
to increase. Every such process as we can study in physics “leaves an
indelible imprint somewhere or other on the progress of events in the
universe considered as a whole.”[14]
[14] Bryan, _Thermodynamics_, p. 195.
We cannot observe a truly isolated system. The earth itself is part of
the solar system, and the latter receives energy from, and radiates
it to the rest of, the universe. Our only isolated system is the
whole universe. We must think of it, in so far as we regard it as
physical, as a finite system: if it is infinite, our speculations
become meaningless. The universe therefore is a system in which energy
tends continually towards degradation. In every process that occurs
in it--that is to say every purely physical process--heat is evolved,
and this heat is distributed by conduction and radiation, and tends
to become universally diffused throughout all its parts. When this
ultimate, uniform distribution of energy will have been attained,
all physical phenomena will have ceased. It is useless to argue that
universal phenomena are cyclical. We vainly invoke the speculations
(founded on rather prematurely developed cosmical physics) of stellar
collisions, light-radiation pressure, the distribution of cosmic
dust, etc. to support our notions of alternate phases of dissipation
and concentration of energy; close analysis will show that all these
processes must be irreversible. The picture physics exhibits to us
is that of the universe as a clock running down; of an ultimate
extinction of all becoming; an universal physical death.
In this conclusion there is nothing that is speculative. It is the
least metaphysical of the great generalisations of science. It
represents simply our experience of the direction in which physical
changes are proceeding. Based upon the most exact methods of science
known to us, nothing seems more certain and more capable of rigorous
mathematical investigation.
And yet we are certain that it is not universally true. For there
must always _have been_ an universe--at least our intellect is
incapable of conceiving beginning. If we suppose a beginning, an
unconditioned creation, at once we leap from science into the rankest
of metaphysics. Holding, then, that the duration of our physical
universe is an infinite one, we see that the ultimate attainment of
energy--dissipation--must have occurred if our physics is true. It does
not matter what new sources of energy modern investigation has shown
to us; nor do the incredibly great lapses of duration necessary for
the depletion of these sources matter. We have eternity to draw upon.
Everywhere in the universe we see diversity and becoming. Is then the
whole problem a transcendental one, or is the second law untrue? We
refuse to regard the problem as insoluble, and we must think of the
second law as true of our physical experience only. But our conception
of the universe shows that it cannot be true, and so we have to seek
for an influence compensatory to it.
If the organism is a mechanism of the physico-chemical kind, it should
therefore conform to the two great principles of energetics established
by the physicists. Now there can be no doubt that the law of
energy-conservation does apply to all the processes observed in animals
and plants. Let us consider the “calorimetric experiments.” An animal,
together with the food and oxygen supplied to it, and the various
substances excreted by it, constitutes a physical system. This system
can be approximately isolated so that no heat enters it from outside,
while the heat that leaves it can be determined quantitatively. The
animal is made to perform mechanical work, and this is measured. The
energy-value of the food ingested by it, and that of the excreta, can
be estimated. All the physical conditions can thus be controlled,
and the results of such experiments show that energy is conserved.
The energy contained in the food is greatly in excess of the energy
contained in the excreta, but the deficit is quantitatively represented
by the work done by the animal, and by the heat lost in conduction
and radiation from its body. The difference between the observed
results and the theoretical ones are within the limits of error of the
experiment. The metabolism of the animal as a whole, then, conforms to
the law of conservation, and the general results of physiology all go
to show that this is also true of chemico-physical changes considered
in detail.
It cannot be shown that the second law, that of the dissipation of
energy, applies to the organism with all the strictness in which
it applies to purely physical systems. If we consider only the
warm-blooded animal we do indeed find that its general metabolism
does proceed in one direction, and that irreversible changes occur.
In the mammal and bird we have organisms which present a superficial
resemblance to the heat-engine, with respect to their chemico-physical
processes, a resemblance, however, which is rather an analogy than an
identity of processes. In the heat-engine we have (1) a mechanism of
parts which do not change in material and relationships to each other
(boiler, cylinder, pistons, cranks, slide-valves, etc.); and (2) a
working substance (the steam).
Energy in the form of the chemical potential of coal and oxygen is
supplied to the mechanism. The coal is oxidised, producing heat. The
heat then expands the working substance (the water in the boiler),
and this working substance--now a gas at high temperature and
pressure--propels the piston and confers kinetic energy on the engine.
Note the essential steps in this process: substances of high chemical
potential (coal and oxygen) suffer transformation into substances of
low chemical potential (carbon dioxide and water), and the difference
of energy appears as high-temperature heat (increased kinetic energy
of water molecules, to be more precise). This heat is then transformed
into mechanical work (the kinetic energy of the molecules of steam
is imparted to the piston of the engine). But in this transformation
only a relatively small proportion (10% to 20%) of the energy
available is transformed into mechanical work: the rest is dissipated
as irrecoverable low-temperature heat, by radiation from boiler,
steam-pipes, engine, and as the heat which passes into the condenser
water.
In the organism in general there is no distinction between the fixed
parts of the mechanism and the working substance. The organism
itself (its muscles, nerves, glands, etc.) _is_ the working
substance. Further, it is not quite certain that there is a necessary
transformation of chemical energy into heat. The source of energy in
the case of the warm-blooded animal is the chemical energy of the food
substances and oxygen taken into its body. These chemical substances
undergo transformations in the alimentary canal and in the metabolic
tissues. The proteids of the food are broken down into amino-substances
in the alimentary canal, and these amino-substances are synthesised
into the specific proteids of the animal’s body. Corresponding changes
occur with the carbohydrates and fats ingested. These rearrangements
of the molecular structure of the foodstuffs are the object of the
processes of digestion and assimilation; and when they are concluded,
a certain proportion of the food taken into the body has become
incorporated with, or has actually become a part of, the living tissues
(muscles, nerves, etc.) of the animal body. This living substance,
compounds of high chemical potential (proteids, carbohydrates, and
fats) undergoes transformation into compounds of low chemical potential
(water, carbon dioxide, and urea). There is a difference of energy, and
this appears as mechanical energy, as the chemical energy required for
glandular activity, and as heat.
We must not, however, conclude that this heat of the warm-blooded
animal is comparable with the waste heat of the steam-engine. The
homoiothermic animal maintains its body at a constant temperature,
which is usually higher than that of the medium in which it lives,
and this constancy of temperature obviously confers many advantages.
Chemical reactions proceed with a velocity which varies with the
temperature, so that in the warm-blooded animal the processes of life
go on almost unaffected by changes in the medium. The animal exhibits
complete activity throughout all the seasons of the year. It does not,
or need not, hibernate, and it can live in climates which are widely
different. We therefore find that the most widely-distributed groups
of land-animals are the warm-blooded mammals and birds, while the
largest and most cosmopolitan marine animals are the warm-blooded
whales. Heat-production in the mammals and birds is therefore a
direct object of the metabolism of the animal; it is a means whereby
the latter acquires a more complete mastery over its environment.
That it is not necessarily a step in the transformation of chemical
into mechanical energy we see by considering the metabolism of the
cold-blooded animals. In these poikilothermic organisms the body
preserves the temperature of the medium. The temperature in such
animals may be a degree, or a fraction of a degree, higher than that of
the environment, but, in the absence of exact calorimetric experiments,
we cannot say what proportion of the energy of the food of these
animals passes into unavailable food energy. Probably it is a very
small fraction of the whole, and we are thus justified in saying that
in the cold-blooded animal chemical energy does not, to a significant
extent, become transformed into heat. The result is, of course, that
the vital processes in these organisms keep pace, so to speak, with the
temperature of the environment, since the chemical reactions of their
metabolism are affected by the external temperature. We find therefore
that hibernation, the formation of resting stages, and a general
slowing down of metabolic processes are more characteristic of the
cold-blooded animal during the colder seasons than of the warm-blooded
animal. The former has not that mastery over the environment attained
by the mammal or bird.
The metabolism of the animal therefore resembles the energy process
of the heat-engine only in the general way, that in both series of
transformations chemical energy descends from a condition of high
potential to a condition of low potential, transforming into mechanical
energy in so doing, and thus performing work. In the heat-engine
chemical energy transforms to heat, and then to mechanical energy, and
of the total quantity transformed a certain large proportion suffers
dissipation by conversion into low-temperature heat. In the animal
organism chemical energy transforms directly to mechanical energy
without passing through the phase of heat. If heat is produced it is
because it is, in a way, available energy, inasmuch as it permits of
the continuance of chemical reactions at a normal rate. The analogy of
the animal with the heat-engine is therefore a false one. It suggests
oxidation of the food-stuffs and heat production, whereas it is not
at all certain that any significant proportion of the energy of the
organism is the result of oxidation: many animal organisms indeed
function in the entire absence of free oxygen. Further, the proportion
of energy dissipated is always small compared with the heat-engine,
and tends to vanish. The second law of thermodynamics does not,
then, restrict the energy-transformations of the animal organism to
the same extent that it restricts the energy-transformations of the
physico-chemical mechanism.
The processes involved in the plant organism differ still more in
their _direction_ from those of a “purely physical” train. To see this
clearly we must consider the imaginary mechanism known as a Carnot
heat-engine.[15] This is a system in which we have (1) a heat-reservoir
at a constant high temperature, (2) a refrigerator at a constant low
temperature, and (3) a working substance which is a gas. Energy is
drawn from the reservoir in the form of heat, and this heat expands the
gas, doing work. The gas contracts, and its heat is then given up to
the refrigerator. The work done is equal to the difference between the
amount of heat taken from the reservoir and the amount given to the
refrigerator.
[15] See appendix, p. 363.
This series of operations is called a direct Carnot cycle. But the
mechanism can be worked backwards. In this case heat passes from
the refrigerator into the working substance, which was at a lower
temperature. The working substance, or gas, is then compressed, as the
result of which operation it is heated to just above the temperature
of the reservoir. The heat it thus acquires is then given up to the
reservoir.
In the direct Carnot cycle, therefore, energy passes from a state of
high potential to a state of low potential and work is done _by_ the
mechanism. In the reversed Carnot cycle energy passes from a state of
low potential to a state of high potential and work is done _on_ the
mechanism. The Carnot engine is thus perfectly reversible. No energy
is dissipated in its working. It is, of course, a purely imaginary
mechanism.
In the metabolism of the green plant carbon dioxide and water are
taken into the tissues of the leaf and are transformed into starch.
But the energy of the compounds, carbon dioxide and water, is much
less than that of the same compounds when built up into starch. Energy
must therefore be derived from some source, and this source is said
to be the ether. Solar radiation is absorbed by the green leaf, and
this energy is employed to produce the chemical transformation. Just
how this is effected we do not positively know, in spite of much
investigation. It is possible that formaldehyde is formed from carbon
dioxide and water, polymerized, and then converted into starch. It is
possible that the absorbed electro-magnetic vibrations are converted
into electricity in the chlorophyll bodies of the leaf, though when
radiation is absorbed in physical experiments it is converted into
heat. We do not know just what are the steps in the transformation,
though it is clear that solar radiation is absorbed and that the
chlorophyll of the leaf is instrumental in converting this energy of
radiation into chemical potential energy. But the important thing to
notice is, that we have here a process closely analogous to that of a
reversed Carnot engine. Energy (that of the carbon dioxide and water)
passes from a state of low potential to a state of high potential (that
of the energy of starch), and work is done _on_ the plant in producing
this transformation.
Work is not done _by_ the green plant. This statement is not, of
course, quite rigidly true, for a certain amount of mechanical work
is done by the plant. Flowers open and close; tendrils may move and
clasp other objects; there is a circulation of protoplasm in the plant
cells, and a circulation of sap in the vessels of stems, etc. Also work
is done against gravity in raising the tissues of the plant above the
soil, while work is also done by the roots in penetrating the soil. But
when compared with the work done by radiation in producing the chemical
transformations referred to above, these other expenditures of energy
must be insignificant. Speaking generally, then, we may describe the
green plant as a system in which available energy is accumulated in
the form of chemical compounds of high potential. It is, further, a
system in which energy becomes transformed without doing mechanical
work, except to a trifling extent, and in which there is no formation
of heat, or at least in which the quantity of heat dissipated is only
perceptible during very restricted phases, is relatively small during
the other phases, and tends to vanish.
Let us now combine the processes of plant and animal; we start with
the latter. In it we have a mechanism which does work. The source of
its energy is the potential chemical energy of its foodstuffs, which
latter reduce down to those substances known as proteids, fats, and
carbohydrates. The energy-value of these compounds is considerable,
that is to say, if they are burned in a stream of oxygen a large
quantity of heat is obtained from their combustion. They are ingested
by the animal, broken down chemically, and rearranged. The proteids
eaten by the animal (say those of beef or mutton or wheat) are acted
upon by the enzymes of the alimentary canal and are decomposed into
their immediate constituents, amino-acids, and then other enzymes
rearrange these amino-acids so as to form proteid again, but proteids
of the same kinds as those characteristic of the tissues. This
decomposition and re-synthesis is carried out also with respect to the
fats and carbohydrates ingested. The result is that the food taken into
the alimentary canal, or at least a part of it, is built up into the
living substance of the animal’s body. The energy expended upon these
processes of digestion and assimilation is probably inconsiderable.
During these processes the animal absorbs available chemical energy.
The energy thus taken into the animal is then transformed. The major
part of it appears as mechanical energy--that of bodily movement, the
movements of heart, lungs, blood, etc.--and heat. Some part of it
becomes nervous energy, by which rather vague term we mean the energy
involved in the propagation of nervous impulses. Some of it is used
in glandular reactions, in the formation of the digestive juices, for
instance. The most of it, however, transforms to mechanical energy
and heat. Just how these energy transformations are effected we do
not know. The heat is, of course, the result of chemical changes,
oxidations, decompositions, or changes of the same kind as that of the
dilution of sulphuric acid by water, but the mechanical energy appears
to result directly from chemical change without the intermediation of
heat. We shall return to this point in a later chapter, and content
ourselves with saying here that the chemical compounds contained in
the metabolic tissues of the animal body undergo transformation from
a state of high to a state of low chemical potential, and that this
difference of potential is represented by the work done and the heat
generated. The proteid, fat, and carbohydrate of the tissues represent
the condition of high potential; and the carbon dioxide, the water, and
the urea, into which these substances are transformed, represent the
condition of low potential.
Let us suppose a Carnot heat-engine in which the temperature of the
reservoir of heat is (say) 120°C., and that of the refrigerator 50°C.
The heat of the refrigerator can still be made a further source of
energy by constituting it the heat reservoir of another Carnot engine
which has a refrigerator at a temperature of 0°C. Our animal organism
may be compared with a Carnot cycle; its energy reservoir is the
proteid, fat, and carbohydrate ingested, and its refrigerator (or
energy sink) is the carbon dioxide and urea excreted. Now the urea of
the higher mammal becomes infected with certain bacteria, which convert
it into ammonium carbonate. Another species of bacteria converts the
ammonia into nitrite, and yet another turns the nitrite into nitrate.
The main process of the animal is therefore combined with several
subsidiary ones.
_Carbohydrate_, _fat_, _proteid_ } Metabolism
break down into } of the animal
↓
Carbon dioxide
Water
Urea------_Urea_ } Metabolism of
Chemical ↓ } urea bacteria
Energy at passes into
high potential ammonium
\ carbonate----_ammonium_ } metabolism
\ _carbonate_ } of nitrifying
\ ↓ bacteria
\ oxidises to
Work done by \ nitrite----_Nitrite_ } Metm. of
the system \ ↓ } nitrifying
\ oxidises } bacteria
\ to nitrate}
chemical energy
at low potential
The arrows show that energy is descending the incline indicated by a
direct Carnot cycle. There is no more work to be obtained from the
carbon dioxide and water excreted by the mammal, but more work can be
obtained from the urea when it is used by bacteria, and “ferments”
to ammonia. Work can again be obtained from the ammonia by bacteria,
which convert it into nitrite, and yet again from the nitrite by other
bacteria, which convert it into nitrate. The nitrate represents the
energy-zero so far as the organisms considered are concerned.
Other nitrogenous residues are contained in the urine of animals,
and several other excretory products may be formed. But in all
these cases we can easily find subsidiary energy-transformations
effected by bacteria, as in the above scheme. This, then, is the
positive, or direct half, of that reversible Carnot cycle with
which we are comparing life. In it energy falls in potential (or
intensity, or level), and in this fall of potential transformations
are produced--exhibit themselves, is perhaps a better way of putting
it. We will consider these transformations later; in the meantime it
should be noted that in this fall of potential is a degradation of
chemical energy. Compounds, carbon dioxide, water, and nitrate are
produced which are chemically inert. It is no use to say that carbon
dioxide may react with (say) glowing magnesium, water with metallic
sodium, and nitrate with (say) glowing carbon. A condition of chemical
equilibrium would result from purely inorganic becoming on our earth in
which there was no metallic sodium or magnesium or incandescent carbon;
in which the metals would become inert oxides, and the carbon would
become dioxide. The formation of these compounds represents a limit to
energy-transformations. Note also that all these energy-transformations
are conservative; the total quantity remains unchanged throughout,
and is the same at the end as at the beginning. But _entropy has been
augmented: unavailable energy has increased at the expense of available
energy_.
Consider now the indirect, or reversed, Carnot cycle. We begin with
the inert matter, resulting from the metabolism of the animal, carbon
dioxide, water, nitrate, and a few more mineral substances. We have the
energy of solar radiation. By virtue of the living chlorophyll plastid
in the cells of the green plant, this solar radiation uses the carbon
dioxide and water as raw materials in the elaboration of starch. At the
same time it absorbs nitrate, with some other inert mineral substances
from the soil, and takes these into its tissues. The starch formed
in the chlorophyll is converted into soluble sugar, which circulates
through the vessels of the plant and is associated with the nitrogenous
salt in the elaboration of proteid. Proteid, oils, fats and resins,
and to a greater extent carbohydrates, are thus built up by the plant
and _accumulate_, for mechanical work is not done by it, nor is heat
dissipated--or at least these processes occur to an insignificant
extent.
} Metabolism --------------> are synthesised to--
Carbon dioxide } of the { proteid
Water } green { fat
Nitrate } plant { carbohydrate
Chemical energy at --------------> Chemical energy
low potential Work done on at high potential.
the system
The “working substance” of our organic cycle has therefore returned to
its original state.
We have considered the process of metabolism in two categories of
organisms, the typical animal and the green plant, and we have
combined these so as to obtain a picture of a reversible cycle of
physico-chemical processes. When we speak of the “organism” in the
most general sense, we mean that it exhibits these two modes of
metabolism. This is, of course, not the case in any actual organism
which we can investigate, or at least the typical modes of behaviour
which characterise animal and plant life are not seen in any one
individual. But we find that there is no absolute distinction between
the two kingdoms. The plant may exhibit a mode of nutrition closely
resembling that of the animal (as in the insectivorous plants), and it
is possible that photo-synthetic process, in the general sense, may
be present in the metabolism of some animals. Certain lower plants,
the zoospores of algæ, exhibit movements identical in character with
those of lower animals. At the base of both kingdoms are organisms,
the Peridinians, for instance, which have much of the structure of the
animal (though cellulose is present in their skeleton), which possess
motile organs, but which also possess a photo-synthetic apparatus,
and exhibit the typical plant mode of nutrition. Further, there are
symbiotic partnerships, that is, associations of plant and animal
in one “individual” form (as, for instance, among the lower worms,
Echinoderms, polyzoa, molluscs, and other groups of animals). In these
cases green algal cells, capable of forming starch from carbon dioxide
and water under the influence of light, become intercalated among
the tissues of the animal. We find, also, that with regard to some
fundamental characters, plant and animal display close similarities:
the structure of the cell, for example, and the highly special mode of
conjugation of the germ-nuclei in sexual reproduction. We must regard
all the distinctive characters of the plant as represented in the
animal and _vice versa_. Why they have become specialised in different
directions is a question that we discuss later.
The organism, then, in so far as we regard it as a physico-chemical
mechanism, as the theatre of energetic happenings, exhibits the
following general characters:--
(1) It slowly accumulates available energy in the form of chemical
compounds of high potential, work being done upon it.
(2) It liberates this energy in relatively rapid, controlled,
“explosive reactions,” transforming into movements carried out by a
sensori-motor system of parts, work being done by it.
(3) In all these transformations the amount of energy which is
dissipated is relatively small, and tends to vanish.
From the point of view, then, of energetic processes these are the
characters of life, using the term in the general sense indicated
above.[16]
[16] This is, of course, the argument of part of Chapter II. of
Bergson’s _Creative Evolution_. The reader will not find the essential
differences between plants and animals stated so clearly anywhere else
in biological literature.
Is there an absolute distinction between the organic mechanism and
the inorganic one? Let us note, for the first time, that the actual
physico-chemical transformations _themselves_, which we study in
inorganic matter, are identical with those which we study in the
organism. Molecules of carbon dioxide, water, nitrate, sodium chloride,
potassium chloride, phosphate, and so on, are just the same in inert
matter as in the organism. Chemical transformations, such as the
hydrolysis of starch, the inversion of cane sugar, or the splitting of
a neutral fat, are certainly just the same processes, whether we carry
them out in the glass vessels of the laboratory, or observe them to
proceed in the living tissues of the animal body. The same molecular
rearrangements, and the same transfers of energy, occur in both series
of events. This, however, is not the material of a distinction: what we
have to find is, whether the _direction_ of a group of physico-chemical
reactions is the same in the organism and in a series of inorganic
processes.
Let us return to the Carnot cycle. This is a series of operations which
occur in an imaginary mechanism in such a manner that the whole series
can be easily reversed. Heat is supplied to the imaginary engine,
which then performs work and yields up its heat to a refrigerator.
Work is then performed on the engine, which thereupon takes heat from
the refrigerator and returns it to the source. The work done _by_
the engine in the direct cycle is equal to the work done _on_ it in
the indirect cycle. The heat taken from the source and given to the
refrigerator in the direct cycle is equal to the heat taken from the
refrigerator and given to the source in the indirect cycle. But it
is a purely imaginary mechanism, and all experience shows not only
that it has not been realised in practice, but that it cannot so be
realised. If it could be realised, we should show that the second law
of thermo-dynamics is not physically true.
Do the energy processes of life realise such a perfectly reversible
cycle of operations? In order to answer this question we must consider
the fate of the energy which is absorbed in the plant metabolic cycle,
and that which is given out in the animal one. Does _all_ the energy of
solar radiation which is absorbed by the plant pass into the form of
the potential chemical energy of the carbohydrates and other substances
manufactured? Does any of the energy of the animal which results from
the metabolism of its body pass into the unavailable form--that is,
into a form in which it cannot be utilised by other organisms? That is
to say, is energy dissipated by the organism?
Undoubtedly it is to some extent, but to a far less extent than in the
inorganic train of processes. Some of the energy of solar radiation
absorbed by the plant must become transformed, by the friction of
whatever movements occur, into low-temperature heat, and some quantity
of heat, however small, is generated by the metabolism of the plant.
Again, some of the heat of the warm-blooded animal must be radiated
into space, or conducted away from its body; and this energy becomes
dissipated--let us assume, at least, that it is so dissipated in the
physical sense. Probably also some quantity of heat is generated by
the metabolism of the cold-blooded animal, though this must be a
very small proportion of the total energy transformed. We see, then,
that the distinction is one of degree, though the difference between
inorganic and organic energetic processes is very great in this
respect; so great that we must regard it as constituting a fundamental
difference, and as indicative of the limitation of the second law when
extended to the functioning of the organism.
But we have also to consider the effect of the work done by the
organism. We consider the nature and meaning of the evolutionary
process in a later chapter, but in the meantime we may state this
thesis: that the process of evolution leads up to man and his activity.
It _leads_, if we regard the process as a _directed_ one; but even if
we regard it as a fortuitous process we still find that man, far more
than any other organism, is the result of it. All the facts of biology
and history show that man dominates the organic world, plant or animal;
that the whole trend of his activity is to eliminate whatever organisms
are inimical, and to foster those that are useful. Already, during the
brief period of his rational activity, the wolf has disappeared from
civilised lands while the dog has been produced. Species after species
of hostile or harmful organisms have been, or are being, destroyed or
changed, while numerous other species have been preserved and altered
for his benefit. In the future we see an organic world subservient to
him either entirely or to an enormous extent.
So also in the inorganic world. Rivers which formerly rushed down
through rapids, dissipating their energy of movement in waste
irrecoverable heat, now pour through turbines and water wheels,
generating electricity and accumulating available energy. Winds
which “naturally” dissipated their mechanical energy in waste heat
now propel ships and windmills. Tides, with their incredibly great
mechanical energy, now simply warm up the crust of the earth by an
infinitesimal fraction of a degree daily, and produce heat which at
once radiates into space. Who doubts that by and by this energy too
will become accumulated for human use? Multitudes of chemical reactions
were potential, so to speak, in the molecules of petroleum, while the
energy which might have produced them ran to waste. But under human
activity this energy became directed and made to produce chemical
reactions formerly existing only in their possibility, and all the
substances of modern organic chemistry came into existence.
The energy, then, of human activity has been directed towards averting
or retarding the progress towards dissipation, or irrecoverable waste,
of cosmic energy--that of the sun’s radiation, and of the motions
of earth and moon. Human activity has accumulated available energy.
The difference of water-level between Niagara and the rapids below
represents available mechanical energy. A few years ago an enormous
quantity of this energy became irredeemably lost in waste heat every
twenty-four hours: now it remains available for work; and this quantity
of work retained is enormously greater than is the human energy which
was expended on erecting the water-power installation there.
The processes studied by physics and chemistry are therefore
irreversible ones. We can conceive a perfectly reversible process,
as in the Carnot heat-engine, but this is a purely intellectual
conception, formed as the limit to a series of operations which
approximate closer and closer to an ideal reversibility. It is a
conception that has no physical reality--a guide to reasoning only. On
the other hand we see that all naturally occurring physical processes
are irreversible and in their sum tend to complete degradation of
energy. Mechanistic biology isolates physico-chemical processes in the
functioning of the organism, and sees that they conform to the law of
dissipation, as well as to that of the conservation of energy.
Yet the organism as a whole, that is, life as a whole, on the earth,
does not conform to the law of dissipation. That which is true of the
isolated processes into which physiology decomposes life is not true of
life. In all inorganic happenings energy becomes unavailable for the
performance of work. Solar radiation falling on sea and land fritters
itself away in waste irrecoverable heat, but falling on the green plant
accumulates in the form of available chemical energy. The total result
of life on the earth in the past has been the accumulation of enormous
stores of energy in the shape of coal and other substances. By its
agency degradation has been retarded. Whenever, says Bergson, energy
descends the incline indicated by Carnot’s law, and where a cause of
inverse direction can retard the descent, there we have life.