THE ESSENTIAL CHARACTERS OF THE LIVING ORGANISM
Before trying to comprehend the physical conditions on any planet which are
essential for the development and maintenance of a varied and complex
system of organic life comparable to that of our earth, we must obtain some
knowledge of what life is, and of the fundamental nature and properties of
the living organism.
Physiologists and philosophers have made many attempts to define 'life,'
but in most cases in aiming at absolute generality they have been vague and
uninstructive. Thus De Blainville defined it as 'The twofold internal
movement of composition and decomposition, at once general and continuous';
while Herbert Spencer's latest definition was 'Life is the continuous
adjustment of internal relations to external relations.' But neither of
these is sufficiently precise, explanatory, or distinctive, and they might
almost be applied to the changes occurring in a sun or planet, or to the
elevation and gradual formation of a continent. One of the oldest
definitions, that of Aristotle, seems to come nearer the mark: 'Life is the
assemblage of the operations of nutrition, growth, and destruction.' But
these definitions of 'life' are unsatisfactory, because they apply to an
abstract idea rather than to the actual living organism. The marvel and
mystery of life, as we know it, resides in the body which manifests it, and
this living body the definitions ignore.
The essential points in the living body, as seen in its higher
developments, are, first, that it consists throughout of highly complex but
very unstable forms of matter, every particle of which is in a continual
state of growth or decay; that it absorbs or appropriates dead matter from
without; takes this matter into the interior of its body; acts upon it
mechanically and chemically, rejecting what is useless or hurtful; and so
transforming the remainder as to renew every atom of its own structure
internal and external, at the same time throwing off, particle by particle,
all the worn-out or dead portions of its own substance. Secondly, in order
to be able to do all this, its whole body is permeated throughout by
branching vessels or porous tissues, by which liquids and gases can reach
every part and carry on the various processes of nutrition and excretion
above referred to. As Professor Burdon Sanderson well puts it: 'The most
distinctive peculiarity of living matter as compared with non-living is,
that it is ever changing while ever the same.' And these changes are the
more remarkable because they are accompanied, and even produced, by a very
large amount of mechanical work--in animals by means of their normal
activities in search of food, in assimilating that food, in continually
renewing and building up their whole organism, and in many other ways; in
plants by building up their structure, which often involves raising tons of
material high into the air, as in forest trees. As a recent writer puts
it: 'The most prominent, and perhaps the most fundamental, phenomenon of
life is what may be described as the _Energy Traffic_ or the function of
_trading in energy_. The chief physical function of living matter seems to
consist in absorbing energy, storing it in a higher potential state, and
afterwards partially expending it in the kinetic or active form.'[11]
Thirdly--and perhaps most marvellous of all--all living organisms have the
power of reproduction or increase, in the lowest forms by a process of
self-division or 'fission,' as it is termed, in the higher by means of
reproductive cells, which, though in their earliest stage quite
indistinguishable physically or chemically in very different species, yet
possess the mysterious power of developing a perfect organism, identical
with its parents in all its parts, shapes, and organs, and so wonderfully
resembling them, that the minutest distinctive details of size, form, and
colour, in hair or feathers, in teeth or claws, in scales, spines, or
crests, are reproduced with very close accuracy, though often involving
metamorphic changes during growth of so strange a nature that, if they were
not familiar to us but were narrated as occurring only in some distant and
almost inaccessible region, would be treated as travellers' tales,
incredible and impossible as those of Sindbad the Sailor.
In order that the substance of living bodies should be able to undergo
these constant changes while preserving the same form and structure in
minute details--that they should be, as it were, in a constant state of
flux while remaining sensibly unchanged, it is necessary that the molecules
of which they are built up should be so combined as to be easily separated
and as easily united--be, as it is termed, _labile_ or flowing; and this is
brought about by their chemical composition, which, while consisting of few
elements, is yet highly complex in structure, a large number of chemical
atoms being combined in an endless variety of ways.
The physical basis of life, as Huxley termed it, is protoplasm, a substance
which consists essentially of only four common elements, the three gases,
nitrogen, hydrogen, and oxygen, with the non-metallic solid, carbon; hence
all the special products of plants and animals are termed carbon-compounds,
and their study constitutes one of the most extensive and intricate
branches of modern chemistry. Their complexity is indicated by the fact
that the molecule of sugar contains 45, and that of stearine no less than
173, constituent atoms. The chemical compounds of carbon are far more
numerous than those of all the other chemical elements combined; and it is
this wonderful variety and the complexity of its possible combinations
which explain the fact, that all the various animal tissues--skin, horn,
hair, nails, teeth, muscle, nerve, etc., consist of the same four elements
(with occasionally minute quantities of sulphur, phosphorus, lime, or
silica, in some of them), as proved by the marvellous fact that these
tissues are all produced as well by the grass-eating sheep or ox as by the
fish or flesh-eating seal or tiger. And the marvel is still further
increased when we consider that the innumerable diverse substances produced
by plants and animals are all formed out of the same three or four
elements. Such are the endless variety of organic acids, from prussic acid
to those of the various fruits; the many kinds of sugars, gums, and
starches; the number of different kinds of oil, wax, etc.; the variety of
essential oils which are mostly forms of turpentines, with such substances
as camphor, resins, caoutchouc, and gutta-percha; and the extensive series
of vegetable alkaloids, such as nicotine from tobacco, morphine from opium,
strychnine, curarine, and other poisons; quinine, belladonna, and similar
medicinal alkaloids; together with the essential principles of our
refreshing drinks, tea, coffee, and cocoa, and others too numerous to be
named here--all alike consisting solely of the four common elements from
which almost our whole organism is built up. If this were not indisputably
proved, it would scarcely be credited.
Professor F.J. Allen considers that the most important element in
protoplasm, and that which confers upon it its most essential properties in
the living organism--its extreme mobility and transposibility--is nitrogen.
This element, though inert in itself, readily enters into compounds when
energy is supplied to it, the most striking illustration of which is the
formation of ammonia, a compound of nitrogen and hydrogen, produced by
electric discharges through the atmosphere. Ammonia, and certain oxides of
nitrogen produced in the atmosphere in the same way, are the chief sources
of the nitrogen assimilated by plants, and through them by animals; for
although plants are continually in contact with the free nitrogen of the
atmosphere, they are unable to absorb it. By their leaves they absorb
oxygen and carbon-dioxide to build up their woody tissues, while by their
roots they absorb water in which ammonia and oxides of nitrogen are
dissolved, and from these they produce the protoplasm which builds up the
whole substance of the animal world. The energy required to produce these
nitrogen-compounds is given up by them when undergoing further changes, and
thus the production of ammonia by electricity in the atmosphere, and its
being carried by rain into the soil, constitute the first steps in that
long series of operations which culminates in the production of the higher
forms of life.
But the remarkable transformations and combinations continually going on in
every living body, which are, in fact, the essential conditions of its
life, are themselves dependent on certain physical conditions which must be
always present. Professor Allen remarks: 'The sensitiveness of nitrogen,
its proneness to change its state of combination and energy, appear to
depend on certain conditions of temperature, pressure, etc., which exist at
the surface of this earth. Most vital phenomena occur between the
temperature of freezing water and 104° F. If the general temperature of the
earth's surface rose or fell 72° F. (a small amount relatively), the whole
course of life would be changed, even perchance to extinction.'
Another important, and even more essential fact, in connection with life,
is the existence in the atmosphere of a small but nearly constant
proportion of carbonic acid gas, this being the source from which the whole
of the carbon in the vegetable and animal kingdoms is primarily derived.
The leaves of plants absorb carbonic acid gas from the atmosphere, and the
peculiar substance, chlorophyll, from which they derive their green colour,
has the power, under the influence of sunlight, to decompose it, using the
carbon to build up its own structure and giving out the oxygen. In the
laboratory the carbon can only be separated from the oxygen by the
application of heat, under which certain metals burn by combining with the
oxygen, thus setting free the carbon. Chlorophyll has a highly complex
chemical structure very imperfectly known, but it is said to be only
produced when there is iron in the soil.
The leaves of plants, so often looked upon as mere ornamental appendages,
are among the most marvellous structures in living organisms, since in
decomposing carbonic acid at ordinary temperatures they do what no other
agency in nature can perform. In doing this they utilise a special group of
ether-waves which alone appear to have this power. The complexity of the
processes going on in leaves is well indicated in the following
quotation:--
'We have seen how green leaves are supplied with gases, water, and
dissolved salts, and how they can trap special ether-waves. The active
energy of these waves is used to transmute the simple inorganic compounds
into complex organic ones, which in the process of respiration are reduced
to simpler substances again, and the potential energy transformed into
kinetic. These metabolic changes take place in living cells full of intense
activities. Currents course through the protoplasm and cell-sap in every
direction, and between the cells which are also united by strands of
protoplasm. The gases used and given off in respiration and assimilation
are floated in and out, and each protoplasm particle burned or unburned is
the centre of an area of disturbance. Pure protoplasm is influenced equally
by all rays: that associated with chlorophyll is affected by certain red
and violet rays in particular. These, especially the red ones, bring about
the dissociation of the elements of the carbonic acid, the assimilation of
the carbon, and the excretion of the oxygen.'[12]
It is this vigorous life-activity ever at work in the leaves, the roots,
and the sap-cells, that builds up the plant, in all its wondrous beauty of
bud and foliage, flower and fruit; and at the same time produces, either as
useful or waste-products, all that wealth of odours and flavours, of
colours and textures, of fibres and varied woods, of roots and tubers, of
gums and oils and resins innumerable, that, taken altogether, render the
world of vegetable life perhaps more varied, more beautiful, more
enjoyable, more indispensable to our higher nature than even that of
animals. But there is really no comparison between them. We _could_ have
plants without animals; we could _not_ have animals without plants. And all
this marvel and mystery of vegetable life, a mystery which we rarely ponder
over because its effects are so familiar, is usually held to be
sufficiently explained by the statement that it is all due to the special
properties of protoplasm. Well might Huxley say, that protoplasm is not
only a substance but a structure or mechanism, a mechanism kept at work by
solar heat and light, and capable of producing a thousand times more varied
and marvellous results than all the human mechanism ever invented.
But besides absorbing carbonic acid from the atmosphere, separating and
utilising the carbon and giving out the oxygen, plants as well as animals
continually absorb oxygen from the atmosphere, and this is so universally
the case that oxygen is said to be the food of protoplasm, without which it
cannot continue to live; and it is the peculiar but quite invisible
structure of the protoplasm which enables it to do this, and also in plants
to absorb an enormous amount of water as well.
But although protoplasm is so complex chemically as to defy exact analysis,
being an elaborate structure of atoms built up into a molecule in which
each atom must occupy its true place (like every carved stone in a Gothic
cathedral), yet it is, as it were, only the starting-point or material out
of which the infinitely varied structures of living bodies are formed. The
extreme mobility and changeability of the structure of these molecules
enables the protoplasm to be continually modified both in constitution and
form, and, by the substitution or addition of other elements, to serve
special purposes. Thus when sulphur in small quantities is absorbed and
built into the molecular structure, proteids are formed. These are most
abundant in animal structures, and give the nourishing properties to meat,
cheese, eggs, and other animal foods; but they are also found in the
vegetable kingdom, especially in nuts and seeds such as grain, peas, etc.
These are generally known as nitrogenous foods, and are very nutritious,
but not so easily digestible as meat. Proteids exist in very varied forms
and often contain phosphorus as well as sulphur, but their main
characteristic is the large proportion of nitrogen they contain, while many
other animal and vegetable products, as most roots, tubers, and grains, and
even fats and oils, are mainly composed of starch and sugar. In its
chemical and physiological aspects protein is thus described by Professor
W.D. Haliburton:--'Proteids are produced only in the living laboratory of
animals and plants; proteid matter is the all-important material present in
protoplasm. This molecule is the most complex that is known; it always
contains five and often six or even seven elements. The task of thoroughly
understanding its composition is necessarily vast, and advance slow. But,
little by little, the puzzle is being solved, and this final conquest of
organic chemistry, when it does arrive, will furnish physiologists with new
light on many of the dark places of physiological science.'[13]
What makes protoplasm and its modifications still more marvellous is the
power it possesses of absorbing and moulding a number of other elements in
various parts of living organisms for special uses. Such are silica in the
stems of the grass family, lime and magnesia in the bones of animals, iron
in blood, and many others. Besides the four elements constituting
protoplasm, most animals and plants contain also in some parts of their
structure sulphur, phosphorus, chlorine, silicon, sodium, potassium,
calcium, magnesium, and iron; while, less frequently, fluorine, iodine,
bromine, lithium, copper, manganese, and aluminium are also found in
special organs or structures; and the molecules of all these are carried
by the protoplasmic fluids to the places where they are required and built
into the living structure, with the same precision and for similar ends as
brick and stone, iron, slate, wood, and glass are each utilised in their
proper places in any large building.[14] The organism, however, is not
built, but grows. Every organ, every fibre, cell, or tissue is formed from
diverse materials, which are first decomposed into their elementary
molecules, transformed by the protoplasm or by special solvents formed from
it, carried to the places where they are needed by the vital fluids, and
there built up atom by atom or molecule by molecule into the special
structures of which they are to form a part.
But even this marvel of growth and repair of every individual organism is
far surpassed by the greater marvel of reproduction. Every living thing of
the higher orders arises from a single microscopic cell, when fertilised,
as it is termed, by the absorption of another microscopic cell derived from
a different individual. These cells are often, even under the highest
powers of the microscope, hardly distinguishable from other cells which
occur in all animals and plants and of which their structure is built up;
yet these special cells begin to grow in a totally different manner, and
instead of forming one particular part of the organism, develop inevitably
into a complete living thing with all the organs, powers, and peculiarities
of its parents, so as to be recognisably of the same species. If the
simple growth of the fully formed organism is a mystery, what of this
growth of thousands of complex organisms each with all its special
peculiarities, yet all arising from minute germs or cells the diverse
natures of which are wholly indistinguishable by the highest powers of the
microscope? This, too, is said to be the work of protoplasm under the
influence of heat and moisture, and modern physiologists hope some day to
learn 'how it is done.' It may be well here to give the views of a modern
writer on this point. Referring to a difficulty which had been stated by
Clerk-Maxwell twenty-five years ago, that there was not room in the
reproductive cell for the millions of molecules needed to serve as the
units of growth for all the different structures in the body of the higher
animals, Professor M'Kendrick says:--'But to-day, it is reasonable from
existing data to suppose that the germinal vesicle might contain a million
of millions of organic molecules. Complex arrangements of these molecules
suited for the development of all the parts of a highly complicated
organism, might satisfy all the demands of the theory of heredity.
Doubtless the germ was a material system through and through. The
conception of the physicist was, that molecules were in various states of
movement; and the thinkers were striving toward a kinetic theory of
molecules and of atoms of solid matter, which might be as fruitful as the
kinetic theory of gases. There were motions atomic and molecular. It was
conceivable that the peculiarities of vital action might be determined by
the kind of motion that took place in the molecules of what we call living
matter. It might be different in kind from some of the motions dealt with
by physicists. Life is continually being created from non-living
material--such, at least, is the existing view of growth by the
assimilation of food. The creation of living matter out of non-living may
be the transmission to the dead matter of molecular motions which are _sui
generis_ in form.' This is the modern physiological view of 'how it may be
done,' and it seems hardly more intelligible than the very old theory of
the origin of stone axes, given by Adrianus Tollius in 1649, and quoted by
Mr. E.B. Tylor, who says:--'He gives drawings of some ordinary stone axes
and hammers and tells how naturalists say that they are generated in the
sky by a fulgureous exhalation conglobed in a cloud by the circumfixed
humour, and are, as it were, baked hard by intense heat, and the weapon
becomes pointed by the damp mixed with it flying from the dry part, and
leaving the other end denser, but the exhalations press it so hard that it
breaks through the cloud and makes thunder and lightning. But--he says--if
this is really the way in which they are generated, it is odd they are not
round, and that they have holes through them. It is hardly to be believed,
he thinks.'[15] And so, when the physiologists, determined to avoid the
assumption of anything beyond matter and motion in the germ, impute the
whole development and growth of the elephant or of man from minute cells
internally alike, by means of 'kinds of motion' and the 'transmission of
motions which are _sui generis_ in form,' many of us will be inclined to
say with the old author--'It is hardly to be believed, I think.'
This brief statement of the conclusions arrived at by chemists and
physiologists as to the composition and structure of organised living
things has been thought advisable, because the non-scientific reader has
often no conception of the incomparable marvel and mystery of the
life-processes he has always seen going on, silently and almost unnoticed,
in the world around him. And this is still more the case now that
two-thirds of our population are crowded into cities where, removed from
all the occupations, the charms, and the interests of country life, they
are driven to seek occupation and excitement in the theatre, the
music-hall, or the tavern. How little do these know what they lose by being
thus shut out from all quiet intercourse with nature; its soothing sights
and sounds; its exquisite beauties of form and colour; its endless
mysteries of birth, and life, and death. Most people give scientific men
credit for much greater knowledge than they possess in these matters; and
many educated readers will, I feel sure, be surprised to find that even
such apparently simple phenomena as the rise of the sap in trees are not
yet completely explained. As to the deeper problems of life, and growth,
and reproduction, though our physiologists have learned an infinite amount
of curious or instructive facts, they can give us no intelligible
explanation of them.
The endless complexities and confusing amount of detail in all treatises on
the physiology of animals and plants are such, that the average reader is
overwhelmed with the mass of knowledge presented to him, and concludes
that after such elaborate researches everything must be known, and that the
almost universal protests against the need of any causes but the
mechanical, physical, and chemical laws and forces are well founded. I
have, therefore, thought it advisable to present a kind of bird's-eye view
of the subject, and to show, in the words of the greatest living
authorities on these matters, both how complex are the phenomena and how
far our teachers are from being able to give us any adequate explanation of
them.
I venture to hope that the very brief sketch of the subject I have been
able to give will enable my readers to form some faint general conception
of the infinite complexity of life and the various problems connected with
it; and that they will thus be the better enabled to appreciate the extreme
delicacy of those adjustments, those forces, and those complex conditions
of the environment, that alone render life, and above all the grand
age-long panorama of the development of life, in any way possible. It is to
these conditions, as they prevail in the world around us, that we will now
direct our attention.
FOOTNOTES:
[11] Professor F.J. Allen: _What is Life?_
[12] Art. 'Vegetable Physiology' in _Chambers's Encyclopædia_.
[13] Address to the British Association, 1902, Section Physiology.
[14] This enumeration of the elements that enter into the structure of
plants and animals is taken from Professor F.J. Allen's paper already
referred to.
[15] _Early History of Mankind_, 2nd ed. p. 227.