THE LIVING ORGANISM
A world for habitation, then, is a world whereon living organisms can
exist that are comparable in intelligence with men. But "men" presuppose
the existence of living organisms of inferior grades. Therefore a world
for habitation must first of all be one upon which it is possible for
living organisms, as such, to exist.
It does not concern us in the present connection how life first came into
existence on this planet. It is sufficient that we know from experience
that life does exist here; and in whatsoever way it was first generated
here, in that same way we may consider that it could have been generated
on another planet.
Nor need any question trouble us as to the precise line of demarkation to
be drawn between inorganic and organic substances, or amongst the latter,
between plants and animals. These are important subjects for discussion,
but they do not affect us here, for we are essentially concerned with the
highest form of organism, the one furthest from these two dividing lines.
It suffices that living organisms do exist here, and exist under
well-defined conditions. Wanting these conditions, they perish. We can, to
a varying degree, determine the physical conditions prevailing upon the
heavenly bodies, and we can ascertain whether these physical conditions
would be favourable, unfavourable, or fatal to the living organism.
What is a living organism? A living organism is such that, though it is
continually changing its substance, its identity, as a whole, remains
essentially the same. This definition is incomplete, but it gives us a
first essential approximation, it indicates the continuance of the whole,
with the unceasing change of the details. Were this definition complete, a
river would furnish us with a perfect example of a living organism,
because, while the river remains, the individual drops of water are
continually changing. There is then something more in the living organism
than the continuity of the whole, with the change of the details.
An analogy, given by Max Verworn, carries us a step further. He likens
life to a flame, and takes a gas flame with its butterfly shape as a
particularly appropriate illustration. Here the shape of the flame remains
constant, even in its details. Immediately above the burner, at the base
of the flame, there is a completely dark space; surrounding this, a bluish
zone that is faintly luminous; and beyond this again, the broad spread of
the two wings that are brightly luminous. The flame, like the river,
preserves its identity of form, while its constituent details--the gases
that feed it--are in continual change. But there is not only a change of
material in the flame; there is a change of condition. Everywhere the gas
from the burner is entering into energetic combination with the oxygen of
the air, with evolution of light and heat. There is change in the
constituent particles as well as change of the constituent particles;
there is more than the mere flux of material through the form; there is
change of the material, and in the process of that change energy is
developed.
A steam-engine may afford us a third illustration. Here fresh material is
continually being introduced into the engine there to suffer change. Part
is supplied as fuel to the fire there to maintain the temperature of the
engine; so far the illustration is analogous to that of the gas flame. But
the engine carries us a step further, for part of the material supplied to
it is water, which is converted into steam by the heat of the fire, and
from the expansion of the steam the energy sought from the machine is
derived. Here again we have change in the material with development of
energy; but there is not only work done in the subject, there is work done
by it.
But the living organism differs from artificial machines in that, of
itself and by itself, it is continuously drawing into itself non-living
matter, converting it into an integral part of the organism, and so
endowing it with the qualities of life. And from this non-living matter it
derives fresh energy for the carrying on of the life of the organism.
The engine and the butterfly gas flame do not give us, any more than the
river, a complete picture of the living organism. The form of the river is
imposed upon it from without; the river is defined by its bed, by the
contour of the country through which it flows. The form and size of the
flame are equally defined by exterior conditions; they are imposed upon it
by the shape of the burner and the pressure of the gas passing through it.
The form of the engine is as its designer has made it. But the form of the
living organism is imposed upon it from within; and, as far as we can
tell, is inherent in it. Here is the wonder and mystery of life: the power
of the living organism to assimilate dead matter, to give it life and
bring it into the law and unity of the organism itself. But it cannot do
this indiscriminately; it is not able thus to convert every dead material;
it is restricted, narrowly restricted, in its action. "One of the chief
characteristics of living matter is found in the continuous range of
chemical reactions which take place between living cells and their
inorganic surroundings. Without cease certain substances are taken up and
disappear in the endless round of chemical reactions in the cell. Other
substances which have been produced by the chemical reactions in living
matter pass out of the cell and reappear in inorganic nature as waste
products of the life process. The whole complex of these chemical
transformations is generally called _Metabolism_. Inorganic matter
contrasts strikingly with living substance. However long a crystal or a
piece of metal is kept in observation, there is no change of the
substance, and the molecules remain the same and in the same number. For
living matter the continuous change of substances is an indispensable
condition of existence. To stop the supply of food material for a certain
time is sufficient to cause a serious lesion of the life process or even
the death of the cell. But the same happens when we hinder the passing out
of the products of chemical transformation from the cell. On the other
hand, we may keep a crystal of lifeless matter in a glass tube carefully
shut up from all exchange of substance with the external world for as many
years as we like. The existence of this crystal will continue without end
and without change of any of its properties. There is no known living
organism which could remain in a dry resting state for an infinitely long
period of time. The longest lived are perhaps the spores of mosses which
can exist in a dry state more than a hundred years. As a rule the seeds of
higher plants show their vital power already weakened after ten years;
most of them do not germinate if kept more than twenty to thirty years.
These experiences lead to the opinion that even dry seeds and spores of
lower plants in their period of rest of vegetation continue the processes
of metabolism to a certain degree. This supposition is confirmed by the
fact that a very slight respiration and production of carbonic acid can be
proved when the seeds contain a small percentage of water. It seems as if
life were weakened in these plant organs to a quite imperceptible degree,
but never, not even temporarily, really suspended.
"Life is, therefore, quite inseparable from chemical reactions, and on the
whole what we call life is nothing else but a complex of innumerable
chemical reactions in the living substance which we call protoplasm."[1]
The essential quality, therefore, of life is continual change, but not
mere change in general. It is that special process of the circulation of
matter which we call metabolism, and this circulation is always connected
with a particular chemical substance--protoplasm.
In this substance five elements are always present and
predominant--carbon, oxygen, nitrogen, hydrogen, and sulphur. The
compounds which these five elements form with each other are most complex
and varied, and they also admit to combination--but in smaller
proportions--some of the other elements, of which phosphorus, potassium,
calcium, magnesium, and iron are the most important.
For protoplasm--using the term in the most general sense--is a chemical
substance, not a mere mixture of a number of chemical elements, nor a mere
mechanical structure. "However differently the various plasma substances
behave in detail, they always exhibit the same general composition as the
other albuminoids out of the five 'organo-genetic elements'--namely in
point of weight, 51-54% carbon, 21-23% oxygen, 15-17% nitrogen, 6-7%
hydrogen, and 1-2% sulphur."[2]
Haeckel, the writer just quoted, describes the plasm, the universal basis
of all the vital phenomena, in the following terms: "In every case where
we have with great difficulty succeeded in examining the plasm as far as
possible and separating it from the plasma-products, it has the appearance
of a colourless, viscous substance, the chief physical property of which
is its peculiar thickness and consistency. The physicist distinguishes
three conditions of inorganic matter--solid, fluid, and gaseous. Active
living protoplasm cannot be strictly described as either fluid or solid in
the physical sense. It presents an intermediate stage between the two
which is best described as viscous; it is best compared to a cold jelly,
or solution of glue. Just as we find the latter substance in all stages
between the solid and the fluid, so we find in the case of protoplasm. The
cause of this softness is the quantity of water contained in the living
matter, which generally amounts to a half of its volume and weight. The
water is distributed between the plasma molecules or the ultimate
particles of living matter in much the same way as it is in the crystals
of salts, but with the important difference that it is very variable in
quantity in the plasm. On this depends the capacity for the absorption or
imbibition in the plasm, and the mobility of its molecules, which is very
important for the performance of the vital actions. However, this capacity
of absorption has definite limits in each variety of plasm; living plasm
is not soluble in water, but absolutely resists the penetration of any
water beyond this limit."[3] And Czapek further tells us that "the most
striking feature of cell life is the fact that an enormous number of
chemical reactions take place within the narrowest space. Most plant cells
do not exceed 0.1 to 0.5 millimetres in diameter. Their greatest volume
therefore can only be an eighth of a cubic millimetre. Nevertheless, in
this minute space we notice in every stage of cell life a considerable
number of chemical reactions which are carried on contemporaneously,
without one disturbing the other in the slightest degree."[4]
It is clear if organic bodies were built up of chemical compounds of small
complexity and great stability that this continuous range of chemical
reactions, this unceasing metabolism, could not take place. It is
therefore a necessary condition for organic substances that they should be
built up of chemical compounds that are most complex and unstable.
"Exactly those substances which are most important for life possess a very
high molecular weight, and consequently very large molecules, in
comparison with inorganic matter. For example: egg-albumin is said to have
the molecular weight of at least 15,000, starch more than 30,000, whilst
the molecular weight of hydrogen is 2, of sulphuric acid and of potassium
nitrate about 100, and the molecular weight of the heaviest metal salts
does not exceed about 300."[5]
To sum up: the living organism, whether it be a simple cell, or the
ordered community of cells making up the perfect plant or animal, is an
entity, a living individual, wherein highly complex and unstable compounds
are unceasingly undergoing chemical reactions, a metabolism essentially
associated with protoplasm. But these complex compounds are, nevertheless,
formed by the combinations of but a few of the elements now known to us.
Many writers on the subject of the habitability of other worlds, from
contemplating the rich and apparently limitless variety of the forms of
life, and the diversity of the conditions under which they exist, have
been led to assume that the basis of life must itself also in like manner
be infinitely broad and infinitely varied. In this they are mistaken. As
we have seen, the elements entering into the composition of organic bodies
are, in the main, few in number. The temperatures at which they can exist
are likewise strictly limited. But, above all, that circulation of matter
which we call Life--the metabolism of vital processes--requires for its
continuance the presence of one indispensable factor--WATER.
Protoplasm itself, as Czapek puts it, is practically an _albumin sol_;
that is to say, it is a chemical substance of which the chief constituents
are albuminous matter and water, and the protoplasm can only take from
without material dissolved in water; it can only eject matter in the same
way. This _osmosis_ is an indispensable condition in the vital process.
And the "streaming" of protoplasm, its continual movement in the cell, can
only be carried on in water.
WATER is the compound of oxygen and hydrogen in the proportion of two
atoms of hydrogen to one of oxygen. It is familiar to us in three states:
solid, liquid, and gaseous, or ice, water, and steam. But it is only in
the liquid state that water is available for carrying on the processes of
life. This fact limits the temperatures at which the organic functions can
be carried on, for water under terrestrial conditions is only liquid for a
hundred degrees; it freezes at 0 deg. Centigrade, it boils at 100 deg. Centigrade.
Necessarily, our experiences are mostly confined within this range, and
therefore we are apt unconsciously to assume that this range is all the
range that is possible, whereas it is but a very small fraction of the
range conceivable, and indeed existing, in cosmical space. In its liquid
state water is a general solvent, and yet pure water is neutral in its
qualities, both characteristics being essential to its usefulness as a
vehicle for the protoplasmic actions. Naturally, this function of water as
a solvent can only exist when water is in the liquid state; solid water,
that is ice, neither dissolves nor flows; and water, when heated to
boiling point, passes into vapour, and so leaves the organism
moistureless, and therefore dead. It is possible to grind a living
organism to a pulp so that the structure of the cells is practically
destroyed, and yet for some reactions which are quite peculiar to life
still to show themselves for some appreciable time. But when the cell-pulp
is heated to the temperature of boiling water, these chemical processes
cannot be longer observed. What is left may then be considered as
definitely dead.
Water is, then, indispensable for the living organism; but there are two
great divisions of such organisms--plants and animals. Animals are
generally, but not universally, free to move, and therefore to travel to
seek their food. But their food is restricted; they cannot directly
convert inorganic matter to their own use; they can only assimilate
organic material. The plant, on the other hand, unlike the animal, can
make use of inorganic material. Plant life, therefore, requires an
abundant supply of water in which the various substances necessary for its
support can be dissolved; it must either be in water, or, if on land,
there must be an active circulation of water both through the atmosphere
and through the soil, so as to bring to it the food that it requires.
Animal life presupposes plant life, for it is always dependent upon it.
Many writers have assumed that life is very widely distributed in
connection with this planet. The assumption is a mistaken one, as has been
well pointed out by Garrett P. Serviss, a charming writer on astronomical
subjects: "On the Earth we find animated existence confined to the surface
of the crust of the globe, to the lower and denser strata of the
atmosphere, and to the film of water that constitutes the oceans. It does
not exist in the heart of the rocks forming the body of the planet nor in
the void of space surrounding it outside the atmosphere. As the Earth
condensed from the original nebula, and cooled and solidified, a certain
quantity of matter remained at its surface in the form of free gases and
unstable compounds, and, within the narrow precincts where these things
were, lying like a thin shell between the huge inert globe of permanently
combined elements below, and the equally unchanging realm of the ether
above, life, a phenomenon depending upon ceaseless changes, combinations
and re-combinations of chemical elements in unstable and temporary union,
made its appearance, and there only we find it at the present time."[6]
"The huge inert globe of permanently combined elements below, and the
equally unchanging realm of the ether above," offer no home for the living
organism; least of all for the highest of such organisms--Man. Both must
be tempered to a condition which will permit and favour continual change,
the metabolism which is the essential feature of life.
"When the earth had to be prepared for the habitation of man, a veil, as
it were, of intermediate being was spread between him and its darkness, in
which were joined, in a subdued measure, the stability and the
insensibility of the earth, and the passion and perishing of mankind.
"But the heavens, also, had to be prepared for his habitation. Between
their burning light,--their deep vacuity, and man, as between the earth's
gloom of iron substance, and man, a veil had to be spread of intermediate
being;--which should appease the unendurable glory to the level of human
feebleness, and sign the changeless motion of the heavens with the
semblance of human vicissitude. Between the earth and man arose the leaf.
Between the heaven and man came the cloud. His life being partly as the
falling leaf and partly as the flying vapour."[7]
The leaf and the cloud are the signs of a habitable world. The leaf--that
is to say, plant life, vegetation--is necessary because animal life is not
capable of building itself up from inorganic material. This step must have
been previously taken by the plant. The cloud, that is to say
water-vapour, is necessary because the plant in its turn cannot directly
assimilate to itself the nitrogen from the atmosphere. The food for the
plant is brought to it by water, and it assimilates it by the help of
water. It is, therefore, upon the question of the presence of water that
the question of the habitability of a given world chiefly turns. In the
physical sense, man is "born of water," and any world fitted for his
habitation must "stand out of the water and in the water."