CHAPTER II

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."