Part 199. The organic realm. A typical living organism may be conceived of as a self-building boat formed of materials taken from the inorganic stream in which it floats, but controlled by an indwelling, immaterial power capable of steering as it chooses. The materials of such a living boat as we have imagined would be continually dissolving into the stream; while, at the same time, fresh inorganic material, admitted by the indwelling power, would be building the structure anew. So long as these materials formed part of the boat or showed signs of having once belonged to its organized structure, we should call them organic; and we should apply the same term to any compounds possessing the same properties. So long as the materials were arranged in a way to permit the indwelling chooser to act through them directly, they would constitute living substance. Until thus controlled they would be simply lifeless substances; after they had passed from this control they would be dead. Before they had been organized and after they had ceased to bear the marks of organization we should call them inorganic.

The materials of which these wonderful boats are made consist chiefly, as we have seen, of the elements carbon, hydrogen, oxygen, and nitrogen. It is perhaps significant that each of the four is preeminent for certain properties which are in marked contrast with what characterizes one or more of the others. Carbon, in a sense, is the most solid of all known substances. It requires the highest temperature to melt it, and in its diamond condition exceeds all other materials in hardness. It is remarkable for the different ways it can combine, and as entering into more compounds than all the other elements taken together. Hydrogen, on the other hand, is of all common elements the most fluid. It requires the utmost cold to freeze it and remains gaseous under the highest pressure. It is remarkable for the ease with which it may be made to pass from one compound to another. Oxygen, also a gas at ordinary temperatures, is preëminent for the stability of its compounds, and for the activity it shows in combining; while nitrogen, similarly gaseous, is in marked contrast as being most difficult to combine and most unstable in combination. We have here, then, three of the most fluid of substances, gaseous at all life-temperatures, combined with the most solid substance known; and among the four we find the readiest combiner, and the most inert; the easiest to displace, and the most firmly grappling; the stablest, and the most unstable, of all common elements. From the interplay of such opposites extraordinary resultants should appear.

If localized wills are to gain progressive expression through masses of matter we should expect that the materials used would have both mobility and fixity. That is to say, we should look for a constant flow of particles, and at the same time relative permanence in the arrangements into which they temporarily enter; for only thus could change be added to change. Furthermore, if such a will were to be free to oppose outside influences as well as to yield to them promptly, the material through which it responded should have unusual stability associated with an instability resembling that of explosive compounds. Accordingly, since the properties of a compound result from the properties of its constituent elements more or less modified by mutual influence, it may not be altogether fanciful to suppose that the solidity of carbon, the fluidity of hydrogen, the stability of oxygen, and the instability of nitrogen may be especially significant as properties which in combination largely account for the almost paradoxical properties of living substance which is characterized by permanence with constant change, and sensitiveness with resistance; and having withal such an exceeding delicacy of balance that an infinitesimal force is sufficient to release energy in one direction rather than another.

Of course a complete explanation of the chemico-physical properties of this living substance, if ever attainable, must be vastly more complex than might appear from the vague suggestions given above as to possible connections between a few important facts. The purpose of these hints is merely to indicate how increasing knowledge of matter may help us to understand the conditions under which life is possible, and so be of profit in our dealings with the world in which we live. It seems only reasonable to assume that the properties inherent in the materials of which all living bodies are composed should make possible and largely determine the activities they all exhibit.

Whatever may be the explanation of the fundamental properties of protoplasm, they are indeed, marvelous to contemplate. Our simile of the living boats would need to be much elaborated before it could well portray the bewildering complexities of action and interaction which go on within the simplest organisms. We said that the materials of each organic craft were being continually lost and continually replaced. But we must remember that often more is added than is lost; then the organism grows. It should be said also that so long as inner impulses control the arrangement of the fresh material, and thus partly determine the character of the growing structure, the arrangements formed usually show progressive fixity, each arrangement determining somewhat the arrangements which follow and rendering them less susceptible of change. Hence, old age with its decreasing mobility and final death is an incidental result of the progressive fixity which makes structure and habit possible. Yet under certain conditions, as we have seen, protoplasm passes into a fixed condition, to all appearance like that of death, but from which it may revive with youth renewed. A similar renewal of mobility distinguishes reproduction from mere growth, and offsets death in the economy of nature. Our living boats, then, grow old, and may die; or, they may become inactive and afterward resume activity with youthful vigor. When they have grown large enough they form out of themselves new boats, similarly invigorated and similarly relieved from the hamperings of old habits or fixed arrangements;-but not entirely, for each is built upon much the same lines as its parent, and in its own building can only modify the design. Yet what wonders may result from an ever so slight power of modification bearing the slightest impress of a choice ! This part may be modified in one way, that in another: and morphological differentiation with physiological division of labor may ensue. What one brief life cannot accomplish, another may; individuality, heredity, adaptation, organic evolution-all are here implied. Such are the powers and potentialities of a mass of living jelly.

Our imagined boats each built and captained by a choosing power are meant to represent living things in general. All plants and all animals, as we have seen, differ from all minerals in having differentiated organs adapted to the needs imposed by the conditions under which they live; and in detaching certain portions of their substance, such as seeds or eggs, capable of developing from infancy to old age by taking in as food suitable materials, transforming them, then building them into their bodies, and finally after utilization, eliminating them as waste products. Such being the characteristics of all living things we should hardly expect any well-marked peculiarities by which all animals can be distinguished from all plants. In fact there is not a single point of difference available for separating sharply the animal from the vegetable kingdom. The Linnaean criterion of feeling we have already found to fail when applied to primitive types. So also the popular criterion of motion or locomotion must be rejected by anyone acquainted with the lower forms of life, which include not only motile plants but fixed animals; and we have only to remember the absence of chlorophyll from many plants to realize that even this highly characteristic vegetable substance does not afford an adequate mark of distinction between the two kingdoms. Not a few organisms behave like plants at one stage, and like animals at another. A considerable number of these vegeto-animal organisms have been claimed alike by botanists and zoologists. The uncertainty in classifying such forms has led to the suggestion that a third organic kingdom, intermediate between the animal and the vegetable, be recognized to include all the kinds in dispute. This suggestion has not met with much favor among naturalists, for instead of lessening the practical difficulties of the case it would really double them by giving us two uncertain boundary lines instead of one. Our best way surely is to meet the difficulty by trying to define as strictly as possible what may be conveniently meant by animal and plant, remembering that whatever definition we frame is sure to be arbitrary.

We know that the great majority of plants organize inorganic material, while the great majority of animals, if not all, have no such power and so must depend upon plants for their food. The raw materials which plants build up into food have only to be absorbed in solution from the water, soil, or air in which they live. The elaborated food of animals, on the other hand, is generally solid and so requires to be dissolved in a digestive cavity within the body before absorption is possible. Plants which can make their own food from materials always at hand have no such need for traveling about in search of food as most animals have; and while food-seeking calls for the special sensitiveness which Linnaeus termed feeling, food-making involves only such manifestations of irritability as might easily escape his notice. A typical plant is thus a sedentary food-maker, a typical animal being a roving eater. It is only when plants lose more or less their power of making food, and animals their power of locomotion that doubt arises as to their kingdom, and then the question has to be decided not so much by rules and definitions, as by evidences of their kinship to undoubted examples of vegetable or animal life.

A Bacterium, for example, is classed as a plant because of resemblances to a Nostoc which outweigh its animal-like motility and dependence upon organic food. If a Bacterium should develop a digestive cavity for the reception of its food we should say it had become an animal.

The most fundamental difference between plants and animals appears thus to lie in the ways they prefer to get their food-the vegetable way being to make the best of what comes to it, the animal choosing rather to capture what plants have made. Returning to our simile of the boats it might be said that the vegetable craft choose to anchor in a stream of materials which they organize into food, while the animal craft navigate the stream and repair their losses entirely from other vessels. A modern revision of the aphorism of Linnaeus, still, however, confessedly inaccurate, might read:-minerals crystallize; plants organize or reorganize materials which they absorb; animals reorganize food which they have swallowed.