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THE WHEEL OF LIFE

I heard them in their sadness say

The earth rebukes the thought of God;

We are but embers wrapped in clay,

A little nobler than the sod.

But I have touched the lips of clay;

Mother, thy rudest sod to me

Is thrilled with fire of hidden day,

And haunted by all mystery.—A.E.

IT has long been known that no definite line of demarcation can be drawn between the animal and the vegetable worlds. There are lowly organisms which cannot be decisively referred either to the one or to the other. It has been more recently shown that the apparently more strongly marked line between the living and the non-living also grows wavering and indistinct in places. Metals are known to respond to stimuli and to show ‘fatigue’ in a manner commonly attributed only to the nervous system of animals,16 while some of the phenomena of crystallization strongly resemble those of vitality.17 Le Dantec has uttered the latest word of physics on this subject, where he insists on the “absence of all essential difference and all absolute discontinuity between living and non-living matter.”18 Indeed, one may say of nature-study in general, that if, as Plato said, the beginning of knowledge is in definitions and classification, the end of it lies in getting rid of them. There is probably no such thing as a universally applicable definition of any group of natural phenomena. There is certainly no condition of matter of which we are entitled to say that Life is impossible without it. Still, natural groups have well-marked central features, even if at their margins they melt into something else. Now the things which in the ordinary sense of the word we call Living are marked by these characters: Their chemical constituents are always compounds of carbon. These compounds are what is called ‘unstable’—they ‘consume’ or disintegrate by combining with oxygen in air or water. In this process organisms obtain the energy necessary for assimilation and growth. The above characters (carbon-compounds, chemical instability, and faculty of assimilation) apply to plants and animals alike. But we find, in general, that plants are able, from inorganic mineral constituents such as carbonic acid, water, nitrates, sulphates, etc.,19 to build up the organic compounds like proteid, albumin, the carbohydrates, alcohol, fat; while animals use for their nourishment not the inorganic substances but only organic compounds already formed by plants or by other animals. A well-developed vegetable world must therefore, it would seem, have preceded the appearance of animal life on the globe.20 It was long believed that these organic compounds of carbon could only be formed by the vital action of living vegetation. One of the epochs in the history of modern chemistry has been the demonstration (first by Wöhler, in 1828,) that many of them can be produced in the laboratory from inorganic chemical constituents. But this is only effected by circuitous and difficult ways, and—a circumstance often overlooked—it only resembles what is accomplished in nature if we include under nature the directive agency represented by the chemist himself, as well as the materials with which he deals.

The characteristic colour of living vegetation is green. This is also the rarest of colours among the higher animals.21 It is due in vegetables to the presence in their cells of grains of the substance known as chlorophyll, which very few animals possess or have need of. It is developed normally under the action of sunlight, and plays a most important part in the economy of the plant. The usual method by which any organism obtains the energy necessary for its vital functions is through the oxidization, i.e. the slow burning, of its substance, by combination with the oxygen of the air. The process is to all intents the same as the more rapid oxidization, under great heat, of coal in a steam-engine. If a plant can obtain sugar, which oxidizes easily in contact with atmospheric oxygen, it has thus a fund of energy to draw on for all the processes of its life. Now sugar is composed of carbon and water. Carbon exists in the air, in combination with oxygen, in the form known as carbonic acid. Chlorophyll, in some way as yet unexplained, enables plants, when acted upon by light, to take in carbonic acid from the air and to disintegrate it into its constituents, carbon and oxygen. The oxygen disappears again in the air, and the released carbon combines with water in the plant to form sugar,22 thus giving the plant its needed store of potential energy. All it does with this energy is to live, grow, and reproduce its kind; till at length a time comes when the assimilative energy weakens relatively to the forces of decay, and the plant dies; it is again resolved into the chemical constituents from which it was built up; but not without having passed on the flame of life to burn afresh in its descendants.

Plants which have no chlorophyll, like certain bacteria and moulds, and which, therefore, cannot decompose the carbonic acid in the air for their nourishment, offer an interesting example of the manner in which Nature contrives to get her way, if not by the normal instruments, then by the utilization of others. They acquire their first store of energy sometimes like animals, from other organic compounds, or they take carbon from acetates and tartrates. The nitro-bacteria appear to depend on ammonia derived from decaying animal matter, and the moulds draw their energy from sugar, which (as in our jams, etc.) they find already formed.

There are other plants, such as the fly-eating Drosera, which feed upon organic substances with the aid of digestive juices, exactly as animals do; and there are animals, such as Hydra and others, of very primitive form,23 which produce chlorophyll and are thereby enabled, like plants, to feed upon carbonic acid. The distinction therefore which has been drawn between the two kingdoms as regards their modes of nourishment must, like other definitions of natural groups, be taken to apply to central and typical forms and not to constitute a distinct boundary line. Allowing for these exceptional cases, we may say broadly that the wheel of life makes its full circle in passing from inorganic matter through plants to animals and thence back to gases and minerals again. The process of taking in fresh matter, transforming it chemically into living tissue, and thus repairing the waste occasioned by the decomposition of the carbon-compounds of that tissue, is technically known as Metabolism. This is the typical and characteristic function of organic life.

Now this function of living matter, or Protoplasm, depends upon two elements: first, its Substance; secondly, its Structure. As regards the former, we are in this serious difficulty, that living matter can never be chemically investigated by any means at present known, for it dies immediately in presence of any of the reagents which are used to ascertain its chemical composition. It is known that there are no elementary substances in living matter which are not also found in the world of inorganic matter, but it is also known that their synthetic combination in living is different from that which obtains in dead tissue,24 and it is precisely through this factor—that of the grouping or synthesis of elements—that the most remarkable forms of energy are developed.

The secret of life, therefore, cannot be stated in terms of chemistry, because we cannot surprise the secret of its chemical synthesis. Even if we could do this we should still be unable to say why certain syntheses should appear in living matter and resolve themselves into others at death.

We find, however, in the investigation of organic tissue (plant or animal) by such means as are available, that one substance is common to all the organic and is never found (as such) in the inorganic world. This is called Proteid. It is composed of five elements—Carbon, Hydrogen, Sulphur, Nitrogen, and Oxygen, which are combined in proportions not at present ascertained. Subject to the limitations just set forth we may say that proteid is the essential stuff of organic tissue. The two other usual (though not, like proteid, universal) constituents of this tissue—the Carbohydrates (sugar, starch, etc.) and the Fats—are, it is believed, formed partly from the products of the metabolism of proteid.

When we come to deal with the essential Structure of life we are in much the same difficulty as that in which we found ourselves in investigating its chemical Substance. We can observe living cells under the microscope, but the most powerful microscope has never reached the limits beyond which we can say that there is no structure. There is another limitation too. The microscope has revealed the fact that all living tissue is made up of cells, but the internal structure of the cell, beyond the fact that it is composed of a fluid substance within which a darker coloured nucleus is usually embedded, could not be ascertained until the recent device of staining the object with aniline dyes had been thought of. Different substances in the cell are found to take these dyes differently, and thus a world of structure of the most singular kind has been revealed in what formerly seemed a simple, semi-transparent fluid. Some parts of this structure hover, as it were, upon the very edge of perceptibility, the most suitable dyes for bringing them under observation not having been as yet discovered. There may be others which no dye can reveal, but which are yet active and necessary parts of the organism. Moreover, here too the cell is killed by the means taken to observe it, and the processes in which its structure is engaged can only as a rule be deduced from the observation of a great number of cells in which their internal movements are arrested at different stages of completion.

It has been practically demonstrated that all organic life must be at least duplex if not multiplex in its constituent elements. In its simplest known form it consists of Protoplasm and Nucleus. We know that the carrying-on of all vital functions depends on peculiar relations existing between these two elements, but what these relations exactly are is still quite obscure. Both protoplasm and nucleus are compounds of proteid with other chemical substances not yet fully determined. Protoplasm is a fluid, and has been shown by the epoch-making observations of Bütschli25 to have a structure resembling that of an exceedingly minute foam. The nucleus usually exists in the form of a single definite body, but it may be scattered through the protoplasm of the organism in little granules. In the lowliest of organisms, the Amœbæ, we have simply a speck of protoplasm containing a nucleus, but with no surrounding wall of the harder substance which protoplasm builds up for itself in the cells belonging to higher forms of life. Such amœboid forms are the white corpuscles in the human blood, whose slow changes of form we can observe under the microscope, and which play so important a part in our economy by feeding on the noxious bacteria which produce the various forms of blood-poisoning and zymotic disease.

A more detailed account of the functions and structure of the cell must be reserved for the next chapter. In considering these and all other phenomena of vitality let me again recall the warning expressed in the taunt of Mephistopheles to the young student: the lines are as true to-day as they were when Goethe wrote them over a hundred years ago:—

“If some living thing you would learn about,

You begin by driving its Spirit out;

There lie the parts of it, one by one,

But the binding Spirit, alas, is gone!”