Parallel Paths: A Study in Biology, Ethics, and Art. T. W. Rolleston
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the universe could never have come into being. It may also be pointed out that the hypothesis itself seems to be irrational. For action means the production of a change of some kind, change in the nature or situation of objects. But if one of our forces is producing changes of a certain kind and the other producing changes of another kind, then they are not contradictory but complementary. The only real antagonism between two ultimate principles must consist in one of them being identified with action, change, life, the other with immobility and death. But a principle of immobility and death, if there could be such a thing, could not also be a principle of action, not even of destructive action, for to act at all would be a contradiction of its own nature. It would begin and end in total inaction, and the field would be clear for the other Power, just as if nothing else existed. It follows that, in the living and moving universe around us, there cannot be any such thing as an active principle of destruction and death. We are obliged to perceive Being under the guise of Becoming and Becoming under the guise of Change and Progression. This is a process taking place in the visible and temporal order of things and capable, under certain conditions, of partial arrest or retrogression. But the Whole, regarded as a whole, can be and can contain nothing but life, and must under all its diversity (which is an aspect of life) be One. It is this unity which alone can make intelligible and rational the diversity of which every study of life must treat. It is my endeavour in the present work to bring into clear light some important aspects of this unity, as revealed in the inter-relations of the parts of which, to our eyes, it seems to be composed.15
CHAPTER II
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