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.