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!”
CHAPTER III
DE MINIMIS
Immense have been the preparations for me,
Faithful and friendly the arms that have help’d me.
******
“Before I was born out of my mother generations guided me,
My embryo has never been torpid, nothing could overlay it.”
Walt Whitman.
There are two functions of organic life which are often confused together, but which it is well to keep distinct in our thought. These are Growth and Development. The mark of growth is that an organism, by assimilation from the outside world, becomes larger than it was. But in development it becomes different from what it was. The history of an embryo in the womb presents a succession of phenomena which, when one comes to realize them, almost stagger thought; for, while remaining the same thing all through, it is continually becoming a different class of thing—first two cells, then one cell, then a fish, a quadruped, ultimately a human being. This is Development. Once born, it is laid hold of by the principle of Growth which lasts until maturity. Now in the groups called Species, as well as in individuals, we observe exactly the same distinction. The members of a species multiply and increase their numbers. This is Growth. But under certain conditions, which we have now to investigate, they vary in type and ultimately give rise to new species differing widely from that from which they sprang. This we call Development or, in the more popular term for the process when applied to species, Evolution.
The investigation of this process in all its details has been the master-impulse of biology ever since the fact of the process was established by the researches of Darwin.
In
