and descending substances; it also contains others in solution and invisible-for, like a lump of jelly (such as the cook serves up shaped by a mould and soaked with flavour and colour), protoplasm can soak up either a large or a small quantity of water, and with the water (that is the important point) all sorts of chemical bodies soluble in water. Just as a lump of quivering calves'-foot jelly (which is a chemical compound of a lower grade than proteids, but like them), when placed in a shallow dish of water coloured red by carmine, does not dissolve in the water, but absorbs the water and the carmine, allowing the coloured water and any chemical bodies in solution in it to diffuse into and become physically, though not chemically, a part of its substance, so protoplasm takes up water and the compounds dissolved by it. Just as a "jelly" of water-holding gelatine can give up its water and become hard and horny, so is protoplasm capable of gradually giving up much of its water, and even in some cases of becoming hard and horny, yet able to return, when remoistened, to its active state. Moreover, a "jelly" can be made to "soak up" or take into itself water and let it pass through its substance, so as to wash out from it all soluble matters. In the same way the protoplasm of a living cell is supplied with nourishing and oxygenating fluids which diffuse into it, and is "washed out," purified, and cleansed of waste or effete chemical compounds by the water which first permeates it, and then diffuses out of it into surrounding watery fluids carrying the excess of soluble chemical bodies with it.

Whilst proteids are the compounds of the highest stage of chemical complexity recognised in protoplasm, and appear to form the bulk of its substance, we must carefully avoid the error (which is not uncommon) of supposing that protoplasm is itself a definite chemical

compound. It is not. Cell-protoplasm includes the nucleus, that denser central body, and is a structure consisting of "proteids" and of many granules and dust-like particles, and of more and of less liquid or watery parts which are less complex in chemical nature than are proteids. Some of the visible granules and invisible liquids present in protoplasm are being built up to the proteid stage of elaboration, whilst some are steps in degradation and decomposition. We have no reason to suppose that the molecules of any proteid known at present to the chemist really are the highest degree of chemical complexity attained to in living protoplasm. Probably there is present a further stage of elaboration, a chemical body even more complex than is "proteid," which is continually attracting the lower chemical compounds to itself and as continually breaking down. This is the ultimate chemical substance of life. It is hidden invisibly in the protoplasm, yet all the chemical changes which go on in the protoplasm of a cell are either leading up to this supreme life-stuff or are leading downwards from it. This ultimate compound, which we suppose to exist but have not demonstrated, has been called "plasmogen." It is this body in which resides the peculiar property of living matter, namely, that of attracting to itself substances containing the socalled "organic" elements-carbon, oxygen, hydrogen, and nitrogen and of acting on them in such a way that they "nourish" it—that is to say, combine chemically with it to form more "plasmogen."

The intermediate steps leading up to plasmogen and the products arising from its incessant breaking down are formed under the influence of this unique chemical body, and by it alone. Chemists have not yet succeeded in making them; only the less elaborate kinds have been "artificially" constructed without the aid of the

living plasmogen. To construct plasmogen itself is a task for the chemists of the distant future. In early geological ages plasmogen came into being; it has gone on ever since "nourishing" itself, maintaining itself, growing and spreading over the earth. It is improbable that the conditions which led to its formation have ever recurred. All subsequent plasmogen has been formed by the growth and increase of that first sample of it, which once in a remote period of the earth's history was built up by chemical conditions, which came to an end as soon as they had produced it.

The only process in nature of which we know, which resembles the "building" action of plasmogen, the ultimate molecule of life, buried in the cell's protoplasm, is the selective action of crystals, which draw to themselves from a solution or magma of all sorts of chemical bodies those molecules of a chemical nature identical with their own, and build them up into special and definite crystalline forms. But there is a very wide gap between this process and even the mere assimilation by living matter of the organic elements, so as to raise them from a lower to a higher grade of chemical complexity of combination. And over and above this we have added, in the case of living material, to the mere power of assimilation and growth the almost unthinkable complications and variations of specific form and quality, and yet further of individual form and quality, which are determined by special complications and variations of the plasmogen, that unique compound concealed in the cell-protoplasm.

We cannot at present, if ever, picture to ourselves adequately the mechanism of plasmogen, though the attempt has been, and must be, made. But we can

watch its workings closely; we can ascertain the conditions which promote, check, or modify its activity; in

fact, we can observe its output and experiment on it in a thousand ways, and so get more and more knowledge of it. We are not led to suppose that it is possessed by a demon, nor that in it resides an elsewhere unknown essence. It is enough for us to satisfy ourselves that its qualities, whilst they can be grouped with the chemical and physical qualities of other bodies, so far transcend them in complexity and in immensity of result-the whole creation of plant and animal life-that their appearance constitutes in effect a new departure, a sudden and, to us, unaccountable acquirement. But then we must remember that it is also an unaccountable thing to us that water suddenly becomes ice at a low temperature, and suddenly becomes vapour at a high temperature, even if we are able to imagine the mechanism which necessitates those changes. We cannot " explain the nature of things. Even though we can classify them and arrange them in order, and more or less satisfactorily guess what their inner mechanism is, we cannot, in our present state of knowledge, trace them in detail to a first beginning. Even though we believe that such a history lies behind us, we ourselves cannot as yet show how exactly every quality and property and form of matter has developed in due order as a matter of necessity during the cooling of the cosmic gas. All we can do is to ascertain, bit by bit, some sequences, some lines of orderly development and interaction, adding thus step by step to our knowledge of what has taken place.

XXI

THE SIMPLEST LIVING THINGS

N old times, if one wanted to compare a man to the humblest and simplest of animals, one called him "a worm." But really a worm is a very elaborate creature, with skin, muscles, blood-vessels, kidneys, nervous system, pharynx, stomach, and an intestine, and is built up by hundreds of thousands of protoplasmic cells. Shakespeare got nearer the mark when he made one of his uncompromising professional "murderers " exclaim, as he stabbed the young Macduff to the heart, "What, you egg!" An egg is a single cell or corpuscle of protoplasm, and the simplest living things are of the same structure---mere units, single corpuscles of protoplasm, often less than the one-thousandth of an inch in diameter, and invisible except with the microscope, though in some cases big enough to be seen by the naked eye as they swim or crawl in a glass of pondwater. Many thousands of kinds of these simplest animals and plants have been carefully recorded, distinguished from one another, and named by naturalists.

Many of these unicellular animals (or "Protozoa ") crawl by a curious irregular flowing movement of the viscid tenacious protoplasm of which they consist. There is no firm coat or cell-wall, only the thinnest pellicle on the surface. The Proteus-animalcule (Fig.