resolved into elements which are called cells. They are not little hollow cases, it is true, although for historical reasons we employ a word that implies such a condition. They are unitary masses of living matter with a peculiar central body or nucleus, and every tissue of every living thing is composed of them.

The cells of bone differ from those of cartilage mainly in the different consistency of the substances secreted by the cells to lie between them; skin cells are soft-walled masses lying close together; even blood is a tissue, although it is fluid and its cells are the corpuscles which float freely in a liquid serum. Thus an organism proves to be a complex mechanism composed of cells as structural units, just as a building is ultimately a collection of bricks and girders and bolts, related to one another in definite ways.

Our analysis reveals the living creature in an entirely new light, not only as a machinelike structure whose parts are marvelously formed and coordinated in material respects, but also as one whose activities or workings are ultimately cellular in origin. Structure and function are inseparable, and if an animal or a plant is an aggregate of cells, then its whole varied life must be the sum total of the lives of its constituent cells. Should these units be subtracted from an animal, one by one, there would be no material organism left when the last cells had been disassociated, and there would be no organic activity remaining when the last individual cell-life was destroyed. All the various things we do in the performance of our daily tasks are done by the combined action of our muscle and nerve and other tissue cells; our life is all of their lives, and nothing more. The cell, then, is the physiological or functional unit, as truly as it is the material element of the organic world. Being combined with countless others, specialized in various ways, relations are established which are like those exhibited by the human beings constituting a nation. In this case the life of the community consists of the activities of the diverse human units that make it up. The farmer, the manufacturer, the soldier, clerk, and artisan do not all work in the same way; they undertake one or another of the economic tasks which they may be best fitted by circumstances to perform. Their differentiation and division of labor are identical with the diversity in structure and in function as well, exhibited by the cells of a living creature. We might speak of the several states as so many organs of our own nation; the commercial or farming or manufacturing communities of a state would be like the tissues forming an organ, made up ultimately of human units, which, like cells, are engaged in similar activities. As the individual human lives and the activities of differentiated economic groups constitute the life of a nation and national existence, so cell-lives make the living of an organism, and the expressions "division of labor" and "differentiation" come to have a biological meaning and application.

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The cell, then, is in all respects the very unit of the organic world. Not only is it the ultimate structural element of all the more familiar animals and plants that we know, as the foregoing analysis demonstrates, but, in the second place, the microscope reveals simple little organisms, like Amoeba, the yeast plant and bacteria, which consist throughout their lives of just one cell and nothing more. Still more wonderful is the fact that the larger complex organisms actually begin existence as single cells. In three ways, therefore,—the analytic, the comparative, and the developmental,—the cell proves to be the "organic individual of the first order." As the ultimate biological unit, its essential nature must possess a profound interest, for in its substance resides the secret of life.

This wonderful physical basis of life is called protoplasm. It contains three kinds of chemical compounds known as the proteins, carbohydrates, and hydrocarbons. Proteins are invariably present in living cells, and are made up of carbon, hydrogen, nitrogen, sulphur, and usually a little phosphorus. The elements are also combined in a very complex chemical way. For example, the substance called hæmoglobin is the protein which exists in the red blood cells and which causes those cells to appear light red or yellow when seen singly. Its chemical formula states the precise number of atoms which enter into the constitution of a single molecule as: C_{600}H_{960}N_{154}FeO_{179}. This is truly a marvelously complex substance when compared with the materials of the inorganic world, like water, for example, which has the formula H_{2}O. And just as the peculiar properties of H_{2}O are given to it by the properties of the hydrogen and the oxygen which combine to form it, just so, the scientist believes, the marvelous properties of protein are due to the assemblage of the properties of the carbon and hydrogen and other elements which enter into its composition.

It would be interesting to see how each one of these elements contributes some particular characteristic to the whole compound. The carbon atom, for example, is prone to combine with other atoms in definite varied ways, and the high degree of complexity which the protein molecule possesses may depend in greater part upon the combining power of its carbon elements. The nitrogen atom makes the protein an extremely volatile compound, so that the latter burns readily in the tissue cells; and the hydrogen and oxygen bring their specific characteristics to the total molecule. And furthermore, it is evident that the great complexity of this constituent, protein, gives to protoplasm its power of doing work, or, in a word, its power of living. In constructing it, much energy has been absorbed and stored up as potential energy, and so, like the stored-up energy in a watch spring or in gunpowder, this may be converted, under proper conditions, into the kinetic energy and the work of actual operation. On account of its peculiar and complex nature, it possesses great capacity for burning or oxidization, thus serving as a source of vital power. It burns in the living tissue just as coal oxidizes in the boiler of an engine; its atoms fly apart and unite with oxygen so as to satisfy their chemical affinities for this substance. If we could only see what happens to the protein molecule when it undergoes oxidization, we would witness a violent explosion, like that of a mass of gunpowder. And the astonishing fact is that this process is actually the same for the living molecule, for exploding gunpowder, and for the fuel which burns in the locomotive boiler. Does this mean that the essential process of what we call life is a chemical one? So it would seem on the basis of this fact alone, but a conclusion must be deferred until we reach a later point.

The second kind of substance which we find in protoplasm is the carbohydrate. A typical member of this group is common sugar, C_{6}H_{12}O_{6}; another sugar has the formula C_{12}H_{22}O_{11}. Starch is again a typical carbohydrate, and its formula is C_{6}H_{10}O_{5}, or some multiple of this. One sees at a glance that these substances agree in having twice as many hydrogen atoms as there are oxygen atoms, the same proportion that the hydrogen bears to the oxygen in the compound water,—a characteristic which makes it easy to remember the general constitution of carbohydrate as compared with the protein. The substances of this second class are obviously much less complex, both as regards the different kinds of atoms and in respect to the numbers of each kind that enter into the formation of a single molecule. Therefore the carbohydrates do not possess so much power or energy as the protein molecule; in short, they are not such good fuels for the living mechanism.

Finally, we find almost always in protoplasm other substances composed of carbon and