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 hydrogen and oxygen which are called hydrocarbons, distinguished from carbohydrates by the fact that the number of oxygen atoms is less than half the number of hydrogen atoms. These substances are the fats and oils of various kinds, less powerful sources of energy than the proteins, but they contain more potential energy than the carbohydrates because they are more oxidizable.
Besides the characteristic substances of these three classes, protoplasm contains certain other chemical compounds, like the various salts of sodium, chlorine, magnesium and potassium, and a few others, which bring the list of chemical elements to the number twelve. We have already noted how strikingly small and restricted is the list of elements composing living matter as compared with the long array of eighty-odd different kinds of chemical atoms existing in the world as a whole.
But an astonishing result is reached through the brief analysis we have just made. It is this: we do not find _peculiar_ kinds of atoms which occur exclusively in living matter; the materials are exactly the same as those of the outer world.
In short, the elements of both the organic and inorganic divisions of the universe prove to be the same. Carbon is carbon, whether it is part of the substance of a living brain cell, or black inert coal, or the glistening diamond, or an incandescent part of the fiery sun. Hydrogen is the same, whether it be a constituent of the ocean, of the air, or of the living muscle fiber. And so it is with all of the other elements of the living mechanism. This starts us upon a line of thought which leads to a significant conclusion, namely, that a living thing which seems so distinct and permanent is after all only a temporary aggregate of elements which come to it from the not-living world; existing for a time in peculiar combinations which render life possible, they pass incessantly away from the living thing and return to the inorganic world.
Every breath we draw sends out particles which were at one time living portions of ourselves; every movement we make involves the destruction of living muscle cells, whose protoplasm breaks down into the ash and gas and fluid wastes which eventually return to the world of dead things. A tree loses its living leaves with each recurring season, and the antlers of the stag are lost annually, to be replaced anew. Indeed the major part of some organisms is itself actually dead. The bones and hair and nails of such an animal as a cat are almost entirely lifeless, even though they are integral and necessary portions of the organism as a whole. They are constructed by living protoplasm which has died in their making.
Thus without going beyond the boundaries of the individual body, these substances have passed from the sphere of life, and are dead. The apparent gap on the other side between the lifeless and living world is equally imaginary, for our living substance is continually replenished and rebuilt from the elements of our dead foods. So, as Huxley says, a living organism is like a flame or a whirlpool, which is an ever changing though seemingly constant individuality. We look at a gas flame, and we see in the flame itself those particles of gas which have come through the pipe to be agitated violently in the higher temperature of the flame as they are oxidized or burnt. These particles immediately pass off as carbonic acid gas and water vapor which are no longer parts of the flame. A fountain is continually replenished by the water which is not-fountain, but which becomes for the time a part of the graceful jet, falling out and away as it leaves the fountain itself.
Just so a living organism is an ever changing, ever renewed, and ever destroyed mass of little particles--the atoms of the inorganic world which combine and come to life for a time, but which return inevitably to the world of lifeless things. This is one of the most fundamental facts of biology. The independence of a living thing like a human being or a crustacean is a product of the imagination. How can we be independent of the environment when we are interlocked in so many ways with inorganic nature? Our very substance with its energies has been wrested from the environment; and as we, like all other living things, must replenish our tissues as we wear out in the very act of living, we cannot cease to maintain the closest possible relations with the environment without surrendering our existence in the battle of life.
From the foregoing discussion, it will be evident, I am sure, that there is ample justification for the biological dictum that a living individual is a mechanism. Not only is the organism composed always of cell units grouped mechanically in tissues and organs and organic systems; not only are the operations which make up its life constant and regular under similar conditions; not only is the whole creature mechanically connected with the inorganic world; but above all the whole activity of a biological individual is concerned necessarily and again mechanically with the acquisition of materials endowed with energy, which materials and energy are mechanically transformed into living matter and its life. Even though an organism is so much more complex than a locomotive, and so plastic, nevertheless, in so far as both are mechanisms, the conception of the evolution of the former may be much more readily understood through a knowledge of the historical transformation of the latter.