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.

* * * * *

What, now, is life? To most people "life seems to be something which enters into a combination of carbon and hydrogen and the other elements, and makes this complex substance, the protoplasm, perform its various activities." Nearly every one finds it difficult to regard life and vitality as anything but actuating principles that exist apart from the materials into which they enter, and which they seem to make alive. According to this general conception, "life is something like an engineer who climbs into the cab of the locomotive and pulls the levers which make it go," as health might supposedly be regarded as something that does not inhere in well-being, but gets into the body to alter it. But is this conception really justified by the facts of animal structure and physiology? Let us recall the steps of our analysis. The living organism is a collection of differentiated parts, the organs; the life of an organism is a series of activities of the several organic systems and organs. If we could take away one organ after another, there would be nothing left after the last part had been subtracted. In a similar manner, the activities of organs prove to be the combined activities of the tissue-cells, and again the truth of this statement will be clear when we imagine the result of taking away one cell after another from organisms like the frog or tree. When the last cell had been withdrawn, there would be nothing left of the frog's structure, and there would be no element of the frog's life. It is true that the particular way the tissue-cells are combined is of primary importance, but it is none the less true that the life of a cell is the kind of element out of which the life of even the most complex organism is built. And we have seen that the essential substance of a cell is a complex chemical compound we call protoplasm, whose elements are identical with chemical substances outside the living world. Is there any ground for supposing that the properties of protoplasm are due to any other causes than those which may be found in the chemical and physical constitution of protoplasm? In brief, is life physics and chemistry? Nowadays the majority of biologists believe that it is. Just as the properties of water are contributed by the elements hydrogen and oxygen which unite to form it, just so the marvelous properties of protoplasm are regarded as the inevitable derivatives of the combined properties of the various chemical elements which constitute protoplasm. Biologists have known for more than a century, since the work of