chemical composition; (2) its universal disintegration and waste by oxidation and its concomitant redintegration by the intussusception of new matter; and (3) "its tendency to undergo cyclical changes." These characteristics serve fairly well to delimit living substances from lifeless material. There are also several other properties that serve the same purpose. I have therefore compiled a somewhat more extensive list of differentia than Professor Huxley did, although most of the following may really be implied in, or deducible from, those which I have just quoted from this authority.

As regards their chemical composition, living organisms are distinguished by the extraordinary complexity of their component molecules. All living matter contains the elements oxygen, hydrogen, carbon, nitrogen, sulphur, and phosphorus. Commonly also it contains potassium, sodium, calcium, magnesium, iron, chlorine, and iodine. More rarely do we find in it the elements copper, manganese, fluorine, and silicon. Every organism includes a certain amount of material which is not really alive, namely, water, inorganic salts, food materials, waste products, and sometimes supporting substances, such as the limy material of our bones, and the cellulose framework of plants. These are the mere adjuncts or accidents of the process of living. The substance that is supposed to be the vehicle of vital activity is a jellylike material called protoplasm, which is sometimes spoken of, in Huxley's famous phrase, as the physical basis of life. Protoplasm is now recognized to be not merely one substance, but a collection of substances of a high degree of chemical complexity. It has thus far resisted accurate chemical analysis, and nothing closely approaching it has ever been fabricated by the skill of the chemist. So far as is known, it is produced only through the agency of life.

One of the most distinctive of the characteristics of living things consists in the kind of change which their living protoplasm undergoes. Protoplasm is subject to a twofold process of waste and repair, which is called metabolism. On the one hand, the food materials are built up into more and more complex sub

stances (anabolism), and these are again broken down (katabolism) into simpler and simpler constituents, which are finally eliminated. When building up predominates over breaking down, there is growth. The growth of living matter takes place not by the addition of materials to the outside merely, as in the growth of most non-living things, but throughout the mass by a process called intussusception. In this growth there is an assimilation by which nutrient material is made over into the kinds of protoplasm characteristic of the organism. Whether we dine on beefsteak, chicken, oysters, or plain bread and cheese, the molecules of these foods are first broken down into a readily absorbable form, and are then taken into the cells of our body and converted into our own peculiar varieties of living tissue. If these same foods should fall to the lot of a dog or a pig, they would be transformed into the somewhat different protoplasms peculiar to these animals. Each kind of protoplasm has the remarkable property of building itself up out of a great variety of different materials. There are as many kinds of protoplasm as there are kinds of organisms, and there are probably millions of different kinds of organisms. This physical basis of life is the scene of continuous transformations in which construction and destruction are always involved. During metabolism many compounds are broken down and oxidized, and then eliminated. By this process of oxidation, and by other processes also, there is yielded a supply of energy which is employed by the organism in carrying on its functions. Oxidation plays a rôle in metabolism similar to that which it performs in burning the coal in the furnace of an engine. It furnishes the energy for running the machine.

The process of metabolism is found in all things that live. It may be said to be the central and basic feature of the life process. Because it furnishes the material and the energy for the business of living, an understanding of the various steps involved in its course would bring us very close to discovering the fundamental secrets of life itself.

There seems to be nothing quite like metabolism in the in

organic world. A crystal may grow if placed in a solution of its own substance, and if other substances are also present in the solution the crystal picks out and adds its own kind of material. There is a resemblance here to growth by assimilation. Of late years several investigations have been carried on with so-called liquid crystals. These are said to grow, not as a solid crystal grows by external accretion, but by incorporating the new material throughout their mass, i. e., by intussusception. But the liquid crystal does not build up its substance out of materials of a different nature; it simply uses its own kind of molecules, that are already available.

There is a closer analogy with the growth of a candle flame. The flame, like a living organism, is the scene of constant chemical transformation. Wax or tallow is drawn into it, volatilized, and burned. Oxidation supplies the energy for the process of combustion, and it plays much the same rôle in the life of an organism. Matter is drawn through the flame, as it is through a living being, and the flame under favorable conditions may grow, the materials composing it being added throughout its mass. If we may speak of the substance of the flame (for like protoplasm it really contains many kinds of substances), the flame may be said to grow by converting its food into its own characteristic material. Here we have something that is at least analogous to metabolism and growth by assimilation. From the chemical standpoint, however, the processes are, for the most part, purely destructive.

Another general feature of living things is their tendency to assume a fairly definite size, which is characteristic of a given kind of organism, although exceedingly variable in different kinds of organisms. This means that each kind of living creature, when it reaches about a certain size, stops growing. Naturalists in describing a species commonly specify its average dimensions, or the limits of size between which it is apt to vary. At the one extreme we have those organisms, the filterable viruses, which pass through all ordinary filters, and are too small to be visible through the most powerful microscope; and at the other, the

gigantic whales, and the mammoth redwood trees of California. This relative constancy of adult size is a very general characteristic of living things, although it is by no means one of their exclusive properties. Many other things, such as atoms and molecules, may be very uniform in size, although in the inorganic world this uniformity may usually be due to other causes than the cessation of growth. No one, so far as I am aware, has ever explained why the atoms of any given element are practically alike, and of nearly, if not quite, the same size.

Along with relative constancy of size, living beings typically exhibit a relative constancy of form. The material of which they are composed undergoes a continual change, without entailing much change of form or structure. Living beings have often been compared to vortices, whose form remains the same, but whose substance is never twice the same. The form of an organism may be retained despite agencies which mutilate or distort it. A salamander may regenerate a leg or a tail, and a Hydra may form a new body from a short piece of its stem. This power of restoration, although varying greatly in extent, is exhibited to a greater or less degree by all living things. Without it, life in any organism would not be possible. Were I to maintain, however, that this fortunate property is an exclusive possession of living organisms, I should probably be reminded that broken crystals are also able to regenerate if they are placed in a solution of their own substance. Some have thought that the regeneration of a crystal and the regeneration of a salamander's tail are fundamentally akin. This, at least, is a debatable point, and illustrates the difficulty of being sure about absolute distinctions between the living and the non-living.

Living beings, however, have a way of changing their form in successive periods of their life-they undergo cyclical changes. The living organism is not a static thing like a crystal; it represents a dynamic equilibrium. What it tends to regenerate, therefore, depends upon the period of its life cycle in which the regenerative activities occur. With higher organisms the cyclical changes involve growth, development, and finally natural death

and dissolution. Life leads to death as the final term of its own natural cɔurse. The life history may be run through quickly. It may have, as with man, a normal period of threescore and ten years; it may be prolonged through many centuries, as in the giant redwood trees of California, some of which were already old when Plato taught in the Academy. Whatever the allotted term of their existence, all higher organisms at least are like clocks, designed to keep going for a given length of time, but inevitably destined to run down and stop.

Correlated with the tendency of life to pass through a cycle of changes, is the power of organisms to give rise to other organisms of the same kind, i. e., the power of reproduction, which insures the perpetuation of life, despite the inevitable death of the individual organism. Some of the simplest living things may not be subject to natural death, since they simply divide into two parts, each of which goes on living; but these organisms pass through a series of changes from one division to the next, slight as these may be in the most primitive types. All organisms reproduce and undergo a process of development. This is one of the most distinctive features of living things, but it may not set them apart absolutely from inorganic objects. Several crystals may arise from the broken fragments of a single crystal, but this kind of reproduction differs from that occurring in an organism by being imposed from without, instead of occurring as a result of the inherent activity of the object. Certainly objects as a rule do not grow, develop, and divide, unless they are alive.

Another universal feature of organisms is irritability, or the property of responding to stimuli. If we happen to touch a hot stove, we respond to the stimulus of heat by jerking back, and perhaps also in other ways. Even plants may respond to stimuli by turning their leaves and stems toward the sun, and by sending their roots toward moisture in the soil. Much of life consists of responding to stimuli in one way or another. The ability to react makes possible the science of psychology, which is concerned with the ways in which organisms, especially animals, behave.