The philosophy of biology

by James Johnstone

In a lifeless earth all energetic processes would tend continually toward a condition of stability. The crust of the earth, that is, the part known to us by direct observation, is made up of rocks and the remains of rocks; materials consisting of compounds of oxygen, silicon, iron, aluminium, sodium, potassium, calcium, and so on. They are substances which would be stable but for the eroding action of water, the gases of the atmosphere, and volcanic activity. But as volcanic activity tends always toward cessation, the oxygen of the atmosphere would gradually disappear, first by its combination with oxidisable substances, and second by its combination with the nitrogen of the atmosphere under the influence of electric discharges. Carbon dioxide would either combine with materials in the rocks, or would remain in the atmosphere along with nitrogen and other inert gases in a stable condition. Water, moved by the tides and winds, would gradually plane down the surface of the land, unless along with other gases it would gradually become dissipated into outer space. We see, then, that the materials of the earth tend to fall into stable combinations, and that they approximate toward conditions in which potential chemical energy becomes reduced to a minimum, the whole energy possessed by matter being that of the motions of the molecules, that is, kinetic energy unavailable for transformations of any kind. It would be an earth devoid of phenomena.

Vegetable life alone would be possible only for a time on an earth such as we know it at present. The green plant depends for its existence on the presence in the soil of mineral substances such as salts of nitric acid and of ammonia, and on the presence of water and carbon dioxide in the atmosphere. The chlorophyllian apparatus is essentially a mechanism whereby these substances become built up into carbohydrates, like starch and sugar; hydrocarbons, like resins and oils; and proteids. The energy necessary for these syntheses is obtained from solar radiation through the agency of the chlorophyll plastids. The green plant would depend for its supply of nitrate or ammonia on the combination of the nitrogen of the atmosphere with oxygen, or on the exhalations from volcanoes, and these are irreversible processes which tend continually toward cessation. The plant requires also carbon dioxide and the amount of this substance in the atmosphere is very limited, while the only inorganic source from which it can be renewed seems to be volcanic activity: this substance also would tend to disappear. A time would therefore come when plant life on the earth would cease to be possible because of the disappearance of the materials on which it depends; but while it did exist its result would be the accumulation of chemical compounds of high potential energy. The result of the metabolism of the plant is the formation of such compounds as cellulose from woody tissues and shed leaves, of other plant carbohydrates, of oils and resins, and of proteids. In the absence of bacteria such substances would persist unchanged: even in an earth tenanted by bacteria such products as oils, lignite, peat, coal, etc., have been able to accumulate throughout geological time. The tendency of plant life is therefore toward the accumulation of compounds of high potential energy, and this process also is irreversible.

Bacterial activity would, of itself, make continued plant life possible on the earth. The essential characters of these organisms are their ability to bring about the most varied energy-transformations. From our present point of view bacteria may be divided into paratrophic, metatrophic, and prototrophic forms. Paratrophic bacteria are those which live as parasites within the living tissues of plants and animals: this mode of life is obligatory, and these organisms are unable to live in the open. The result of their activity is the breaking down of protoplasmic substance. Metatrophic bacteria are those that produce putrefaction and fermentation of organic compounds. They may be parasitic in their mode of life, but most of them live in soil, in water, and in the cavities of the animal body—the mouth, alimentary canal, nose, and vagina. Proteids are decomposed by them into simple chemical compounds such as amido-acids, and then these substances, along with carbohydrates, are fermented so as ultimately to form water, carbonic acid, and salts of nitric acid. These bacteria obtain their energy from the conversion of chemical compounds of high potential energy into compounds of low potential energy. Prototrophic bacteria are never parasites, nor do they live in the cavities of the bodies of animals: they always live in the open. They carry on still further the action of the putrefactive bacteria by converting ammonia into nitrous acid, and nitrous acid into nitric acid. Others reverse this series of changes by reducing nitric acid to nitrous acid, nitrous acid to ammonia, and ammonia to free nitrogen. Others again oxidise sulphuretted hydrogen to sulphuric acid, others ferrous hydrate to ferric hydrate, while it has recently been shown that some bacteria are apparently able to oxidise the carbon of coal to carbonic acid. Some are able to oxidise the free nitrogen of the atmosphere into nitrous and nitric acids. How precisely the energy necessary for these transformations is obtained is not at all clearly understood, and it may be possible that some of the prototrophic bacteria obtain their energy by making use of the un-co-ordinated kinetic energy of the medium in which they live. From our point of view the net result of the activity of the predominant species of bacteria which inhabit the earth is that they reverse the processes which are the manifestations of the metabolism of plants and animals. The result of the metabolism of plants is the accumulation of stores of high potential compounds such as carbohydrates, and the depletion of the terrestrial stores of carbon dioxide and other materials necessary for the continued existence of the plants themselves. The result of the metabolism of the bacteria is the break-down of this accumulation of such compounds as carbohydrates, and the replenishing of the stores of carbon dioxide and nitrogenous mineral substance on which the plant depends. If bacteria are present, the life process becomes a reversible one.

Plant life and bacterial life are thus complementary to each other, for, on the whole, the energetic processes of the green plant proceed in the opposite direction to those of the bacteria. An organic world consisting of green plants and bacteria would therefore be one capable of permanent existence. Now, so far, we need only consider these various kinds of organisms as living protoplasmic substances in which energy-transformations of different types proceed. The bacterium is simply a cell containing a nucleus, and the green plant need only be a nucleated cell containing a chlorophyll plastid: this is, indeed, all that it is in the case of a Diatom or a Peridinian. The morphology of the green plant is only accessory to the chlorophyllian apparatus. Neglecting the reproductive apparatus, the higher green plant consists essentially of the chlorophyllian cells in the parenchyma of the leaf, for roots and stomata are only organs for the absorption of water and mineral salts from the soil and carbon dioxide from the atmosphere; while the tissues of the trunk, stems, and branches are, in the main, apparatus for the conduction of these raw materials through the body of the plant, and, of course, the nutritive substances into which they are elaborated. All the innumerable variations of form in the plant (apart from the structure of the flower or other reproductive organ) are adaptations which provide for the absorption and distribution of these substances; or for the mechanical support of the plant body; or are non-adaptive variations, pure luxuries, so to speak.

More than this is represented by the structure of the animal body, but we must first of all consider the points of difference between plant and animal regarded merely as apparatus in which energy-transformations occur. In the green plant energy is accumulated in the form of high potential chemical compounds, but in the animal energy is expended. Inorganic mineral substances are built up by the plant into carbohydrate, proteid, and fat or oil, but in the animal body carbohydrate, proteid, and fat are dissociated into water, carbonic acid, and urea (or some other nitrogenous excretory substance); and the urea or other analogous substance is broken down by bacteria into nitrate, water, and carbon dioxide. The metabolic activities of the animal are said to be “analytic” or destructive, while those of the plant are said to be “synthetic” or constructive, but these contrasting terms hardly describe accurately the essential nature of the activities of the two kinds of organisms. What further constitutes “animality”? It is purposeful mobility, and the energy-transformations that occur are the means whereby this mobility is attained. The plant is essentially immobile, for such movements as the turning of leaves toward the light, the down-growth of roots, the up-growth of stems, the twining of tendrils round supporting objects, and the opening and closing of flowers are only the movements of parts of the plant organism. They are constant, directed responses to external stimuli—real tropisms—and the extension of this kind of response so as to describe in general the movements of animals is only an instance of the insufficient analysis of facts. The movements of the typical green plant are therefore movements of its parts, they are few in number, they belong to a few simple types, and they are evoked by simple external physical changes in the medium. The movements of the typical animal are movements of the organism as a whole; they are infinitely varied in their nature; they are evoked by individualised stimuli and they are continually being modified by the experience of the organism.

The bodily structure of the animal is the means whereby this purposeful mobility is attained and the energy-transformations directed; and the greater and more varied the movements of the animal, the more complex is its structure. In respect of the manner in which the energy-transformations are effected, that is, in respect of the material means whereby energy falls from a state of high potential to a state of low potential, the morphology of the animal is similar to that of the plant, that is, the energy-transformations are the functions of nucleated cells. But in the plant the kinetic energy of solar radiation passes into the potential energy of chemical compounds which become stored in the body of the plant; while in the animal the potential energy of ingested chemical compounds passes into the kinetic energy of the movements of the animal itself. How exactly it moves, how this kinetic energy is employed is determined by the sensori-motor system.

It is the existence of the sensori-motor system that makes the animal an animal. What, then, is the sensori-motor system? It is the skeleton and muscles, that is, the organs of locomotion, aggression, prehension, and mastication; the peripheral sensory and motor nerves; and the central nervous system or brain. The skeleton of an animal, whether it be the carapace or exoskeleton of a crustacean, or the vertebral column, limb-girdles, and limb-bones of a vertebrate, is a rigid and fixed series of supports to which the muscles are attached. Organs of locomotion are, for instance, the appendages of a crustacean, the wings of a bird or insect, the tail and fins of a fish, or the limbs of a vertebrate. Organs of aggression are the mandibles of a spider or blood-sucking fly, the chelate claws of a crab or lobster, the jaws of a fish, or the claws and teeth of a terrestrial vertebrate. Organs of prehension and mastication are in the main also those of aggression. All these parts consist of modified skeletal structures, teeth, claws, etc., attached to muscles which originate in the rigid parts of the skeleton. When we speak of the movements of an animal we speak of the motions of such parts as we have mentioned; other parts do indeed move—the heart pulsates, the lungs dilate and contract, and the blood and other fluids circulate through closed vessels; but these are movements of the parts of the animal, and are comparable rather with those movements of the plant organism that we have considered. They are not to be regarded as examples of the mobility of the animal in the sense of the exercise of its sensori-motor system.

A central and peripheral nervous system is, of course, bound up with a motor system. Receptor organs, eyes, olfactory, auditory, tactile organs of sense, and so on, are the means whereby the animal is affected by changes in its environment—it need not be cognisant of, or become aware of, or perceive these impressions on its receptor organs. These stimuli are transmitted along the sensory, or afferent, nerves to the central nervous system: this is the way in. The effector nervous organs are the motor plates, that is, the nervous structures in the muscles in which the nerves terminate. The motor nerves are the efferent paths, the way out from the central nervous system.

The central nervous system is essentially the organ for the integration of the activities of the whole body. It is the “seat of multitudinous synapses,” a description which better than any other applies to the morphology of the brain of the vertebrate animal. We have already considered what is meant by the term “reflex action,” it is the series of processes which occur when a “reflex arc” becomes functionally active. A reflex arc consists of (1) a receptor organ, say a tactile corpuscle in the skin; (2) an afferent nerve fibre; (3) a nerve cell in the brain or spinal cord; (4) an efferent nerve fibre; and (5) an effector nerve organ, say a motor plate in a muscle fibre. The series of processes involved in a reflex action consist of the stimulation of the receptor organ, the passage of the afferent impulse into the brain or cord, the passage of the impulse through a series of cells in the nerve centre forming a synapse, the transmission of the impulse through the efferent nerve fibre into the effector organ in the muscle and the stimulation of the latter to an act of contraction. This is a purely schematic description of the structures and processes forming a reflex action and arc: in reality the path both into and out from the central nervous system is interrupted again and again, and at each place of interruption there are alternative paths. The interruptions occur at the synapses. At a synapse the nervous impulse passes through an arborescence of fine nervous twigs, into which the fibre breaks up, into a similar arborescence, and these two arborescences are not in actual physical contact: the impulse leaps over a gap. At numerous places in both brain and cord there are alternative synapses and at these places the impulse may travel in more than one direction.