The Principles of Biology Vol. I/Chapter I.5

§ 27. We habitually distinguish between a live object and a dead one, by observing whether a change which we make in the surrounding conditions, or one which Nature makes in them, is or is not followed by some perceptible change in the object. By discovering that certain things shrink when touched, or fly away when approached, or start when a noise is made, the child first roughly discriminates between the living and the not-living; and the man when in doubt whether an animal he is looking at is dead or not, stirs it with his stick; or if it be at a distance, shouts, or throws a stone at it. Vegetal and animal life are alike primarily recognized by this process. The tree that puts out leaves when the spring brings increase of temperature, the flower which opens and closes with the rising and setting of the sun, the plant that droops when the soil is dry and re-erects itself when watered, are considered alive because of these induced changes; in common with the acorn-shell which contracts when a shadow suddenly falls on it, the worm that comes to the surface when the ground is continuously shaken, and the hedgehog that rolls itself up when attacked.

Not only, however, do we look for some response when an external stimulus is applied to a living organism, but we expect a fitness in the response. Dead as well as living things display changes under certain changes of condition: instance, a lump of carbonate of soda that effervesces when dropped into sulphuric acid; a cord that contracts when wetted; a piece of bread that turns brown when held near the fire. But in these cases, we do not see a connexion between the changes undergone and the preservation of the things that undergo them; or, to avoid any teleological implication—the changes have no apparent relations to future events which are sure or likely to take place. In vital changes, however, such relations are manifest. Light being necessary to vegetal life, we see in the action of a plant which, when much shaded, grows towards the unshaded side, an appropriateness which we should not see did it grow otherwise. Evidently the proceedings of a spider which rushes out when its web is gently shaken and stays within when the shaking is violent, conduce better to the obtainment of food and the avoidance of danger than were they reversed. The fact that we feel surprise when, as in the case of a bird fascinated by a snake, the conduct tends towards self-destruction, at once shows how generally we have observed an adaptation of living changes to changes in surrounding circumstances.

A kindred truth, rendered so familiar by infinite repetition that we forget its significance, must be named. There is invariably, and necessarily, a conformity between the vital functions of any organism and the conditions in which it is placed—between the processes going on inside of it and the processes going on outside of it. We know that a fish cannot live long in air, or a man under water. An oak growing in the ocean and a seaweed on the top of a hill, are incredible combinations of ideas. We find that each kind of animal is limited to a certain range of climate; each kind of plant to certain zones of latitude and elevation. Of the marine flora and fauna, each species is found only between such and such depths. Some blind creatures flourish in dark caves; the limpet where it is alternately covered and uncovered by the tide; the red-snow alga rarely elsewhere than in the arctic regions or among alpine peaks.

Grouping together the cases first named, in which a particular change in the circumstances of an organism is followed by a particular change in it, and the cases last named, in which the constant actions occurring within an organism imply some constant actions occurring without it; we see that in both, the changes or processes displayed by a living body are specially related to the changes or processes in its environment. And here we have the needful supplement to our conception of Life. Adding this all-important characteristic, our conception of Life becomes—The definite combination of heterogeneous changes, both simultaneous and successive, in correspondence with external co-existences and sequences. That the full significance of this addition may be seen, it will be necessary to glance at the correspondence under some of its leading aspects.Speaking of "the general idea of life" M. Comte says:—"Cette idée suppose, en effet, non-seulement celle d'un être organisé de manière à comporter l'état vital, mais aussi celle, non moins indispensable, d'un certain ensemble d'influences extérieures propres à son accomplissement. Une telle harmonie entre l'être vivant et le milieu correspondant, caractérise evidemment la condition fondamentale de la vie." Commenting on de Blainville's definition of life, which he adopts, he says:—"Cette lumineuse définition ne me paraît laisser rien d'important à désirer, si ce n'est une indication plus directe et plus explicite de ces deux conditions fondamentales co-relatives, nécessairement inséparables de l'état vivant, un organisme déterminé et un milieu convenable." It is strange that M. Comte should have thus recognized the necessity of a harmony between an organism and its environment, as a condition essential to life, and should not have seen that the continuous maintenance of such inner actions as will counterbalance outer actions, constitutes life.

[When the original edition was published Dr. J. H. Bridges wrote to me saying that in the Politique Positive, Comte had developed his conception further. On p. 413, denying "le prétendu antagonisme des corps vivants envers leurs milieux inorganiques," he says "au lieu de ce conflit, on a reconnu bientôt que cette relation nécessaire constitue une condition fondamentale de la vie réelle, dont la notion systématique consiste dans une intime conciliation permanente entre la spontanéité intérieure et la fatalité extérieure." Still, this "conciliation permanente" seems to be a "condition" to life; not that varying adjustment of changes which life consists in maintaining. In presence of an ambiguity, the interpretation which agrees with his previous statement must be chosen.]

§ 28. Neglecting minor requirements, the actions going on in a plant pre-suppose a surrounding medium containing at least carbonic acid and water, together with a due supply of light and a certain temperature. Within the leaves carbon is being appropriated and oxygen given off; without them, is the gas from which the carbon is taken, and the imponderable agents that aid the abstraction. Be the nature of the process what it may, it is clear that there are external elements prone to undergo special re-arrangements under special conditions. It is clear that the plant in sunshine presents these conditions and so effects these re-arrangements. And thus it is clear that the changes which primarily constitute the plant's life, are in correspondence with co-existences in its environment.

If, again, we ask respecting the lowest protozoon how it lives; the answer is, that while on the one hand its substance is undergoing disintegration, it is on the other hand absorbing nutriment; and that it may continue to exist, the one process must keep pace with, or exceed, the other. If further we ask under what circumstances these combined changes are possible, there is the reply that the medium in which the protozoon is placed, must contain oxygen and food—oxygen in such quantity as to produce some disintegration; food in such quantity as to permit that disintegration to be made good. In other words—the two antagonistic processes taking place internally, imply the presence externally of materials having affinities that can give rise to them.

Leaving those lowest animal forms which simply take in through their surfaces the nutriment and oxygenated fluids coming in contact with them, we pass to those somewhat higher forms which have their tissues slightly specialized. In these we see a correspondence between certain actions in the digestive sac, and the properties of certain surrounding bodies. That a creature of this order may continue to live, it is necessary not only that there be masses of substance in the environment capable of transformation into its own tissue, but also that the introduction of these masses into its stomach, shall be followed by the secretion of a solvent fluid which will reduce them to a fit state for absorption. Special outer properties must be met by special inner properties.

When, from the process by which food is digested, we turn to the process by which it is seized, the same general truth faces us. The stinging and contractile power of a polype's tentacle, correspond to the sensitiveness and strength of the creatures serving it for prey. Unless that external change which brings one of these creatures in contact with the tentacle, were quickly followed by those internal changes which result in the coiling and drawing up of the tentacle, the polype would die of inanition. The fundamental processes of integration and disintegration within it, would get out of correspondence with the agencies and processes without it, and the life would cease.

Similarly, when the creature becomes so large that its tissue cannot be efficiently supplied with nutriment by mere absorption through its lining membrane, or duly oxygenated by contact with the fluid bathing its surface, there arises a need for a distributing system by which nutriment and oxygen may be carried throughout the mass; and the functions of this system, being subsidiary to the two primary functions, form links in the correspondence between internal and external actions. The like is obviously true of all those subordinate functions, secretory and excretory, that facilitate oxidation and assimilation.

Ascending from visceral actions to muscular and nervous actions, we find the correspondence displayed in a manner still more obvious. Every act of locomotion implies the expenditure of certain internal forces, adapted in amounts and directions to balance or out-balance certain external forces. The recognition of an object is impossible without a harmony between the changes constituting perception, and particular properties co-existing in the environment. Escape from enemies implies motions within the organism, related in kind and rapidity to motions without it. Destruction of prey requires a special combination of subjective actions, fitted in degree and succession to overcome a group of objective ones. And so with those countless automatic processes constituting instincts.

In the highest order of vital changes the same fact is equally manifest. The empirical generalization that guides the farmer in his rotation of crops, serves to bring his actions into concord with certain of the actions going on in plants and soil. The rational deductions of the educated navigator who calculates his position at sea, form a series of mental acts by which his proceedings are conformed to surrounding circumstances. Alike in the simplest inferences of the child and the most complex ones of the man of science, we find a correspondence between simultaneous and successive changes in the organism, and co-existences and sequences in its environment.

§ 29. This general formula which thus includes the lowest vegetal processes along with the highest manifestations of human intelligence, will perhaps call forth some criticisms which it is desirable here to meet.

It may be thought that there are still a few inorganic actions included in the definition; as, for example, that displayed by the mis-named storm-glass. The feathery crystallization which, on a certain change of temperature, takes place in its contained solution, and which afterwards dissolves to reappear in new forms under new conditions, may be held to present simultaneous and successive changes that are to some extent heterogeneous, that occur with some definiteness of combination, and, above all, occur in apparent correspondence with external changes. In this case vegetal life is simulated to a considerable extent; but it is merely simulated. The relation between the phenomena occurring in the storm-glass and in the atmosphere respectively, is not a correspondence at all, in the proper sense of the word. Outside there is a thermal change; inside there is a change of atomic arrangement. Outside there is another thermal change; inside there is another change of atomic arrangement. But subtle as is the dependence of each internal upon each external change, the connexion between them does not, in the abstract, differ from the connexion between the motion of a straw and the motion of the wind that disturbs it. In either case a change produces a change, and there it ends. The alteration wrought by some environing agency on this or any other inanimate object, does not tend to induce in it a secondary alteration which anticipates some secondary alteration in the environment. But in every living body there is a tendency towards secondary alterations of this nature; and it is in their production that the correspondence consists. The difference may be best expressed by symbols. Let A be a change in the environment, and B some resulting change in an inorganic mass. Then A having produced B, the action ceases. Though the change A in the environment is followed by some consequent change a in it; no parallel sequence in the inorganic mass simultaneously generates in it some change b that has reference to the change a. But if we take a living body of the requisite organization, and let the change A impress on it some change C; then, while in the environment A is occasioning a, in the living body C will be occasioning c; of which a and c will show a certain concord in time, place, or intensity. And while it is in the continuous production of such concords or correspondences that Life consists, it is by the continuous production of them that Life is maintained.

The further criticism to be expected concerns certain verbal imperfections in the definition, which it seems impossible to avoid. It may fairly be urged that the word correspondence will not include, without straining, the various relations to be expressed by it. It may be asked:—How can the continuous processes of assimilation and respiration correspond with the co-existence of food and oxygen in the environment? or again:—How can the act of secreting some defensive fluid correspond with some external danger which may never occur? or again:—How can the dynamical phenomena constituting perception correspond with the statical phenomena of the solid body perceived? The only reply is, that we have no word sufficiently general to comprehend all forms of this relation between the organism and its medium, and yet sufficiently specific to convey an adequate idea of the relation; and that the word correspondence seems the least objectionable. The fact to be expressed in all cases is that certain changes, continuous or discontinuous, in the organism, are connected after such a manner that in their amounts, or variations, or periods of occurrence, or modes of succession, they have a reference to external actions, constant or serial, actual or potential—a reference such that a definite relation among any members of the one group, implies a definite relation among certain members of the other group.

§ 30. The presentation of the phenomena under this general form, suggests that our conception of Life may be reduced to its most abstract shape by regarding its elements as relations only. If a creature's rate of assimilation is increased in consequence of a decrease of temperature in the environment, it is that the relation between the food consumed and the heat produced, is so re-adjusted by multiplying both its members, that the altered relation in the environment between the quantity of heat absorbed from, and radiated to, bodies of a given temperature, is counterbalanced. If a sound or a scent wafted to it on the breeze prompts the stag to dart away from the deer-stalker, it is that there exists in its neighbourhood a relation between a certain sensible property and certain actions dangerous to the stag, while in its body there exists an adapted relation between the impression this sensible property produces, and the actions by which danger may be escaped. If inquiry has led the chemist to a law, enabling him to tell how much of any one element will combine with so much of another, it is that there has been established in him specific mental relations, which accord with specific chemical relations in the things around. Seeing, then, that in all cases we may consider the external phenomena as simply in relation, and the internal phenomena also as simply in relation; our conception of Life under its most abstract aspect will be—The continuous adjustment of internal relations to external relations.

While it is simpler, this formula has the further advantage of being somewhat more comprehensive. To say that it includes not only those definite combinations of simultaneous and successive changes in an organism, which correspond to co-existences and sequences in the environment, but also those structural arrangements which enable the organism to adapt its actions to actions in the environment, is going too far; for though these structural arrangements present internal relations adjusted to external relations, yet the continuous adjustment of relations cannot be held to include a fixed adjustment already made. Life, which is made up of dynamical phenomena, cannot be described in terms that shall at the same time describe the apparatus manifesting it, which presents only statical phenomena. But while this antithesis serves to remind us that the distinction between the organism and its actions is as wide as that between Matter and Motion, it at the same time draws attention to the fact that, if the structural arrangements of the adult are not properly included in the definition, yet the developmental processes by which those arrangements were established, are included. For that process of evolution during which the organs of the embryo are fitted to their prospective functions, is the gradual or continuous adjustment of internal relations to external relations. Moreover, those structural modifications of the adult organism which, under change of climate, change of occupation, change of food, bring about some re-arrangement in the organic balance, may similarly be regarded as progressive or continuous adjustments of internal relations to external relations. So that not only does the definition, as thus expressed, comprehend all those activities, bodily and mental, which constitute our ordinary idea of Life; but it also comprehends both those processes of development by which the organism is brought into general fitness for such activities, and those after-processes of adaptation by which it is specially fitted to its special activities.

Nevertheless, so abstract a formula as this is scarcely fitted for our present purpose. Reserving it for use where specially appropriate, it will be best commonly to employ its more concrete equivalent—to consider the internal relations as "definite combinations of simultaneous and successive changes;" the external relations as "co-existences and sequences;" and the connexion between them as a "correspondence."