CHAPTER IV.
THE TWOFOLD CONDITIONING OF VITAL PHENOMENA. IRRITABILITY.
Appearance of internal activity of the living being—Vital phenomena regarded as a reaction of the ambient world.—§ 1. Extrinsic conditions—The optimum law.—§ 2. Intrinsic conditions—The structure of organs and apparatus—How experiment attacks the phenomena of life. Generalization of the law of inertia—Irritability.
Instability. Mutability. The Appearance of Internal Activity of the Living Being.—One of the most remarkable characteristics of the living being is its instability. It is in a state of continual change. The simplest of the elementary beings, the plastid, grows and goes on growing and becoming more complex, until it reaches a stage at which it divides, and thus rejuvenated it commences the upward march which leads it once again to the same segmentation. Its evolution is thus betrayed by its growth, by the variations of form which correspond to it, and by its division.
If it be a question of beings higher in organization than the cellular element the evolutionary character of this mutability becomes more obvious. The being is formed, it grows; then in most cases, after having passed through the stages of youth and adult age, it grows old, declines and dies, and is disorganized after having gone through what we may call an ideal trajectory. This march in a fixed direction with its points of departure, its degrees, and its termination, is a repetition of the path that the ancestors of the living being have already followed.
Here, then, is a characteristic fact of vitality, or rather there are two facts. The one consists in this morphological and organic evolution, the negation of immutability, the negation of the indefinite maintenance of a permanent state or form which is regarded, on the contrary, as the condition of inert, fixed stable bodies, eternally at rest. The other consists in the repetition, realized by this evolution, of the similar evolution of its ancestors; this is a fact of heredity. Finally, evolution is always in a cycle—that is to say, that it comes to an end which brings the course of things to their point of departure.
This kind of internal activity of the living being is so striking, that not only does it serve us to differentiate the living being from the inert body, but it gives rise to the illusion of a kind of internal demon, vital force, manifested by the more or less apparent acts of the life of relation, of the motricity, of the displacement, or by the less obvious acts of vegetative life.
Vital Phenomena regarded as a Reaction of the Ambient World. Their Twofold Conditioning.—In reality, as the doctrine of energetics teaches us, the phenomena of vitality are not the effect of a purely internal activity. They are a reaction of the environment. “The idea of life,” says Auguste Comte, “constantly assumes the necessary correlation of two indispensable elements:—an appropriate organism and a suitable environment. It is from the reciprocal action of these two elements that all vital phenomena inevitably result.” The environment furnishes the living being with three things:—its matter, its energy, and the exciting forces of its vitality. All vital manifestation results from the conflict of two factors: the extrinsic factor which provokes its appearance; the intrinsic factor, the very organization of the living body, which determines its form. Bichat and Cuvier saw in the phenomena of life the exclusive intervention of a principle of action entirely internal, checked rather than aided by the universal forces of nature. The exact opposite is true. The protozoan finds the stimuli of its vitality in the aquatic medium which is its habitat. The really living particles of the metazoan—that is to say, its cells, its anatomical elements—meet these stimuli in the lymph, in the interstitial liquids which bathe them and which form their real external environment.
Auguste Comte thoroughly understood this truth, and has clearly expressed it in the passage we have just quoted. Claude Bernard has fully developed it and given it its classical form.
In order to manifest the phenomena of vitality, the elementary being, the protoplasmic being, requires from the external world certain favourable conditions; these it finds there, and they may be called the stimuli, or extrinsic conditions of vitality. This being possesses no initiative or spontaneity in itself, it has only a faculty of entering into action when an external stimulus provokes it. This subjection of the living matter is called irritability. The term expresses that life is not solely an internal attribute, but an internal principle of action.