Page:Popular Science Monthly Volume 84.djvu/583

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T is customary to say that irritability, or the capability of responding to stimuli, is an essential characteristic of living beings. Whether this is true or not of the lowest organisms—certain bacteria or the filterable viruses—there is no doubt that it is preeminently so of the higher, and especially of those leading free and active lives, like most animals. If this were not the case it is difficult to see how such organisms could maintain themselves in their surroundings and continue to behave as living beings—i. e., show their characteristic activities, grow, and eventually reproduce themselves. At least it is clear that in order to do this they must react to the changes continually taking place in their particular environment in such a way as to favor their continued existence in that environment; if, for instance, any animal failed to respond to the presence of food—material that can serve it as source of energy—by capturing and incorporating enough to replace its own normal loss of substance, quite obviously its life would soon come to an end. And if it reacted in the same way to the poisonous or otherwise injurious substances in its surroundings as to food, and incorporated both classes of material indifferently, the same result would follow. Evidently there is needed some power of active and selective response to the changing conditions of the environment if the living organism is to continue to live; it must preserve a certain equilibrium with its surroundings; the materials and energy which it appropriates from those surroundings must in the long run at least equal those which it inevitably loses to them in the normal course of its vital processes. This is the physiological interpretation of Spencer's dictum that all life involves a continued adjustment between internal and external relations. The organism must continually alter its activities in correspondence with altered conditions in its surroundings, and in such a way as to preserve this adjustment—avoiding conditions likely to disturb or destroy the vital equilibrium and tending to place itself in those favorable to its continuance. Accordingly, we may say that living organisms in general, and especially animals, exhibit two broad classes of reactions, first those of a defensive or protective kind, including avoiding reactions and inhibitions of various kinds, and second, the more active group of what we may call self-seeking or acquisitive reactions; of these the chief are the reactions of food-seeking,