With a very powerful stimulus, such as a strong chemical, this reaction makes the impression of being violent and disordered, as indeed may the reactions of a human being under similar conditions. But with a moderate stimulus the reaction

may be very delicate. This may be illustrated by the behavior of Paramecia within an area of water containing carbon dioxide. Part of the reaction under these conditions was described

Fig. 9. Diagram of the swinging of the anterior end about a large circle, in reacting to a strong stimulus. The revolution on the long axis has entirely ceased. in one of my earlier paper (JENNINGS, 1899, p. 331), though without a full appreciation of its real significance. The Paramecium, swimming slowly within the area of carbon dioxide, comes near to the edge of the area, where it receives water containing none of the gas in solution. This change acts as a very mild stimulus; the organism merely stops and swings its anterior end gently toward the aboral side, "trying" a new direction. If the water now received is still without the carbon dioxide, the Paramecium swings its anterior end still farther, at the same time continuing to revolve on the long axis, which changes the direction of swinging. As soon as the water it receives contains carbon dioxide, it swims ahead, changing its

course only when it again receives water without the gas in solution. The reaction under such conditions is a very delicate one, keeping the animal in close touch with the environmental conditions. The behavior does not impress one as a definite "reflex"; the Paramecium is seen merely to change its course a little after trying several slightly differing directions.

The behavior of Paramecium in swinging its anterior end about in a circle is essentially similar to the "feeling about,' "searching," or "trial" of a higher organism. We know, of course, no more of subjective qualities in any organism outside the self than we do in Paramecium. If we describe the "feeling about" or "searching" of any higher animal in a purely objective way, we shall find that the description takes essentially the same form as for Paramecium. Under certain conditions. the organism performs certain movements, which subject it to certain environmental changes. As long as the conditions remain of essentially the same character, it continues these movements. As soon as these movements induce conditions differing in a certain way, the movements stop. This description fits equally well the movements of a cat trying to escape from a cage (see THORNDIKE, 1898), of a dog searching for a bone, and of Paramecium reacting to carbon dioxide. In its method the behavior seems fundamentally similar throughout.

The behavior of Paramecia under such "repellent" stimuli follows then, perhaps, as effective a general formula as could be devised. When stimulated it performs movements which. take it away from the source of stimulus, and direct it successively in many ways, until the stimulation ceases. Reaction of this character is essentially that of "trial and error" as we find it in higher animals. From this standpoint the behavior may be summed up as follows: When there is "error" the organism "tries" various directions or methods of action till one is found in which the "error" ceases. These relations have been brought out by the author for lower organisms in general in a previous paper (JENNINGS, 1904, b).

We must ask here the question whether the reaction method of Paramecium above described should or should not

be called a reflex-a term which I have applied to it in previous papers. The question which interests us here is not whether an act performed without the intervention of a nervous system may properly be called a reflex; it may be strongly doubted whether the anatomical structure of organisms forms a proper basis for classification of types of behavior. But does the reaction method described fall in the concept of a reflex, judged merely as a type of behavior?

A reflex is commonly described as a fixed and invariable method of response to a definite stimulus. It is rare, however, that such definitions are found to be rigidly maintainable for given instances; the excellent discussion of HOBHOUSE (1901) shows how the reflex concept must be modified and its limits effaced, till it flows easily into other behavior types, before it can be applied to the phenomena actually found in animal behavior. Such a process of softening down is certainly necessary before we can make the reflex concept apply to the avoiding reaction of Paramecium. This reaction is composed of three factors, which may vary more or less independently of one another, in such a way that an absolutely unlimited number of combinations may result, all fitting the common reaction type. The possible variations may be expressed as follows: If the Paramecium be taken as a center about which a sphere is described, with a radius several times the length of the animal, then as a result of the avoiding reaction the Paramecium may traverse the peripheral surface of this sphere at any point, moving at the time either backward or forward. In other words, the reaction may carry it in any one of the unlimited number of directions leading from its position as a center. While the direction of turning is absolutely defined by the structure of the animal, yet the combination of this turning with the revolution on the long axis permits the animal to reach any conceivable position with relation to the enviroment. In other words, Paramecium, in spite of its curious limitations as to method of movement, is as free to vary its relations to the environment in response to a stimulus as an organism of its form and structure could conceivably be.

Again, the reaction at times keeps the organism in the closest possible touch with the environment, continuing as long as certain conditions continue, increasing in effectiveness as the conditions causing it increase in intensity, and ceasing when the conditions causing it cease, maintaining the organism throughout in certain relations with the source of stimulation. Altogether, I believe that the following admission must be made. If we consider the reaction of Paramecium a reflex, it is because we are convinced beforehand that such an organism can show only reflexes. If the actions of Paramecium did belong to some higher type of behavior, there could be little objective evidence of this, beyond what we already have.

In Paramecium the reaction has not been shown to be modifiable by previous experience, so that from this criterion the behavior retains the characteristics of a reflex. But in a close relative, Stentor, such modification by experience has been demonstrated (JENNINGS, 1902), so that it may be presumed that technical difficulties alone have thus far prevented our observing it in Paramecium.

The effectiveness of the method of reacting by "trial and error" that we have described above for Paramecium depends upon the power of discrimination of the reacting organism. By discrimination" of stimuli we mean, in an objective study of behavior, that the organism reacts differently to the different stimuli in question. In this sense Paramecium discriminates acids from alkalies and salts, and these again from sugar. Furthermore, it discriminates different strengths of solution, reacting differently, for example, with relation to weak and to strong acids. On the other hand, it does not effectively discriminate different acid substances, save in so far as one is stronger than another. Thus it swims into weak carbonic acid, which is harmless, and likewise into weak sulphuric acid and copper sulphate, which kill it. It does not markedly discriminate a ten per cent sugar solution from water, hence it swims readily into such a sugar solution and is killed by the osmotic action. Thus in re

Details as to the facts cited are given in my previous papers on Paramecium here we are concerned only with the interpretation of these facts.

gard to powerful acid substances and to sugar solution it makes what we would call in ourselves a "mistake." In higher animals we recognize that the power of accurate discrimination is one of the "higher" powers, becoming more secure as development progresses. We cannot, therefore, be surprised that it should not be perfect in so low an organism, nor that such organisms, through lack of discrimination of injurious and noninjurious agents, often react in a way that leads to their destruction. Any organism reacting by the method of trial and error" is subject to the possibility of destruction in some of the "trials."

This method of "trial and error," based on the "avoiding reaction" above described, plays a large part in the behavior of Paramecium. Through it are produced the "negative" reactions to agents of all sorts, as well as the collections formed in certain chemicals, in regions of optimum temperature, and the like. On the other hand, there exist certain reactions in which the final relation to the environment is brought about in a more direct way-notably "positive thigmotaxis" and certain features of the reaction to the electric current. These reactions will be taken up later.

II. NATURE OF STIMULATION.

Just what is the nature of the stimulation which produces this reaction by "trial and error" in Paramecium? An examination of the facts shows that as a general rule the effective stimuli consist of some change in the conditions, or, what is the same thing to the organism, of some change in the relation of the organism to the conditions. Change is the essential feature in producing the chief reactions of Paramecium.

This statement requires of course some qualification in detail. A change may be nearly instantaneous, while the consequent reaction of the animal of course requires time, and must, therefore, continue for a certain period after the change has been completed. If the animal is suddenly subjected to a onefourth per cent solution of common salt, it continues to react for a short time after the instant of the change, though if the