the infusoria, some change in the relation of the organism to the environment. It is usually, or perhaps universally, a change in the intensity of action of a stimulating agent on the body as a whole or on its most sensitive portion. The organism, having been in one condition, passes to another, and it is the transition that acts as the effective stimulation. In one of the two classes of reactions to be distinguished, the effective stimulation is due to the fact that as the organism progresses from one region to another it meets different conditions, and to the changes thus caused it reacts. This is the case under natural conditions with the reactions to mechanical stimuli, to heat and cold, to chemicals, to osmotic pressure, and to light when passing to a region of greater or less intensity of illumination. The cause of reaction is analogous to that in our own case in passing from a region of moderate temperature to a hot or cold region. The differences determining reaction are here arranged along the axis of the course, so that it is the backward or forward movement that brings them into action. The reaction is a change of movement such as to carry the organism successively in many directions-a series of trial movements. As soon as one of these movements carries the organism away from the stimulating agent that is, in such a direction that the changes to which it subjects the organism lead toward the optimum instead of away from it-the reaction ceases, since the cause for it has ceased. The organism therefore continues in that direction. The position of the body has little or no effect on the production of the change that causes stimulation, or on the release from stimulation. The organism might retreat from the stimulating agent backward or forward or sideways; in the one case as in the other it would be relieved of the stimulating changes. The different individuals may swim away directly or obliquely, their

alleled under any other conditions. As a result of this interference the reaction of the ciliates to the electric current takes in many respects a different character from the rest of their behavior. I wish therefore to emphasize the fact that the general relations set forth in the text do not apply to the reactions of the ciliates to this agent. Save for the forced cathodic reversal of cilia the response to the electric current would fall in our second class of reactions, as it actually does in the flagellates and rotifers.

various paths crossing at all sorts of angles; the only requirement is that the path shall on the whole lead away from the greatest (or least) intensity of the stimulating agent. Hence when many specimens react in this way their paths need not be parallel, and as a rule no marked orientation results. When a common orientation of many individuals does occur, it is produced through "exclusion, "-through the fact that under the given conditions movement in any direction but one causes the changes which act as effective stimuli, so that all are forced to move in that direction. The behavior of Oxytricha in reacting to heat, as shown in Fig. 7 of the author's paper on Reaction to Heat and Cold' will serve as a type for this reaction method. It is, so far as the infusoria are concerned, the basis for apobatic taxis (ROTHERT), phobotaxis (PFEFFER), and -pathy (DAVENPORT), and might farther be classed, from certain points of view, as kinesis or -metry.

The second class of reactions includes those in which the changes that act as stimuli are brought about by a swerving toward one side or the other, while the movements in the axis of progression have no such effect. In the infusoria movements from side to side are of course a regular part of the locomotion. In the reactions to the effects of water currents, of centrifugal force, of gravity, and of light rays coming from one side, the lateral movements of unoriented animals induce marked changes. In a water current or under the action of a centrifugal force, or of gravity, lateral movements meet with less resistance in one direction, greater resistance in another direction, and these changes of resistance act as stimuli. In light coming from one side the sensitive anterior end is more illuminated as the organism swerves towards one side; less illuminated as it swerves toward the other, and these changes act as stimuli. The reaction is the same as in the first class; the organism changes its course by a series of trial movements. It continues these movements till it comes into a position in which it is no longer subjected to the changes that act as stimuli. Such a position is found only when the axis of the course coincides with the direction of ac

Carnegie Institution of Washington, Publication 16, 1904, p. 16.

tion of the impinging force; in other word, when the organism is oriented. The anterior end will be directed toward the source of the stimulating agent or away from it, according as it is the decrease or increase in intensity that is the effective stimulus1. The transference of the organism along the axis of progress has in these cases no effect on the relation of the organism to the stimulating agent, so that so far as this reaction is concerned backward progression might take place as well as forward. Under these stimuli a certain orientation of the organism, not a certain direction of progress, is the essential result of the reaction. This class of reactions forms the basis, so far as the infusoria are concerned, for taxis proper (MASSART, DAVENPORT, etc.), strophic taxis (ROTHERT), and topotaxis (PFEFFER).

Thus in the first class of reactions the essential result is a certain direction of progression-toward a region of greater or of less intensity-while in the second class the essential result is the orientation. But in both classes the nature of the stimulus is the same and the reaction is the same. The stimulus is some change in the relation of the organism to the surrounding conditions-a change in the intensity of action of some agent. The response is a motor reaction that consists of a series of trial movements. The response continues in each case until the effective stimulus comes to an end, then the usual motion is resumed. In the first class this does not produce orientation (save sometimes by "exclusion"), because stimulation comes to an end without it. In the second class it does produce orientation, because stimulation does not come to an end without it. The difference between the two classes depends on the peculiar difference in the distribution of the stimulating agent, not upon different ways of reacting on the part of the organism. The reaction is by "trial" movements continued till a cessation of the effective stimulation is brought about; this will lead to orientation or not, just as the conditions require.

H. S. JENNINGS.

'For details as to this relation, see this Journal, 14, 1904, pp. 470, 472, and 478; Carnegie Institution, Publication 16, 1904, p. 60.

THE PROBLEM OF INSTINCT.

In the whole field now awaiting the student of mental life in animals, there is nothing of more cardinal interest and importance, both for animal psychology in itself and for general psychology, than the group of problems centering about the development of the conative life. And of these problems none promise results so significant in themselves or so generally illuminative of the mechanism of mind, as the problem of instinct. The question of the origin of instincts, ranks as a classic. Many prominent naturalists and psychologists have taken a distinctly anti-evolutional view of the matter, claiming intelligence and volition as the source of instincts. They make a good case; at least there are facts of introspection which point unmistakably in this direction. On the other hand, the facts at our command point to an evolution of mental processes as well as of physical structure. Instinct is clearly the conative life of lower forms. They seem to have no other conative life. Higher forms have both instinctive and intelligent conation. But it is still an open question whether the instinct is the result of the automatization of volitional processes, or whether it is the unconscious reaction which has been selected by the environment and is thus the precursor which prepares the way for volition.

The adaptation to the environment, through complex and yet very fundamental instincts, as well as the fact of habit formation in the individual life, make for the view that choice is a factor in the formation of instinct. On the other hand, the gen. eral order of evolution and development would place the simple, the instinct, before the more complicated volition. Volitions may be comparatively simple and instincts may be quite complex. But the general plan of the instinct marks it as an earlier form than the volition. We certainly have the indubitable evidence of introspection for this order in ontogenesis. And we see the same order in the mental development of the child. The same is true in the phylogenetic series; but the further we go from a ner

vous system like our own, the greater becomes the merely analogical element, and the less trust we can place in the inference.

But indisputable factual evidence can be brought to bear for the definite answering of this question of the origin of instinct as to whether instinct in its simplest lines originates through the automatization of volitional acts or not. That is, we can obtain definite data concerning the functional and genetic relations of instinctive and volitional processes in animals which have both. The method we propose is simply this:Take an animal with well defined but comparatively simple instincts. Make a careful study of its instincts, both as they come forward in the growing animal and as they function in the adult animal. Then devise a set of experiments, for the express purpose of testing the intelligence of the adult animal, its power and capacity for dealing with new conditions. In this way it will be quite definitely ascertained whether the present intelligence is such as would have sufficed for the determination of the behavior of the same animal which now constitutes its instinct. For it is a reasonable and necessary assumption that if instinct arose thus through the automatization of volitional acts, the present intelligence of the animal should be equal to dealing as efficiently with similarly complex situations. For the function of automatization is not to do away with intelligence by rendering it useless, but to make possible its further advance by providing for the ready performance of that which has been learned already. This is the way it does work in our own development.

Of course great care and skill will be required in devising the experimental means of this test and in carrying them out. Such criticisms as those made by Professor MILLS (Science, N.S., 191, 745.) of Dr. THORNDIKE in particular and of the labor

as he calls them, in general, must be kept carefully in mind. We must assure ourselves in every possible way, as by checks and counter-checks and all possible variations, that the stimulus is acting upon the animal in the way that we think it is, that it is really attending in animal fashion to the situation as we are thinking the situation, and that it is really feeling the