creased, swerve more rapidly toward the dorsal side and swim less rapidly forward, and then proceed as before but in a new direction. In course of this the anterior end of the body describes a circle while the posterior end moves but little and is virtually a pivot for the freer end. "The reaction which occurs when the illumination is changed is really an accentuation of a certain feature of the usual movements." Now if the change in illumination has been slight, the animal will slowly resume its forward movement swerving less noticeably toward the dorsal side, so that presently it will be moving along its former spiral course. But if the modified illumination is now too great, the animal while describing the pivotal movement will find its anterior (and more sensitive) end turned toward the light at some points of its circular course and away from it at others; since now the light is supraoptimal, the animal will cominence the forward movement while its anterior end is turned from the light, so that its new spiral course as it is gradually resumed will be directed at least somewhat away from the source of light. If this change in course is not sufficient to make the animal recede from the source of light, the animal will either tack again in the same way as before, or else change its orientation gradually but in a way very comparable with the previous. That is, the organism will swerve farther than normally toward the dorsal side and this will cause its anterior end to point successively in many different directions. In some of these positions the anterior end is directed more nearly toward the source of light, in other positions farther away from it. In the latter case the swinging toward the dorsal side becomes less marked; hence the succeeding phase of the swing, which carries the anterior end toward the light, is less pronounced; The anterior end therefore does not swing so far in the direction toward the light as in the preceding phase it swung away from the light. But as a result of such swerving as does occur the anterior end is now directed more away from the source of light than before. Thus this gradual change of orientation is effected by means of the same principles as the former, more sudden change. In either case the direction of the rays of light is not a determining feature, and the orientation is effected by a definite sort of "motor response."

This description applies to the movements of orientation of ciliates and flagellates in general. The ciliate studied was Stentor caeruleus; the flagellates were Euglena viridis, Cryptomonas ovata (sometimes called Chilomonas ovata), and a species of Chlamydomonas.

Third Paper. REACTIONS TO STIMULI IN CERTAIN ROTifera.
The author studies the Rotatoria, a group of metazoa much re-

sembling the infusoria in their mode of life, in order to find whether the reaction method of the metazoa differs radically in character from that of infusoria. The following free-swimming Rotifera, which progress through the water in the same manner as the ciliate infusoria, were studied; several species of the Rattulidae and of the Cathypnadae, two or three species of Euchlanis, Ploesoma lenticulare, Anuroea cochlearis, and Brachionus pala. The stimuli used were chemicals, heat, light, and the constant electric current. The reactions of all these Rotifera are essentially the same as those of the ciliate infusoria. Fourth Paper. THE THEORY OF TROPISMS.

This paper forms an admirable summary and theoretical discussion of the undoubtedly careful and exact observations described in the foregoing three papers. And it is to be said, further, that in each paper the author evinces the rare combination of qualities-a lively theoretical interest combined with the power of impartial observation.

The paper aims utterly to destroy the theory of the several "-taxes." Firstly, the theory of LOEB and others that the direction of ray (in light and electricity) is the determining factor in orienentation is untenable, because all the organisms studied react only by swerving toward a structurally defined side of their own bodies, even although this movement causes them to turn directly toward the rays that they seek to avoid. The theory that the intensity of stimulation effects orientation by a local effect on such parts of the body as it reaches is untenable, because in many, if indeed not most, organisms the anterior end is the only part sensitive enough to be effected by the stimuli (that is, the threshold of orientation is the threshold of stimulation of the anterior end); and because, as in the previous case, the response is invariably a perfectly definite "motor reaction" toward an anatomically defined side of the body of the organism. The defenders of the theory of tropisms will hardly call in question Professor JENNINGS' patently exact and unbiased observations, nor his methods, which are of the most careful, nor yet the range of organisms which he has chosen to study, which is far greater than that of any other individual experimenter. But they may possibly ask Professor JENNINGS what it is if not intensity of stimulation that causes the organisms, while circling on their anterior ends as pivots, to dart forward more or less suddenly at that point of the circle where the anterior end receives least of the undesirable stimulus; and if this cause is the intensity, whether his observations have not demonstrated a phototaxis of the anterior end. But even if this were to be the case, Professor JENNINGS would have effected a most important refinement on the old

theory, and contributed vastly to our knowledge of the behavior of lower organisms.

Fifth Paper. PHYSIOLOGICAL STATES AS DETERMINING FACTORS IN THE BEHAVIOR OF LOWER ORGANISMS.

In unicellular organisms as in multicellular the forms of reaction to stimuli exhibited clearly indicate the importance of internal factors (physiological states) as determining conditions. "To the same stimuli, under the same external conditions, the same individuals react at different times in radically different ways, (thus) showing the existence of different physiological states of the organism, which determine the nature of the reactions," p. 126. JENNINGS adduces numerous observations of the behavior of various unicellular organisms and flatworms in proof of the falsity of the prevalent notion that the reactions of these animals are determined almost wholly by external conditions. As a matter of fact all organisms, so far as reactions are concerned, are influenced by both external and internal conditions, and it is only because of the apparent invariableness of the reactions of the lower animals that they are so commonly spoken of as automata which are at the mercy of their surroundings in a quite different way than are the higher animals.

The paper serves the good purpose of calling attention to the fact that reactions are more often determined by the total state of the organism than by the local action of a stimulus.

Sixth Paper. THE MOVEMENTS AND REACTIONS OF AMBA.

This paper, like the earlier ones of the volume, rudely shakes the foundations of certain current theories of reaction, and it totally destroys certain explanations of the reactions of simple organisms which have been pretty generally accepted by physiologists. Unfortunately for the peace of mind of certain of the earlier investigators of the movements and reactions of Amoeba, the author was not satisfied to accept what had already been done in this field of research as the basis of his work. He chose instead to begin at the beginning and to attempt to verify the descriptions already given.

RHUMBLER and BÜTSCHLI state that in the normal locomotion of Amoeba there is a forward moving current of endosarc in the middle axis which flows outward at the anterior end of the organism, then backward along the surface. According to these physiologists, the protoplasmic currents within a moving Amoeba are essentially like the movements produced in a drop of water by lowering the surface tension at some point. The axial current moves toward the point of decreased tension. Consequently RHUMBLER and BÜTSCHLI Conclude

that the movements of Amoeba are due to changes in surface tension. JENNINGS, on the basis of careful observation of the movements of several species of Amoeba, states that there are no side or backward currents, but that the process occurs as follows: "In an advancing Amoeba substance flows forward on the upper surface, rolls over at the anterior edge, coming in contact with the substratum, then remains quiet until the body of the Amoeba has passed over it. It then moves upward at the posterior end, and forward again on the upper surface, continuing in rotation as long as the Amoeba continues to progress. The motion of the upper surface is congruent with that of the endosarc, the two forming a single stream " p. 148.

Accurate and detailed descriptions are given of the processes of formation and withdrawal of pseudopodia, and it is shown that contractility is a capacity of the ectosarc. "Altogether, then, our results lead us to look upon Amoeba as an elastic and contractile sac, containing fluid. In locomotion one side actively stretches out, becomes attached to the substratum, and draws the remainder of the sac after it in a rolling movement. The primary phenomena are the stretching out of one side, the elasticity, and the contractility of the outer layer" P. 172.

Under the subject of reactions, the author presents the results of extensive studies of the influence of different stimuli upon Amoeba. He distinguishes three classes of reaction: the positive, the negative, and a complex feeding reaction. Several interesting instances of the pursuit of food are cited in support of the author's opinion that the behavior is by no means so simple and uniform as is commonly supposed.

After redescribing the protoplasmic movements of Amoeba in the light of his own observations, the author discusses the physics of locomotion with results which may most fitly be expressed in his own words. "Putting all our results together we must conclude that the movements and reactions of Amoeba have as yet by no means been resolved into their physical components. Amoeba is a drop of fluid which moves in its usual locomotion in much the same way as inorganic drops move under the influence of similarly directed forces. But what these forces are is by no means clear. When we take into consideration the currents as they actually exist, local decrease in surface tension breaks down completely as an explanation for the locomotion and other movements. The locomotion taken by itself might be explained as due to the adhesion of the fluid protoplasm to solids,

taken in connection with the surface tension of the fluids, but this explanation fails when we consider the formation of free pseudopodia, and discover that all the processes concerned in locomotion can take place without adhesion to the substratum." 225.

Seventh Paper. THE METHOD OP TRIAL AND ERROR IN THE BEHAVIOR OF LOWER ORGANISMS.

In higher animals we speak of learning by the method of trial and error, and in unicellular organisms we may speak of the trial and error method of reaction. In the former instance the animal when first confronted by a certain situation exhibits a number of reactions; one of these reactions brings satisfaction and therewith the trial reactions cease. The next time the same situation is presented the animal, if it has learned, omits the useless or harmful trials and performs only the appropriate reaction. In the latter instance, namely, in case of unicellular organisms, each re-presentation of the situation brings about, so far as yet observed, the same sort of trial and error series of reacWhereas the mammals after a number of experiences omit the majority of the trials, the protozoa apparently have to go through the whole gamut of tests each time.

Trial may then be defined as the process of testing different portions of the environment, and error is descriptive of those trials which result in negative reaction. Now, the problem proposed by the author is, How is error distinguished by the organism? "In ourselves the stimuli which induce the negative reaction bring about the subjective state known as pain, and popularly we consider that the drawing back is due to pain. Is there ground for this view?" p. 248. This problem, JENNINGS insists, is one throughout the animal series.

Finally, the method of trial and error which the author believes to be the prevalent mode of reaction among lower organisms is not in agreement with the principles of reaction which are at the bottom of the tropism schema. For the tropism is a fixed way of reacting in response to a certain stimulus, and it has no place for the trials which have been demonstrated to occur.

Whether this is fair to the advocates of the tropism formulae for reactions would be hard to say. Certainly some of them will at once contend that JENNINGS by merely breaking up the stereotyped reaction. into a number of parts has not altered the nature of the reaction.

The author's insistance throughout this volume upon the importance of internal conditions in lower organisms is certainly justified by the facts he presents.